geophysical characterization of the precambrian …...magnetic properties of the sub-phanerozoic...

Geophysical Characterization of the Precambrian Basement in Western Saskatchewan

Jiakang Li 1 and Igor Morozov 1

Li, J. and Morozov, I. (2008): Geophysical characterization of the Precambrian basement in western Saskatchewan; in Summary of Investigations 2008, Volume 1, Saskatchewan Geological Survey, Saskatchewan Ministry of Energy and Resources, Misc. Rep. 2008-4.1, CD-ROM, Paper A-8, 18p.

Abstract Regional gravity and high-resolution aeromagnetic maps were compiled for an area that includes western Saskatchewan and the eastern edge of Alberta between 49º to 58ºN and 105º to 111°30'W. This study provides a tectonic framework of Precambrian basement rocks buried beneath Phanerozoic sedimentary rocks. Data analysis methods that were applied are principally the same as those used in the Targeted Geoscience Initiative Phase 2 (TGI-2) Project and included new high-resolution attributes, inversion, and feature extraction calibrated by using well-log data. As a result, we constructed multiple attribute maps that help reveal the complex geological structure of the Precambrian basement in the study area.

A detailed geophysical interpretation was carried out south of the exposed Canadian Shield in western Saskatchewan. Nine primary structural elements previously recognized in the study area were confirmed: 1) the Rae Craton, 2) the Hearne Craton, 3) the Hearne-Reindeer-Sask Boundary Zone, 4) the Glennie Zone, 5) the Sask-Reindeer Boundary Zone, 6) the Sask Craton, 7) the Great Falls Tectonic Zone, 8) the Wyoming Craton, and 9) an unnamed zone. Within these primary structural elements, domains and sub-domains have been differentiated based on distinctive combinations of their geophysical attributes.

Mapping peak gradients of magnetic-source pseudo-gravity and their correlation with gravity, highlighted the details of the structure of the crystalline basement. Based on these geophysical features, the Hearne Craton is subdivided into seven domains: Virgin River, Mudjatik, Makwa, West Battleford, East Battleford, Wathaman, and Swift Current. Only one domain (the Clearwater) of the Rae Craton is discussed. The Sask Craton includes three domains (SC-1, SC-1a, and Humboldt) within the study area. The internal patterns of geophysical fields within each of these domains are described. The magnetic source structural feature and pattern map could be particularly important in relating its structural elements to the fault-block structure of the study area.

Finally, we have integrated data from this study with data from the earlier TGI-2 study of the Phanerozoic-covered portions of eastern Saskatchewan and western Manitoba to provide maps of geophysical fields within this broader area.

Keywords: basement, geophysics, gravity, magnetic, basement topography, western Manitoba, Precambrian, Saskatchewan.

1. Introduction Geophysical mapping of the Precambrian basement in the southern two-thirds of Saskatchewan provides important clues to understanding the tectonics and evolution of the overlying Phanerozoic strata, and, therefore, the generation and migration of hydrocarbons. In several recent projects, the first of which was the Targeted Geoscience Initiative (TGI-2 Williston Basin Project Working Group, 2008), we have studied geophysical attributes of the Saskatchewan and Manitoba portions of the Western Canada Sedimentary Basin. The emphasis was on advanced processing and interpretation, extraction of new structural features and parameters, and providing a seamless coverage of this unusually large area.

In the Geophysical Framework of Western Saskatchewan (GFWS) Project study presented here, our goals were to provide an interpretation of the Precambrian basement within western Saskatchewan, and to integrate this interpretation with that already provided in the TGI-2 study, which covered eastern Saskatchewan and western Manitoba. The main objective was to highlight structures within the crystalline basement, particularly those related to the regional and local tectonics and those that may be related to hydrocarbon traps and migration pathways.

1 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2.

Saskatchewan Geological Survey 1 Summary of Investigations 2008, Volume 1

The GFWS Project was originally defined to cover an area of approximately 375 000 km2 in western Saskatchewan (latitude 49º to 58ºN; longitude 105º to 110ºW). To emphasize the structural relationships in the western parts of this region, the area of geophysical data analysis was expanded to 111º30'W in eastern Alberta. This expanded study covers an area of more than 512 500 km2 (Figure 1). The detailed interpretation of the data in this paper is, however, limited to western Saskatchewan, particularly the area south of the exposed Canadian Shield boundary.

Our ability to learn about the Precambrian basement underlying Phanerozoic strata is limited by the availability of direct and indirect observations of basement rocks. Basement depth is one of the most important crustal parameters which can be established from deep wells and geophysical investigations. However, as the resolution and sensitivity of the different methods to the absolute depths and to their spatial variations vary, different techniques need to be combined. In a previous study, Burwash et al. (1994) produced a map of the basement of the Western Canada Sedimentary Basin on the basis of drill-hole information alone. Reliable lithological data from drill holes reaching the bedrock are unevenly distributed and completely absent over large portions of the study area (Figure 2). In contrast, the available aeromagnetic and gravity data have near-continuous coverage, and are particularly useful for regional structural analysis. They therefore form the basis of our interpretation.

Based on recently surveyed high-resolution aeromagnetic and gravity data, Miles et al. (1997) performed regional gravity and magnetic interpretations in Saskatchewan. Kreis et al. (2004) identified primary structural elements and domains based on gravity and magnetic data within the province. In the earlier TGI-2 study, we used new, high-resolution processing methods to confirm and update the definitions of the primary elements and domain boundaries, to identify smaller scale features, and to estimate the basement topography by using several inversion methods. In order to provide seamless geophysical maps, similar processing methods and filtering parameters were applied in the GFWS study. However, in this paper, we also attempted to establish a more rigorous classification of the observed features of anomalies and to look more closely at the similarities and dissimilarities in the magnetic and gravity anomalies.

Below, following an outline of the data processing and interpretation methods, we describe the structures observed within the primary structural elements and domains and discuss the observed structures and the inverted basement topography. In the final section, we provide basement maps and summarize the structure within a broader area from 96° to 112ºW. The structural trends and patterns within western Saskatchewan and their continuation into Alberta are seen more clearly on these large-scale maps.

This paper is based on Li and Morozov (2007b). A complete set of processed high-resolution geophysical maps resulting from this project is available in PDF format at the website <http://seisweb.usask.ca/reports/West_SK/>. Selected images from this report are reproduced below, and other maps from this site are referenced below as the “West_SK” maps. In addition, for more detail on the area encompassed by the TGI-2 study, we refer the reader to Li and Morozov (2007a), and to the geological maps and database available in TGI-2 Williston Basin Project Working Group (2008).

2. Data and Methods Gravity and aeromagnetic datasets were compiled from the Geological Survey of Canada (GSC) databases, and the borehole data were largely available from the Petroleum Technology Research Centre’s (PTRC) Regional Stratigraphic Framework of Western Saskatchewan Project – Phases I (Marsh and Heinemann, 2005) and II (Marsh and Heinemann, 2006).

The raw aeromagnetic data sets were provided by the GSC as 200 m grids. Our previous experience showed that a 500 m-spacing grid set produced good-resolution, suitably sized data sets, and such gridding was, therefore, selected for this and our TGI-2 studies. Gravity data included merged regional and mineral-exploration data sets which were interpolated into a common 1 km grid. Using 1 km gravity grids improves map resolution, particularly for maps based on derivative processing.

Gravity and aeromagnetic signatures most closely reflect the tectonic and structural character of the area, and potential-field data also provide the necessary spatial continuity of coverage. Gravity data have been successfully used to map crustal-scale and basin-scale structures. Due to steeper distance dependence and higher susceptibility to the metamorphic and deformational processes within crystalline rocks, magnetic anomalies are the most sensitive indicators of the structural character of Precambrian basement covered by sedimentary rocks. Inherent limitations (mostly ambiguities in depth resolution) in potential-field mapping and inversion procedures need, however, to be calibrated and constrained using information available from wireline logs, cores, and seismic data.

The magnetic character of rocks depends on their composition and deformational and metamorphic history. Because of their magnetization, Precambrian basement rocks can be sensed through a cover of Phanerozoic sediments, where


http://seisweb.usask.ca/reports/West_SK/

Figure 1 - Crustal blocks in the study area: primary structural elements (solid black lines), domains (dashed lines south of the Canadian Shield edge; dotted lines north of the shield edge and in Alberta), and interpreted sub-domains (grey lines). Domain names are italicized. Background shows the magnetic field illustrated in Figure 3, lightened for clarity. Edge of the exposed Canadian Shield is shown in purple. For a larger and more detailed image, see West_SK Map 21 in Li and Morozov (2007b); BZ = Boundary Zone.


SaskatchewanAlberta

SaskCraton

WyomingCraton

Unnamed

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atFal

lsTe

cton

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Figure 2 - Precambrian basement elevation (relative to sea level) from interpolated deep-well picks. Well locations are shown by black dots. Data for most of these boreholes are available in Marsh and Heinemann (2005, 2006). Interpolation was performed by using the minimum-tension splines in GMT programs (Wessel and Smith, 1995). Contour interval is 100 m. See also West_SK Map 19 in Li and Morozov (2007b). Also shown are the outlines of tectonic blocks identified in Figure 1: primary structural elements (solid black lines), domains (dashed lines south of the Canadian Shield edge; dotted lines north of the shield edge and in Alberta), and interpreted sub-domains (grey lines). The green contours outline selected anomalies summarized in Table 1. Gravity and magnetic anomalies that could originate from common sources are shown by green solid lines, and anomalies with discordant gravity and magnetic patterns, by green dotted lines. Edge of the exposed Canadian Shield is shown in purple. SCH = Swift Current High.


