gravity, morphology, and bedrock depth of the rathdrum...

Gravity, Morphology, and Bedrock

Depth of the Rathdrum Prairie, Idaho

Guy W. AdemaRoy M. BreckenridgeKenneth F. Sprenke

Idaho Geological SurveyUniversity of IdahoMoscow, Idaho 83844-3014

Technical Report 07-2ISBN 1-55756-514-6

Contents

Abstract ...........................................................................................................................1Introduction .................................................................................................................... 1Geologic Setting ............................................................................................................. 2 Basement Rocks .............................................................................................. 2 Tertiary Geology ............................................................................................. 3 Quaternary Geology ........................................................................................ 4Previous Geophysical Investigations ............................................................................. 7 Seismic Surveys .............................................................................................. 7 Gravity Surveys ............................................................................................... 9Gravity Data Collection ................................................................................................. 9 Purves Data ..................................................................................................... 9 Cady and Meyer Data .................................................................................... 21 New Observations ......................................................................................... 21Gravity Data Reduction ............................................................................................... 23 Instrument Calibration Correction ................................................................ 23 Tidal and Instrument Drift Corrections ......................................................... 23 Terrain Corrections ........................................................................................ 24 Latitude and Elevation Corrections ............................................................... 25 Bouguer Corrections ..................................................................................... 25 Correlation of Data Sets ................................................................................ 25 Removal of Regional Trend .......................................................................... 26Gravity Data Modeling ................................................................................................ 27Gravity Model Interpretation ....................................................................................... 30Conclusion ................................................................................................................... 34Acknowledgments ........................................................................................................ 34References .................................................................................................................... 34

Figures

Figure 1. Location of the Rathdrum Prairie ................................................................... 2Figure 2 Maximum extent of the Cordilleran ice sheet ................................................. 4Figure 3. Maximum terminal extent of the Lake Pend Oreille lobe .............................. 5Figure 4. Locations of geophysical work performed on the Rathdrum Prairie ............. 7Figure 5. Seismic reflection profile ................................................................................ 8Figure 6. Locations of eight gravity measurements ..................................................... 26Figure 7. Bouguer anomaly map of the Rathdrum Prairie ........................................... 27Figure 8. Bouguer gravity map showing regional trend .............................................. 28Figure 9. Residual Bouguer gravity map of the Rathdrum Prairie .............................. 29Figure 10a. The Idaho Road (Washington) profile ....................................................... 30Figure 10b. The Corbin Road profile ........................................................................... 31Figure 10c. The Idaho Road (Idaho) profile ................................................................ 31Figure 10d. The Idaho Highway 41 profile .................................................................. 32Figure 10e. The Hayden Avenue profile ...................................................................... 32

Tables

Table 1. Principal Gravity Station Data .................................................................. 10-20Table 2. Principal Facts for Primary Benchmark .................................................... 22-23Table 3. Digital Elevation Models ............................................................................... 25Table 4. Modeled Aquifer Characteristics .................................................................... 33

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Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho

Guy W. Adema1

Roy M. Breckenridge1

Kenneth F. Sprenke2

ABSTRACT

The Rathdrum Prairie overlies part of a regional ground-water source, known as the Rathdrum Prairie-Spokane Valley aquifer, that covers 1,055 square km in Spokane County, Washington, and Kootenai and Bonner counties, Idaho. The aquifer is considered a sole-source water supply for the greater Coeur d’Alene and Spokane metropolitan areas. The 615-square-km part in Idaho is called the Rathdrum Prairie aquifer. It extends from Lake Pend Oreille south and west to the Idaho-Washington state line. The aquifer occupies a glacially scoured trough filled with highly permeable, coarse-grained, catastrophically deposited glacial outwash.

Five geologic cross-sections of the valley have been created using 630 gravity measurements, 146 of which were collected specifically for our study to complement existing data. The data were modeled in 2¾ dimensions, and the regional trend caused by crustal thickening to the east has been removed. The models present a generally smooth valley floor with an incised channel in the ancestral, subsurface valley in the western half of the prairie. The bedrock-sediment interface appears to be slightly sloped to the east with the deepest point between Idaho Road (Idaho) and Hayden Avenue where sediments may extend over 350 m below the surface. Westward, near the state line, the sediments appear to thin to a thickness of 216 m.

INTRODUCTION

The Rathdrum Prairie-Spokane Valley aquifer (Figure 1) is a valley-fill aquifer that extends from the southern end of Lake Pend Oreille, south to Coeur d’Alene Lake, and west to Spokane. The valley is bound by Mount Spokane to the north, the Mica Peak uplands to the south, and 1Idaho Geological Survey, University of Idaho, Moscow, ID 83844 2Department of Geological Sciences, University of Idaho, Moscow, ID 83844

the Coeur d’Alene Mountains to the east. The aquifer is identified as the sole-source water supply for the greater Coeur d’Alene and Spokane metropolitan areas. Our study will focus on the 615-square-km part in Idaho, the Rathdrum Prairie aquifer.

In 1978, the aquifer was designated “sole source,” which qualified it for protection under the 1974 Federal Safe Drinking Water Act. Since then, the area’s rapid growth in both population and irrigation demand continues to strain the aquifer’s capability. Consequently, to answer civic concerns about the aquifer requires reconstructing the broader geologic history of the region. This must begin with a clear understanding of the subsurface geology of the Rathdrum Prairie; however, developing an accurate predictive model of its structure has been difficult because of its complex history. Nonetheless, knowing what the limits are for this critical aquifer depends on scientific analysis that measures its actual extent, recharge potential, and future expectations. Determining the depth and geometry of the bedrock surface is the first step. Our study brings together new geophysical information and an improved interpretation technique to more fully analyze the basin geometry and its influence on hydrologic assumptions.

The Rathdrum Prairie aquifer is primarily composed of valley-fill deposits of glaciofluvial origin. Extensive gravels, deposited by catastrophic floods from Glacial Lake Missoula, fill the ancestral valley of the Rathdrum River (Breckenridge, 1989). The Missoula Floods, as these events are known today, were caused by the periodic impoundments and sudden releases of water from Glacial Lake Missoula, which was reestablished many times during the Pleistocene when the Pend Oreille lobe of the Cordilleran ice sheet repeatedly blocked the Clark Fork River (Bretz and others, 1956; Bretz, 1959). The latest

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Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, IdahoAdema, Breckenridge, and Sprenke

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periods of lake-emptying cycles occurred from 12,000 to 17,000 years ago (Waitt and Atwater, 1989). Proglacial and flood processes deposited clasts with lithologies derived from local terranes that include the Precambrian Belt Series, the Cretaceous and Tertiary plutons and associated rocks, the lower Paleozoic sediments, and the Miocene basalt (Breckenridge, 1997). Some evidence suggests the aquifer overlies the Latah Formation, which is characterized by thick units of shale and clay with some sands and gravels (Newcomb and others, 1953; Pardee and Bryan, 1926). The Latah beds are thought to lie unconformably over pre-Tertiary sediments, metasediments, and granitic rocks. Intercalated with the Latah Formation, and near the margins of the aquifer, lies Miocene basalt of the Columbia River Basalt Group (Breckenridge and Othberg, 1998a and 1998b).

Our study employs gravity techniques to determine the subsurface geology. Previous gravity surveys (Purves, 1969; Hammond, 1974; Cady and Meyer, 1976a) and seismic refraction and reflection studies (Newcomb and others, 1973; Gerstel and Palmer, 1994) have focused on relatively small sections of the aquifer and do not produce an overall subsurface view of the Rathdrum Prairie. Our study measured the gravity at 146 new locations on the prairie to complement the same number of existing ones.

These new data target coverage to specific profiles and validate the previous work. The combined data set was reduced and modeled with techniques not applied in the earlier studies.

The thickening of the continental crust beneath the northern Rocky Mountains (Winston and others, 1989; Harrison and others, 1972) east of the Rathdrum Prairie imposes a regional trend on gravity data. The regional gradient is steep and obscures near-surface contributions to the gravity field. The effects of this gradient were not incorporated in the most extensive previous gravity survey of the prairie (Purves, 1969). The models developed in our investigation account for the regional trend and, as a result, more accurately define the basin geometry.

GEOLOGIC SETTING

BASEMENT ROCKS

The country rocks surrounding, and presumably underlying, the Rathdrum Prairie are mostly Precambrian, but intrusions as young as Eocene are also present. These pre-Miocene igneous and metamorphic units are described in Lewis and others (2002). The units include low-grade metasedimentary rocks of the

Figure 1. Location of the Rathdrum Prairie, Idaho (from Wyman, 1994).

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Belt Supergroup and high-grade (amphibolite facies) metamorphic rocks whose protolith is either the Belt Supergroup or the basement rocks that predate the Belt metasedimentary rocks. The high-grade rocks are exposed in the Priest River metamorphic core complex west of the Rathdrum Prairie. Deformed granitic rock (orthogneiss) of probable Cretaceous age is included in the core complex. Plutonic rocks of Cretaceous age are also present as intrusions within the low-grade Belt Supergroup. Relatively undeformed Eocene igneous rocks are exposed as plutons northwest of the Rathdrum Prairie. A few Eocene rhyolite and dacite dikes are also present.

The pre-Miocene igneous and metamorphic units generally act as an aquiclude and constitute the impervious basement to the Rathdrum Prairie aquifer. Because of their complexity and local variation, the pre-Miocene rocks are treated as one unit for gravity modeling purposes. Differentiating the various host rocks and intrusions and assigning unique physical properties to each require more comprehensive data than are available.

TERTIARY GEOLOGY

The rocks of Tertiary age include the Latah Formation and the Columbia River Basalt Group. These rocks were deposited on a mature erosional surface that developed during the Late Cretaceous and Early Tertiary. The geography included ridge crests and mountain tops with gently rounded forms (Molenaar, 1988) and rugged canyons with depths exceeding 600 m (Connors, 1976). The west-flowing, Early Tertiary, ancestral Rathdrum River occupied a valley under the present-day Spokane Valley and Rathdrum Prairie (Savage, 1967).

During the Miocene, flows of the Columbia River Basalt Group spread northeastward from the Columbia Plateau and filled these deep canyons. The basalt dammed drainageways, including the Rathdrum River, and these dams allowed lakes to form in which the sand, silt, and clay beds of the Latah Formation were deposited. The Latah sediments consist predominantly of white, yellow, orange, and brown lacustrine silt and clay, along with some fluvially deposited sand and gravel units (McKiness, 1988). The older basalt flows did not extend to the eastern and northern Rathdrum Prairie, and a relatively thick section of Latah Formation is believed to have formed at these locations, as evidenced in three water-well logs of Hammond (1974). Pardee and Bryan (1926) suggest that over 500 m of Latah sediments accumulated in the Spokane area. Such deposits probably extended throughout the Lake Pend Oreille basin, up the

Clark Fork Valley, and perhaps into Montana (Conners, 1976).

Younger Miocene basalt flows eventually overrode the entire Rathdrum Prairie region. The separate events and long interludes created alternating layers of basalt and Latah Formation interbeds, as observed by Hammond (1974) in well sections. The Latah sediments were deposited in lakes formed where the basalt flows impounded westward-flowing drainage systems. Kiver and Stradling (1989) suggest that basalt and Latah sediments filled the valley to a present-day elevation of 730 m. Breckenridge and Othberg (1998a, 1998b) delineate where these younger basalt flows filled the embayments of the region. The increasing elevation of basalt flows in the Miocene caused lake levels to the east to rise accordingly. Eventually, levels were high enough to force the drainage pattern to alter from a westerly to a northerly direction along the margin of the basalt (Connors, 1976; Savage, 1965). The new drainage flowed north along the Purcell Trench through what is now Lake Pend Oreille and then westward to the Columbia River. The Latah sediments of the Rathdrum-Spokane valley may have been completely covered by one or more basalt flows during the final extrusions of the Columbia River basalt.

The downcutting from the late Miocene to the early Pleistocene removed as much as 180 m of Latah sediments (Anderson, 1927). Developing drainages probably eroded much of the exposed Latah beds and some of the marginal basalt. The Coeur d’Alene drainage developed along the basalt marginal valley now occupied by Coeur d’Alene Lake. Those sediments, protected by overlying basalt flows, were largely unaffected by this erosional cycle, but a substantial amount of basalt may have been eroded during this time. The thickness of remaining Latah sediments is difficult to estimate owing to the cover of Pleistocene drift and the scarcity of evidence from boreholes. So far, researchers have not determined the extent of the Latah sediments. Anderson (1940) discovered a 300-m-thick bed of Latah Formation below an exposed basalt flow in a well west of Hayden Lake. Similarly, seismic velocities, intermediate to those of bedrock and gravels, were interpreted by Newcomb (1953) to represent Latah Formation near the state line. He estimated the thickness of these sediments to be 300 m.

The Missoula Floods eroded much of the Columbia River Basalt Group and Latah Formation, significantly altering the geologic features that had developed during the Miocene. Many Tertiary geologic and geomorphic features were either destroyed or obscured.

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QUATERNARY GEOLOGY

The glacial and interglacial periods began between 2 Ma and 3 Ma in this region. The Cordilleran glacier complex was the dominant geomorphic force during the Pleistocene in North America. The epoch began with high alpine glaciers in the Cascades, Coast Range, and northern Rocky Mountains. As these glaciers coalesced in major valleys, the thickening ice eventually formed a massive sheet at least 2,200 m thick, over 3,400 km long, and over 800 km wide (Conners, 1976). A series of massive glacial lobes occupied the major north-south trending valleys in northern Idaho and produced the primary landscaping event of the period. Erosion by the advancing ice lobes deepened valleys, smoothed bedrock

exposure, and removed large amounts of rock material from the valleys extending north of the main Rathdrum Prairie (Conners, 1976; Savage, 1964).

Determining the southernmost extents to the various lobes of the Cordilleran ice sheet is difficult owing to the lack of exposed glacial and proglacial features. Richmond (1986) has compiled the stratigraphy and chronology of well-documented glacial deposits. The current consensus on the southernmost margin of the ice sheet is shown on Figure 2, as summarized in Richmond (1986) and modified by Breckenridge (1989). The locations shown are revisions of previous estimates taken from numerous studies of Quaternary deposits throughout the northwest.

Figure 2. Maximum extent of the Cordilleran ice sheet (Breckenridge, 1989). BRL = Bull River lobe; FL = Flathead lobe; PTL = Purcell Trench lobe; PRL = Priest River lobe; TRL = Thompson River lobe. Crosshatched area = Glacial Lake Missoula. Arrows show routes of major flood outbursts.

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Of interest to our study is the Lake Pend Oreille lobe, sometimes referred to as the Rathdrum lobe. This tongue of the Cordilleran ice sheet extended southward between the Selkirk Range and Cabinet Mountains. Flint (1937) and Bretz and others (1956) originally mapped the Lake Pend Oreille lobe well onto the Columbia Plateau. Weis and Richmond (1965) extended it through the Rathdrum Prairie-Spokane Valley to the present location of Spokane. A more refined estimate of the southern extent of the lobe is shown in Figure 3, taken from Breckenridge (1989) who mapped it as extending only to the present southern shore of Lake Pend Oreille.

Oscillations of the Lake Pend Oreille lobe significantly affected the formation of the Rathdrum Prairie-Spokane Valley aquifer. Most sediments in the valley are thought to have been deposited catastrophically. With each major advance of the lobe in late Wisconsin time, an ice dam formed at Clark Fork and impounded the Clark Fork River to create another glacial Lake Missoula. The ice dams eventually became unstable, producing periodic failures and recurrent, sudden flooding of the downstream reaches. With each failure, as much as 2.1 x 107 m3sec-1 of water was released (Baker, 1973). These events are now referred to as the Missoula Floods. The

Figure 3. Maximum terminal extent of the Lake Pend Oreille lobe of the Cordilleran ice sheet (from Breckenridge, 1989).

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water released from Lake Missoula flowed forcefully across the Rathdrum Prairie, scoured the region now known as the Channeled Scabland, and continued down the Columbia River to the Willamette Valley and the Pacific Ocean.

Evidence for the Missoula Floods was introduced by J Harlan Bretz in the 1920s (Bretz: 1923, 1925, 1928a, 1928b, 1928c, 1930a, 1930b, 1932). Bretz identified erosional and depositional features in the Channeled Scabland. For him, the only explanation for these unique landforms was a relatively brief, but enormous flood, which he called the Spokane Flood. Bretz investigated such examples as erratic boulders, deeply notched cliffs, streamlined loess hills, gravel deposits, and the Portland delta.

Scientists were slow to accept his thesis. For years, most denounced the idea as outrageous and physically impossible. Prominent among the old guard attacking this perceived heresy was R.F. Flint (1935, 1936, 1938) who, in rebuffing Bretz, decreed the scablands merely the result of normal proglacial discharge. The controversy finally prodded J.T. Pardee to show Bretz his own work on a large Pleistocene lake in western Montana. Pardee (1910) had been studying this evidence of a flood source, which he named Lake Missoula, before Bretz presented the idea of massive floods. Finally, Pardee (1942) published his work on glacial Lake Missoula. The paper strongly complemented Bretz’s great flood hypothesis and represents a much-cited example of how the collaboration of evidence advances ideas in science. Both men agreed that the Lake Pend Oreille lobe of the Wisconsin ice sheet had impounded a huge lake and that the lake had drained catastrophically. Later, this idea would be enlarged to include several episodes of ice dams and floods.

Evidence described by Pardee (1942) included severely scoured constrictions in the lake basin, huge bars of current-transported debris, and giant ripple marks with heights of 16 m and spacings of 160 m. Bretz and others (1956) and Bretz (1959) provided revisions to his original work that suggested several flood episodes. Chambers (1971, 1984) followed with detailed descriptions of the sedimentation cycles of the rhythmically bedded floor sediments in the Clark Fork valley. He interpreted them as evidence of multiple filling and flooding sequences.

Present work is looking at whether sedimentary variations between rythmite beds represent distinct flood events or are merely products of different energy levels from a single event, as well as at the timing of each depositional feature (Breckenridge and Othberg,

1998c). The most recent flood events are thought to have occurred between 17,200 and 11,000 years ago (Waitt, 1985). The number of flooding episodes is unknown, but at least fifty have been associated with depositional rythmites (Waitt, 1980). Other researchers have found evidence for numerous episodes in other localities (Sieja, 1959; Chambers, 1971, 1984; Breckenridge and Othberg, 1998c).

Several theories compete to explain the catastrophic ice dam failures and have been summarized by Breckenridge (1989). Those theories include jökulhlaup releases (Waitt, 1985), flotation of the ice (Thorarinsson, 1939), the enlargement of subglacial tunnels by water (Liestol, 1956), the deformation of ice by water pressure (Glenn, 1954), and the enlargement of tunnels by icebergs (Aitkenhead, 1960). Whatever the mechanism of ice-dam failure, the geomorphic expression of the Missoula Floods has been well documented and serves as the most recent significant event affecting the composition of the Rathdrum Prairie-Spokane Valley aquifer.

The widely accepted geologic model of the Rathdrum Prairie has the ancestral Rathdrum River valley first being filled with Miocene basalt and Latah sediments and then with Missoula Flood deposits. The Missoula Floods eroded much of the basalt and Latah sediments and also obscured their exposures with extensive flood deposits. These flood deposits are principally composed of fine to coarse gravels of glaciofluvial origin derived from glacial outwash of the Purcell Trench lobe. The provenance of the gravels includes Precambrian Belt Supergroup rocks, Tertiary and Cretaceous plutons, lower Paleozoic sediments, and Miocene basalt (Breckenridge and others, 1997).

Coarser gravels are found centrally in the valley, and finer sands and gravels are near the margins. Some of the sands and gravels are classified as eddy and pendant bar deposits (Breckenridge and Othberg, 1998a and 1998b). The high-energy depositional environment resulted in cross-bedded gravel deposits with intercalated layers of finer sands and clays. Discriminating these sand and gravel layers with gravity methods is impractical, but their existence is visible in local gravel pits. The gravel structure is further complicated by the occasional occurrence of clast cementation (Breckenridge and others, 1997). A primarily calcium carbonate cement varies in development, from minimal clast rinds to near complete matrix filling. The cement was found to have fine amounts of angular silica, perhaps carried downward by water infiltrating through surface ash. The coatings, which must have originally formed in the zone of a fluctuating water table, are now found in the vadose

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zone. The cemented gravels and intercalated fine beds provide complicated hydrologic characteristics in the aquifer that may adversely affect hydrologic modeling.

The gravels are mantled with thin volcanic ash and loess deposits. The distribution of these eolian sediments varies greatly and is difficult to quantify. Deposits seldom reach a meter in thickness and in most places only a few centimeters (R.M. Breckenridge and K.L. Othberg, oral commun., 1998). Some of the material is thought to percolate into the matrix of the underlying gravels. The surficial geologic mapping of Breckenridge and Othberg (1998a and 1998b) did not include ash and loess.

PREVIOUS GEOPHYSICALINVESTIGATIONS

Geophysical data from two previous studies have been included in our research to create a more complete model of the subsurface geology of the Rathdrum Prairie. We incorporated 484 gravity measurements

from Cady (1976) and Purves (1969) that complemented ours. Results from other geophysical investigations also helped in clarifying the subsurface conditions and in developing the model.

SEISMIC SURVEYS

Newcomb and others (1953) completed two seismic refraction surveys near Spokane, Washington, in May and June of 1951 to locate the base of the glacial outwash aquifer. This geophysical investigation of the Rathdrum Prairie aquifer obtained stratigraphic and hydrologic information about the Spokane Valley. The study also sought to determine the type of material underlying the aquifer and to locate the bedrock of the ancestral valley. One survey trended north-south across the Spokane Valley east of the Idaho-Washington border; the other trended east-west across the Hillyard trough, north of Spokane. The survey near the Washington-Idaho border (Figure 4) provided the primary bedrock “ties” for the gravity modeling in our report.

Figure 4. Locations of geophysical work performed on the Rathdrum Prairie.

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Newcomb and others (1953) based their interpretation on limited reverse seismic refraction data. Their few refractions indicated a V-shaped valley with a maximum aquifer thickness of about 480 m where glacial outwash contacts the Latah Formation. The contact corresponds to an elevation of about 530 m and occurs with the inference of Pardee and Bryan (1926) about bedrock depth in this region. Newcomb and others (1953) interpreted five subsurface units from their refractions: (1) soil and subsoil of the glacial outwash, (2) unsaturated glacial outwash, (3) saturated glacial outwash, (4) Latah Formation with intercalated igneous rocks, and (5) granitic rock. The velocity obtained for the Latah Formation was so high that Newcomb and others (1953) reasonably presumed that basalt sills and dikes were present. These sills and dikes have also been found in the Latah Creek vicinity (Pardee and Bryan, 1925) and in the highway cuts southeast of Coeur d’Alene (Conners, 1976). Newcomb and others (1953) were limited by the technology of

their time in the amount of data that could be obtained and interpreted. The geologic interfaces shown in their model were based on only a few data points.

Seismic reflections on the Rathdrum Prairie by Gerstel and Palmer (1994) followed the lines of Newcomb and others (1953) on Idaho Road (Washington) so that results of the two studies could be compared (Figure 4). (Because Idaho Road lies in both Washington and Idaho, its location will be identified with the state name following in parentheses.) Their unconventional technique used a single point, zero-offset shot-receiver procedure and a pneumatic acoustic source. They claimed to find an undulating bedrock surface 160 m deep, with an average bedrock-sediment interface at about 475 m above sea level. Their interpretation revealed a much more U-shaped valley floor than previously thought (Figure 5). The velocities used for reflection processing were obtained from cross-hole seismic techniques and

Figure 5. Seismic reflection profile (top) and interpretation (bottom) of Gerstel and Palmer (1994). An unconventional single point receiver technique, with a pneumatic acoustic source, was used.

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were unavailable for us to compare with the refraction velocities measured by Newcomb and others (1953).

GRAVITY SURVEYS

Bonini (1963) conducted the first regional gravity study of Idaho and produced a reconnaissance Bouguer anomaly map of the state using over 1,200 observations. The study was part of one on gravity anomalies, isostatic equilibrium, and tectonic features of the northwestern United States. Bonini correlated the Idaho batholith with negative Bouguer anomalies (as low as -236 mGals), the Snake River Plain with a broad gravity high, and the Columbia River Plateau as a relative gravity high. In northern Idaho and western Montana, he identified a modest variation in the Bouguer anomaly that generally followed a north-south trend. Bonini concluded that the pattern reflected the broad structural fabric and variations in density values of different members of the Belt Supergroup rocks. The survey was not detailed enough to identify individual features or units that the gravity variations were attributed to. Though no subsurface geological modeling was performed, Bonini (1963) laid the groundwork for future gravity work in Idaho.

Purves (1969) performed an extensive gravity survey of the Spokane Valley-Rathdrum Prairie to identify subsurface stratigraphic units, possible areas of underflow impedance, and glacial conditions. Over 743 gravity measurements were taken on 16 profiles in Idaho and Washington. All standard corrections, including terrain, were made, and measurements were tied to the extended gravity control network of North America established by Wollard and Behrendt (1961). The survey by Purves (1969) provides the starting point for our study, and selected data from that work are modeled with ours.

A significant finding of Purves (1969) was the existence of a probable west-northwest trending subsurface drainage divide within the Rathdrum Prairie basin about 3.2 km west of the Washington-Idaho state line. He suggests that this drainage divide was produced by the terminal position of the maximum glacial advance of a proposed Hayden Lake lobe and the recognized Pend Oreille lobe. He claimed that the subsurface configuration east of the divide, in the Rathdrum Prairie, appeared to be dominantly influenced by glacial erosive processes, with the U-shaped trough filled almost exclusively with glacial material and minimal basalt. He cites irregular geohydrological evidence near the proposed flow divide that suggests the ultimate glacial terminus existed west of this divide. His survey was inconclusive about the existence of any Latah Formation in the Rathdrum Prairie section. West of this divide, in the Spokane Valley section

of the aquifer, he claimed that the basement configuration appeared to be a fluvially dissected erosional valley with erosional terraces composed of either a complex basement or basalt.

Purves (1969) never numerically modeled his data because of the difficulty in running computations before the common use of computers. Instead, he relied on the simple supposition that, in a relatively simple geologic system, the Bouguer gravity field will generally mimic the underlying bedrock shape. The data from his study were used extensively in our study and provide the effective starting point for our investigation.

In 1969, Hammond (1974) began a gravity survey and subsequent remodeling of the northern Rathdrum Prairie. This U.S. Geological Survey study, undertaken in cooperation with the then Idaho Department of Water Administration, evaluated previous estimates of (1) the quantity of underflow moving toward the Rathdrum Prairie from the Athol area across a line extending about 10 km northwestward from Chilco and (2) the quantity of water being recharged to the aquifer by Lake Pend Oreille. A detailed gravity survey was conducted to define the configuration of the bedrock surface and to calculate the thickness of fill from the southern end of Lake Pend Oreille south to the three Chilco channels, which were the focus of the study. Unfortunately, details about the gravity data reduction and modeling were not included in the report. Depth and Bouguer anomaly contours produced by Hammond (1974) only overlap the very northern section of the region encompassed by our study, but show a trend that continues to Lake Pend Oreille.

GRAVITY DATA COLLECTION

Our investigation combines gravity data from three main sources: Purves (1969); Cady and Meyer (1976), which includes the data of Bonini (1963) and Hammond (1974); and our own measurements taken in 1997. Data collected for our study was carefully planned to fill gaps in the coverage of previous studies. The combined result is an extensive data set that allows for more comprehensive modeling and interpretation. The three data sets are assimilated as a final numerical adjustment. Principal facts for all gravity data are given in Table 1.

PURVES DATA

In 1966, Purves (1969) collected over 743 gravity measurements on transects of the Rathdrum Prairie and Spokane Valley. Of these, 276 stations on 5 profiles were used in our study (Figure 4). Though Purves did the

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Field descriptions:

Count Arbitrarily assigned. Point_ID Descriptive identification for each point. Alpha prefix describes data source. A = Adema, P = Purves (1969), D = Cady and Meyer (1976a) DD_lat Degree-decimal latitude, WGS84 geiod. DD_long Degree-decimal longitude, WGS84 geiod. Easting UTM zone 11 easting value, presented in meters.

Northing UTM zone 11 northing value, presented in meters. Elevation Elevation above MSL, presented in meters. Obs_Grav Observed station gravity, presented in mGal. Corrected only for drift. C.B.A._1 Bouguer Anomaly, ρ=2.67 kg/m3.

C.B.A._2 Residual Bouguer anomaly, ρ=2.67 kg/m3, regional trend correction of 0.11 mGal per km east.

Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 21 AB285 47.8022 -116.9156 506,323.36 5,294,103.18 665.1 980,649.10 -92.38 -4.422 AG601 47.6770 -116.7979 515,179.26 5,280,195.86 651.7 980,632.59 -100.83 -3.133 AG602 47.6773 -116.7914 515,658.67 5,280,227.97 651.0 980,631.27 -102.33 -4.104 AG603 47.6773 -116.7881 515,908.83 5,280,228.65 659.1 980,629.50 -102.51 -4.015 AG604 47.6773 -116.7805 516,471.92 5,280,230.43 667.8 980,627.90 -102.42 -3.306 AG605 47.6772 -116.7762 516,805.57 5,280,231.24 668.7 980,627.50 -102.66 -3.187 AG606 47.6772 -116.7723 517,097.47 5,280,232.29 669.0 980,627.70 -102.38 -2.578 AG607 47.6772 -116.7673 517,472.87 5,280,233.17 665.5 980,628.60 -102.13 -1.909 AG608 47.6772 -116.7618 517,889.71 5,280,234.41 665.3 980,629.14 -101.59 -0.9110 AG609 47.6769 -116.7567 518,265.29 5,280,204.81 662.4 980,630.56 -100.65 0.4511 AH101 47.7593 -116.9584 503,122.67 5,289,315.06 659.9 980,643.52 -95.81 -11.3812 AH102 47.7593 -116.9533 503,497.46 5,289,315.33 661.7 980,642.02 -96.97 -12.1213 AH103 47.7594 -116.9479 503,913.80 5,289,346.43 662.0 980,641.51 -97.44 -12.1314 AH104 47.7593 -116.9428 504,288.48 5,289,315.92 662.4 980,641.16 -97.72 -12.0015 AH105 47.7593 -116.9370 504,725.73 5,289,316.28 661.1 980,641.81 -97.33 -11.1316 AH106 47.7593 -116.9316 505,121.23 5,289,316.57 667.5 980,640.33 -97.56 -10.9317 AH107 47.7592 -116.9264 505,516.74 5,289,316.87 675.3 980,637.82 -98.55 -11.4818 AH108 47.7592 -116.9211 505,912.25 5,289,317.46 679.4 980,636.44 -99.15 -11.6419 AH109 47.7592 -116.9159 506,307.76 5,289,317.76 684.4 980,635.13 -99.47 -11.5320 AH110 47.7592 -116.9128 506,536.89 5,289,318.11 682.2 980,635.39 -99.65 -11.4621 AH111 47.7592 -116.9075 506,932.40 5,289,318.40 679.7 980,635.15 -100.40 -11.7822 AH112 47.7591 -116.9038 507,202.98 5,289,318.81 680.9 980,634.33 -100.98 -12.0623 AH113 47.7590 -116.9007 507,432.00 5,289,288.39 683.0 980,633.45 -101.45 -12.2824 AH114 47.7590 -116.8930 508,014.91 5,289,288.96 685.9 980,632.25 -102.08 -12.2625 AH115 47.7590 -116.8885 508,347.95 5,289,289.47 688.5 980,631.63 -102.16 -11.9826 AH116 47.7590 -116.8839 508,702.01 5,289,290.01 689.6 980,631.19 -102.41 -11.8427 AH117 47.7589 -116.8788 509,076.50 5,289,290.57 688.4 980,630.92 -102.93 -11.9428 AH118 47.7589 -116.8722 509,576.21 5,289,291.33 687.3 980,630.57 -103.49 -11.9529 AH119 47.7589 -116.8669 509,971.72 5,289,291.93 686.6 980,630.34 -103.86 -11.8930 AH120 47.7590 -116.8621 510,325.78 5,289,292.78 687.5 980,629.51 -104.53 -12.1731 AH121 47.7590 -116.8564 510,763.04 5,289,293.44 689.5 980,628.31 -105.33 -12.4932 AH122 47.7590 -116.8506 511,199.98 5,289,294.40 692.2 980,627.49 -105.64 -12.3233 AH123 47.7590 -116.8441 511,678.98 5,289,295.13 692.9 980,627.21 -105.77 -11.9234 AH124 47.7590 -116.8390 512,074.48 5,289,296.04 697.5 980,626.04 -106.04 -11.7635 AH125 47.7589 -116.8334 512,490.71 5,289,296.97 697.6 980,625.15 -106.90 -12.1636 AH126 47.7589 -116.8295 512,782.31 5,289,297.41 699.2 980,624.51 -107.22 -12.1637 AH127 47.7589 -116.8257 513,052.89 5,289,298.13 701.0 980,623.76 -107.62 -12.2638 AH128 47.7588 -116.8213 513,385.93 5,289,298.94 702.0 980,623.47 -107.70 -11.9839 AH129 47.7588 -116.8172 513,698.25 5,289,299.72 702.0 980,623.40 -107.77 -11.7040 AH130 47.7587 -116.8133 513,989.74 5,289,269.38 701.0 980,623.48 -107.88 -11.4941 AH131 47.7587 -116.8085 514,343.81 5,289,270.23 700.4 980,623.20 -108.26 -11.48

Table 1. Principal Gravity Station Data.

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11


Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 242 AH132 47.7587 -116.8050 514,614.39 5,289,270.94 700.7 980,622.81 -108.60 -11.5343 AH133 47.7587 -116.7996 515,009.89 5,289,272.15 698.5 980,623.14 -108.68 -11.1744 AH134 47.7587 -116.7937 515,467.86 5,289,273.15 699.3 980,622.91 -108.74 -10.7245 AH135 47.7586 -116.7913 515,634.53 5,289,273.71 699.2 980,623.02 -108.63 -10.4346 AH136 47.7587 -116.7884 515,863.36 5,289,274.36 697.2 980,623.54 -108.51 -10.0647 AH137 47.7587 -116.7856 516,071.47 5,289,274.98 694.8 980,624.05 -108.46 -9.7848 AH138 47.7587 -116.7825 516,300.61 5,289,275.63 693.2 980,624.16 -108.65 -9.7249 AH139 47.7587 -116.7786 516,591.91 5,289,276.37 695.0 980,623.77 -108.65 -9.4050 AH140 47.7587 -116.7754 516,841.77 5,289,277.06 693.7 980,624.17 -108.50 -8.9851 AH141 47.7586 -116.7724 517,049.88 5,289,277.68 689.0 980,624.96 -108.58 -8.8252 AH142 47.7587 -116.7681 517,383.22 5,289,278.80 690.3 980,624.54 -108.73 -8.6153 AH143 47.7585 -116.7654 517,591.34 5,289,279.42 698.2 980,622.89 -108.79 -8.4454 AH144 47.7585 -116.7630 517,757.71 5,289,279.67 699.4 980,622.93 -108.50 -7.9755 AH145 47.7585 -116.7607 517,924.38 5,289,280.22 699.4 980,623.00 -108.42 -7.7056 AH146 47.7586 -116.7577 518,153.52 5,289,280.88 699.3 980,623.21 -108.19 -7.2357 AH147 47.7586 -116.7566 518,236.70 5,289,281.31 699.3 980,623.15 -108.22 -7.1658 AM301 47.7767 -116.7781 516,628.05 5,291,283.22 700.8 980,622.32 -110.62 -11.3359 AM302 47.7729 -116.7782 516,608.22 5,290,850.81 698.3 980,622.57 -110.51 -11.2460 AM303 47.7681 -116.7783 516,609.69 5,290,326.11 700.2 980,622.20 -110.09 -10.8261 AM304 47.7638 -116.7783 516,611.08 5,289,862.96 697.8 980,622.97 -109.39 -10.1262 AM305 47.7604 -116.7784 516,612.38 5,289,461.67 698.1 980,623.26 -108.71 -9.4463 AM306 47.7569 -116.7784 516,613.49 5,289,091.15 691.2 980,624.60 -108.39 -9.1264 AM307 47.7544 -116.7783 516,614.02 5,288,813.26 689.5 980,624.93 -108.16 -8.8865 AM308 47.7508 -116.7783 516,615.32 5,288,411.96 687.3 980,626.23 -106.94 -7.6666 AN501 47.6860 -116.7937 515,489.46 5,281,215.57 669.5 980,627.95 -102.80 -4.7667 AN502 47.6860 -116.7913 515,656.13 5,281,216.13 672.2 980,626.59 -103.68 -5.4668 AN503 47.6860 -116.7883 515,885.27 5,281,216.78 675.4 980,625.29 -104.37 -5.9069 AN504 47.6859 -116.7855 516,093.88 5,281,186.32 677.3 980,624.80 -104.48 -5.7870 AN505 47.6859 -116.7811 516,427.54 5,281,187.13 676.9 980,625.58 -103.78 -4.7171 AN506 47.6858 -116.7778 516,677.69 5,281,188.12 675.5 980,626.37 -103.26 -3.9172 AN507 47.6859 -116.7738 516,969.60 5,281,188.87 674.8 980,627.58 -102.14 -2.4873 AN508 47.6858 -116.7714 517,157.30 5,281,189.46 669.4 980,629.11 -101.64 -1.7774 AN509 47.6858 -116.7686 517,365.71 5,281,190.08 666.1 980,630.07 -101.28 -1.1875 AN510 47.6858 -116.7661 517,553.41 5,281,190.67 666.3 980,630.46 -100.81 -0.5076 AN511 47.6859 -116.7631 517,782.55 5,281,191.32 664.8 980,630.48 -101.00 -0.4477 AP001 47.7021 -116.9498 503,772.11 5,282,986.70 649.4 980,641.74 -93.81 -8.6678 AP002 47.7845 -116.9369 504,723.30 5,292,125.66 660.0 980,648.02 -93.37 -7.1879 AP004 47.7813 -116.9370 504,723.60 5,291,786.21 662.6 980,646.32 -94.35 -8.1580 AP005 47.7784 -116.9370 504,723.91 5,291,446.47 664.1 980,644.99 -95.15 -8.9681 AP006 47.7755 -116.9371 504,703.31 5,291,137.77 662.7 980,644.34 -95.85 -9.6782 AP007 47.7720 -116.9371 504,703.70 5,290,736.47 668.7 980,641.75 -96.99 -10.8183 AP008 47.7693 -116.9372 504,703.81 5,290,427.81 668.8 980,640.96 -97.53 -11.3584 AP009 47.7655 -116.9373 504,704.20 5,290,026.21 666.5 980,640.79 -97.84 -11.6785 AP010 47.7634 -116.9373 504,704.53 5,289,779.40 666.3 980,640.75 -97.72 -11.5586 AP011 47.7606 -116.9373 504,704.64 5,289,470.73 661.5 980,642.05 -97.12 -10.9587 AP012 47.7585 -116.9373 504,704.79 5,289,254.39 661.1 980,641.78 -97.28 -11.1188 AP013 47.7557 -116.9373 504,705.20 5,288,945.72 660.0 980,641.24 -97.79 -11.6289 AP014 47.7530 -116.9373 504,705.31 5,288,637.05 660.9 980,640.01 -98.60 -12.4290 AP015 47.7500 -116.9373 504,705.62 5,288,297.61 668.1 980,637.45 -99.49 -13.3291 AP016 47.7471 -116.9373 504,706.04 5,287,988.64 670.6 980,636.76 -99.45 -13.2792 AP017 47.7447 -116.9373 504,706.26 5,287,710.75 674.1 980,636.48 -98.80 -12.6293 AP018 47.7416 -116.9372 504,706.56 5,287,371.31 684.3 980,634.42 -98.62 -12.4494 AP019 47.7386 -116.9372 504,706.56 5,287,031.56 681.2 980,634.89 -98.49 -12.3195 AP020 47.7359 -116.9372 504,706.97 5,286,722.90 679.7 980,635.15 -98.27 -12.0996 AP021 47.7331 -116.9372 504,707.08 5,286,414.23 679.4 980,635.02 -98.21 -12.0397 AP022 47.7301 -116.9372 504,707.38 5,286,074.79 678.3 980,634.92 -98.24 -12.0698 AP023 47.7269 -116.9371 504,707.69 5,285,735.04 676.0 980,634.99 -98.32 -12.1499 AP024 47.7242 -116.9371 504,708.10 5,285,426.38 674.6 980,635.12 -98.20 -12.02100 AP025 47.7214 -116.9372 504,708.21 5,285,117.71 672.4 980,635.47 -98.02 -11.84