Elevation ASL,metres

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BA

Table 1 - Selected geophysical anomalies identified in western Saskatchewan (Figures 2 to 5).

present. Magnetic-field anomalies observed in western Saskatchewan are thus largely due to variations in the magnetic properties of the sub-Phanerozoic basement.

To improve the resolution and also to sharpen the geophysical contacts critical for the definition of structural framework of the study region, our data processing generally focused on enhancement of the short-wavelength features (Li and Morozov, 2005). In addition to the standard methods, such as interpolation, filtering, taking several orders of horizontal and vertical derivatives, reduction of magnetic field to the pole, and analytic signal, we also used special techniques to target accurate determination of basement topography and anomaly-source distribution. These methods included:

1) Band-pass filtering; 2) Tilt derivative (TDR); 3) Magnetic source pseudo-gravity (PG); 4) Horizontal-gradient maxima of pseudo-gravity; 5) Isostatic gravity residual; and 6) Euler deconvolution (a method for estimation of the positions of magnetic sources at depth). The attribute maps derived by these methods provided the basis for interpretations described below. The same methods were also used in the TGI-2 study, allowing seamless merging of the maps from both studies.

For consistent interpretation, structural features need to be classified and the corresponding terminology defined. For an object as complex as potentially faulted and deformed crystalline basement covered by up to ~2.5 km of sedimentary rocks that extends over 500 000 km2, a comprehensive quantitative classification of structural features is, unfortunately impossible. The classification used in this paper is, therefore, qualitative. In our discussion below, we thus differentiate: a) the primary structural elements, such as cratons, and tectonic and boundary zones; b) domains; c) sub-domains; d) lineaments and other structural patterns; and e) the relation of the gravity and magnetic fields to common sources. We also broadly refer to a), b), and c) and some smaller scale features as crustal blocks simply to emphasize their structural contrasts with their neighbours.

The existing definitions of primary structural elements and domains are mostly based on rock types, tectonic history, and other geological evidence derived from exposed areas of the Canadian Shield, yet their delineation in the areas covered by the sediments is mostly based on geophysical data. We do not consider primary geological data in this study. Our classification purely relies on geophysical evidence and principally addresses structures at scales smaller than those of the known primary structural elements and domains. With some limited exceptions discussed below, these larger scale blocks were defined in previous geological investigations and are not revised here.


Anomaly Numbers in

Figures 2 to 5

Quality (G-gravity, M-magnetic;

H-high, L-low)

Intensity

Shape

Same Sources of Gravity and Magnetic Anomalies

Clear

Boundary 1 GL; ML medium round yes undetermined2 GH; MH mid-high group yes partly 3 GL; MH medium single yes partly 4 GL; ML medium near round yes partly 5 Mixed medium group yes sub-domain 6 Mixed strong circular group yes sub-domain 7 GH; MH strong body yes sub-domain 8 GL; MH strong narrow elongated no yes 9 GH; MH mid-low ridge, group similar yes 10 GL; ML weak round similar no 11 GH; ML weak single undetermined sub-domain 12 GH; MH mid-low group undetermined yes 13 GL; ML low group yes small contrast 14 GH; MH medium group maybe not 15 GL; MH mid-strong small band nearly yes 16 GL; MH mid narrow elongated nearly yes

WY1 GH; MH mid-strong small area yes yes

Figure 3 - Reduced to the pole total magnetic field (relative to background); see also West_SK Map 10 in Li and Morozov (2007b). Selected localized anomalies discussed in the text and summarized in Table 1 are labelled and contoured in green. Gravity and magnetic anomalies that could originate from common sources are shown by solid green lines, and anomalies with discordant gravity and magnetic patterns, by dotted green lines. Also shown are the outlines of tectonic blocks identified in Figure 1: primary structural elements (solid black lines), domains (dashed lines south of the Canadian Shield edge; dotted lines north of the shield edge and in Alberta), and interpreted sub-domains (grey lines). Edge of the exposed Canadian Shield is shown in purple.


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The primary structural elements are the largest scale structures in the study area and are considered to be related to plate-forming tectonic crustal features such as terranes and ancient orogenic belts, each having a distinct movement history and structure. The boundaries of these primary structural elements are indicated with solid black lines in our maps (e.g., Figure 1 and West_SK Map 21). Tectonic domains form a secondary structural level within four of the primary structural elements in the study area, and their interpreted boundaries are shown in dashed black lines on our maps (e.g., Figure 1) within the areas covered by sediments, where the definition of domains is primarily based on geophysical data. Within the exposed Canadian Shield, domain boundaries are defined by geological outcrops and are shown as dotted lines in our maps (Figure 1 and below; Kreis et al., 2004). In a few places, dotted black lines are used to extend the interpretation into Alberta, but only where the geophysical pattern appears undoubtedly continuous (Figure 1).

The geophysical definition of domains is mainly based on the contrasts in their potential-field attributes, such as amplitudes, spatial trends, texture, and continuity of the anomalies. This includes signs of the anomalies, their intensities, trend directions, and internal patterns. As aeromagnetic anomalies are more directly related than gravity anomalies to the magnetized Precambrian basement and also provide higher spatial resolution, we mainly rely on them in delineating the domains. Also, because gravity and magnetic field anomalies inherently represent spatial integration of the physical properties of the rocks, spatial derivatives of these fields typically better correspond to block boundaries and other structural features. In particular, the aeromagnetic tilt derivative (Verduzco et al., 2004) is highly suitable for mapping basement structures underneath shallow sedimentary basins and shows distinct advantages over other derivative-based attributes. As different geophysical attributes reflect only certain aspects of the physical causes of these anomalies, combining the observations from various attribute maps and dismissing the effects of noise and processing artefacts enable structural patterns to be identified in a unified way.

At smaller scales, sub-domains are differentiated as parts of domains with distinctive patterns of geophysical fields, including lineaments (preferred orientations) or other structural arrangements of geophysical anomalies, such as circular or elongated shapes. Sub-domains, which are outlined by grey lines on our maps (e.g., Figure 1), are potentially associated with relatively localized fracturing, deformation, or metamorphism within a particular domain. The relation of the gravity and magnetic fields to common sources is summarized for 17 selected anomalies in Table 1; these anomalies are also illustrated in Figures 2 to 5.

Lineaments and structural patterns (Figure 4) play the most important role in defining the geophysical domains and, particularly, sub-domains. In addition, they are sometimes also important in defining primary structural elements (e.g., the Great Falls Tectonic Zone). Structural trends within the crustal blocks, truncation of these trends at block boundaries, and characteristic patterns of suture zones can be recognized in the geophysical maps and used for defining the various structures. We will further discuss the relationships of lineaments and structural patterns to the basement structures and the techniques for their identification (leading to the map in Figure 4) below (Lineaments and Structural Patterns).

Definitions of subtle features, particularly lineaments and the relation of the gravity and magnetic fields to common sources, are difficult to formalize and to apply uniformly to the whole study area. This problem will be the subject of further research. At present, however, an interpretative classification of selected, distinctive geophysical anomalies that are key to the definition of some sub-domains, domains, and primary structural elements is useful.

In the geological structural analysis and domain definition, the amplitudes, trends, and shapes of the interior features of the geophysical anomalies are commonly the critical attributes determining the classification. Single-attribute analysis is often prone to uncertain or inconclusive discrimination in potential-field interpretation, and multiple attributes need to be used to produce reliable and geologically meaningful conclusions.

In particular, in the study area, it is important to correlate the gravity and magnetic attributes for meaningful recognition of smaller anomalies (Table 1). The magnetic basement is assumed to coincide with the surface of Precambrian crystalline rocks or igneous rocks of younger age; Phanerozoic sedimentary rocks should generally be non-magnetic and are assumed to produce little or no change in the magnetic field. This assumption is generally supported by the weak correlation of the patterns of magnetic field anomalies with the edge of the Canadian Shield noted above. However, magnetization of the basement rocks is strongly variable, depending in part on their mineralogical composition and the age of metamorphism and cooling history. Adding further complication, the tills may contain reworked magnetic rocks. Consequently, the depths of effective magnetic sources may not automatically coincide with the top of the basement. Consideration of the gravity anomalies helps in identifying true basement-related features (Table 1).

3. Interpretation of Geophysical Maps In the study area (49° to 58°N; 106º to 110ºW), the dominant trend of all geophysical fields changes from southeast–northwest south of about 51ºN to south-north and progressively to southwest-northeast north of ~55ºN


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Figure 4 - Magnetic structural feature lines extracted from magnetic-field derivatives. The edge of the Canadian Shield, and the magnetic-field background as in Figure 1; contours of selected anomalies (Table 1) as in Figures 2, 3, and 5. For labels of the crustal blocks, see Figure 1.


(Figure 1). Within this general trend, the primary structural elements are recognized primarily by coherent contrasts extending across most of the study area. On top of this general pattern, a hierarchy of crustal blocks can be distinguished (Figure 1).