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Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2101 AP026 47.7184 -116.9371 504,708.52 5,284,777.96 671.1 980,635.55 -97.89 -11.71102 AP027 47.7157 -116.9371 504,708.74 5,284,500.07 669.1 980,635.84 -97.72 -11.54103 AP028 47.7132 -116.9371 504,708.96 5,284,222.18 667.5 980,636.30 -97.30 -11.12104 AP029 47.7108 -116.9372 504,709.17 5,283,944.60 663.5 980,637.45 -96.67 -10.49105 AP030 47.7080 -116.9371 504,709.59 5,283,635.63 659.9 980,638.80 -95.69 -9.51106 AP031 47.7047 -116.9369 504,730.49 5,283,265.44 664.2 980,638.12 -95.15 -8.95107 AP032 47.7021 -116.9371 504,709.99 5,282,987.52 657.0 980,639.98 -94.31 -8.13108 AP033 47.6982 -116.9375 504,689.55 5,282,555.11 663.5 980,638.78 -93.64 -7.48109 AP034 47.6958 -116.9382 504,627.30 5,282,277.43 670.4 980,637.16 -93.41 -7.32110 AP035 47.7885 -116.9369 504,723.13 5,292,588.81 656.6 980,649.20 -93.11 -6.91111 AP036 47.7909 -116.9369 504,722.80 5,292,835.62 658.9 980,649.48 -92.51 -6.31112 AP037 47.7939 -116.9369 504,722.48 5,293,175.37 665.0 980,649.47 -91.40 -5.21113 AP038 47.7956 -116.9369 504,722.23 5,293,360.62 687.2 980,645.22 -91.50 -5.30114 AP039 47.7969 -116.9368 504,742.90 5,293,514.84 722.7 980,637.10 -92.50 -6.28115 AP040 47.7994 -116.9368 504,742.67 5,293,792.73 724.5 980,637.52 -92.34 -6.13116 AP041 47.8025 -116.9367 504,742.66 5,294,132.48 723.5 980,639.20 -91.07 -4.86117 AR401 47.7480 -116.7973 515,200.39 5,288,099.33 694.7 980,624.71 -106.87 -9.15118 AR402 47.7534 -116.7973 515,198.84 5,288,685.88 696.9 980,623.83 -107.81 -10.09119 AR403 47.7586 -116.7972 515,197.28 5,289,272.43 699.2 980,622.92 -108.75 -11.03120 AR404 47.7661 -116.7971 515,195.09 5,290,106.10 702.5 980,622.71 -109.00 -11.29121 AS201 47.7824 -116.7648 517,624.93 5,291,934.36 736.6 980,617.25 -109.09 -8.70122 AS202 47.7800 -116.7648 517,625.76 5,291,656.47 734.9 980,617.23 -109.27 -8.88123 AS203 47.7779 -116.7648 517,626.40 5,291,409.66 729.3 980,618.25 -109.17 -8.78124 AS204 47.7758 -116.7648 517,627.14 5,291,193.62 729.3 980,617.78 -109.44 -9.05125 AS205 47.7735 -116.7647 517,627.78 5,290,946.51 722.4 980,618.45 -109.88 -9.49126 AS206 47.7707 -116.7648 517,628.81 5,290,637.85 705.8 980,621.43 -109.92 -9.53127 AS207 47.7685 -116.7648 517,629.45 5,290,390.73 702.0 980,622.09 -109.82 -9.42128 AS208 47.7670 -116.7648 517,630.01 5,290,205.47 699.8 980,622.57 -109.62 -9.23129 AS209 47.7654 -116.7649 517,609.84 5,290,020.18 698.3 980,622.85 -109.49 -9.12130 AS210 47.7633 -116.7650 517,610.59 5,289,804.15 698.9 980,622.60 -109.41 -9.03131 AS211 47.7611 -116.7650 517,611.22 5,289,557.34 699.0 980,622.55 -109.21 -8.84132 AS212 47.7587 -116.7651 517,612.06 5,289,279.45 699.2 980,622.72 -108.78 -8.40133 AS213 47.7568 -116.7652 517,592.09 5,289,063.08 697.7 980,623.14 -108.48 -8.12134 AS214 47.7542 -116.7654 517,592.92 5,288,785.19 696.9 980,623.50 -108.02 -7.67135 AS215 47.7518 -116.7653 517,593.75 5,288,507.60 686.0 980,625.78 -107.54 -7.19136 AS216 47.7490 -116.7652 517,594.47 5,288,198.63 685.0 980,626.25 -106.41 -6.06137 AS217 47.7478 -116.7652 517,594.94 5,288,075.23 724.7 980,618.42 -106.90 -6.54138 AV701 47.6734 -116.7861 516,055.96 5,279,797.11 652.0 980,632.79 -100.26 -1.60139 AV702 47.6765 -116.7857 516,076.08 5,280,136.59 661.3 980,629.53 -101.99 -3.31140 AV703 47.6837 -116.7858 516,073.80 5,280,939.17 670.7 980,626.49 -103.88 -5.20141 AV704 47.6914 -116.7856 516,092.15 5,281,803.65 677.2 980,624.40 -105.41 -6.71142 AV705 47.6968 -116.7857 516,069.89 5,282,390.17 680.2 980,624.03 -105.68 -7.01143 AV706 47.7030 -116.7856 516,088.59 5,283,100.47 684.0 980,623.86 -105.68 -6.98144 AV707 47.7098 -116.7862 516,044.95 5,283,841.14 680.5 980,624.55 -106.30 -7.65145 AV708 47.7159 -116.7866 516,001.38 5,284,520.26 683.3 980,624.66 -106.19 -7.59146 AV709 47.7225 -116.7863 516,020.19 5,285,261.33 681.0 980,625.28 -106.61 -7.98147 D7928 47.6267 -116.7667 517,531.48 5,274,614.81 680.2 980,624.38 -98.44 1.85148 D7942 47.6392 -116.9137 506,489.17 5,275,981.08 723.7 980,625.19 -88.97 -0.83149 D7961 47.6467 -116.8883 508,386.83 5,276,817.03 677.7 980,633.69 -90.30 -0.07150 D7962 47.6468 -116.8877 508,428.58 5,276,817.09 677.8 980,633.88 -90.14 0.14151 D7972 47.6573 -117.0398 497,017.34 5,277,985.06 681.1 980,637.50 -86.96 -9.24152 D7977 47.6583 -116.9138 506,466.09 5,278,111.24 810.9 980,607.50 -91.62 -3.50153 D7984 47.6663 -116.7702 517,247.00 5,279,028.63 648.2 980,635.19 -97.51 2.46154 D7985 47.6663 -116.8633 510,260.46 5,279,011.92 748.4 980,618.75 -94.56 -2.27155 D7987 47.6668 -116.8298 512,783.82 5,279,048.05 651.2 980,638.32 -93.55 1.52156 D7992 47.6698 -117.0917 493,118.46 5,279,377.43 634.2 980,646.50 -89.56 -16.13157 D8006 47.6763 -116.8107 514,199.41 5,280,131.60 675.9 980,628.88 -99.27 -2.65158 D8010 47.6787 -116.8168 513,760.92 5,280,377.75 707.6 980,625.00 -97.15 -1.02159 D8012 47.6798 -116.8048 514,657.19 5,280,503.43 654.6 980,633.32 -99.53 -2.41

12


13


Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2160 D8013 47.6803 -116.8248 513,155.84 5,280,561.48 735.9 980,621.00 -95.92 -0.45161 D8015 47.6818 -116.8052 514,615.01 5,280,719.40 647.9 980,634.32 -99.81 -2.74162 D8021 47.6837 -116.8067 514,510.06 5,280,935.27 655.8 980,633.44 -99.00 -2.04163 D8026 47.6848 -116.7798 516,532.21 5,281,064.19 663.1 980,629.00 -102.81 -3.62164 D8027 47.6863 -116.8117 514,134.25 5,281,243.07 647.9 980,634.13 -96.62 -0.08165 D8032 47.6868 -116.7948 515,405.89 5,281,276.99 661.6 980,629.50 -102.89 -4.94166 D8037 47.6878 -116.7913 515,655.58 5,281,401.39 673.8 980,625.19 -104.96 -6.74167 D8038 47.6887 -116.7848 516,155.63 5,281,495.38 673.8 980,624.38 -105.85 -7.08168 D8044 47.6908 -116.7748 516,905.49 5,281,744.25 671.9 980,625.94 -104.79 -5.19169 D8045 47.6908 -116.7798 516,530.09 5,281,743.37 672.2 980,625.19 -105.51 -6.32170 D8047 47.6917 -116.9138 506,461.78 5,281,815.84 648.8 980,639.50 -95.08 -6.97171 D8050 47.6928 -116.7637 517,738.40 5,281,963.07 658.5 980,630.75 -101.86 -1.35172 D8051 47.6933 -116.7682 517,383.96 5,282,023.47 661.6 980,629.00 -103.70 -3.57173 D8052 47.6933 -116.7717 517,133.79 5,282,022.79 672.2 980,626.63 -104.22 -4.37174 D8053 47.6943 -116.8718 509,629.88 5,282,098.24 648.8 980,638.88 -95.79 -4.20175 D8058 47.6963 -116.8923 508,087.14 5,282,342.71 648.5 980,639.69 -95.65 -5.75176 D8059 47.6963 -116.9783 501,625.87 5,282,337.47 749.0 980,623.44 -91.64 -8.85177 D8068 47.7017 -116.8103 514,234.22 5,282,941.34 664.6 980,631.32 -101.89 -5.23178 D8070 47.7018 -116.7867 516,005.77 5,282,945.86 682.9 980,623.63 -106.03 -7.42179 D8073 47.7033 -116.7848 516,151.17 5,283,131.34 656.1 980,633.00 -101.23 -2.46180 D8074 47.7033 -116.9483 503,876.16 5,283,110.26 648.5 980,641.38 -94.59 -9.32181 D8075 47.7053 -116.8698 509,773.58 5,283,333.43 656.4 980,633.88 -101.12 -9.37182 D8076 47.7053 -116.8498 511,274.25 5,283,336.01 660.7 980,630.69 -103.51 -10.11183 D8086 47.7098 -116.8917 508,126.60 5,283,824.85 673.2 980,631.38 -100.58 -10.64184 D8087 47.7098 -116.9598 503,021.58 5,283,819.85 648.8 980,642.69 -93.97 -9.64185 D8089 47.7108 -116.9167 506,251.28 5,283,946.02 665.8 980,634.07 -99.68 -11.80186 D8091 47.7117 -116.8933 508,001.23 5,284,040.70 669.2 980,632.00 -100.63 -10.83187 D8094 47.7133 -116.9798 501,521.12 5,284,220.70 656.4 980,641.69 -94.01 -11.34188 D8095 47.7148 -116.9567 503,250.28 5,284,375.67 661.3 980,639.13 -95.75 -11.18189 D8096 47.7148 -117.0000 499,999.98 5,284,374.70 647.6 980,643.00 -94.62 -13.62190 D8097 47.7153 -116.8498 511,271.85 5,284,447.57 688.1 980,623.44 -106.50 -13.10191 D8098 47.7153 -116.9418 504,375.47 5,284,438.02 662.2 980,637.57 -97.27 -11.45192 D8100 47.7158 -116.8067 514,501.32 5,284,516.46 671.9 980,625.19 -107.90 -10.95193 D8102 47.7163 -117.0223 498,333.30 5,284,560.17 639.3 980,646.25 -93.17 -14.00194 D8104 47.7167 -116.9783 501,625.15 5,284,591.07 651.8 980,642.13 -94.84 -12.06195 D8108 47.7187 -116.9578 503,166.82 5,284,807.92 662.9 980,638.88 -96.14 -11.66196 D8109 47.7187 -116.9583 503,125.07 5,284,807.86 664.0 980,638.44 -96.36 -11.92197 D8111 47.7193 -116.8288 512,833.49 5,284,882.92 687.2 980,623.57 -106.93 -11.81198 D8116 47.7228 -116.9577 503,166.65 5,285,271.08 673.2 980,636.44 -96.95 -12.46199 D8117 47.7232 -117.0223 498,333.33 5,285,331.99 642.4 980,645.32 -94.17 -15.00200 D8124 47.7298 -116.9733 501,999.66 5,286,042.34 651.8 980,640.25 -98.01 -14.81201 D8125 47.7302 -116.7647 517,642.50 5,286,130.37 683.8 980,624.07 -107.57 -7.16202 D8126 47.7302 -116.8923 508,081.80 5,286,109.16 676.5 980,630.82 -102.74 -12.85203 D8127 47.7303 -116.8718 509,623.30 5,286,111.50 678.6 980,628.69 -104.48 -12.89204 D8128 47.7303 -116.8068 514,497.35 5,286,121.64 687.8 980,625.75 -105.59 -8.64205 D8129 47.7303 -116.8288 512,830.92 5,286,117.89 693.3 980,625.38 -104.93 -9.82206 D8130 47.7303 -116.9583 503,124.56 5,286,104.38 664.6 980,638.50 -97.32 -12.88207 D8131 47.7308 -116.8498 511,268.62 5,286,176.16 693.9 980,625.32 -104.93 -11.53208 D8132 47.7308 -116.9133 506,498.78 5,286,168.92 674.4 980,634.44 -99.58 -11.43209 D8133 47.7308 -117.0223 498,333.58 5,286,165.67 643.9 980,647.07 -92.82 -13.65210 D8142 47.7353 -117.0223 498,333.83 5,286,659.60 643.9 980,648.44 -91.83 -12.66211 D8143 47.7358 -116.9578 503,165.78 5,286,721.78 659.4 980,639.32 -98.02 -13.54212 D8147 47.7383 -116.9478 503,915.43 5,287,000.20 678.3 980,635.57 -98.31 -13.01213 D8150 47.7398 -116.9903 500,728.77 5,287,153.20 649.0 980,643.25 -96.45 -14.65214 D8153 47.7417 -116.9578 503,165.36 5,287,370.19 655.5 980,640.69 -97.96 -13.48215 D8156 47.7442 -117.0677 494,918.88 5,287,649.58 651.2 980,649.75 -89.83 -14.42216 D8157 47.7442 -117.0677 494,918.88 5,287,649.58 651.2 980,649.75 -89.83 -14.42217 D8158 47.7443 -116.7942 515,430.41 5,287,667.60 684.1 980,626.07 -107.13 -9.16218 D8159 47.7447 -116.8717 509,620.53 5,287,716.99 683.5 980,628.13 -105.41 -13.82