In interpreting the small-scale magnetic patterns, we paid special attention to correlating the edge of the exposed Canadian Shield in the geophysical attribute maps. As expected, the exposed edge of the shield is not a major feature, but it can still be identified by interruptions of the complex smaller scale magnetic patterns associated with magnetized, shallow metamorphic basement rocks. In the vertical derivative (West_SK Map 12) and magnetic structure pattern maps (Figure 4 and West_SK Map 18), the shield edge is generally consistent with geophysical attributes of the smaller scale basement block structure, although some significant differences are also present in places. Within the Rae Craton, the edge of the shield correlates with some of the magnetic anomalies and gravity gradients. In the Virgin River and Mudjatik domains, the edge of the shield is close to the differential line of their small-scale magnetic patterns. In the northern part of the Battleford domains, maps of gravity gradients also show a change in their high-frequency characters near the edge of the shield. In the magnetic maps, the difference is subtle, but appears detectable in the magnetic structural pattern map (Figure 4). In the cumulative Saskatchewan-Manitoba maps (West_SK Maps 24 to 29), the edge of the Canadian Shield within the Superior Province can be followed quite clearly when viewed in detail. However, it appears that the contrast across the exposed edge of the shield is still primarily related to the loss of resolution when imaging rocks covered by the sedimentary basin south of the edge.

a) Primary Structural Elements Boundaries of the primary structural elements show strong gradients and clear contrasts distinguished in both gravity and magnetic maps, with each element having clear interior features that differentiate it from its neighbours.

Conventional approaches to emphasizing the potential-field gradients usually use the first-order horizontal and vertical derivatives. The second vertical derivative of gravity field can, however, be more consistent with geological structural boundaries, as was shown in previous regional gravity and magnetic interpretations (Chandler, 1985; Li and Morozov, 2006). Magnetic pseudo-gravity is also often used to equalize the magnetic and gravity responses, essentially achieved by integrating the magnetic field in space. However, such integration also reduces the frequency content of the data (e.g., makes the magnetic field appear like gravity, which is commonly lower resolution). To overcome this loss of resolution and to highlight the boundaries of the primary structural elements within the basin, we used an alternate approach in this study and computed the second vertical derivative of gravity (i.e., to make the gravity field appear similar to the magnetic field).

Our primary structural element analysis within the study area is mainly based on the attribute maps correlated with the interpretive geophysical map of Saskatchewan by Kreis et al. (2004). The primary structural elements in western Saskatchewan are determined as follows: the Rae Craton, the Hearne Craton, the Hearne-Reindeer-Sask Boundary Zone, the Glennie Zone, the Sask-Reindeer Boundary Zone, the Sask Craton, the Great Falls Tectonic Zone, and the Wyoming Craton (Figure 1). A relatively small area of an unnamed zone has also been identified in southwestern Saskatchewan.

The Rae Craton includes only one domain that is well defined within the GFWS area, the Clearwater Domain. The adjacent Western Granulite (north of the edge of the Canadian Shield) and Firebag domains also have distinct magnetic field patterns, but their contacts with the Clearwater Domain are difficult to delineate (Figures 1, 3, and 4).

The Hearne Craton is the largest primary structural element within the GFWS study area. It shows a prominent northeast trend and widens toward the southwest (Figure 1). Its intensity, shape, and the character of anomalies suggest that it contains a few areas with distinctive structural styles. Based on these features, the Hearne Craton is subdivided into six well defined domains. Given in north-south order, these domains are: Virgin River, Mudjatik, Makwa, West Battleford, East Battleford, and Swift Current (Figure 1).

Only the western portion of the Sask Craton is included in the GFWS study area, and the eastern part was discussed in the TGI-2 study (the two study areas overlap between W105º and 106º). The Sask Craton is subdivided into six domains, three of which (SC-1, SC-1a, and Humboldt) are named in Figure 1.

The Hearne-Reindeer-Sask Boundary Zone, Sask-Reindeer Boundary Zone, Glennie Zone, and the Great Falls Tectonic Zone are narrow, extended structures in the eastern portion of the study area. We have included the Wathaman Domain in the Hearne-Reindeer-Sask Boundary Zone as suggested by Kreis et al. (2004) based on geological evidence. No domain structures have been defined for the Sask-Reindeer Boundary Zone, the Glennie Zone or the Great Falls Tectonic Zone. These zones need to be considered in the context of the broader area including the combined GFWS and TGI-2 Project areas (West_SK Maps 24 to 30).


Geophysical Characterization of the Precambrian Basement in Western Saskatchewan

Jiakang Li 1 and Igor Morozov 1

Li, J. and Morozov, I. (2008): Geophysical characterization of the Precambrian basement in western Saskatchewan; in Summary of Investigations 2008, Volume 1, Saskatchewan Geological Survey, Saskatchewan Ministry of Energy and Resources, Misc. Rep. 2008-4.1, CD-ROM, Paper A-8, 18p.

Abstract Regional gravity and high-resolution aeromagnetic maps were compiled for an area that includes western Saskatchewan and the eastern edge of Alberta between 49º to 58ºN and 105º to 111°30'W. This study provides a tectonic framework of Precambrian basement rocks buried beneath Phanerozoic sedimentary rocks. Data analysis methods that were applied are principally the same as those used in the Targeted Geoscience Initiative Phase 2 (TGI-2) Project and included new high-resolution attributes, inversion, and feature extraction calibrated by using well-log data. As a result, we constructed multiple attribute maps that help reveal the complex geological structure of the Precambrian basement in the study area.

A detailed geophysical interpretation was carried out south of the exposed Canadian Shield in western Saskatchewan. Nine primary structural elements previously recognized in the study area were confirmed: 1) the Rae Craton, 2) the Hearne Craton, 3) the Hearne-Reindeer-Sask Boundary Zone, 4) the Glennie Zone, 5) the Sask-Reindeer Boundary Zone, 6) the Sask Craton, 7) the Great Falls Tectonic Zone, 8) the Wyoming Craton, and 9) an unnamed zone. Within these primary structural elements, domains and sub-domains have been differentiated based on distinctive combinations of their geophysical attributes.

Mapping peak gradients of magnetic-source pseudo-gravity and their correlation with gravity, highlighted the details of the structure of the crystalline basement. Based on these geophysical features, the Hearne Craton is subdivided into seven domains: Virgin River, Mudjatik, Makwa, West Battleford, East Battleford, Wathaman, and Swift Current. Only one domain (the Clearwater) of the Rae Craton is discussed. The Sask Craton includes three domains (SC-1, SC-1a, and Humboldt) within the study area. The internal patterns of geophysical fields within each of these domains are described. The magnetic source structural feature and pattern map could be particularly important in relating its structural elements to the fault-block structure of the study area.

Finally, we have integrated data from this study with data from the earlier TGI-2 study of the Phanerozoic-covered portions of eastern Saskatchewan and western Manitoba to provide maps of geophysical fields within this broader area.

Keywords: basement, geophysics, gravity, magnetic, basement topography, western Manitoba, Precambrian, Saskatchewan.

1. Introduction Geophysical mapping of the Precambrian basement in the southern two-thirds of Saskatchewan provides important clues to understanding the tectonics and evolution of the overlying Phanerozoic strata, and, therefore, the generation and migration of hydrocarbons. In several recent projects, the first of which was the Targeted Geoscience Initiative (TGI-2 Williston Basin Project Working Group, 2008), we have studied geophysical attributes of the Saskatchewan and Manitoba portions of the Western Canada Sedimentary Basin. The emphasis was on advanced processing and interpretation, extraction of new structural features and parameters, and providing a seamless coverage of this unusually large area.

In the Geophysical Framework of Western Saskatchewan (GFWS) Project study presented here, our goals were to provide an interpretation of the Precambrian basement within western Saskatchewan, and to integrate this interpretation with that already provided in the TGI-2 study, which covered eastern Saskatchewan and western Manitoba. The main objective was to highlight structures within the crystalline basement, particularly those related to the regional and local tectonics and those that may be related to hydrocarbon traps and migration pathways.

1 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2.


The GFWS Project was originally defined to cover an area of approximately 375 000 km2 in western Saskatchewan (latitude 49º to 58ºN; longitude 105º to 110ºW). To emphasize the structural relationships in the western parts of this region, the area of geophysical data analysis was expanded to 111º30'W in eastern Alberta. This expanded study covers an area of more than 512 500 km2 (Figure 1). The detailed interpretation of the data in this paper is, however, limited to western Saskatchewan, particularly the area south of the exposed Canadian Shield boundary.

Our ability to learn about the Precambrian basement underlying Phanerozoic strata is limited by the availability of direct and indirect observations of basement rocks. Basement depth is one of the most important crustal parameters which can be established from deep wells and geophysical investigations. However, as the resolution and sensitivity of the different methods to the absolute depths and to their spatial variations vary, different techniques need to be combined. In a previous study, Burwash et al. (1994) produced a map of the basement of the Western Canada Sedimentary Basin on the basis of drill-hole information alone. Reliable lithological data from drill holes reaching the bedrock are unevenly distributed and completely absent over large portions of the study area (Figure 2). In contrast, the available aeromagnetic and gravity data have near-continuous coverage, and are particularly useful for regional structural analysis. They therefore form the basis of our interpretation.

Based on recently surveyed high-resolution aeromagnetic and gravity data, Miles et al. (1997) performed regional gravity and magnetic interpretations in Saskatchewan. Kreis et al. (2004) identified primary structural elements and domains based on gravity and magnetic data within the province. In the earlier TGI-2 study, we used new, high-resolution processing methods to confirm and update the definitions of the primary elements and domain boundaries, to identify smaller scale features, and to estimate the basement topography by using several inversion methods. In order to provide seamless geophysical maps, similar processing methods and filtering parameters were applied in the GFWS study. However, in this paper, we also attempted to establish a more rigorous classification of the observed features of anomalies and to look more closely at the similarities and dissimilarities in the magnetic and gravity anomalies.