14


1�


Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2219 D8162 47.7448 -116.8072 514,451.62 5,287,727.06 705.8 980,623.82 -105.20 -8.31220 D8163 47.7448 -116.8287 512,848.27 5,287,723.10 706.4 980,624.88 -104.04 -8.91221 D8164 47.7448 -116.8498 511,265.63 5,287,719.79 689.6 980,626.57 -105.78 -12.39222 D8165 47.7448 -116.8923 508,079.64 5,287,714.65 681.4 980,631.00 -102.95 -13.06223 D8166 47.7448 -116.9138 506,455.26 5,287,712.49 680.5 980,633.07 -101.04 -12.94224 D8167 47.7448 -116.9367 504,747.70 5,287,710.81 671.0 980,636.13 -99.74 -13.51225 D8170 47.7452 -116.9578 503,165.28 5,287,771.49 658.2 980,640.38 -98.06 -13.57226 D8181 47.7503 -117.0078 499,416.95 5,288,326.46 647.5 980,647.75 -93.06 -12.70227 D8182 47.7503 -117.0078 499,416.95 5,288,326.46 647.5 980,647.75 -93.06 -12.70228 D8185 47.7517 -117.0683 494,877.99 5,288,483.20 649.7 980,650.38 -90.09 -14.72229 D8186 47.7517 -117.0683 494,877.99 5,288,483.20 649.7 980,650.38 -90.09 -14.72230 D8187 47.7522 -116.9583 503,122.95 5,288,543.24 658.8 980,641.32 -97.63 -13.19231 D8191 47.7563 -116.9583 503,122.78 5,289,006.39 659.4 980,641.75 -97.44 -13.00232 D8193 47.7577 -117.0937 492,984.24 5,289,164.08 654.6 980,653.57 -86.03 -12.75233 D8194 47.7583 -116.7898 515,759.35 5,289,243.12 693.3 980,623.57 -109.19 -10.85234 D8196 47.7588 -117.0617 495,378.46 5,289,285.33 650.6 980,652.57 -88.00 -12.08235 D8197 47.7588 -117.0617 495,378.46 5,289,285.33 650.6 980,652.57 -88.00 -12.08236 D8199 47.7593 -116.8718 509,617.77 5,289,322.17 686.3 980,630.50 -103.80 -12.22237 D8200 47.7597 -116.9367 504,746.36 5,289,378.16 660.3 980,641.63 -97.70 -11.47238 D8201 47.7598 -116.9583 503,122.59 5,289,376.92 658.8 980,643.50 -96.10 -11.67239 D8203 47.7633 -116.9358 504,808.44 5,289,779.55 665.2 980,640.50 -98.19 -11.90240 D8205 47.7652 -117.0118 499,125.84 5,289,993.67 670.4 980,646.63 -90.88 -10.84241 D8206 47.7652 -117.0118 499,125.84 5,289,993.67 670.4 980,646.63 -90.88 -10.84242 D8207 47.7652 -117.0118 499,125.84 5,289,993.67 670.5 980,646.82 -90.67 -10.63243 D8215 47.7697 -116.9348 504,891.12 5,290,489.64 668.0 980,640.75 -97.95 -11.57244 D8216 47.7698 -116.7638 517,691.75 5,290,514.54 702.1 980,621.75 -110.25 -9.79245 D8217 47.7718 -117.0618 495,379.58 5,290,705.57 651.2 980,654.00 -87.49 -11.58246 D8218 47.7718 -117.0618 495,379.58 5,290,705.57 651.2 980,654.00 -87.49 -11.58247 D8223 47.7737 -116.8498 511,259.47 5,290,930.76 694.8 980,628.25 -105.68 -12.29248 D8224 47.7738 -116.8717 509,615.10 5,290,958.43 687.5 980,631.82 -103.53 -11.95249 D8225 47.7738 -116.8933 507,991.94 5,290,955.97 686.3 980,633.94 -101.61 -11.82250 D8227 47.7742 -116.9358 504,807.57 5,290,983.44 664.0 980,642.75 -97.09 -10.81251 D8228 47.7743 -116.9148 506,389.30 5,290,984.93 676.8 980,637.32 -100.09 -12.07252 D8229 47.7743 -116.9363 504,765.83 5,290,983.38 664.0 980,642.88 -96.97 -10.73253 D8233 47.7757 -116.8092 514,297.35 5,291,184.31 704.5 980,624.07 -108.13 -11.40254 D8236 47.7767 -116.9658 502,559.84 5,291,259.76 652.4 980,649.88 -91.95 -8.13255 D8237 47.7767 -116.9658 502,559.84 5,291,259.76 652.4 980,649.88 -91.95 -8.13256 D8243 47.7803 -117.0383 497,128.37 5,291,661.35 668.6 980,650.13 -88.65 -10.81257 D8244 47.7803 -117.0383 497,128.37 5,291,661.35 668.6 980,650.13 -88.65 -10.81258 D8247 47.7817 -116.9363 504,765.16 5,291,817.05 663.4 980,646.07 -94.46 -8.22259 D8249 47.7828 -116.9797 501,518.96 5,291,938.58 732.6 980,635.50 -91.53 -8.86260 D8250 47.7828 -116.9797 501,518.96 5,291,938.58 732.6 980,635.50 -91.53 -8.86261 D8256 47.7858 -116.9417 504,369.48 5,292,279.61 657.6 980,648.13 -93.77 -7.96262 D8257 47.7858 -116.9417 504,369.48 5,292,279.61 657.6 980,648.13 -93.77 -7.96263 D8265 47.7883 -116.7638 517,685.44 5,292,582.86 741.1 980,615.63 -110.46 -10.01264 D8266 47.7883 -116.7853 516,083.31 5,292,578.30 704.2 980,624.25 -109.09 -10.39265 D8267 47.7883 -116.8068 514,481.18 5,292,574.04 703.0 980,625.82 -107.80 -10.87266 D8268 47.7883 -116.8283 512,858.33 5,292,570.37 697.5 980,629.63 -105.07 -9.93267 D8269 47.7883 -116.8502 511,214.45 5,292,566.96 693.3 980,631.44 -104.07 -10.73268 D8270 47.7883 -116.8713 509,633.34 5,292,563.95 696.9 980,631.50 -103.27 -11.68269 D8271 47.7883 -116.8933 507,989.46 5,292,561.46 678.9 980,638.57 -99.64 -9.85270 D8272 47.7883 -116.9363 504,764.76 5,292,557.79 657.3 980,649.32 -92.85 -6.61271 D8275 47.7887 -116.9148 506,387.43 5,292,590.42 664.3 980,643.94 -97.04 -9.02272 D8283 47.7937 -116.9268 505,492.36 5,293,145.15 662.8 980,648.75 -92.75 -5.70273 D8284 47.7937 -116.9268 505,492.36 5,293,145.15 662.8 980,648.75 -92.75 -5.70274 D8285 47.7947 -117.0617 495,381.45 5,293,267.85 732.0 980,637.63 -89.27 -13.35275 D8286 47.7947 -117.0617 495,381.45 5,293,267.85 732.0 980,637.63 -89.27 -13.35276 D8287 47.7947 -117.0617 495,381.45 5,293,267.85 732.0 980,637.63 -89.27 -13.35277 D8291 47.7957 -117.1223 490,846.63 5,293,396.87 718.0 980,642.88 -89.08 -18.15

14


1�


Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2278 D8292 47.7958 -116.9598 503,016.60 5,293,390.34 718.0 980,638.82 -89.08 -4.76279 D8293 47.7958 -116.9598 503,016.60 5,293,390.34 772.2 980,638.82 -89.90 -5.58280 D8294 47.7958 -116.9797 501,518.67 5,293,389.59 772.2 980,630.19 -89.90 -7.23281 D8296 47.7958 -116.9797 501,518.67 5,293,389.59 703.3 980,630.19 -93.38 -10.71282 D8298 47.7963 -116.9383 504,618.36 5,293,453.10 648.2 980,640.25 -83.29 2.79283 D8299 47.7968 -116.9798 501,518.70 5,293,482.22 648.2 980,650.63 -83.29 -0.62284 D8304 47.7968 -116.9798 501,518.70 5,293,482.22 664.0 980,650.63 -92.59 -9.92285 D8305 47.8023 -116.9148 506,385.83 5,294,102.97 664.0 980,649.13 -92.59 -4.57286 D8309 47.8023 -116.9148 506,385.83 5,294,102.97 738.1 980,649.13 -109.55 -21.53287 D8310 47.8028 -116.7637 517,701.24 5,294,188.38 701.5 980,618.44 -106.57 -6.10288 D8311 47.8028 -116.8068 514,477.17 5,294,179.53 701.5 980,628.63 -106.57 -9.64289 D8312 47.8028 -116.8068 514,477.17 5,294,179.53 704.5 980,628.63 -104.62 -7.70290 D8313 47.8028 -116.8283 512,854.62 5,294,175.55 704.5 980,630.00 -104.62 -9.48291 D8314 47.8028 -116.8283 512,854.62 5,294,175.55 689.9 980,630.00 -103.72 -8.58292 D8315 47.8028 -116.8503 511,211.36 5,294,172.14 689.9 980,633.69 -103.72 -10.39293 D8316 47.8028 -116.8503 511,211.36 5,294,172.14 680.8 980,633.69 -99.37 -6.04294 D8317 47.8028 -116.8717 509,609.83 5,294,169.41 680.8 980,639.75 -99.37 -7.80295 D8318 47.8028 -116.8717 509,609.83 5,294,169.41 723.1 980,639.75 -91.05 0.52296 D8322 47.8028 -116.9352 504,846.38 5,294,163.41 723.1 980,639.19 -91.19 -4.86297 D8323 47.8048 -116.9363 504,763.05 5,294,379.32 723.1 980,639.00 -91.19 -4.95298 D8330 47.8048 -116.9363 504,763.05 5,294,379.32 669.2 980,639.00 -92.93 -6.69299 D8331 47.8103 -116.8993 507,549.34 5,294,999.96 669.2 980,648.38 -92.93 -3.62300 D8335 47.8103 -116.8993 507,549.34 5,294,999.96 674.1 980,648.38 -93.35 -4.05301 D8338 47.8117 -116.8967 507,736.38 5,295,154.43 679.9 980,647.19 -89.52 -0.01302 D8344 47.8128 -117.0088 499,334.57 5,295,272.72 735.9 980,648.88 -90.01 -9.74303 D8345 47.8133 -116.9468 503,992.82 5,295,335.84 735.9 980,637.88 -90.01 -4.61304 D8347 47.8133 -116.9468 503,992.82 5,295,335.84 703.9 980,637.88 -105.95 -20.55305 D8348 47.8173 -116.8068 514,473.16 5,295,784.71 703.9 980,630.07 -105.95 -9.02306 D8349 47.8173 -116.8068 514,473.16 5,295,784.71 686.9 980,630.07 -103.32 -6.40307 D8350 47.8173 -116.8282 512,851.22 5,295,781.03 686.9 980,636.00 -103.32 -8.18308 D8351 47.8173 -116.8282 512,851.22 5,295,781.03 683.2 980,636.00 -99.91 -4.77309 D8352 47.8173 -116.8498 511,250.00 5,295,777.69 683.2 980,640.07 -99.91 -6.53310 D8353 47.8173 -116.8498 511,250.00 5,295,777.69 676.2 980,640.07 -96.13 -2.75311 D8354 47.8173 -116.8713 509,628.06 5,295,774.63 676.2 980,645.00 -96.13 -4.54312 D8355 47.8173 -116.8713 509,628.06 5,295,774.63 707.9 980,645.00 -108.06 -16.47313 D8356 47.8178 -116.7867 515,970.09 5,295,850.35 707.9 980,627.07 -108.06 -9.49314 D8360 47.8178 -116.7867 515,970.09 5,295,850.35 684.1 980,627.07 -93.58 4.98315 D8361 47.8192 -116.8848 508,629.40 5,295,989.15 684.1 980,645.82 -93.58 -3.09316 D8364 47.8192 -116.8848 508,629.40 5,295,989.15 750.9 980,645.82 -109.17 -18.68317 D8374 47.8247 -116.7747 516,861.97 5,296,624.74 705.2 980,617.94 -108.21 -8.67318 D8380 47.8283 -116.7698 517,235.14 5,297,027.21 687.2 980,627.50 -104.25 -4.29319 D8381 47.8318 -116.8067 514,469.14 5,297,390.19 693.6 980,636.32 -93.32 3.60320 D8382 47.8318 -116.8713 509,625.26 5,297,380.11 693.6 980,645.38 -93.32 -1.73321 D8391 47.8318 -116.8713 509,625.26 5,297,380.11 732.6 980,645.38 -86.52 5.07322 D8398 47.8373 -117.0908 493,202.59 5,297,993.59 693.9 980,640.82 -101.98 -28.46323 D8420 47.8462 -116.8068 514,465.11 5,298,995.68 695.4 980,638.57 -99.61 -2.70324 D8421 47.8593 -116.7792 516,518.50 5,300,452.23 694.5 980,641.69 -100.08 -0.91325 D8424 47.8597 -116.7797 516,476.67 5,300,513.72 694.5 980,641.44 -101.63 -2.51326 D8426 47.8607 -116.7638 517,660.58 5,300,640.74 699.4 980,640.00 -104.69 -4.26327 D8427 47.8608 -116.7502 518,678.90 5,300,643.81 694.5 980,635.88 -101.51 0.04328 D8428 47.8608 -116.7637 517,681.60 5,300,640.78 695.4 980,640.13 -101.10 -0.65329 D8429 47.8608 -116.7708 517,141.37 5,300,639.04 706.4 980,640.38 -99.60 0.26330 D8430 47.8608 -116.8068 514,460.89 5,300,631.94 703.9 980,639.82 -99.75 -2.84331 D8431 47.8612 -116.8018 514,834.87 5,300,663.58 694.5 980,640.19 -99.50 -2.18332 D8432 47.8613 -116.7863 515,977.62 5,300,697.61 701.8 980,642.25 -100.28 -1.71333 D8433 47.8613 -116.7918 515,582.72 5,300,696.40 703.0 980,640.07 -100.19 -2.04334 D8434 47.8613 -116.7963 515,229.57 5,300,695.57 703.0 980,639.94 -100.44 -2.68335 D8437 47.8613 -116.7963 515,229.57 5,300,695.57 710.9 980,639.69 -98.40 -0.65336 D8440 47.8643 -116.8067 514,460.07 5,301,002.46 742.0 980,640.44 -91.13 5.78

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Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2337 D8445 47.8663 -116.8572 510,678.35 5,301,241.11 724.1 980,641.19 -95.55 -2.81338 D8446 47.8678 -116.8487 511,322.14 5,301,396.57 715.8 980,640.75 -97.70 -4.24339 D8447 47.8678 -116.8333 512,464.78 5,301,398.91 719.5 980,640.38 -97.57 -2.86340 D8448 47.8678 -116.8387 512,069.88 5,301,398.31 719.5 980,639.75 -97.38 -3.10341 D8449 47.8678 -116.8387 512,069.88 5,301,398.31 724.1 980,639.94 -95.73 -1.45342 D8450 47.8678 -116.8483 511,342.86 5,301,396.60 726.8 980,640.57 -93.16 0.32343 D8452 47.8678 -116.8533 510,968.98 5,301,396.03 706.4 980,642.57 -97.55 -4.48344 D8453 47.8682 -116.8068 514,458.98 5,301,465.30 703.6 980,642.50 -96.83 0.08345 D8454 47.8682 -116.8222 513,295.62 5,301,462.63 703.6 980,643.69 -96.82 -1.19346 D8455 47.8682 -116.8237 513,191.71 5,301,462.47 707.3 980,643.69 -97.74 -2.23347 D8456 47.8682 -116.8282 512,838.56 5,301,461.63 714.3 980,641.94 -96.94 -1.82348 D8460 47.8682 -116.8433 511,716.64 5,301,459.32 703.9 980,641.25 -95.94 -2.05349 D8462 47.8683 -116.8152 513,814.84 5,301,463.72 705.8 980,644.57 -100.09 -3.90350 D8463 47.8688 -116.7998 514,978.11 5,301,528.55 706.4 980,640.13 -97.74 -0.26351 D8467 47.8698 -116.8068 514,458.61 5,301,619.79 704.8 980,642.44 -99.43 -2.53352 D8468 47.8728 -116.7697 517,220.44 5,301,967.07 708.2 980,641.32 -98.99 0.95353 D8470 47.8728 -116.7923 515,537.87 5,301,962.08 707.3 980,641.13 -96.22 1.88354 PCR01 47.7578 -116.9899 514,457.21 5,302,082.94 642.7 980,626.61 -91.74 5.17355 PCR02 47.7533 -116.9902 500,749.38 5,289,160.02 640.9 980,627.87 -93.09 -11.27356 PCR03 47.7519 -116.9902 500,728.71 5,288,666.06 645.4 980,629.48 -93.57 -11.77357 PCR04 47.7506 -116.9902 500,728.77 5,288,511.58 646.3 980,630.51 -94.29 -12.48358 PCR05 47.7492 -116.9899 500,728.82 5,288,357.40 645.7 980,631.19 -94.99 -13.19359 PCR06 47.7475 -116.9899 500,749.60 5,288,202.94 647.0 980,632.18 -95.58 -13.76360 PCR07 47.7461 -116.9899 500,749.55 5,288,017.68 647.0 980,632.86 -96.14 -14.31361 PCR08 47.7447 -116.9902 500,749.60 5,287,863.50 647.6 980,633.24 -96.25 -14.43362 PCR09 47.7433 -116.9902 500,728.94 5,287,708.98 648.2 980,633.55 -96.30 -14.50363 PCR10 47.7419 -116.9899 500,728.99 5,287,554.80 647.6 980,633.23 -95.98 -14.18364 PCR11 47.7403 -116.9899 500,749.76 5,287,400.35 647.6 980,633.74 -96.35 -14.53365 PCR12 47.7389 -116.9899 500,749.71 5,287,215.09 648.8 980,634.45 -96.66 -14.84366 PCR13 47.7375 -116.9899 500,749.77 5,287,060.60 648.2 980,634.85 -97.07 -15.24367 PCR14 47.7361 -116.9899 500,749.82 5,286,906.42 644.5 980,634.60 -97.50 -15.68368 PCR15 47.7344 -116.9899 500,749.88 5,286,751.93 644.5 980,635.08 -97.85 -16.03369 PCR16 47.7331 -116.9899 500,749.82 5,286,566.67 645.7 980,635.48 -97.86 -16.04370 PCR17 47.7317 -116.9899 500,749.87 5,286,412.49 648.2 980,695.76 -98.26 -16.43371 PCR18 47.7303 -116.9899 500,749.93 5,286,258.01 650.0 980,637.15 -98.32 -16.50372 PCR19 47.7289 -116.9899 500,749.99 5,286,103.52 650.9 980,637.61 -98.45 -16.62373 PCR20 47.7272 -116.9899 500,749.73 5,285,949.34 652.4 980,638.03 -98.40 -16.57374 PCR21 47.7258 -116.9899 500,749.98 5,285,764.08 652.4 980,638.17 -98.41 -16.59375 PCR22 47.7244 -116.9899 500,750.04 5,285,609.59 650.9 980,637.77 -98.22 -16.39376 PCR23 47.7231 -116.9899 500,750.09 5,285,455.41 648.8 980,637.21 -98.00 -16.17377 PCR24 47.7217 -116.9899 500,749.84 5,285,300.92 646.3 980,636.53 -97.71 -15.89378 PCR25 47.7203 -116.9899 500,749.89 5,285,146.74 645.4 980,636.01 -97.25 -15.42379 PCR26 47.7186 -116.9899 500,749.95 5,284,992.26 645.4 980,635.63 -96.73 -14.90380 PCR27 47.7172 -116.9896 500,749.89 5,284,806.99 645.7 980,634.86 -95.76 -13.93381 PCR28 47.7161 -116.9899 500,770.97 5,284,652.54 643.3 980,634.25 -95.56 -13.71382 PCR29 47.7144 -116.9899 500,750.11 5,284,529.11 645.7 980,633.93 -94.56 -12.74383 PCR30 47.7131 -116.9899 500,750.06 5,284,343.84 646.7 980,633.73 -94.04 -12.21384 PCR31 47.7114 -116.9899 500,750.11 5,284,189.66 645.4 980,633.60 -94.04 -12.22385 PCR32 47.7100 -116.9899 500,750.05 5,284,004.40 644.5 980,633.41 -93.93 -12.10386 PCR33 47.7086 -116.9896 500,750.11 5,283,849.92 644.2 980,633.07 -93.51 -11.68387 PCR34 47.7072 -116.9896 500,770.88 5,283,695.76 644.8 980,633.40 -93.55 -11.70388 PCR35 47.7061 -116.9899 500,770.93 5,283,541.28 642.4 980,633.29 -93.85 -12.00389 PCR36 47.7061 -116.9904 500,750.07 5,283,417.84 639.3 980,632.59 -93.83 -12.00390 PCR37 47.7042 -116.9902 500,729.35 5,283,417.81 637.2 980,631.61 -93.18 -11.38391 PCR38 47.7031 -116.9899 500,729.49 5,283,201.78 629.9 980,630.48 -93.56 -11.76392 PHL01 47.7508 -117.0063 500,750.38 5,283,078.10 648.8 980,630.02 -93.13 -11.31393 PHL02 47.7522 -117.0063 499,521.08 5,288,388.17 649.7 980,629.95 -92.99 -12.51394 PHL03 47.7536 -117.0063 499,521.02 5,288,542.65 651.8 980,630.13 -93.10 -12.62395 PHL04 47.7550 -117.0063 499,521.28 5,288,696.84 662.2 980,632.26 -92.75 -12.27