Below, following an outline of the data processing and interpretation methods, we describe the structures observed within the primary structural elements and domains and discuss the observed structures and the inverted basement topography. In the final section, we provide basement maps and summarize the structure within a broader area from 96° to 112ºW. The structural trends and patterns within western Saskatchewan and their continuation into Alberta are seen more clearly on these large-scale maps.

This paper is based on Li and Morozov (2007b). A complete set of processed high-resolution geophysical maps resulting from this project is available in PDF format at the website <http://seisweb.usask.ca/reports/West_SK/>. Selected images from this report are reproduced below, and other maps from this site are referenced below as the “West_SK” maps. In addition, for more detail on the area encompassed by the TGI-2 study, we refer the reader to Li and Morozov (2007a), and to the geological maps and database available in TGI-2 Williston Basin Project Working Group (2008).

2. Data and Methods Gravity and aeromagnetic datasets were compiled from the Geological Survey of Canada (GSC) databases, and the borehole data were largely available from the Petroleum Technology Research Centre’s (PTRC) Regional Stratigraphic Framework of Western Saskatchewan Project – Phases I (Marsh and Heinemann, 2005) and II (Marsh and Heinemann, 2006).

The raw aeromagnetic data sets were provided by the GSC as 200 m grids. Our previous experience showed that a 500 m-spacing grid set produced good-resolution, suitably sized data sets, and such gridding was, therefore, selected for this and our TGI-2 studies. Gravity data included merged regional and mineral-exploration data sets which were interpolated into a common 1 km grid. Using 1 km gravity grids improves map resolution, particularly for maps based on derivative processing.

Gravity and aeromagnetic signatures most closely reflect the tectonic and structural character of the area, and potential-field data also provide the necessary spatial continuity of coverage. Gravity data have been successfully used to map crustal-scale and basin-scale structures. Due to steeper distance dependence and higher susceptibility to the metamorphic and deformational processes within crystalline rocks, magnetic anomalies are the most sensitive indicators of the structural character of Precambrian basement covered by sedimentary rocks. Inherent limitations (mostly ambiguities in depth resolution) in potential-field mapping and inversion procedures need, however, to be calibrated and constrained using information available from wireline logs, cores, and seismic data.

The magnetic character of rocks depends on their composition and deformational and metamorphic history. Because of their magnetization, Precambrian basement rocks can be sensed through a cover of Phanerozoic sediments, where


http://seisweb.usask.ca/reports/West_SK/

Figure 1 - Crustal blocks in the study area: primary structural elements (solid black lines), domains (dashed lines south of the Canadian Shield edge; dotted lines north of the shield edge and in Alberta), and interpreted sub-domains (grey lines). Domain names are italicized. Background shows the magnetic field illustrated in Figure 3, lightened for clarity. Edge of the exposed Canadian Shield is shown in purple. For a larger and more detailed image, see West_SK Map 21 in Li and Morozov (2007b); BZ = Boundary Zone.


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Figure 2 - Precambrian basement elevation (relative to sea level) from interpolated deep-well picks. Well locations are shown by black dots. Data for most of these boreholes are available in Marsh and Heinemann (2005, 2006). Interpolation was performed by using the minimum-tension splines in GMT programs (Wessel and Smith, 1995). Contour interval is 100 m. See also West_SK Map 19 in Li and Morozov (2007b). Also shown are the outlines of tectonic blocks identified in Figure 1: primary structural elements (solid black lines), domains (dashed lines south of the Canadian Shield edge; dotted lines north of the shield edge and in Alberta), and interpreted sub-domains (grey lines). The green contours outline selected anomalies summarized in Table 1. Gravity and magnetic anomalies that could originate from common sources are shown by green solid lines, and anomalies with discordant gravity and magnetic patterns, by green dotted lines. Edge of the exposed Canadian Shield is shown in purple. SCH = Swift Current High.



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Table 1 - Selected geophysical anomalies identified in western Saskatchewan (Figures 2 to 5).

present. Magnetic-field anomalies observed in western Saskatchewan are thus largely due to variations in the magnetic properties of the sub-Phanerozoic basement.

To improve the resolution and also to sharpen the geophysical contacts critical for the definition of structural framework of the study region, our data processing generally focused on enhancement of the short-wavelength features (Li and Morozov, 2005). In addition to the standard methods, such as interpolation, filtering, taking several orders of horizontal and vertical derivatives, reduction of magnetic field to the pole, and analytic signal, we also used special techniques to target accurate determination of basement topography and anomaly-source distribution. These methods included:

1) Band-pass filtering; 2) Tilt derivative (TDR); 3) Magnetic source pseudo-gravity (PG); 4) Horizontal-gradient maxima of pseudo-gravity; 5) Isostatic gravity residual; and 6) Euler deconvolution (a method for estimation of the positions of magnetic sources at depth). The attribute maps derived by these methods provided the basis for interpretations described below. The same methods were also used in the TGI-2 study, allowing seamless merging of the maps from both studies.

For consistent interpretation, structural features need to be classified and the corresponding terminology defined. For an object as complex as potentially faulted and deformed crystalline basement covered by up to ~2.5 km of sedimentary rocks that extends over 500 000 km2, a comprehensive quantitative classification of structural features is, unfortunately impossible. The classification used in this paper is, therefore, qualitative. In our discussion below, we thus differentiate: a) the primary structural elements, such as cratons, and tectonic and boundary zones; b) domains; c) sub-domains; d) lineaments and other structural patterns; and e) the relation of the gravity and magnetic fields to common sources. We also broadly refer to a), b), and c) and some smaller scale features as crustal blocks simply to emphasize their structural contrasts with their neighbours.

The existing definitions of primary structural elements and domains are mostly based on rock types, tectonic history, and other geological evidence derived from exposed areas of the Canadian Shield, yet their delineation in the areas covered by the sediments is mostly based on geophysical data. We do not consider primary geological data in this study. Our classification purely relies on geophysical evidence and principally addresses structures at scales smaller than those of the known primary structural elements and domains. With some limited exceptions discussed below, these larger scale blocks were defined in previous geological investigations and are not revised here.


Anomaly Numbers in

Figures 2 to 5

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Boundary 1 GL; ML medium round yes undetermined2 GH; MH mid-high group yes partly 3 GL; MH medium single yes partly 4 GL; ML medium near round yes partly 5 Mixed medium group yes sub-domain 6 Mixed strong circular group yes sub-domain 7 GH; MH strong body yes sub-domain 8 GL; MH strong narrow elongated no yes 9 GH; MH mid-low ridge, group similar yes 10 GL; ML weak round similar no 11 GH; ML weak single undetermined sub-domain 12 GH; MH mid-low group undetermined yes 13 GL; ML low group yes small contrast 14 GH; MH medium group maybe not 15 GL; MH mid-strong small band nearly yes 16 GL; MH mid narrow elongated nearly yes

WY1 GH; MH mid-strong small area yes yes

Figure 3 - Reduced to the pole total magnetic field (relative to background); see also West_SK Map 10 in Li and Morozov (2007b). Selected localized anomalies discussed in the text and summarized in Table 1 are labelled and contoured in green. Gravity and magnetic anomalies that could originate from common sources are shown by solid green lines, and anomalies with discordant gravity and magnetic patterns, by dotted green lines. Also shown are the outlines of tectonic blocks identified in Figure 1: primary structural elements (solid black lines), domains (dashed lines south of the Canadian Shield edge; dotted lines north of the shield edge and in Alberta), and interpreted sub-domains (grey lines). Edge of the exposed Canadian Shield is shown in purple.


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The primary structural elements are the largest scale structures in the study area and are considered to be related to plate-forming tectonic crustal features such as terranes and ancient orogenic belts, each having a distinct movement history and structure. The boundaries of these primary structural elements are indicated with solid black lines in our maps (e.g., Figure 1 and West_SK Map 21). Tectonic domains form a secondary structural level within four of the primary structural elements in the study area, and their interpreted boundaries are shown in dashed black lines on our maps (e.g., Figure 1) within the areas covered by sediments, where the definition of domains is primarily based on geophysical data. Within the exposed Canadian Shield, domain boundaries are defined by geological outcrops and are shown as dotted lines in our maps (Figure 1 and below; Kreis et al., 2004). In a few places, dotted black lines are used to extend the interpretation into Alberta, but only where the geophysical pattern appears undoubtedly continuous (Figure 1).

The geophysical definition of domains is mainly based on the contrasts in their potential-field attributes, such as amplitudes, spatial trends, texture, and continuity of the anomalies. This includes signs of the anomalies, their intensities, trend directions, and internal patterns. As aeromagnetic anomalies are more directly related than gravity anomalies to the magnetized Precambrian basement and also provide higher spatial resolution, we mainly rely on them in delineating the domains. Also, because gravity and magnetic field anomalies inherently represent spatial integration of the physical properties of the rocks, spatial derivatives of these fields typically better correspond to block boundaries and other structural features. In particular, the aeromagnetic tilt derivative (Verduzco et al., 2004) is highly suitable for mapping basement structures underneath shallow sedimentary basins and shows distinct advantages over other derivative-based attributes. As different geophysical attributes reflect only certain aspects of the physical causes of these anomalies, combining the observations from various attribute maps and dismissing the effects of noise and processing artefacts enable structural patterns to be identified in a unified way.