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Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2396 PHL05 47.7564 -117.0066 499,521.22 5,288,851.32 664.9 980,632.44 -92.44 -11.96397 PHL06 47.7581 -117.0066 499,500.44 5,289,005.78 670.7 980,633.81 -92.63 -12.18398 PHL07 47.7594 -117.0066 499,500.50 5,289,190.73 674.7 980,634.32 -92.31 -11.86399 PHL08 47.7608 -117.0066 499,500.44 5,289,345.22 674.7 980,633.86 -91.93 -11.48400 PHL09 47.7619 -117.0068 499,500.38 5,289,499.70 671.3 980,632.15 -91.08 -10.63401 PHL10 47.7628 -117.0087 499,500.52 5,289,623.11 670.7 980,631.64 -90.90 -10.45402 PHL11 47.7636 -117.0104 499,354.60 5,289,715.82 671.3 980,631.31 -90.55 -10.26403 PHL12 47.7644 -117.0120 499,229.70 5,289,808.26 671.3 980,631.26 -90.58 -10.43404 PHL13 47.7650 -117.0139 499,104.79 5,289,901.01 671.7 980,631.83 -91.15 -11.14405 PHL14 47.7658 -117.0158 498,959.37 5,289,962.65 668.6 980,631.41 -91.50 -11.64406 PHL15 47.7669 -117.0175 498,813.44 5,290,055.36 668.6 980,631.36 -91.54 -11.85407 PHL16 47.7672 -117.0191 498,688.65 5,290,178.88 669.2 980,631.08 -91.14 -11.59408 PHL17 47.7672 -117.0224 498,563.83 5,290,209.77 675.6 980,632.18 -90.82 -11.40409 PHL18 47.7658 -117.0224 498,313.97 5,290,210.00 681.7 980,632.94 -90.02 -10.87410 PHL19 47.7644 -117.0224 498,314.02 5,290,055.52 684.8 980,633.05 -89.33 -10.19411 PHL20 47.7636 -117.0224 498,313.78 5,289,901.03 687.2 980,633.74 -89.40 -10.25412 PHL21 47.7636 -117.0202 498,313.75 5,289,808.70 676.8 980,632.37 -90.35 -11.20413 PHL22 47.7636 -117.0180 498,480.42 5,289,808.65 671.7 980,632.21 -91.34 -12.01414 PHL23 47.7636 -117.0150 498,647.09 5,289,808.60 676.2 980,633.71 -91.81 -12.30415 PHL24 47.7631 -117.0137 498,875.93 5,289,808.34 677.7 980,633.97 -91.61 -11.85416 PHL25 47.7619 -117.0120 498,979.92 5,289,746.64 682.3 980,635.39 -91.86 -11.98417 PHL26 47.7603 -117.0120 499,105.02 5,289,623.12 685.1 980,636.58 -92.31 -12.29418 PHL27 47.7589 -117.0120 499,104.96 5,289,437.86 685.1 980,637.00 -92.62 -12.61419 PHL28 47.7575 -117.0117 499,104.71 5,289,283.67 680.2 980,636.22 -92.84 -12.83420 PHL29 47.7558 -117.0117 499,125.49 5,289,129.22 666.5 980,632.83 -92.34 -12.30421 PHL30 47.7539 -117.0117 499,125.74 5,288,943.96 662.8 980,631.78 -91.86 -11.82422 PHL31 47.7522 -117.0117 499,125.57 5,288,727.92 653.0 980,629.59 -91.71 -11.67423 PHL32 47.7508 -117.0117 499,125.52 5,288,542.66 650.6 980,629.14 -91.66 -11.63424 PHR01 47.7106 -117.1107 499,125.57 5,288,388.18 636.9 980,624.02 -85.98 -5.94425 PHR02 47.7092 -117.1107 491,685.93 5,283,917.51 635.7 980,623.57 -85.68 -13.83426 PHR03 47.7075 -117.1107 491,685.67 5,283,763.32 634.2 980,623.23 -85.55 -13.69427 PHR04 47.7058 -117.1107 491,685.31 5,283,578.06 633.5 980,623.62 -85.98 -14.13428 PHR05 47.7042 -117.1107 491,685.26 5,283,392.80 632.9 980,624.00 -86.37 -14.52429 PHR06 47.7022 -117.1107 491,684.89 5,283,207.54 632.3 980,624.54 -86.93 -15.08430 PHR07 47.7006 -117.1107 491,684.73 5,282,991.50 631.7 980,624.98 -87.39 -15.54431 PHR08 47.6989 -117.1107 491,684.36 5,282,806.24 631.1 980,625.62 -88.04 -16.19432 PHR09 47.6975 -117.1107 491,684.00 5,282,620.98 630.2 980,626.00 -88.51 -16.66433 PHR10 47.6961 -117.1107 491,683.75 5,282,466.80 629.3 980,626.45 -89.09 -17.23434 PHR11 47.6947 -117.1107 491,683.50 5,282,312.31 629.0 980,626.86 -89.45 -17.60435 PHR12 47.6933 -117.1107 491,683.55 5,282,157.83 629.6 980,627.61 -89.95 -18.10436 PHR13 47.6919 -117.1104 491,683.30 5,282,003.64 628.4 980,627.97 -90.43 -18.58437 PHR14 47.6906 -117.1104 491,724.79 5,281,849.22 627.7 980,628.30 -90.83 -18.93438 PHR15 47.6892 -117.1104 491,724.54 5,281,694.73 627.4 980,628.49 -90.87 -18.97439 PHR16 47.6878 -117.1104 491,724.29 5,281,540.55 626.5 980,628.55 -90.99 -19.09440 PHR17 47.6864 -117.1104 491,724.04 5,281,386.07 625.3 980,628.36 -90.92 -19.02441 PHR18 47.6850 -117.1104 491,723.79 5,281,231.58 620.4 980,627.46 -90.99 -19.09442 PHR19 47.6822 -117.1101 491,723.54 5,281,077.40 619.5 980,626.60 -90.05 -18.15443 PHR20 47.6806 -117.1101 491,744.06 5,280,768.46 618.3 980,626.48 -90.08 -18.16444 PHR21 47.6792 -117.1101 491,743.69 5,280,583.50 621.0 980,626.93 -89.79 -17.87445 PHR22 47.6778 -117.1101 491,743.44 5,280,429.01 622.9 980,627.37 -89.69 -17.77446 PHR23 47.6764 -117.1101 491,743.19 5,280,274.52 621.7 980,627.41 -89.88 -17.96447 PHR24 47.6750 -117.1101 491,742.94 5,280,120.34 623.5 980,627.79 -89.72 -17.81448 PHR25 47.6736 -117.1098 491,742.69 5,279,965.86 629.0 980,628.74 -89.34 -17.42449 PHR26 47.6722 -117.1098 491,763.46 5,279,811.40 630.5 980,629.17 -89.31 -17.37450 PHR27 47.6714 -117.1098 491,763.21 5,279,657.22 631.4 980,629.18 -88.98 -17.04451 PHR28 47.6697 -117.1098 491,763.18 5,279,564.59 633.2 980,629.17 -88.44 -16.50452 PIH01 47.8103 -116.8926 491,762.82 5,279,379.33 671.7 980,630.15 -93.72 -21.78453 PIH02 47.8086 -116.8926 508,048.76 5,295,000.42 666.5 980,629.77 -94.36 -4.51454 PIH03 47.8072 -116.8926 508,049.01 5,294,815.16 666.8 980,630.73 -95.13 -5.28

18


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Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2455 PIH04 47.8058 -116.8926 508,049.07 5,294,660.97 668.0 980,631.91 -95.90 -6.05456 PIH05 47.8044 -116.8926 508,049.43 5,294,506.49 669.8 980,633.07 -96.53 -6.67457 PIH06 47.8031 -116.8926 508,049.49 5,294,352.31 671.3 980,633.60 -96.59 -6.74458 PIH07 47.8017 -116.8926 508,049.85 5,294,197.82 673.5 980,634.49 -96.88 -7.02459 PIH08 47.8000 -116.8926 508,049.91 5,294,043.34 672.6 980,635.19 -97.65 -7.79460 PIH09 47.7986 -116.8926 508,050.16 5,293,858.07 673.8 980,635.65 -97.70 -7.85461 PIH10 47.7972 -116.8926 508,050.52 5,293,703.90 674.7 980,636.09 -97.81 -7.96462 PIH11 47.7958 -116.8926 508,050.58 5,293,549.41 669.5 980,636.39 -99.14 -9.28463 PIH12 47.7944 -116.8926 508,050.94 5,293,395.23 672.0 980,636.61 -98.68 -8.82464 PIH13 47.7928 -116.8926 508,051.00 5,293,240.74 672.0 980,636.97 -98.91 -9.06465 PIH14 47.7914 -116.8926 508,051.25 5,293,055.48 672.3 980,637.35 -99.12 -9.26466 PIH15 47.7900 -116.8926 508,051.61 5,292,901.30 674.4 980,638.35 -99.52 -9.67467 PIH16 47.7883 -116.8926 508,051.67 5,292,746.82 678.1 980,639.46 -99.71 -9.86468 PIH17 47.7869 -116.8926 508,051.92 5,292,561.56 681.4 980,640.43 -99.83 -9.97469 PIH18 47.7856 -116.8929 508,052.28 5,292,407.07 684.8 980,641.18 -99.72 -9.86470 PIH19 47.7842 -116.8929 508,031.62 5,292,252.86 686.3 980,641.78 -99.86 -10.03471 PIH20 47.7828 -116.8929 508,031.98 5,292,098.38 687.8 980,642.39 -100.00 -10.17472 PIH21 47.7814 -116.8929 508,032.04 5,291,944.19 688.1 980,642.85 -100.27 -10.44473 PIH22 47.7797 -116.8929 508,032.40 5,291,789.71 688.1 980,643.32 -100.61 -10.77474 PIH23 47.7783 -116.8929 508,032.65 5,291,604.45 688.1 980,643.65 -100.81 -10.97475 PIH24 47.7769 -116.8929 508,032.70 5,291,450.27 686.9 980,643.73 -101.02 -11.19476 PIH25 47.7756 -116.8929 508,033.07 5,291,295.78 686.0 980,643.84 -101.22 -11.38477 PIH26 47.7739 -116.8929 508,033.13 5,291,141.30 685.1 980,643.84 -101.30 -11.46478 PIH27 47.7725 -116.8929 508,033.38 5,290,956.04 683.5 980,643.67 -101.33 -11.50479 PIH28 47.7711 -116.8929 508,033.73 5,290,801.86 681.7 980,643.74 -101.68 -11.85480 PIH29 47.7697 -116.8929 508,033.79 5,290,647.37 682.6 980,644.10 -101.71 -11.88481 PIH30 47.7683 -116.8929 508,034.15 5,290,493.19 682.6 980,644.40 -101.75 -11.91482 PIH31 47.7667 -116.8929 508,034.21 5,290,338.70 683.2 980,644.73 -101.95 -12.11483 PIH32 47.7653 -116.8929 508,034.46 5,290,153.44 683.2 980,645.12 -102.21 -12.37484 PIH33 47.7639 -116.8929 508,034.82 5,289,998.96 683.8 980,645.32 -102.15 -12.31485 PIH34 47.7625 -116.8929 508,034.88 5,289,844.78 682.9 980,645.24 -102.15 -12.31486 PIH35 47.7611 -116.8929 508,035.24 5,289,690.29 680.8 980,645.14 -102.39 -12.55487 PIH36 47.7594 -116.8929 508,035.29 5,289,536.11 681.1 980,645.26 -102.51 -12.67488 PIH37 47.7581 -116.8929 508,035.54 5,289,350.85 683.8 980,645.75 -102.05 -12.21489 PIH38 47.7567 -116.8929 508,035.90 5,289,196.37 685.1 980,645.91 -101.81 -11.97490 PIH39 47.7553 -116.8929 508,035.96 5,289,042.18 684.8 980,646.06 -101.90 -12.06491 PIH40 47.7539 -116.8929 508,036.32 5,288,887.70 683.8 980,646.00 -101.91 -12.07492 PIH41 47.7522 -116.8929 508,036.38 5,288,733.21 682.0 980,645.78 -101.98 -12.14493 PIH42 47.7508 -116.8929 508,036.63 5,288,547.95 681.1 980,645.78 -102.05 -12.21494 PIH43 47.7494 -116.8932 508,036.98 5,288,393.77 680.5 980,645.93 -102.20 -12.36495 PIH44 47.7481 -116.8932 507,995.60 5,288,239.23 679.0 980,646.36 -102.84 -13.05496 PIH45 47.7467 -116.8932 507,995.65 5,288,085.04 679.0 980,646.43 -102.79 -12.99497 PIH46 47.7453 -116.8932 507,996.02 5,287,930.56 679.0 980,646.66 -102.88 -13.09498 PIH47 47.7436 -116.8932 507,996.07 5,287,776.38 679.0 980,646.73 -102.82 -13.02499 PIH48 47.7422 -116.8932 507,996.32 5,287,591.12 679.0 980,646.99 -102.95 -13.15500 PIH49 47.7408 -116.8932 507,996.68 5,287,436.63 679.0 980,647.15 -102.97 -13.18501 PIH50 47.7394 -116.8932 507,996.74 5,287,282.15 678.7 980,647.37 -103.13 -13.33502 PIH51 47.7381 -116.8932 507,997.09 5,287,127.97 678.1 980,647.39 -103.40 -13.60503 PIH52 47.7364 -116.8932 507,997.15 5,286,973.48 677.4 980,647.50 -103.27 -13.47504 PIH53 47.7350 -116.8932 507,997.71 5,286,788.22 676.5 980,647.50 -104.04 -14.24505 PIH54 47.7336 -116.8932 507,997.76 5,286,634.04 675.0 980,647.32 -103.37 -13.57506 PIH55 47.7322 -116.8932 507,998.12 5,286,479.56 674.7 980,647.26 -103.25 -13.45507 PIH56 47.7306 -116.8932 507,998.17 5,286,325.38 673.8 980,647.12 -103.18 -13.38508 PIH57 47.7292 -116.8932 507,998.42 5,286,140.11 673.2 980,646.80 -102.87 -13.07509 PIH58 47.7278 -116.8932 507,998.78 5,285,985.63 672.9 980,646.68 -102.68 -12.89510 PIH59 47.7264 -116.8932 507,998.84 5,285,831.14 672.9 980,646.60 -102.48 -12.68511 PIH60 47.7250 -116.8932 507,999.19 5,285,676.97 672.9 980,646.42 -102.16 -12.36512 PIH61 47.7233 -116.8932 507,999.25 5,285,522.48 673.2 980,646.36 -101.86 -12.06513 PIH62 47.7219 -116.8932 507,999.50 5,285,337.22 672.6 980,646.24 -101.73 -11.93

18


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Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2514 PIH63 47.7206 -116.8932 507,999.85 5,285,183.04 672.0 980,646.05 -101.52 -11.72515 PIH64 47.7192 -116.8932 507,999.91 5,285,028.55 670.7 980,645.56 -101.14 -11.34516 PIH65 47.7178 -116.8932 508,000.27 5,284,874.37 668.9 980,645.12 -100.95 -11.15517 PIH66 47.7161 -116.8932 508,000.32 5,284,719.89 668.0 980,644.94 -100.83 -11.03518 PIH67 47.7147 -116.8932 508,000.57 5,284,534.63 666.5 980,645.05 -101.14 -11.34519 PIH68 47.7133 -116.8932 508,000.93 5,284,380.14 666.5 980,645.36 -101.32 -11.52520 PIH69 47.7122 -116.8934 508,000.98 5,284,225.96 670.7 980,646.17 -101.05 -11.25521 PIH70 47.7114 -116.8937 508,001.15 5,284,102.55 672.3 980,646.38 -100.78 -10.98522 PIH71 47.7103 -116.8940 507,980.40 5,284,009.89 670.7 980,646.03 -100.65 -10.87523 PIH72 47.7094 -116.8943 507,959.85 5,283,886.15 669.2 980,645.25 -100.07 -10.32524 PIH73 47.7086 -116.8934 507,939.10 5,283,793.49 669.8 980,645.28 -99.78 -10.05525 PIH74 47.7072 -116.8929 508,001.84 5,283,700.96 667.7 980,644.44 -99.29 -9.48526 PIH75 47.7061 -116.8929 508,043.64 5,283,546.84 664.0 980,643.01 -98.49 -8.64527 PIH76 47.7044 -116.8929 508,043.80 5,283,423.43 661.6 980,641.71 -97.57 -7.72528 PIH77 47.7031 -116.8929 508,044.05 5,283,238.17 663.7 980,641.46 -96.67 -6.82529 PIH78 47.7017 -116.8929 508,044.41 5,283,083.69 662.8 980,641.47 -96.73 -6.88530 PIH79 47.6997 -116.8926 508,044.46 5,282,929.20 662.2 980,641.36 -96.60 -6.75531 PIH80 47.6983 -116.8929 508,065.62 5,282,713.20 658.5 980,640.51 -96.43 -6.56532 PIH81 47.6967 -116.8929 508,044.95 5,282,558.98 649.4 980,638.25 -96.08 -6.23533 PIR01 47.7439 -117.0470 508,045.20 5,282,373.72 665.5 980,631.25 -89.71 0.14534 PIR02 47.7422 -117.0464 496,480.69 5,287,617.52 665.5 980,630.76 -89.12 -11.99535 PIR03 47.7408 -117.0464 496,522.38 5,287,432.33 665.5 980,630.91 -89.15 -11.97536 PIR04 47.7394 -117.0464 496,522.13 5,287,277.84 657.3 980,629.98 -89.96 -12.78537 PIR05 47.7381 -117.0464 496,522.19 5,287,123.35 649.4 980,628.22 -89.87 -12.69538 PIR06 47.7364 -117.0464 496,521.94 5,286,969.17 647.0 980,627.99 -90.03 -12.85539 PIR07 47.7353 -117.0464 496,521.88 5,286,783.91 643.6 980,627.97 -90.63 -13.46540 PIR08 47.7336 -117.0464 496,521.74 5,286,660.50 643.6 980,628.48 -91.03 -13.85541 PIR09 47.7322 -117.0464 496,521.69 5,286,475.24 642.4 980,628.64 -91.30 -14.13542 PIR10 47.7308 -117.0464 496,521.75 5,286,320.76 642.4 980,629.07 -91.61 -14.43543 PIR11 47.7294 -117.0464 496,521.50 5,286,166.27 642.4 980,629.53 -91.94 -14.76544 PIR12 47.7278 -117.0462 496,521.55 5,286,012.09 642.1 980,629.71 -92.06 -14.88545 PIR13 47.7261 -117.0464 496,542.21 5,285,826.86 641.8 980,629.84 -92.12 -14.93546 PIR14 47.7247 -117.0462 496,521.13 5,285,641.56 640.5 980,629.72 -92.15 -14.97547 PIR15 47.7233 -117.0462 496,541.91 5,285,487.11 640.5 980,630.00 -92.31 -15.11548 PIR16 47.7217 -117.0462 496,541.96 5,285,332.93 638.7 980,629.73 -92.33 -15.13549 PIR17 47.7203 -117.0462 496,541.60 5,285,147.66 635.4 980,629.49 -92.70 -15.50550 PIR18 47.7186 -117.0462 496,541.66 5,284,993.18 636.0 980,629.66 -92.60 -15.41551 PIR19 47.7172 -117.0462 496,541.60 5,284,807.92 634.8 980,629.84 -92.92 -15.73552 PIR20 47.7156 -117.0462 496,541.35 5,284,653.74 634.8 980,630.13 -93.07 -15.88553 PIR21 47.7142 -117.0462 496,541.29 5,284,468.48 636.3 980,630.27 -92.74 -15.54554 PIR22 47.7125 -117.0462 496,541.35 5,284,313.99 637.5 980,630.34 -92.40 -15.21555 PIR23 47.7111 -117.0462 496,541.29 5,284,128.73 637.5 980,630.13 -92.24 -15.05556 PIR24 47.7094 -117.0462 496,541.04 5,283,974.55 638.4 980,630.02 -91.52 -14.32557 PIR25 47.7081 -117.0462 496,540.98 5,283,789.29 639.3 980,629.88 -91.03 -13.84558 PIR26 47.7067 -117.0462 496,540.73 5,283,634.80 639.3 980,629.78 -90.77 -13.58559 PIR27 47.7050 -117.0462 496,540.78 5,283,480.62 639.6 980,629.51 -90.29 -13.10560 PIR28 47.7036 -117.0462 496,540.73 5,283,295.36 639.6 980,629.07 -89.82 -12.63561 PIR29 47.7019 -117.0462 496,540.48 5,283,140.87 639.3 980,628.83 -89.49 -12.30562 PIR30 47.7003 -117.0462 496,540.42 5,282,955.61 638.4 980,628.34 -89.05 -11.86563 PIR31 47.6967 -117.0470 496,540.36 5,282,770.35 628.7 980,626.10 -88.64 -11.45564 PIR32 47.6953 -117.0467 496,477.67 5,282,369.26 630.2 980,626.61 -88.67 -11.54565 PIR33 47.6936 -117.0467 496,498.44 5,282,214.81 629.6 980,626.50 -88.56 -11.42566 PIR34 47.6925 -117.0464 496,498.08 5,282,029.54 630.2 980,626.37 -88.18 -11.03567 PMA01 47.6697 -117.1098 496,518.96 5,281,906.17 631.4 980,629.14 -88.81 -11.64568 PMA02 47.6706 -117.1074 491,762.82 5,279,379.33 631.4 980,629.60 -89.33 -17.39569 PMA03 47.6708 -117.1052 491,929.83 5,279,471.60 630.8 980,629.27 -89.20 -17.07570 PMA04 47.6714 -117.1033 492,096.61 5,279,502.33 630.8 980,629.04 -89.01 -16.71571 PMA05 47.6714 -117.1011 492,242.79 5,279,563.80 631.7 980,629.94 -89.71 -17.25572 PMA06 47.6714 -117.0986 492,409.46 5,279,563.74 632.3 980,630.62 -90.25 -17.60

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Count Point_ID DD_lat DD_long Easting Northing Elevation Obs Grav C.B.A. 1 C.B.A. 2573 PMA07 47.6714 -117.0964 492,597.16 5,279,563.42 633.8 980,631.41 -90.70 -17.85574 PMA08 47.6714 -117.0943 492,764.14 5,279,563.37 636.0 980,632.19 -90.97 -17.93575 PMA09 47.6714 -117.0921 492,930.82 5,279,563.01 639.0 980,633.00 -91.10 -17.88576 PMA10 47.6714 -117.0902 493,076.77 5,279,562.93 640.2 980,633.70 -91.53 -18.14577 PMA11 47.6714 -117.0883 493,222.73 5,279,562.54 641.8 980,633.78 -91.27 -17.73578 PMA12 47.6714 -117.0861 493,368.68 5,279,562.46 643.9 980,633.89 -90.91 -17.20579 PMA13 47.6714 -117.0842 493,535.66 5,279,562.41 644.2 980,633.55 -90.49 -16.60580 PMA14 47.6714 -117.0817 493,681.61 5,279,562.02 647.6 980,633.59 -89.76 -15.71581 PMA15 47.6714 -117.0795 493,869.31 5,279,562.00 648.5 980,633.30 -89.27 -15.01582 PMA16 47.6714 -117.0773 494,035.99 5,279,561.64 648.5 980,632.97 -88.92 -14.48583 PMA17 47.6714 -117.0754 494,202.97 5,279,561.60 648.2 980,632.50 -88.52 -13.90584 PMA18 47.6714 -117.0732 494,348.92 5,279,561.51 648.8 980,631.66 -87.52 -12.74585 PMA19 47.6714 -117.0710 494,494.87 5,279,561.43 647.9 980,630.85 -86.77 -11.82586 PMA20 47.6714 -117.0689 494,661.55 5,279,561.07 649.4 980,631.16 -86.44 -11.31587 PMA21 47.6714 -117.0667 494,828.53 5,279,561.02 646.0 980,630.49 -86.51 -11.20588 PMA22 47.6714 -117.0653 494,995.20 5,279,560.97 649.4 980,630.97 -86.24 -10.75589 PMR01 47.6700 -117.0885 495,099.72 5,279,560.83 644.5 980,634.42 -91.19 -15.58590 PMR02 47.6683 -117.0885 493,347.70 5,279,408.25 648.8 980,635.64 -91.31 -17.63591 PMR03 47.6669 -117.0885 493,347.65 5,279,222.99 653.0 980,636.66 -91.25 -17.57592 PMR04 47.6656 -117.0885 493,347.40 5,279,068.50 653.3 980,637.08 -91.46 -17.78593 PMR05 47.6642 -117.0885 493,347.14 5,278,914.32 651.8 980,637.23 -91.81 -18.13594 PMR06 47.6625 -117.0885 493,346.89 5,278,759.83 645.1 980,635.89 -91.82 -18.14595 PMR07 47.6611 -117.0885 493,346.83 5,278,574.57 641.5 980,635.11 -91.72 -18.03596 PMR08 47.6597 -117.0885 493,346.58 5,278,420.39 641.8 980,635.42 -91.83 -18.15597 PMR09 47.6583 -117.0885 493,346.33 5,278,265.90 642.1 980,635.39 -91.58 -17.90598 PMR10 47.6567 -117.0885 493,346.38 5,278,111.42 636.0 980,634.18 -91.57 -17.88599 PMR11 47.6556 -117.0885 493,346.01 5,277,926.46 632.0 980,632.97 -91.11 -17.43600 PNL01 47.7439 -117.0470 493,345.87 5,277,802.75 665.5 980,631.24 -89.91 -16.23601 PNL02 47.7439 -117.0489 496,480.69 5,287,617.52 665.5 980,631.38 -90.09 -12.96602 PNL03 47.7439 -117.0511 496,335.04 5,287,617.61 666.2 980,631.51 -90.19 -13.22603 PNL04 47.7439 -117.0533 496,168.37 5,287,617.66 666.2 980,631.41 -90.10 -13.32604 PNL05 47.7439 -117.0555 496,001.70 5,287,617.71 971.0 980,631.68 -90.38 -13.78605 PNL06 47.7439 -117.0574 495,835.33 5,287,617.76 665.8 980,631.81 -90.57 -14.15606 PNL07 47.7439 -117.0593 495,689.37 5,287,617.84 664.3 980,632.06 -91.14 -14.89607 PNL08 47.7439 -117.0615 495,564.44 5,287,617.96 662.2 980,631.50 -91.05 -14.93608 PNL09 47.7439 -117.0637 495,397.77 5,287,618.31 651.8 980,628.89 -90.74 -14.80609 PNL10 47.7439 -117.0656 495,231.40 5,287,618.37 650.6 980,628.29 -90.41 -14.66610 PNL11 47.7439 -117.0678 495,085.44 5,287,618.45 651.5 980,628.21 -90.13 -14.53611 PNL12 47.7456 -117.0678 494,919.08 5,287,618.50 651.5 980,628.23 -90.15 -14.74612 PNL13 47.7469 -117.0678 494,919.13 5,287,803.76 650.9 980,627.95 -90.12 -14.71613 PNL14 47.7483 -117.0678 494,919.38 5,287,958.25 650.6 980,627.71 -90.08 -14.66614 PNL15 47.7497 -117.0678 494,919.33 5,288,112.43 650.6 980,627.63 -90.13 -14.72615 PNL16 47.7511 -117.0678 494,919.57 5,288,266.92 650.3 980,627.46 -90.16 -14.74616 PNL17 47.7525 -117.0678 494,919.52 5,288,421.40 650.0 980,627.37 -90.25 -14.84617 PNL18 47.7539 -117.0680 494,919.77 5,288,575.59 649.4 980,627.06 -90.20 -14.78618 PNL19 47.7553 -117.0680 494,898.99 5,288,730.04 649.1 980,626.78 -90.11 -14.72619 PNL20 47.7569 -117.0680 494,899.24 5,288,884.53 648.8 980,626.62 -90.15 -14.76620 PNL21 47.7583 -117.0680 494,899.30 5,289,069.48 649.4 980,626.46 -89.98 -14.59621 PNL22 47.7597 -117.0689 494,899.54 5,289,223.97 648.5 980,626.13 -89.97 -14.58622 PNL23 47.7608 -117.0699 494,837.33 5,289,378.36 647.6 980,625.99 -90.14 -14.82623 PNL24 47.7622 -117.0710 494,753.98 5,289,501.95 648.2 980,625.89 -90.01 -14.78624 PNL25 47.7633 -117.0721 494,671.04 5,289,656.31 648.5 980,625.79 -89.96 -14.82625 PNL26 47.7644 -117.0732 494,587.69 5,289,779.89 648.5 980,625.34 -89.64 -14.60626 PNL27 47.7656 -117.0743 494,504.64 5,289,903.48 648.8 980,625.15 -89.51 -14.56627 PNL28 47.7669 -117.0754 494,442.31 5,290,027.09 648.8 980,625.08 -89.56 -14.67628 PNL29 47.7683 -117.0757 494,359.07 5,290,181.45 648.8 980,624.52 -88.99 -14.19629 PNL30 47.7700 -117.0762 494,317.58 5,290,335.87 650.0 980,624.17 -88.41 -13.66630 PNL31 47.7711 -117.0765 494,297.22 5,290,521.11 650.6 980,623.34 -87.21 -12.48

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standard gravity reductions to his data, we chose to work directly from his original data to ensure that identical corrections were applied to all observations. Purves’ original data and accompanying notes were provided to us by the Washington State Department of Natural Resources (Stephen Palmer, written commun., 1997).

This information included the following data: gravity readings, elevations, times, locations, temperatures, base readings, and meter constants. It also contained Purves’ notes on how he collected and recorded his findings. He logged horizontal positions using an automobile’s odometer and marked in tenths of a mile. He noted vertical positions with an altimeter, and these were verified daily against local benchmarks at numerous places. He monitored elevation drift due to changes in barometric pressure by allowing no more than a 0.03 percent disparity in elevations between local benchmarks. His correction correlated to a vertical accuracy of ± 8 cm (approx.). The horizontal locations were plotted by us on a topographic map from the details of starting points for each line. We digitally sampled the plotted points to obtain coordinates for each gravity station. Therefore, the accuracy of the station positions can be no better than 1/10 of a mile according to the odometer. Because of the Rathdrum Prairie’s relatively consistent trends and the substantial amount of additional data being considered, we are confident that Purves’ locations are sufficient for our study. We have no reason to suspect inconsistent levels of precision or accuracy in the data.

CADY AND MEYER DATA

In their geologic study of uranium deposits in the region, Cady and Meyer (1976a) compiled the principal facts for 2,077 gravity stations around Spokane, Washington. From these data, they constructed a Bouguer gravity anomaly map of the Okanagan, Sandpoint, Ritzville, and Spokane 1° x 2° quadrangles (Cady and Meyer, 1976b). The principal facts for the stations were obtained from various sources, including USGS open-file reports and the U.S. Department of Defense’s gravity library. Data from investigations by Bonini (1963) and Hammond (1975) were included; data from Purves (1969) were not. The Cady and Meyer (1976a) principal facts were obtained in digital form from Hittelman and others (1994), a CD-ROM collection of significant gravity data sets.

We used 206 stations from this data set (Figure 4). They are spaced generally one per square mile over the study area to complement the profiles measured by Purves (1969) and us. Unfortunately, raw gravity data were unavailable. Corrections for drift, latitude, free-air,

Bouguer, and terrain were applied to standard gravity measurements but could not be checked.

NEW OBSERVATIONS

In 1997, we undertook 146 gravity measurements along three main transects in the Rathdrum Prairie as well as at the mouths of Hayden Lake and Coeur d’Alene Lake (Figure 4). The primary transects were along Idaho Road (Idaho) and Hayden Avenue with five smaller transects near the mouths of Hayden Lake and Coeur d’Alene Lake.

Measurements were read with a Lacoste and Romberg model G gravity meter (no. 1069). A standard base plate was used. At least two measurements were taken for each station, with the requirement that they agree to within 0.01 mGal. These were read at the base station at least three times a day to monitor instrument drift. The base station was a U.S. Geological Survey first-order, second-class benchmark—designation P285 on the Rathdrum 7.5-minute quadrangle, near the corner of Idaho Highway 53 and Greensferry Road, about 0.6 km from Rathdrum. Principal facts for the benchmark are given in Table 2. All gravity measurements are thought to be precise to at least 0.01 mGal before correction.

Coordinates and elevations for each gravity station were obtained using the differential Global Positioning System (GPS) technique. Leica SR 399 receivers were used, with the previously described benchmark as a coordinate tie to the geodetic system. With these techniques, horizontal and vertical accuracy is at least ± 2 cm (J.S. Oldow, oral commun., 1997). Normally, a 2-cm-vertical variation should result in no more than a 0.01 mGal gravity variation. Measurements were recorded on level road surfaces, with the GPS receiver and gravity meter at the same elevation.

The field survey was designed with two goals: to cover previously uncovered sections of the Rathdrum Prairie, and to enable a meaningful comparison and check of Purves’ (1969) data. The Idaho Road (Idaho) profile fits between two profiles of Purves (1969), Corbin Road and Idaho Highway 41. These three profiles, on the western side of the aquifer, are constrained to the north and south by bedrock exposure, which reduces the uncertainty of the profile’s geometry at these locations. The Idaho Road (Idaho) profile provides a meaningful comparison with the data of Purves (1969), indirectly verifying their validity. The Hayden Avenue profile is oriented east-west and designed to tie the north-south profiles and improve the model of the east-west trend. The difficulty, to be discussed later, is its orientation with the regional trend

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The NGS Data Sheet DATABASE = Sybase ,PROGRAM = datasheet, VERSION = 5.70Starting Datasheet Retrieval.1 National Geodetic Survey, Retrieval Date = NOVEMBER 29, 1998 SV0371 ***************************************************************** SV0371 DESIGNATION - P 285 SV0371 PID - SV0371 SV0371 STATE/COUNTY- ID/KOOTENAI SV0371 USGS QUAD - RATHDRUM (1986) SV0371 SV0371 *CURRENT SURVEY CONTROL SV0371 ___________________________________________________________________ SV0371* NAD 83(1986)- 47 48 08. (N) 116 54 56. (W) SCALED SV0371* NAVD 88 - 665.103 (meters) 2182.09 (feet) ADJUSTED SV0371 ___________________________________________________________________ SV0371 GEOID HEIGHT- -17.41 (meters) GEOID96 SV0371 DYNAMIC HT - 665.142 (meters) 2182.22 (feet) COMP SV0371 MODELED GRAV- 980,649.1 (mgal) NAVD 88 SV0371 SV0371 VERT ORDER - FIRST CLASS II SV0371 SV0371.The horizontal coordinates were scaled from a topographic map and have SV0371.an estimated accuracy of +/- 6 seconds. SV0371 SV0371.The orthometric height was determined by differential leveling SV0371.and adjusted by the National Geodetic Survey in June 1991. SV0371 SV0371.The geoid height was determined by GEOID96. SV0371 SV0371.The dynamic height is computed by dividing the NAVD 88 SV0371.geopotential number by the normal gravity value computed on the SV0371.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45 SV0371.degrees latitude (G = 980.6199 gals.). SV0371 SV0371.The modeled gravity was interpolated from observed gravity values. SV0371 SV0371; North East Units Estimated Accuracy SV0371;SPC ID W - 682,440. 712,690. MT (+/- 180 meters Scaled) SV0371 SV0371 SUPERSEDED SURVEY CONTROL SV0371 SV0371 NGVD 29 - 663.931 (m) 2178.25 (f) ADJ UNCH 1 2 SV0371 SV0371.Superseded values are not recommended for survey control. SV0371.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums. SV0371.See file format.dat to determine how the superseded data were derived. SV0371 SV0371_MARKER: DD = SURVEY DISK SV0371_SETTING: 7 = SET IN TOP OF CONCRETE MONUMENT (ROUND) SV0371_STAMPING: P 285 1944 P.C. 395+90.8 40.00 SV0371_STABILITY: C = MAY HOLD, BUT OF TYPE COMMONLY SUBJECT TO SV0371+STABILITY: SURFACE MOTION SV0371

Table 2. Principal Facts for Primary Benchmark.

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SV0371 HISTORY - Date Condition Recov. By SV0371 HISTORY - 1944 MONUMENTED IDDT SV0371 HISTORY - 1944 GOOD NGS SV0371 HISTORY - 1980 GOOD SV0371 SV0371 STATION DESCRIPTION SV0371 SV0371’DESCRIBED BY NATIONAL GEODETIC SURVEY 1944 SV0371’1.0 MI SW FROM RATHDRUM. SV0371’1.0 MILE SOUTHWEST ALONG STATE HIGHWAY NO. 53 FROM THE NORTHERN SV0371’PACIFIC RAILROAD STATION AT RATHDRUM, ABOUT 0.1 MILE NORTHWEST OF THE SV0371’TRACK, 89 FEET WEST OF CENTER-LINE OF A SIDE ROAD LEADING SOUTH AND SV0371’ACROSS THE TRACK, 40 FEET SOUTHEAST OF CENTER-LINE OF HIGHWAY, 2 FEET SV0371’NORTHEAST OF REFERENCE POST. A IDAHO STATE HIGHWAY BRONZE SV0371’RIGHT-OF-WAY DISK SET IN TOP OF A CONCRETE POST PROJECTING ABOUT 0.4 SV0371’FOOT ABOVE THE GROUND. SV0371 SV0371 STATION RECOVERY (1980) SV0371 SV0371’RECOVERED 1980 SV0371’RECOVERED IN GOOD CONDITION.

of the gravity field and its location over a rather distinct non-two-dimensional basin. Typical spacing was about 300 m between gravity stations. This spacing was based on an analysis of the profiles presented in Purves (1969). Small-scale variations identified in Bouguer gravity profiles of Purves (1969) are equally prominent when sampled every 300 m, as opposed to the approximate 150-m spacing that he typically used. Predictive forward modeling also confirmed the efficacy of a 300-m spacing in identifying the bedrock-sediment interface that has been presumed to exist at depths by previous authors (Newcomb, 1953; Hammond, 1974; Gerstel and Palmer, 1994).

GRAVITY DATA REDUCTION

Because our survey is using gravity measurements from the two previous studies, various reductions to all these field measurements are necessary to properly model and interpret the combined gravity data. Reductions must be consistent for each data set in the final product to ensure that artifacts from inconsistent reduction do not appear as anomalies on the final gravity map.

INSTRUMENT CALIBRATION CORRECTION

Every gravity meter has a unique calibration function that is determined shortly after its production. This calibration allows readings to be converted to a milliGal (mGal) scale. This scale is accurate, however, only in

a relative sense and should not be confused with an absolute gravity measurement. For our study, the tie to the local standard was a final step of the data reduction.

The meter readings were converted to mGal with a calibration table provided by Lacoste & Romberg specific to our gravity meter G-1069 for a given gravity meter temperature. Purves (1969) used a standard Worden gravity meter and had to multiply readings by the meter constant of 0.09869-mGal/scale division. Original information on meter readings was not available for the data from Cady and Meyer (1976a).

TIDAL AND INSTRUMENT DRIFT CORRECTIONS

All instrumental readings in the field require a correction to compensate for the effects of instrument drift and earth tides. Instrument drift may occur with the Lacoste & Romberg gravity meter even though its integral parts are housed in a vacuum at a fixed temperature. Readings are corrected for the tidal drift caused by the variable forces applied to the earth by the sun and the moon.

After the tidal drift was removed, instrument drift was removed by calibrating the measurements to a fixed value at the previously introduced benchmark P285. This instrument drift was never greater than 0.10 mGal over one day. Data from Cady and Meyer (1976a) were received already corrected for drift, but with no way to verify the accuracy of those calculations. Purves (1969)

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properly corrected for combined tidal and instrument drift values as much as 0.10 mGal/day.