At smaller scales, sub-domains are differentiated as parts of domains with distinctive patterns of geophysical fields, including lineaments (preferred orientations) or other structural arrangements of geophysical anomalies, such as circular or elongated shapes. Sub-domains, which are outlined by grey lines on our maps (e.g., Figure 1), are potentially associated with relatively localized fracturing, deformation, or metamorphism within a particular domain. The relation of the gravity and magnetic fields to common sources is summarized for 17 selected anomalies in Table 1; these anomalies are also illustrated in Figures 2 to 5.

Lineaments and structural patterns (Figure 4) play the most important role in defining the geophysical domains and, particularly, sub-domains. In addition, they are sometimes also important in defining primary structural elements (e.g., the Great Falls Tectonic Zone). Structural trends within the crustal blocks, truncation of these trends at block boundaries, and characteristic patterns of suture zones can be recognized in the geophysical maps and used for defining the various structures. We will further discuss the relationships of lineaments and structural patterns to the basement structures and the techniques for their identification (leading to the map in Figure 4) below (Lineaments and Structural Patterns).

Definitions of subtle features, particularly lineaments and the relation of the gravity and magnetic fields to common sources, are difficult to formalize and to apply uniformly to the whole study area. This problem will be the subject of further research. At present, however, an interpretative classification of selected, distinctive geophysical anomalies that are key to the definition of some sub-domains, domains, and primary structural elements is useful.

In the geological structural analysis and domain definition, the amplitudes, trends, and shapes of the interior features of the geophysical anomalies are commonly the critical attributes determining the classification. Single-attribute analysis is often prone to uncertain or inconclusive discrimination in potential-field interpretation, and multiple attributes need to be used to produce reliable and geologically meaningful conclusions.

In particular, in the study area, it is important to correlate the gravity and magnetic attributes for meaningful recognition of smaller anomalies (Table 1). The magnetic basement is assumed to coincide with the surface of Precambrian crystalline rocks or igneous rocks of younger age; Phanerozoic sedimentary rocks should generally be non-magnetic and are assumed to produce little or no change in the magnetic field. This assumption is generally supported by the weak correlation of the patterns of magnetic field anomalies with the edge of the Canadian Shield noted above. However, magnetization of the basement rocks is strongly variable, depending in part on their mineralogical composition and the age of metamorphism and cooling history. Adding further complication, the tills may contain reworked magnetic rocks. Consequently, the depths of effective magnetic sources may not automatically coincide with the top of the basement. Consideration of the gravity anomalies helps in identifying true basement-related features (Table 1).

3. Interpretation of Geophysical Maps In the study area (49° to 58°N; 106º to 110ºW), the dominant trend of all geophysical fields changes from southeast–northwest south of about 51ºN to south-north and progressively to southwest-northeast north of ~55ºN


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Figure 4 - Magnetic structural feature lines extracted from magnetic-field derivatives. The edge of the Canadian Shield, and the magnetic-field background as in Figure 1; contours of selected anomalies (Table 1) as in Figures 2, 3, and 5. For labels of the crustal blocks, see Figure 1.


(Figure 1). Within this general trend, the primary structural elements are recognized primarily by coherent contrasts extending across most of the study area. On top of this general pattern, a hierarchy of crustal blocks can be distinguished (Figure 1).

In interpreting the small-scale magnetic patterns, we paid special attention to correlating the edge of the exposed Canadian Shield in the geophysical attribute maps. As expected, the exposed edge of the shield is not a major feature, but it can still be identified by interruptions of the complex smaller scale magnetic patterns associated with magnetized, shallow metamorphic basement rocks. In the vertical derivative (West_SK Map 12) and magnetic structure pattern maps (Figure 4 and West_SK Map 18), the shield edge is generally consistent with geophysical attributes of the smaller scale basement block structure, although some significant differences are also present in places. Within the Rae Craton, the edge of the shield correlates with some of the magnetic anomalies and gravity gradients. In the Virgin River and Mudjatik domains, the edge of the shield is close to the differential line of their small-scale magnetic patterns. In the northern part of the Battleford domains, maps of gravity gradients also show a change in their high-frequency characters near the edge of the shield. In the magnetic maps, the difference is subtle, but appears detectable in the magnetic structural pattern map (Figure 4). In the cumulative Saskatchewan-Manitoba maps (West_SK Maps 24 to 29), the edge of the Canadian Shield within the Superior Province can be followed quite clearly when viewed in detail. However, it appears that the contrast across the exposed edge of the shield is still primarily related to the loss of resolution when imaging rocks covered by the sedimentary basin south of the edge.

a) Primary Structural Elements Boundaries of the primary structural elements show strong gradients and clear contrasts distinguished in both gravity and magnetic maps, with each element having clear interior features that differentiate it from its neighbours.

Conventional approaches to emphasizing the potential-field gradients usually use the first-order horizontal and vertical derivatives. The second vertical derivative of gravity field can, however, be more consistent with geological structural boundaries, as was shown in previous regional gravity and magnetic interpretations (Chandler, 1985; Li and Morozov, 2006). Magnetic pseudo-gravity is also often used to equalize the magnetic and gravity responses, essentially achieved by integrating the magnetic field in space. However, such integration also reduces the frequency content of the data (e.g., makes the magnetic field appear like gravity, which is commonly lower resolution). To overcome this loss of resolution and to highlight the boundaries of the primary structural elements within the basin, we used an alternate approach in this study and computed the second vertical derivative of gravity (i.e., to make the gravity field appear similar to the magnetic field).

Our primary structural element analysis within the study area is mainly based on the attribute maps correlated with the interpretive geophysical map of Saskatchewan by Kreis et al. (2004). The primary structural elements in western Saskatchewan are determined as follows: the Rae Craton, the Hearne Craton, the Hearne-Reindeer-Sask Boundary Zone, the Glennie Zone, the Sask-Reindeer Boundary Zone, the Sask Craton, the Great Falls Tectonic Zone, and the Wyoming Craton (Figure 1). A relatively small area of an unnamed zone has also been identified in southwestern Saskatchewan.

The Rae Craton includes only one domain that is well defined within the GFWS area, the Clearwater Domain. The adjacent Western Granulite (north of the edge of the Canadian Shield) and Firebag domains also have distinct magnetic field patterns, but their contacts with the Clearwater Domain are difficult to delineate (Figures 1, 3, and 4).

The Hearne Craton is the largest primary structural element within the GFWS study area. It shows a prominent northeast trend and widens toward the southwest (Figure 1). Its intensity, shape, and the character of anomalies suggest that it contains a few areas with distinctive structural styles. Based on these features, the Hearne Craton is subdivided into six well defined domains. Given in north-south order, these domains are: Virgin River, Mudjatik, Makwa, West Battleford, East Battleford, and Swift Current (Figure 1).

Only the western portion of the Sask Craton is included in the GFWS study area, and the eastern part was discussed in the TGI-2 study (the two study areas overlap between W105º and 106º). The Sask Craton is subdivided into six domains, three of which (SC-1, SC-1a, and Humboldt) are named in Figure 1.

The Hearne-Reindeer-Sask Boundary Zone, Sask-Reindeer Boundary Zone, Glennie Zone, and the Great Falls Tectonic Zone are narrow, extended structures in the eastern portion of the study area. We have included the Wathaman Domain in the Hearne-Reindeer-Sask Boundary Zone as suggested by Kreis et al. (2004) based on geological evidence. No domain structures have been defined for the Sask-Reindeer Boundary Zone, the Glennie Zone or the Great Falls Tectonic Zone. These zones need to be considered in the context of the broader area including the combined GFWS and TGI-2 Project areas (West_SK Maps 24 to 30).


No geological domains and sub-domains are identified within the studied part of the Wyoming Craton in Saskatchewan. Also, the relationship of the Unnamed Zone (Figure 1) to the primary structural elements and to the West Battleford Domain is unclear at this time.

b) Tectonic Domains within the Primary Structural Elements Kreis et al. (2004) used gravity and magnetic fields and their vertical derivatives to define the basic basement blocks in Saskatchewan, which included domains identified within four of the primary structural elements in our study area (Figures 1, 3, and 4). In the following, we propose further refinement of this classification by using the new multiple attribute maps which lead us to extending the domain and inner sub-domain structures.

Rae Craton

The Clearwater Domain, toward its southern end, is dissected by a narrow negative magnetic anomaly. From their magnetic structure patterns (Figure 4) and gravity attributes (not shown in this paper), the two resultant parts of the domain show small but distinct geophysical variations (see anomalies #15 and #16 in Table 1). However, we have not classified them as geophysical sub-domains.

Hearne Craton

The Virgin River Domain extends in a northeast-southwest direction and is located close to 109º45'W in the northern part of the study area. Its high, generally smooth magnetic intensity, nearly axis-symmetric magnetic amplitude distribution and southwest-northeast trend distinguish it from the neighbouring Mudjatik Domain. Near 109ºW and south from the edge of the exposed Canadian Shield, its magnetic intensity appears to decrease and to become less smooth, perhaps due to the effect of the overlying sedimentary rocks.

The Mudjatik Domain shows a northeast-southwest trend as a whole but, in comparison to the Virgin River Domain, appears to be fragmented. This pattern is characteristic of the Hearne Craton. Identification of its eastern edge (between 108º and 109ºW) is difficult, but it can be constrained by using the combined vertical gravity gradient and magnetic attribute maps. In terms of the magnetic field intensity, differentiation of sub-domains is difficult (Figure 3). However, the domain is subdivided into two sub-domains because the orientations and spatial scale-lengths of their internal trends are completely different (Figure 3). Correlation with the vertical derivative of gravity (West_SK Map 4) also supports the existence of these two sub-domains.

The Makwa Domain is represented by two negative- and two positive-polarity magnetic anomaly areas (West_SK Map 21). Based on subtle magnetic differences and on the characters of gravity derivatives, each of the two negative magnetic anomaly areas is subdivided into two sub-domains. Consequently, six sub-domains are recognized within the Makwa Domain (Figure 1).

The West Battleford Domain trends north-northeast–south-southwest in the north-eastern portion of the study area and north-south to north-northwest–south-southeast in the south (Figure 1). Two sub-domains are identified in this area. The southern sub-domain could, however, possibly be defined as two sub-domains with a break at 54º30'N as the difference in the magnetic patterns of its two parts clearly indicates an internal north-south contrast with a relatively sharp transition between the two areas.

The East Battleford Domain, like the West Battleford Domain, trends north-northeast–south-southwest in the northeast, and north-south in the south. It follows the Hearne-Reindeer-Sask Boundary Zone, showing clear boundaries with its neighbours. The magnetic field intensity within its northern part is very strong. In its southern part, the intensity is low to medium, and the central positive anomaly is surrounded by negative anomalies. A total of eight sub-domains are proposed within the East Battleford Domain (Figure 1). Based on the differences between the northern and southern parts of the domain, it could probably be also subdivided into two different domains.

The magnetic anomalies within this domain are very complex (Figures 3 and 4), and their magnitudes do not suggest a clear definition of sub-domains. The sub-domains are principally defined from the smaller scale, internal trends and patterns of the magnetic and gravity fields.

The Swift Current Domain has a west-east–trending northern boundary and is clearly truncated to the west (near W109º) by the West Battleford Domain (Figure 1).

Sask Craton

Domain SC-1, identified in our TGI-2 report, has now been subdivided into SC-1 and SC-1a by adding a new north-northeast–trending boundary (Figure 1). This domain boundary is based on the vertical gravity gradient and


magnetic structure pattern as well as other attributes and corresponds to a gravity-low belt within the western part of the Sask Craton. Because of the prominent and coherent characters of their geophysical contrasts (Figure 1), we assign domain-level status to the SC-1, SC-1a, and Humboldt blocks.

Hearne-Reindeer-Sask Boundary Zone

The Wathaman Domain has a characteristic internal magnetic anomaly pattern (Figure 4) which follows the strike of the Hearne-Reindeer-Sask Boundary Zone, but shows sharp contrasts with the eastern part of the zone and with the adjacent easternmost sub-domain of the East Battleford Domain. From the strong contrasts of gravity and magnetic fields, the Wathaman Domain, including its northern extension within the Canadian Shield, appears to show more affinity to the Hearne Craton (Figures 1 and 3) than to the Hearne-Reindeer-Sask Boundary Zone (HRSZ). By its size, shape, and the character of its positive magnetic anomaly, this domain is similar to three out of the four northeastern sub-domains identified within the East Battleford Domain above (Figures 1, 3, and 4). Nevertheless, in this paper, we have included this domain in the HSRZ as suggested by Kreis et al. (2004) based on geological evidence.

Sask-Reindeer Boundary Zone, Great Falls Tectonic Zone, Glennie Zone, and Unnamed Zone

No domains have been recognized in these four primary structural elements.

Wyoming Craton

No geological domains and sub-domains are identified within the studied part of the Wyoming Craton in Saskatchewan. However, several local magnetic structures and four blocks were identified in this area. In particular, two strong positive magnetic anomalies occupy its northern and eastern parts, and the southern part shows two negative anomalies (Figure 3; Table 1).

c) Selected Geophysical Anomalies and Further Subdivision of Crustal Blocks In addition to the primary-element and domain classification above, the data indicate a number of smaller crustal blocks with pronounced geophysical signatures. In most cases, these blocks represent the contrasts that are important for delineating their enclosing structural elements and domains. For example, within the Wyoming Craton, several local magnetic structures and four blocks were identified; in particular, two strong positive magnetic anomalies occupy its northern and eastern parts, and the southern part shows two negative anomalies (Figure 3).

The individual geological significance of each of these smaller crustal blocks within the GFWS area remains to be investigated, but, as a first step, we have attempted a simple classification of their geometric shapes, textures, and mutual relationships between their gravity and magnetic fields. Table 1 summarizes the characters of 17 identified gravity and magnetic anomalies (see Figures 2 to 5 for locations). The gravity and magnetic anomalies were subdivided into four types according to their characters indicated as “Shape” in Table 1. By their shapes, the anomalies are subdivided into “round” or “elongated.” Among the latter, we occasionally emphasize “ridge”-like anomalies, which are also characterized by distinctive field gradients directed across their axes. Further, within both shape types, we differentiate the anomalies into “single” (or isolated) features and “groups” representing clusters of multiple positive and negative anomalies. When combined, these two characteristics may give rise to “group of ridges” or “group of elongated” descriptions. For the last of these forms, “band” can also be used as a more descriptive term. Finally, for some anomalies, the column “Shape” in Table 1 also contains descriptive attributes related to their size (e.g., “small”, “narrow”, or “body” for the distinctive shape of anomaly #7). Although the simple classification above is subjective and interpretive, it provides basic definitions that are useful in the present discussion and can be refined and formalized for further quantitative analysis in the future.

d) Lineaments and Structural Patterns Lineaments and spatial patterns of geophysical attributes are the most important attributes in gravity and magnetic interpretation. Because geophysical mapping is sensitive to contrasts in source-rock properties, the primary contribution to the observed potential fields should come from the strongest contrast, which is the sediment-basement boundary. The post-depositional sediment-basement contact is not affected by erosion, and therefore structural patterns related, for example, to faulting remain imprinted in it and could be revealed from potential fields. Alignment of anomalies in geophysical maps provides important information for structural analysis. An open question remains, however, as to whether or not all or most magnetic and gravity lineaments correspond to features such as faults, folds, and/or contrasts in metamorphic histories of the basement, and structures at the sediment-basement boundary.

In contrast to the Williston–Blood Creek Basin discussed by Thomas (1974), where a clear criss-crossing block pattern was identified, structures within both the GFWS and TGI-2 study areas can be best described as having


curved trends with varying angles. Structures are commonly elongated and sub-parallel such that most areas are characterized by distinctive patterns of structural feature lines. Only rarely, however, do entire domains or even sub-domains have well defined dominant directions.

Elongated and sub-parallel structures within the basement are theoretically best delineated by their gravity and magnetic gradients, as these gradients reflect the positions of the actual contrasts in physical properties (hence likely lithological changes) within the basement. A domain within the basement can thus be subdivided into sub-domain blocks with similar sub-parallel trends. Some of the domains discussed above were distinguished by using these characters of sub-parallel trends within corresponding areas.

Maximum horizontal gravity gradients generally occur above lithological contacts. Points of maximum gradients form lines drawn along the ridges enclosing gradient highs, and these lines are useful in locating structural trends or contacts (Cordell and Grauch, 1985). This interpretation method, which can be described as the pure “lineament” approach, was used in our recent TGI-2 study.

By contrast to the pure lineament method that simply follows zeros and highs of the horizontal gradient, the new “structural pattern” method used here distinguishes domains and blocks by similar geometries of their magnetic sources (background lines in Figure 4). The observed field patterns could thus be linear, curved or mosaic, suggesting that similar types of subsurface structures are being mapped. The trend-pattern technique used in our TGI-2 interpretation is still incorporated in this classification by providing the general structure of the magnetic field, as well as the lineament orientation.

The magnetic structural patterns are quite different for all crustal blocks within the study area (Figure 4). Thus, the clear extended south-trending pattern of the Hearne-Reindeer-Sask Boundary Zone separates the Hearne Craton from the western part of the Sask Craton, the Glennie Zone, and the Sask-Reindeer Boundary Zone. Even for the Great Falls Tectonic Zone, the boundaries of which would otherwise be difficult to identify, the internal structural and texture patterns clearly distinguish it from the Swift Current Domain and the Wyoming Craton (Figure 4). Lineaments and structural patterns were also critical to the definitions of some of the domains and sub-domains discussed above. Another, albeit somewhat speculative, example of a structural pattern potentially recognized in the Euler basement depth map is described below.

4. Basement Topography The Precambrian basement elevation (relative to sea level) map, based on 76 boreholes penetrating the basement (Figure 2 and West_SK Map 19), provides an important reference for regional analysis. Because of long distances between the wells, however, it lacks details of potential basement faulting and crustal block structure. Although inversion of gravity and magnetic data for source depths is difficult, we tried using the Euler deconvolution method to estimate the depth of magnetic sources in this study. The Euler method should be most sensitive to the top of the magnetic basement, and particularly to basement highs, and may therefore help us to analyze the basement topography even though it is not completely quantitative and is prone to artefacts.

Euler deconvolution is a technique that estimates the locations of magnetic sources in three dimensions (Reid et al., 1990) by inverting the following relation for the position of the source (x0, y0, z0):

NzyxTz

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−+∂∂

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− ),,(ln)(),,(ln)(),,(ln)( 000 , (1)

where T is the magnetic field and N is the Euler structural index. Note that because the magnetic field is entering this equation only through lnT, the method is sensitive to the shapes of anomalies (related to their depths), but not to the absolute magnitude of the field (which is generally proportional to the magnetization). For estimation of the Precambrian basement depth from magnetic data, Euler deconvolution was constrained by using a smoothed starting basement-depth model (Figure 2). Two important parameters of Euler deconvolution were the choice of a 3.5 km-wide window and structural index of N = 0.5, as recommended by other researchers (Williams et al., 2005) and suggested from our previous studies (Li and Morozov, 2006).