TERRAIN CORRECTIONS

The Rathdrum Prairie is a relatively flat plain with elevation variations generally less than 50 m. These local variations in topography, as well as the surrounding hills, have an impact on the gravity and must be considered with a terrain correction procedure. The nearby hills give an upward component of gravitational attraction that counteracts a part of the downward pull exerted by the rest of the earth. Conversely, the surrounding valleys below station elevations, which can be modeled as holes in the theoretically continuous slab that extends to the datum, produce a smaller downward pull than is calculated with the Bouguer correction.

In the past, these corrections followed the method developed by Hammer (1939) that calculated the gravitational effect of flat-topped, cylindrical sections or sloping planes, both based on elevations manually read from topographic maps. Other methods developed from Hammer’s original work include one for improved sloping planes (Sandberg, 1958) and another for improved conical prisms (Olivier and Simard, 1981). The accuracy of these methods depends on how closely a particular geometrical model conforms to the actual terrain near a gravity station. Today, data manipulation by computers, combined with the availability of low-price 30-m digital elevation models (DEMs) produced by the U.S. Geological Survey’s (USGS) National Cartographic Information Center, allows for a more accurate and less arduous method of terrain correction. Methods developed by Plouff (1966) and Cogbill (1990) that use the exhaustive surface coverage provided by DEMs have an accuracy that depends mostly on how well the DEM represents the terrain near the gravity station. Corrections calculated using the DEMs are potentially more accurate than hand procedures because of the more subjective manual methods.

Because data from the three sources (ours; Purves, 1969; Cady and Meyer, 1976a) are to be compared with each other and presented as one product, the same terrain correction technique would ideally be applied uniformly. This would avoid any inconsistencies between manual estimation methods and automated DEM-based techniques. Unfortunately, nonterrain corrected data from Cady and Meyer (1976a) were unavailable and therefore not re-corrected in relation to the other data sets. Furthermore, parameters of the terrain corrections performed on the data from Cady and Meyer (1976a), including Hammer zones and correction densities,

could not be obtained. The DEM-based, automated procedure is composed of three software routines: one that combines and reorganizes local 1:24,000 DEMs, a second that uses the reduced DEMs and any additional data provided by the user (GPS points, etc.) to calculate “inner-zone” terrain corrections, and a third that applies the “outer-zone” terrain corrections from a specially produced DEM that provides regional coverage.

The digital elevation models supplied by the USGS are preprocessed for use by the terrain correction software. A routine called DEMREAD, written by Alan H. Cogbill at the Los Alamos National Laboratory, performs the required algorithms that combine numerous DEMs into one large, project specific DEM. The preprocessing enhances the execution speed of the terrain correction program and reduces the data storage space of the DEM file for that program to about 30 percent of the ASCII data size provided by the USGS. Table 3 lists the USGS 7.5-minute DEMs (30-m x 30-m data spacing cast on a Universal Transverse Mercator projection) that were input to DEMREAD for combination and reduction to binary format. Surface coverage was required to a radius of 2,000 m around each gravity station. This radius for inner-zone corrections is suggested by Cogbill (1997) for regions of nonextreme topographic relief. All of the DEMs used are classified as level 1 or 2, correlating to a vertical RMS error of 7-15 m or 3 m, respectively. As a check of the DEMs’ vertical accuracy, twenty GPS measurements were taken within 2.5 m of DEM data points. Assuming that the GPS measurements were accurate in the vertical direction within 0.05 m, the average vertical error of the DEM points that were near the GPS measurements was 3.4 m. Although this variation appears large, it agrees with the 3-m RMS error reported by the USGS for level 1 DEMs and is well below the 7- to 15-m RMS error reported for level 2 DEMs.

The terrain corrections are decomposed into two parts: inner-zone and outer-zone corrections. Inner-zone corrections account for topographic variations very close to the gravity stations (5 m) out to a specified distance (2,000 m), whereas outer-zone corrections are based on coarser terrain data, and broader surface fitting methods are calculated. The inner-zone corrections use a procedure explained in Cogbill (1990) that takes advantage of the USGS 7.5-minute DEMs. The inner zone is further divided into an inner zone (radius of 250 m) and medium zone (radius 250-2,000 m). A continuously differentiable surface is fit to the elevation data within the smaller inner zone. This surface is integrated numerically to obtain that portion of the overall terrain correction. Mathematically, the algorithm effectively integrates a line element between the station and the medium zone, repeated every 3

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degrees of azimuth. In the medium zone, the terrain effect is calculated by numerically integrating the elevation data using a rectangular integration rule. Cogbill (1990) estimates that integration errors are always less than 0.001 mGal, but notes that the calculated corrections can be no better than the elevation data used to represent the terrain about each gravity station. The accuracy of the calculated corrections is also limited by the inherent uncertainty of the estimated mean terrain density.

The inner-zone corrections from all three data sets were conducted using the method of Cogbill (1990), through the software package, INNERTC, which he developed. The terrain corrections ranged from approximately 0.02 mGal in the proximal regions of the prairie to ± 1.20 mGal in the surrounding hills. A correction density (r) of 2.68 g-cm-3 was used.

Outer-zone corrections were calculated using a method developed by Plouff (1966) and modified by Alan Cogbill in his software package, OUTERTC. The correction procedure fits a multiquadric surface to elevation data from USGS 3-arcsec DEMs, and it calculates the effect of this surface on each gravity measurement. A compilation of the entire set of USGS 3-arcsec DEMs was provided by Cogbill, with the accompanying program MAPFILE that selectively removes the nonrequired DEMs and formats the remaining DEMs for input to OUTERTC. Outer-zone corrections cover the distance from the outermost radius used in INNERTC (4,000 m for our study), to a maximum radius of 111 km. The standard maximum radius of 167 km would have been preferred, but significant complications arise with the integration of Canadian DEMs. A correction density of r=2670 kg-m-3 was used. The earth’s curvature is incorporated for elevation data

more than 18 km from each gravity station. Outer terrain corrections ranged from approximately 0.17 mGal for points in the central part of the prairie to 0.78 mGal for points in the surrounding hills.

LATITUDE AND ELEVATION CORRECTIONS

Because the earth is not a perfect, nonrotating sphere but has an equatorial bulge and significant rotation, the effects of latitude must be considered so that gravity adjustments can be compared. Centrifugal acceleration due to rotation, maximum at the equator and minimum at the poles, acts to oppose gravitational acceleration. Conversely, polar flattening acts to increase gravity at the poles by making the geoid closer to the earth’s center of mass. The latitude adjustment is calculated by differentiating the Geodetic Reference System (GRS 1967) formula.

Because gravity varies inversely with distance from the center of the earth, it is necessary to apply the free-air correction, which reduces all readings to a datum surface. The correction is 0.3086 mGal/m (Dobrin, 1976).

BOUGUER CORRECTIONS

The Bouguer correction removes the effect of a presumed infinite slab of material between the horizontal plane of each station and a datum plane. The correction factor is -0.112 mGal/m above the datum, assuming an average density for crustal rocks of 2,670 kg-m-3 (Dobrin, 1976).

By applying the reductions listed above to the field measured data, the Bouguer anomaly is produced. The Bouguer anomaly is the observed value of gravity minus the theoretical value at the latitude and elevation of the observation point. This allows for variations in the Bouguer anomaly to be interpreted and modeled in relation to variations in subsurface geologic features.

The Bouguer anomaly map produced by our study is shown in Figure 6. The Bouguer anomaly has a fairly straightforward basinal appearance, except for a significant overriding, east-west regional trend.

CORRELATION OF DATA SETS

The data from all three sources must be properly correlated. The data from Cady and Meyer (1976a) are referenced to GRS 1967. The data from Purves (1969) were referenced to the control network of North America

USGS DEM LevelAthol, ID 1Fernan Lake, ID 2Greenacres, WA 2Hayden Lake, ID 1Hayden, ID 2Liberty Lake, WA-ID 2Mica Bay, ID 1Mica Peak, ID 1Coeur d’Alene, ID 1Mt. Coeur d’Alene, ID 2Bayview, ID 1Newman Lake, ID 1Post Falls, ID 2Rathdrum, ID 2Rockford Bay, ID 1Spirit Lake East, ID 1

Table 3. Digital Elevation Models.

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established by Wollard and Behrendt (1961). No control point was available for our study, so this correlation is used to reference the data to GRS 1967. To insure consistency between the three data sets, eight locations were identified where gravity measurements were taken for all three stidies. The correlations of Cady and Meyer (1976a) to our data and to Purves (1969) are shown in Figure 6. The data sets, as represented by these points, are evidently well correlated and only require the addition of a constant to reference our data and those of Purves (1969) to the GRS 1967 system.

REMOVAL OF REGIONAL TREND

Any pattern seen on the Bouguer anomaly map is the sum of the attractions of local sources and broader, more distant regional sources. A regional trend that affects Bouguer anomaly because of changes in crustal thickness is apparent in Figure 7. This poses a difficult problem to gravity modeling of the Spokane Valley-Rathdrum Prairie aquifer. The thickening of the continental crust beneath the northern Rocky Mountains (Winston and

Figure 6. Locations of eight gravity measurements overlap between the three data sets. By correlating these points, it is possible to indirectly reference the data of Purves (1969) and ours to GRS 1967, to which the data of Cady and Meyer (1976a) are referenced.

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others, 1989; Harrison and others, 1972), east of the study area, causes a regional decrease in the Bouguer anomaly of 0.8 to 1.9 mGal per km eastward. This trend can be clearly observed on a portion of the Bouguer anomaly map of Idaho shown in Figure 8. Bankey and others (1985) is a complete Bouguer anomaly map of Idaho produced through a compilation of existing data sources. The collection of data from Cady and Meyer (1976a) and Hammond (1974) is that included on the part covering the Rathdrum Prairie, but data sets peripheral to the study area allow for an interpolation of the regional trend. Ten measurements were averaged to estimate a regional trend of 1.1 mGal per km east. Though this correction is subjective, it appears to reasonably estimate the effect of crustal thickening. The trend was simplified to an E-W orientation. Variations in this correction will significantly alter models of east-west oriented profiles, such as the Hayden Avenue profile.

A Bouguer gravity map that is adjusted to compensate for the regional trend is shown in Figure 9. Compared with the Bouguer anomaly map in Figure 7, the trend-adjusted gravity apparently has a basinlike appearance. The method used respects the general geologic model of the subsurface and maintains the simplicity and reproducibility of the corrections.

GRAVITY DATA MODELING

The overall geologic model of the Rathdrum Prairie has been described as an ancestral valley that was filled with various sediments during the late Tertiary and Quaternary periods. The basin-fill geology has generally been inferred from Bouguer anomaly (Figure 7). Unfortunately, this general impression cannot predict the specific depths and morphologies of buried surfaces.

Figure 7. Bouguer anomaly map of the Rathdrum Prairie based on our data and those from Purves (1969) and Cady and Meyer (1976a).

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Figure 8. Bouguer gravity map of the Rathdrum Prairie showing a regional trend of +1.1 mGal/km east.

With a detailed gravity survey, however, small variations in the local gravity field can be accurately detected. These anomalies can presumably be attributed to buried geologic surfaces that can be readily modeled. The limiting factor to the models is determining the densities of the geologic units. Because the densities of surface rocks can be accurately measured and mapped, doing so provides a starting point for estimating subsurface densities.

Numerous studies have documented the surface densities of rocks in and around the Rathdrum Prairie. The rocks can be generally categorized into four main units: (1) basement rocks of the Belt Supergroup and Cretaceous igneous intrusions, (2) Tertiary basalt, (3) Latah sediments, and (4) gravels of the Missoula

Floods. Measured densities of the basement rocks range from 2.64 to 2.80 g-cm-3, with an average of 2.67 g-cm-3 (Purves, 1969; Birch, 1942; Harrison and others, 1972). The densities of the Columbia River basalt fall into a very broad range from 2.78 g-cm-3 for vesicular samples to 3.21 g-cm-3 for massive samples. Latah sediments have a measured dry density of 1.09-1.62 g-cm-3 and a saturated density of 2.13-2.42 (Hosterman, 1960; Purves, 1969). The gravels of the Missoula Floods vary widely in their densities because of the influence of different particle sizes on porosity, as well as the presence of any cementation. By using a source rock density of 2.60-2.80 g-cm-3 and porosity ranging from 20 percent for cemented gravels to 40 percent for coarse uncemented gravels, a dry bulk density range of 1.56 to 2.24 g-cm-3 can be inferred. Given this dry density and porosity range, the saturated gravel density range is calculated at 1.96 to 2.44 g-cm-3. Measuring the densities of surface samples is quite simple, but extending these densities to subsurface gravels should be done with caution.

The density information introduced above is difficult to model. The contrast in density between bedrock and basalt is very small, as is also that between Latah sediments and flood gravels. Adding to this problem is the uncertainty about the locations of basalt dikes and remaining basalt deposits, and the location and quantity of Latah sediments. A complex geologic environment probably exists beneath the prairie, considering the numerous reworking episodes that have occurred. Differentiating the basalt deposits from bedrock or the Latah sediments from gravels is nearly impossible through a gravity model because an infinite number of combinations may create the same signature. Because of the locally complex setting and small density contrasts, the geologic models have been simplified to elements that will materially affect gravity signatures.

The rocks of the Belt Supergroup, Cretaceous intrusions, other dikes and sills, and Miocene basalt have all been combined for the model and are referred to as bedrock, with a density of 2.67 g-cm-3. Latah sediments and flood gravels are modeled as dry sediments with a density of 1.7 g-cm-3, and the intermingled sands and clays are modeled as saturated sediments with a density of 2.1 g-cm-3. The justification for these densities is twofold: They are well within the range of average measured and calculated densities, and they provide the most realistic model when the seismic refraction bedrock tie of Newcomb and others (1953) is imposed. Obviously, this will oversimplify the actual geology in many places, but will allow a more realistic and probable test of the proposed geologic model without the complication of determining contacts between units of similar densities.

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Figure 9. Residual Bouguer gravity map of the Rathdrum Prairie.

The modeling densities used by Hammond (1974) could not be procured for comparison.

The theoretical gravity field of five proposed cross-sections was modeled using the software GM-SYS of Northwest Geophysical Associates in Corvallis, Oregon. It uses the methods of Talwani and others (1959) and Talwani and Heirtzler (1964) to calculate the gravity model response. A powerful feature of the program is its capability to calculate 2¾-dimensional models from the routines of Rasmussen and Pedersen (1979). A 2¾-dimension model allows for the extension of each discrete model along its perpendicular axis to a user-defined interface where an infinite half-slab of the same profile shape is modeled with user-defined density values. This is useful in a basin-type structure, such as

the Rathdrum Prairie, because it gives some control over the 3-dimensional subsurface structure that ultimately defines the gravity field. The 2¾-dimensional modeling technique normally overestimates basement depth, unless the basin segment of the model is chosen to be narrower in the direction normal to the profile than the actual basin, in which case the depth would be underestimated. Adding additional uncertainty to non-3-dimensional models is the elevation gradient of the basement walls in the basin. In less than three dimensions, it is impossible to exactly and correctly identify the perpendicular extent of each modeled profile segment.

Five profiles were modeled on the basis of trend-adjusted Bouguer gravity; their locations are shown on Figure 4. Two of the models, Hayden Avenue and Idaho

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Road (Idaho), were based on our data. Three models are based on data collected by Purves (1969) and re-corrected as part of our study: Idaho Road (Washington), Corbin Road, and Idaho Highway 41. The profiles and calculated and measured gravity values are shown in Figure 10(a-e). All four north-south profiles are shown at the same scale for reliable comparison. The profiles are based on surficial mapping by Breckenridge and Othberg (1998a, 1998b) and other geologic and geophysical data previously mentioned. The depth tie for these models is the seismic refraction profile of Newcomb and others (1953). The seismic reflection profile of Gerstel and Palmer (1994) confirms the depths of Newcomb and others (1953) in the chosen location. The deepest point on Newcomb and others’ profile has been fixed as the deepest point on the Idaho Road (Washington) model. No existing wells penetrate to bedrock in the proximal part of the prairie, most only extend a short distance beneath the water table. The ground-water levels shown in the

models have been interpolated from well-inventory data. Profile element extensions in the third dimension, for 2¾-dimensional modeling, were estimated individually for each profile.

GRAVITY MODEL INTERPRETATION

The gravity profiles in the preceding section represent the possible subsurface structure of the Rathdrum Prairie. Understanding this morphology will aid in interpreting the region’s geologic history. No attempt has been made to identify specific hydrologic boundaries within the sediments, as such divisions would be overwhelmingly subjective because of the low density contrasts involved. The models are based on the existing pool of geological hypotheses for the region. The reversed seismic refraction profile of Newcomb and others (1953) was the only bedrock tie used for these data; therefore, the modeled depths to bedrock are not definitive. Additionally,

Figure 10a. The Idaho Road (Washington) profile is at the western end of the Rathdrum Prairie. At this point, the valley appears to be generally V-shaped, with a small bench on the southern edge of the profile. A deeper section in the middle may be the result of fluvial erosion. The valley is only half as wide at this location, as it is 8 km to the west at Idaho Road (Idaho). Based on the refraction profile of Newcomb and others (1953), the maximum depth to bedrock in this profile has been fixed to a depth of 152 m below the ground surface at the indicated position. The maximum aquifer thickness is 216 m, and the lowest elevation of the bedrock-sediment interface is 426 m.

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Figure 10b. The Corbin Road profile is 5 km east of the Idaho Road (Washington) profile. This profile is characterized by a generally smooth floor and an apparent embedded valley on the north edge, with shallow benches on both sides of the profile. The southern edge shows a small depression in the present location of the Spokane River. It is not possible to determine, through gravity modeling, how much of the subsurface marginal rocks is Miocene basalt rimrock. The maximum thickness of the aquifer is 263 m and the lowest elevation of the bedrock-sediment interface 384 m.

Figure 10c. The Idaho Road (Idaho) profile is significantly wider than the two profiles to its west. It is characterized by what appears to be an incised river channel in the center of the valley (Rathdrum River?) and a perched depression on the north side. The maximum sediment thickness is 332 m, and the lowest elevation of the bedrock-sediment interface is 337 m. This model does not conclusively prove or disprove that this location was overridden by the Cordilleran ice sheet.

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Figure 10d. The Idaho Highway 41 profile is the widest of the north-south profiles. The incised valley evident on the Idaho Road (Idaho) profile is less apparent here. Instead, there is a significant feature on the southern side of the valley. The interface is characterized by a smooth, undulating surface. Because of the wide, smooth profile, Cordilleran glaciation possibly did override this location. The maximum aquifer thickness is 356 m, and lowest elevation of the bedrock-sediment interface is 319 m.

Figure 10e. The Hayden Avenue profile is the only east-west profile. This makes accurate modeling particularly difficult because of the east-west regional trend, causing the shape of this profile to be strongly affected by the trend correction factor. The general bedrock surface has a smooth, undulating character similar to the Idaho Highway 41 profile. Evidence of Pleistocene glaciation across this profile is inconclusive. The valley appears to be deeper on the eastern side, but that may be an influence of the regional trend. The maximum aquifer thickness is 283 m, and the lowest elevation of the bedrock-sediment interface is 395 m.

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variations in sediment densities and anomalies within the metamorphic and crystalline bedrock can cause variations in the gravity field that would lead to grossly misinterpreting the shown depths. Table 4 presents the modeled aquifer thickness and bedrock elevations.