Because the Euler method relies on the gradients of the magnetic field, the resulting depth readings relate primarily to the areas of basement heterogeneities identified as distinct sources of the field. These depth values from adjacent spatial windows are further interpolated to produce a map (Figure 5 and West_SK Map 20) showing the positions of magnetic sources. Thus, by the nature of this interpolation favouring high depth values, only the local maxima in the resulting maps should be taken into consideration. In the subsequent interpretation, we are most interested in the relative local variations and positions of the boundaries. Estimation of the absolute depth proved unreliable, so our model relies mainly on the constraints from well log and seismic data. Compared to the Precambrian elevation map,


No geological domains and sub-domains are identified within the studied part of the Wyoming Craton in Saskatchewan. Also, the relationship of the Unnamed Zone (Figure 1) to the primary structural elements and to the West Battleford Domain is unclear at this time.

b) Tectonic Domains within the Primary Structural Elements Kreis et al. (2004) used gravity and magnetic fields and their vertical derivatives to define the basic basement blocks in Saskatchewan, which included domains identified within four of the primary structural elements in our study area (Figures 1, 3, and 4). In the following, we propose further refinement of this classification by using the new multiple attribute maps which lead us to extending the domain and inner sub-domain structures.

Rae Craton

The Clearwater Domain, toward its southern end, is dissected by a narrow negative magnetic anomaly. From their magnetic structure patterns (Figure 4) and gravity attributes (not shown in this paper), the two resultant parts of the domain show small but distinct geophysical variations (see anomalies #15 and #16 in Table 1). However, we have not classified them as geophysical sub-domains.

Hearne Craton

The Virgin River Domain extends in a northeast-southwest direction and is located close to 109º45'W in the northern part of the study area. Its high, generally smooth magnetic intensity, nearly axis-symmetric magnetic amplitude distribution and southwest-northeast trend distinguish it from the neighbouring Mudjatik Domain. Near 109ºW and south from the edge of the exposed Canadian Shield, its magnetic intensity appears to decrease and to become less smooth, perhaps due to the effect of the overlying sedimentary rocks.

The Mudjatik Domain shows a northeast-southwest trend as a whole but, in comparison to the Virgin River Domain, appears to be fragmented. This pattern is characteristic of the Hearne Craton. Identification of its eastern edge (between 108º and 109ºW) is difficult, but it can be constrained by using the combined vertical gravity gradient and magnetic attribute maps. In terms of the magnetic field intensity, differentiation of sub-domains is difficult (Figure 3). However, the domain is subdivided into two sub-domains because the orientations and spatial scale-lengths of their internal trends are completely different (Figure 3). Correlation with the vertical derivative of gravity (West_SK Map 4) also supports the existence of these two sub-domains.

The Makwa Domain is represented by two negative- and two positive-polarity magnetic anomaly areas (West_SK Map 21). Based on subtle magnetic differences and on the characters of gravity derivatives, each of the two negative magnetic anomaly areas is subdivided into two sub-domains. Consequently, six sub-domains are recognized within the Makwa Domain (Figure 1).

The West Battleford Domain trends north-northeast–south-southwest in the north-eastern portion of the study area and north-south to north-northwest–south-southeast in the south (Figure 1). Two sub-domains are identified in this area. The southern sub-domain could, however, possibly be defined as two sub-domains with a break at 54º30'N as the difference in the magnetic patterns of its two parts clearly indicates an internal north-south contrast with a relatively sharp transition between the two areas.

The East Battleford Domain, like the West Battleford Domain, trends north-northeast–south-southwest in the northeast, and north-south in the south. It follows the Hearne-Reindeer-Sask Boundary Zone, showing clear boundaries with its neighbours. The magnetic field intensity within its northern part is very strong. In its southern part, the intensity is low to medium, and the central positive anomaly is surrounded by negative anomalies. A total of eight sub-domains are proposed within the East Battleford Domain (Figure 1). Based on the differences between the northern and southern parts of the domain, it could probably be also subdivided into two different domains.

The magnetic anomalies within this domain are very complex (Figures 3 and 4), and their magnitudes do not suggest a clear definition of sub-domains. The sub-domains are principally defined from the smaller scale, internal trends and patterns of the magnetic and gravity fields.

The Swift Current Domain has a west-east–trending northern boundary and is clearly truncated to the west (near W109º) by the West Battleford Domain (Figure 1).

Sask Craton

Domain SC-1, identified in our TGI-2 report, has now been subdivided into SC-1 and SC-1a by adding a new north-northeast–trending boundary (Figure 1). This domain boundary is based on the vertical gravity gradient and


magnetic structure pattern as well as other attributes and corresponds to a gravity-low belt within the western part of the Sask Craton. Because of the prominent and coherent characters of their geophysical contrasts (Figure 1), we assign domain-level status to the SC-1, SC-1a, and Humboldt blocks.

Hearne-Reindeer-Sask Boundary Zone

The Wathaman Domain has a characteristic internal magnetic anomaly pattern (Figure 4) which follows the strike of the Hearne-Reindeer-Sask Boundary Zone, but shows sharp contrasts with the eastern part of the zone and with the adjacent easternmost sub-domain of the East Battleford Domain. From the strong contrasts of gravity and magnetic fields, the Wathaman Domain, including its northern extension within the Canadian Shield, appears to show more affinity to the Hearne Craton (Figures 1 and 3) than to the Hearne-Reindeer-Sask Boundary Zone (HRSZ). By its size, shape, and the character of its positive magnetic anomaly, this domain is similar to three out of the four northeastern sub-domains identified within the East Battleford Domain above (Figures 1, 3, and 4). Nevertheless, in this paper, we have included this domain in the HSRZ as suggested by Kreis et al. (2004) based on geological evidence.

Sask-Reindeer Boundary Zone, Great Falls Tectonic Zone, Glennie Zone, and Unnamed Zone

No domains have been recognized in these four primary structural elements.

Wyoming Craton

No geological domains and sub-domains are identified within the studied part of the Wyoming Craton in Saskatchewan. However, several local magnetic structures and four blocks were identified in this area. In particular, two strong positive magnetic anomalies occupy its northern and eastern parts, and the southern part shows two negative anomalies (Figure 3; Table 1).

c) Selected Geophysical Anomalies and Further Subdivision of Crustal Blocks In addition to the primary-element and domain classification above, the data indicate a number of smaller crustal blocks with pronounced geophysical signatures. In most cases, these blocks represent the contrasts that are important for delineating their enclosing structural elements and domains. For example, within the Wyoming Craton, several local magnetic structures and four blocks were identified; in particular, two strong positive magnetic anomalies occupy its northern and eastern parts, and the southern part shows two negative anomalies (Figure 3).

The individual geological significance of each of these smaller crustal blocks within the GFWS area remains to be investigated, but, as a first step, we have attempted a simple classification of their geometric shapes, textures, and mutual relationships between their gravity and magnetic fields. Table 1 summarizes the characters of 17 identified gravity and magnetic anomalies (see Figures 2 to 5 for locations). The gravity and magnetic anomalies were subdivided into four types according to their characters indicated as “Shape” in Table 1. By their shapes, the anomalies are subdivided into “round” or “elongated.” Among the latter, we occasionally emphasize “ridge”-like anomalies, which are also characterized by distinctive field gradients directed across their axes. Further, within both shape types, we differentiate the anomalies into “single” (or isolated) features and “groups” representing clusters of multiple positive and negative anomalies. When combined, these two characteristics may give rise to “group of ridges” or “group of elongated” descriptions. For the last of these forms, “band” can also be used as a more descriptive term. Finally, for some anomalies, the column “Shape” in Table 1 also contains descriptive attributes related to their size (e.g., “small”, “narrow”, or “body” for the distinctive shape of anomaly #7). Although the simple classification above is subjective and interpretive, it provides basic definitions that are useful in the present discussion and can be refined and formalized for further quantitative analysis in the future.

d) Lineaments and Structural Patterns Lineaments and spatial patterns of geophysical attributes are the most important attributes in gravity and magnetic interpretation. Because geophysical mapping is sensitive to contrasts in source-rock properties, the primary contribution to the observed potential fields should come from the strongest contrast, which is the sediment-basement boundary. The post-depositional sediment-basement contact is not affected by erosion, and therefore structural patterns related, for example, to faulting remain imprinted in it and could be revealed from potential fields. Alignment of anomalies in geophysical maps provides important information for structural analysis. An open question remains, however, as to whether or not all or most magnetic and gravity lineaments correspond to features such as faults, folds, and/or contrasts in metamorphic histories of the basement, and structures at the sediment-basement boundary.

In contrast to the Williston–Blood Creek Basin discussed by Thomas (1974), where a clear criss-crossing block pattern was identified, structures within both the GFWS and TGI-2 study areas can be best described as having


curved trends with varying angles. Structures are commonly elongated and sub-parallel such that most areas are characterized by distinctive patterns of structural feature lines. Only rarely, however, do entire domains or even sub-domains have well defined dominant directions.

Elongated and sub-parallel structures within the basement are theoretically best delineated by their gravity and magnetic gradients, as these gradients reflect the positions of the actual contrasts in physical properties (hence likely lithological changes) within the basement. A domain within the basement can thus be subdivided into sub-domain blocks with similar sub-parallel trends. Some of the domains discussed above were distinguished by using these characters of sub-parallel trends within corresponding areas.