Profile

Maximum Sediment Thickness (meters)

Lowest Elevation of

Bedrock (meters)

Idaho Road (Washington) 216 426

Corbin Road 263 384

Idaho Road (Idaho) 332 337

Idaho Highway 41 356 319

Hayden Avenue 283 411

Table 4. Modeled Aquifer Characteristics

control provided by the seismic refraction of Newcomb and others (1953). Bedrock ties act to constrain the model. The valley at this point is relatively constricted and V-shaped. This suggests that the ancestral valley was subject to fluvial erosion. It is not possible to determine whether a major glacial advance overrode this profile, but it seems unlikely from the geomorphic evidence of Breckenridge (1989) and Waitt and Thorson (1983). Along this profile, Newcomb and others (1953) interpreted the lower half of the aquifer as Latah sediments intercalated with Miocene basalt.

Five kilometers to the east is the Corbin Road profile. This profile has a smooth bedrock surface with what appears to be an incised stream channel on the north side. This feature may be the remnant of a flow on the north side of the valley during the Miocene when the region was impounded by the Columbia River Basalt Group. Similarly, the apparent hump may be a large basalt deposit that was not eroded during the Pleistocene. Shallow benches exist on both sides of the valley.

Five more kilometers to the east, the valley widens significantly where the Idaho Road (Idaho) profile is located. The bedrock surface shows a large feature channel in the center of the profile. Perhaps this is the location of the ancestral Rathdrum River. Modeling such extreme variations in bedrock effectively with gravity data is difficult because of the broad influence that relatively deep features have on their surrounding morphology. This may be the cause for the rise to the right of the incised channel. Aside from the channel, however, this profile has a consistent depth and a smooth bedrock-sediment interface.

Two more kilometers to the east is the Idaho Highway 41 profile. This incised stream channel is less evident in the Idaho Highway 41 profile. The valley is broad and smooth with few large variations in the bedrock-sediment interface. Possibly the Pend Oreille lobe did advance this far west, but evidence for or against such an idea is not contained completely in this profile.

The Hayden Avenue profile was quite difficult to model. It is oriented perpendicularly to the regional trend and therefore significantly affected by any adjustments to the trend correction. The valley appears to be deepest on the eastern side and shallower on the west. This is likely an effect in the difficulty of modeling a basin in less than three dimensions. The profile forms an acute angle with the north valley wall for about a kilometer on the western end. The shallow bedrock on the north side of the profile acts to increase the apparent gravity on that end, which leads to a shallower modeled depth

The data contained in Table 2 reveal an interesting feature that was first presented by Purves (1969), though he noticed it only in Bouguer anomaly profiles. The elevation of the aquifer’s base rises significantly beneath Corbin Road, effectively thinning the sediments by about 80 m compared to the neighboring profiles. Hypotheses vary for explaining this feature. Purves (1969) suggests that the local bedrock high is evidence that the advancement of the Pend Oreille lobe stopped just east of the Idaho Road (Washington) profile. His explanation is possible but not overwhelmingly evident from these gravity data alone. A major basalt remnant may exist in the locality of the bedrock high. It would have had to survive Pleistocene erosion and would have acted as a shielding mechanism from the Missoula Floods for the Latah sediments to the west that were identified by Newcomb and others (1953). The Latah sediments may have been overridden by younger basalt flows that slowed their erosion. A localized dike or intrusion in the country rock could also explain the feature. If such an event occurred and the intrusion was a significantly higher density, the modeled feature could be a misinterpretation. These very simplified hypotheses are based on a feature that has only been identified with gravity methods. Its existence should be studied further.

Discussions of each profile are useful in attempting to decipher the paleogeomorphology of the Rathdrum Prairie. It should be reiterated that small-scale variations in a modeled surface are a non-unique solution to a complex physical situation. Many of these variations could quite easily appear somewhat different in another model based on the same data.

The most reliable of the models is the Idaho Road (Washington) profile. This profile has the limited bedrock

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to bedrock. This profile should be limited to the depth to bedrock near the center of the profile, where the edge effect is minimized.

Analysis of the minimum elevations of the bedrock-sediment interface suggests that a low point exists between Idaho Road (Idaho) and Hayden Avenue, as has been previously suggested by Purves (1969). Unfortunately, the reliability of depths modeled becomes less accurate when the profiles are further from the location of the seismic refraction tie. Furthermore, the adjustment for the regional trend is a notable subjective action that would also affect this conclusion.

CONCLUSION

Understanding the thickness and subsurface geology of the Rathdrum Prairie aquifer is critical to interpreting the local and regional geologic history that will ultimately lead to ensuring better aquifer management. To that end, we have incorporated new gravity data with a sizable existing gravity data set to create a broader base from which to interpret the region’s subsurface structure. From this information, we have developed the geometry for each of the five modeled profiles as well as the plausible depth and thickness relationships. The results provide a much clearer picture of the aquifer’s structure and extent.

The sediments that compose the Rathdrum Prairie have a maximum thickness of about 356 m in the center of the valley on Idaho Road (Idaho) between Post Falls and Rathdrum. The aquifer thins to about 215 m only 10 km to the west at the Idaho-Washington state line. The bedrock-sediment interface is generally smooth east of Idaho Road (Idaho). From this point to the west, an incised channel becomes apparent in the ancestral valley floor. The lower boundary of the aquifer remains at a generally consistent elevation across the study area, with a shallow slope to the east. A low point in the elevation of the bedrock-sediment interface is presumed to exist between Idaho Road (Idaho) and Hayden Avenue. This conclusion should be interpreted with caution, as the regional trend correction ultimately determines the magnitude of the interface’s slope in the east-west direction.

Gravity modeling has its limitations. With the existing data, it cannot establish definitive depths to the bedrock-sediment interface or differentiate their various geologic units. The next step in studying the aquifer further should be more seismic investigations, especially on the eastern half of the prairie, and more well drilling that penetrates bedrock.

Two primary factors limit the reliability of these results. First, the models were created using 2¾ dimensional modeling techniques. Any model produced with 3-dimensional techniques will be more accurate. A 3-dimensional inversion of the basin would more clearly define the aquifer boundaries. Second, only one control point was used for our study. Future studies must include two kinds of data gathering: (1) seismic and gravity surveys to map the subsurface depths and structure; and (2) well logs to distinguish geologic units and confirm structure, rock densities, and areal and depth measurements.

ACKNOWLEDGMENTS

This study was funded by the Idaho Geological Survey. The gravity meter and GPS equipment necessary to our research were provided by Dr. John Oldow of the University of Idaho. Dr. Oldow provided insights on the details of GPS and gravity surveying and training in the use of the equipment. Dr. Stephen Palmer at the Washington Department of Natural Resources provided the original data of Purves (1969). Loudon Stanford at the Idaho Geological Survey gave helpful advice on manipulating USGS DEM and DRG data. The late Dan Weisz was an excellent field assistant, and thanks should also go to his parents, who allowed a GPS station to be placed in their yard on the Rathdrum Prairie.

REFERENCES

Aitkenhead, N., 1960, Observation on the drainage of a glacier-dammed lake in Norway: Journal of Glaciology, v. 3, p. 607-609.

Anderson, A.L., 1927, Some Miocene and Pleistocene drainage changes in northern Idaho: Idaho Bureau of Mines and Geology Pamphlet 18, 29 p.

———, 1940, Geology and metalliferous deposits of Kootenai County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 53, 31 p.

Bankey, V., M. Webring, D.R. Mabey, M.D. Kleinkopf, and E.H. Bennett, 1985, Complete Bouguer gravity anomaly map of Idaho: U.S. Geological Survey Miscellaneous Field Studies Map, MF-1773.

Baker, V.R., 1973, Paleohydrology and sedimentology of Lake Missoula flooding in eastern Washington: Geological Society of America Special Paper 144, 79 p.

Birch, F., (ed.), 1942, Handbook of Physical Constants: Geological Society of America Special Paper 36, 319 p.

Bonini, W.E., 1963, Gravity anomalies in Idaho: Idaho Bureau of Mines and Geology Pamphlet 132, 39 p.

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Breckenridge, R.M., 1989, Glacial Lake Missoula and the Channeled Scablands, chapter 3: Lower glacial Lake Missoula and Clark Fork ice dams, T310, in Glacial Geology and Geomorphology of North America, v. 1, 28th International Geologic Congress Field Trip Guidebook: American Geophysical Union, Washington, D.C., 72 p.

Breckenridge, R.M., and K.L. Othberg, 1998a, Surficial geologic map of the Post Falls and part of the Liberty Lake quadrangles, Kootenai County, Idaho: Idaho Geological Survey Surficial Geologic Map 98-1, 1:24,000.

———, 1998b, Surficial geologic map of the Rathdrum and part of the Newman Lake quadrangles, Kootenai County, Idaho: Idaho Geological Survey Surficial Geologic Map 98-2, 1:24,000.

———, 1998c, Chronology of Glacial Lake Missoula Floods; paleomagnetic evidence from Pleistocene lake sediments near Clark Fork, Idaho: Geological Society of America Abstracts with Programs, v. 30, no. 6, p. 30.

Breckenridge, R.M., K.L. Othberg, J.A. Welhan, and C.R. Knowles, 1997, Geologic characteristics of the Rathdrum aquifer: Inland Northwest Water Resources Conference, p. 23.

Bretz, J H., 1923, The channeled scablands of the Columbia Plateau: Journal of Geology, v. 31, p. 617-649.

———, 1925, The Spokane flood beyond the channeled scablands: Journal of Geology, v. 33, p. 97-115, 236-259.

———, 1928a, Bars of channeled scabland: Geological Society of America Bulletin, v. 39, p. 643-702.

———, 1928b, The channeled scabland of eastern Washington: Geographical Review, v. 18, p. 446-477.

———, 1928c, Alternate hypothesis for channeled scabland: Journal of Geology, v. 36, p. 193-223, 312-341.

———, 1930a, Lake Missoula and the Spokane flood (Abs.): Geological Society of America Bulletin, v. 41, p. 92-93.

———, 1930b, Valley deposits immediately west of the Channeled Scabland: Journal of Geology, v. 38, p. 385-422.

———, 1932, The Grand Coulee: American Geographical Society Special Publication 15, 89 p.

———, 1959, Washington’s Channeled Scabland: Washington Department of Conservation, Division of Mines and Geology Bulletin 45, 57 p.

Bretz, J H., H.T.U. Smith, and G.E. Neff, 1956, Channeled scabland of Washington: New data and interpretations: Geological Society of America Bulletin, v. 67, p. 957-1049.

Cady, J.W., and R.R. Meyer, 1976a, Principal facts for

gravity stations in the Okanogan, Sandpoint, Ritzville, and Spokane 1° x 2° quadrangles, northeastern Washington and northern Idaho: U.S. Geological Survey Open-File Report 76-290, 57 p.

———, 1976b, Bouguer gravity anomaly map of the Okanogan, Sandpoint, Ritzville, and Spokane 1° by 2° quadrangles, northeastern Washington and northern Idaho: U.S. Geological Survey Geophysical Investigation Map GP-914.

Chambers, R.L., 1971, Sedimentation in glacial Lake Missoula: University of Montana M.S. thesis, 100 p.

———, 1984, Sedimentary evidence for multiple Glacial Lakes Missoula: Montana Geological Field Conference on Northwestern Montana, Kalispell, Montana, p. 189-199.

Cogbill, A.H., 1990, Gravity terrain corrections calculated using digital elevation models: Geophysics, v. 55, no. 1, p. 102-106.

Conners, J.A., 1976, Quaternary history of northern Idaho and adjacent areas: University of Idaho Ph.D. thesis, 504 p.

Costa, J.E., 1988, Floods from dam failures in V.R. Baker, R.C. Kochel, and P.C. Patton, eds., Flood Geomorphology: John Wiley and Sons, New York, p. 439-463.

Dobrin, M.B., 1960, Introduction to geophysical prospecting: McGraw-Hill Book Company, Inc., New York, p. 169-262.

Flint, R.F., 1935, Glacial features of the southern Okanogan region: Geological Society of America Bulletin, v. 46, p. 169-194.

———, 1936, Stratified drift and deglaciation in eastern Washington: Geological Society of America Bulletin, v. 47, p. 1849-1884.

———, 1937, Pleistocene drift border of eastern Washington: Geological Society of America Bulletin, v. 48, p. 203-232.

———, 1938, Origin of the Cheney-Palouse scabland tract: Geological Society of America Bulletin, v. 49, p. 461-524.

Gerstel, W.J., and S.P. Palmer, 1994, Geologic and geophysical mapping of the Spokane Aquifer— Relevance to growth management: Washington Geology, v. 22, no. 2, p. 18-24.

Glenn, J.W., 1954, The stability of ice-dammed lakes and other water filled holes in glaciers: Journal of Glaciology, v. 2, p. 316-318.

Hammer, S., 1939, Terrain corrections for gravimeter stations: Geophysics, v. 4, p. 184-194.

Hammond, R.E., 1974, Ground-water occurrence and movement in the Athol area and the northern Rathdrum Prairie, northern Idaho: Idaho Department of Water Administration, Water Information Bulletin 35, 18 p.

Harrison, J.E., M.D. Kleinkopf, and J.D. Obradovich,

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1972, Tectonic events at the intersection between the Hope fault and the Purcell trench, northern Idaho: U.S. Geological Survey Professional Paper 719, 24 p.

Hittelman, A., D. Dater, R. Buhman, and S. Racey, 1994, Gravity CD-ROM—1994 Editions and User’s Manual, revised, 1996: U.S. Department of Commerce, National Geophysical Data Center, Boulder, Colorado.

Hosterman, J.W., 1960, Investigations of some clay deposits in parts of Washington and Idaho: U.S. Geological Survey Bulletin 1091, 146 p.

Kiver, E.P., and D.F. Stradling, 1989, Glacial Lake Missoula and the Channeled Scablands, chapter 4: The Spokane Valley and northern Columbia Plateau, T310, in Glacial Geology and Geomorphology of North America, v. 1, 28th International Geologic Congress Field Trip Guidebook: American Geophysical Union, Washington, D.C., 72 p.

Lewis, R.S., R.F. Burmester, R.M. Breckenridge, M.D. McFaddan, and J.D. Kauffman, 2002, Geologic map of the Coeur d’Alene 30 x 60 minute quadrangle, Idaho: Idaho Geological Survey Geologic Map 33.

Liestol, O., 1955, Glacier-dammed lakes in Norway: Norsk Geographisk Tidsskrift, v. 15, p. 122-149.

McKiness, J.P., 1988, The Quaternary system of the eastern Spokane Valley and the lower, northern slopes of the Mica Peak uplands, eastern Washington and northern Idaho: University of Idaho M.S. thesis, 100 p.

Molenaar, D., 1988, The Spokane aquifer, Washington: Its geologic origin and water-bearing quality characteristics: U.S. Geological Survey Water Supply Paper 2265, 74 p.

Newcomb, R.C., and others, 1953, Seismic cross sections across the Spokane River Valley and the Hillyard trough, Idaho and Washington: U.S. Geological Survey Open-File Report, 16 p.

Olivier, R.J., and R.G. Simard, 1981, Improvement of the conic prism model for terrain correction in rugged topography: Geophysics, v. 46, p. 1054-1056.

Pardee, J.T., 1910, The glacial Lake Missoula: Journal of Geology, v. 18, p. 376-386.

———, 1942, Unusual currents in glacial Lake Missoula: Geological Society of America Bulletin, v. 53, p. 1569-1600.

Pardee, J.T., and Kirk Bryan, 1926, Geology of the Latah Formation in relation to the lavas of the Columbia Plateau near Spokane, Washington: U.S. Geological Survey Professional Paper 140-A, p. 1-16.

Plouff, Donald, 1966, Digital terrain corrections based upon geographic coordinates: Geophysics, v. 31, p. 1208.

Purves, W.J., 1969, The stratigraphic control of ground water through the Spokane Valley: Washington State University M.S. thesis, 213 p.

Rasmussen, R., and L.B. Pedersen, 1979, End corrections in potential field modeling: Geophysical Prospecting, v. 27, p. 749-760.

Richmond, G.M., 1986, Tentative correlation of deposits of the Cordilleran Ice Sheet in the northern Rocky Mountains, in V. Sibrava, D.Q. Bowen, and G.M. Richmond, eds., Quaternary Glaciations in the Northern Hemisphere: A Report of the International Geological Correlation Programe, Project 24, International Union of Geological Sciences and UNESCO: Quaternary Science Reviews, v. 5, p. 129-144.

Sandberg, C.H., 1958, Terrain corrections for an inclined plane in gravity computations: Geophysics, v. 23, p. 701-711.

Savage, C.N., 1965, Geologic history of Pend Oreille Lake region in north Idaho: Idaho Bureau of Mines and Geology Pamphlet 134, 18 p.

———, 1967, Geology and mineral resources of Bonner County: Idaho Bureau of Mines and Geology County Report 6, p. 131.

Sieja, D.M., 1959, Clay mineralogy of glacial Lake Missoula varves, Missoula Co., Montana: University of Montana M.S. thesis, 52 p.

Talwani, Manik, and J.R. Heirtzler, 1964, Computation of magnetic anomalies caused by two-dimensional bodies of arbitrary shape, in G.A. Parks, ed., Computers in the Mineral Industries, Part 1: Stanford University Publication, Geological Sciences, v. 9, p. 464-480.

Talwani, Manik, J.L. Worzel, and M. Landisman, 1959, Rapid gravity computations for two-dimensional bodies with application to the Mendocino submarine fracture zone: Journal of Geophysical Research, v. 64, p. 49-59.

Thorarinsson, S., 1939, The ice-dammed lakes of Iceland with particular reference to their values as indicators of glacier oscillations: Geografiska Annaler, v. 21, Ht. 3-4, p. 216-242.

Waitt, R.B., 1980, About forty last-Glacial Lake Missoula jökulhlaups through southern Washington: Journal of Geology, v. 88, p. 653-679.

———, 1985, Case for periodic, colossal jökulhlaups from Pleistocene Glacial Lake Missoula: Geological Society of America Bulletin, v. 96, p. 1271-1286.

Waitt, R.B., and B.F. Atwater, 1989, Glacial Lake Missoula and the Channeled Scablands, chapter 5: Stratigraphic and geomorphic evidence for dozens of last-glacial floods, T310, in Glacial Geology and Geomorphology of North America, v. 1, 28th International Geologic Congress Field Trip Guidebook: American Geophysical Union, Washington, D.C., 72 p.

Waitt, R.B., and R.M. Thorson, 1983, The Cordilleran ice sheet in Washington, Idaho, and Montana, in

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Late-Quaternary Environments of the United States, v. 1: University of Minnesota Press, p. 53-70.

Weis, P.L., and G.M. Richmond, 1965, Maximum extent of late Pleistocene Cordilleran glaciation in northeastern Washington and Idaho: U.S. Geological Survey Professional Paper 525-c, p. 128-132.

Winston, D., R.J. Horodyski, and J.W. Whipple, 1989, Middle Proterozoic Belt Supergroup, western Montana, in Volcanism and Plutonism of Western North America, v. 2: 28th International Geologic Congress, American Geophysical Union, p. 1-103.

Wollard, G.P., and J.C. Behrendt, 1961, An evaluation of the gravity control network in North America: Geophysics, v. 50, p. 866-881.

Wyman, S.A., 1994, The potential for heavy metal migration from sediments of Coeur d’Alene Lake into the Rathdrum Prairie aquifer, Kootenai County, Idaho: University of Idaho M.S. thesis, 141 p.

gravity, morphology, and bedrock depth of the rathdrum...

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