Maximum horizontal gravity gradients generally occur above lithological contacts. Points of maximum gradients form lines drawn along the ridges enclosing gradient highs, and these lines are useful in locating structural trends or contacts (Cordell and Grauch, 1985). This interpretation method, which can be described as the pure “lineament” approach, was used in our recent TGI-2 study.

By contrast to the pure lineament method that simply follows zeros and highs of the horizontal gradient, the new “structural pattern” method used here distinguishes domains and blocks by similar geometries of their magnetic sources (background lines in Figure 4). The observed field patterns could thus be linear, curved or mosaic, suggesting that similar types of subsurface structures are being mapped. The trend-pattern technique used in our TGI-2 interpretation is still incorporated in this classification by providing the general structure of the magnetic field, as well as the lineament orientation.

The magnetic structural patterns are quite different for all crustal blocks within the study area (Figure 4). Thus, the clear extended south-trending pattern of the Hearne-Reindeer-Sask Boundary Zone separates the Hearne Craton from the western part of the Sask Craton, the Glennie Zone, and the Sask-Reindeer Boundary Zone. Even for the Great Falls Tectonic Zone, the boundaries of which would otherwise be difficult to identify, the internal structural and texture patterns clearly distinguish it from the Swift Current Domain and the Wyoming Craton (Figure 4). Lineaments and structural patterns were also critical to the definitions of some of the domains and sub-domains discussed above. Another, albeit somewhat speculative, example of a structural pattern potentially recognized in the Euler basement depth map is described below.

4. Basement Topography The Precambrian basement elevation (relative to sea level) map, based on 76 boreholes penetrating the basement (Figure 2 and West_SK Map 19), provides an important reference for regional analysis. Because of long distances between the wells, however, it lacks details of potential basement faulting and crustal block structure. Although inversion of gravity and magnetic data for source depths is difficult, we tried using the Euler deconvolution method to estimate the depth of magnetic sources in this study. The Euler method should be most sensitive to the top of the magnetic basement, and particularly to basement highs, and may therefore help us to analyze the basement topography even though it is not completely quantitative and is prone to artefacts.

Euler deconvolution is a technique that estimates the locations of magnetic sources in three dimensions (Reid et al., 1990) by inverting the following relation for the position of the source (x0, y0, z0):

NzyxTz

zzzyxTy

yyzyxTx

xx =∂∂

−+∂∂

−+∂∂

− ),,(ln)(),,(ln)(),,(ln)( 000 , (1)

where T is the magnetic field and N is the Euler structural index. Note that because the magnetic field is entering this equation only through lnT, the method is sensitive to the shapes of anomalies (related to their depths), but not to the absolute magnitude of the field (which is generally proportional to the magnetization). For estimation of the Precambrian basement depth from magnetic data, Euler deconvolution was constrained by using a smoothed starting basement-depth model (Figure 2). Two important parameters of Euler deconvolution were the choice of a 3.5 km-wide window and structural index of N = 0.5, as recommended by other researchers (Williams et al., 2005) and suggested from our previous studies (Li and Morozov, 2006).

Because the Euler method relies on the gradients of the magnetic field, the resulting depth readings relate primarily to the areas of basement heterogeneities identified as distinct sources of the field. These depth values from adjacent spatial windows are further interpolated to produce a map (Figure 5 and West_SK Map 20) showing the positions of magnetic sources. Thus, by the nature of this interpolation favouring high depth values, only the local maxima in the resulting maps should be taken into consideration. In the subsequent interpretation, we are most interested in the relative local variations and positions of the boundaries. Estimation of the absolute depth proved unreliable, so our model relies mainly on the constraints from well log and seismic data. Compared to the Precambrian elevation map,


Figure 5 - Precambrian basement elevation from Euler deconvolution of magnetic-field data. A larger scale image can be viewed in West_SK Map 20. Structural boundaries and anomalies are indicated as in Figure 2. Dotted yellow lines indicate the inferred northwest-southeast and southwest-northeast trend pattern in basement depth anomalies.



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however, the Euler method provides additional information leading to further discussion of the basement structure below.

The general trend of the Precambrian basement depth in the well-based model shows a southwesterly dip direction starting from the edge of the Precambrian Shield and turning south near N51º. A notable basement high appears near 51°N/108ºW and a “plateau” near 50°N/109ºW (in the southern part of the Battleford domains). Two additional localized highs are located near the southern edge of the map between 49°N/108° to 110ºW (Figure 2). This could be expected, as the well coverage is very limited, particularly in the northeastern part of the study area. Examination of the well-data coverage shown in Figure 2 suggests that the (relatively) extended basement “plateaus” near 50°N/109ºW and north of 51ºN may have actually resulted from the interpolation procedure. A more detailed well sampling of the basement depth is only available near about 50º30'N/108°W, near our anomalies #2, #4, and #5, where a relatively complex structure is shown in Figure 2. This structure is well documented as the “Swift Current High” (SCH in Figure 2; Kreis et al., 2004); however, its shape could also be influenced by spatial interpolation with sparse and highly variable well sampling.

The Euler depth model (Figure 5) shows similar general trends, but also suggests that the basement could be significantly more heterogeneous and likely more structurally variable than suggested by the well-pick interpolation (Figure 2). By comparing the different depth levels in the two maps, we found that the 1500 m-depth contour in the Euler source depth map between 51°N/105ºW and about 53°N/111º30'W corresponds best to the Precambrian basement topography from well data.

In addition to the similarity of the general trend, the Euler map shows localized highs near 52°20'N/107º45'W, 50°30'N/108º10'W, 53°15'N/111ºW, and 54°N/110º50'W, and several shallow zones in the southern part of the study area. Such highs may be associated with faulting that also affects the sedimentary cover, in which case they may potentially be related to structural hydrocarbon traps. In addition, somewhat speculatively, note that in several areas in the deeper part of the basin, criss-crossing patterns of northeast-southwest– and northwest-southeast–trending groups of structural highs and lows can be recognized (dotted yellow lines in Figure 5). This is similar to feature trends identified from remote sensing and surface mapping, and could be associated with basement block structures or faulting (Thomas, 1974; Brown and Brown, 1987). The high in the vicinity of 49°N/108°W (Figure 5) may be a reflection of the northerly extension of the Bowdoin Dome in Montana, but may also be partially attributed to edge effects. There are no known structures in the vicinity of the strong high that stretches between 49°N/105° to 106°30'W (Figure 5) and, therefore, this anomaly is attributed to edge effects.

As mentioned above, Euler deconvolution is more reliable for shallower sources, which means that the basement depth variations in Figure 5 should be better resolved in the northeastern part of the study area. The depth pattern in this area appears to correlate well with structural features. Thus, within the Hearne-Reindeer-Sask Boundary Zone, a series of localized structures forms a general trend of deeper magnetic anomalies (Figure 5). By comparison, the reduced to the pole of residual total magnetic field map (Figure 3) shows only a strong gradient belt in this area, without noticeable deep or shallow details.

Finally, the Euler deconvolution method as a technique for inverting for magnetic source distribution is limited by its equation (1). The solution is found by inverting for the source position (x0, y0, z0) for each spatial window and averaging the resulting z0’s (Williams et al., 2005). Note that when, for example, the source of the anomaly is a linear structure extended along the x direction, the gradient of lnT in this equation equals 0 and the resulting x0 is unconstrained. However, the value of x0 still has to be assigned, which is done by regularization included in the inversion. Thus, for linear anomalies (which should be dominant in the study area and also correspond to the selection of the structural index N = 0.5 above), regularization of the algorithm plays an important role in the solution and is intricately imprinted in the resulting image. Such hidden sensitivity of the result to the underlying algorithm is common to many modern, involved numerical inversion methods. In addition, the magnitude of the anomaly T is not included in the averaging of the magnetic source positions (x0, y0, z0), and therefore the image in Figure 5 can be viewed as representing averages of possible depths to magnetic sources without regard for their magnitudes or variance in the estimates among the adjacent windows. This may result in the somewhat scattered appearance of the image in the deeper parts of the basin (Figure 5). These limitations of the deconvolution method could potentially be overcome by different inversion techniques based on spatial “wavelets,” which will be addressed in future research.

5. New Maps of Geophysical Basement Structure under Eastern Alberta, Saskatchewan, and Western Manitoba

Data from this project have been combined with the results from our earlier TGI-2 study to provide coverage of an area that includes eastern Alberta, Saskatchewan, and western Manitoba (49° to 58°N; 96° to 112ºW). Geophysical maps of the Precambrian basement of this large area reveal the patterns of basement structures (West_SK Maps 23 to 30) which are significantly more complex than the earlier Precambrian-depth maps based on the borehole data


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(Burwash et al., 1994). Similar to the analysis above, in addition to the main gravity and magnetic anomaly maps, the following secondary attribute maps are particularly useful for structural analysis of the basement: 1) magnetic structural patterns and definitions of the structural units and domains in the entire area (Figure 6 and West_SK Maps 29, 30, and 31); 2) second vertical derivative of gravity anomaly (Figure 7 and West_SK Map 26); and 3) magnetic tilt derivative (Figure 8 and West_SK Map 28).

In addition to the nine primary structural elements identified in the GFWS study (Figure 1), TGI-2 data have provided identification of three additional primary elements: the Reindeer Zone, the Superior Province, and the Churchill-Superior Boundary Zone (Figure 6). Combined gravity and magnetic geophysical attributes reveal boundaries of these elements and detailed structural patterns within the basement that help define tectonic domains and sub-domains. In addition, maps which include eastern Alberta, illustrate the continuation of structural style from western Saskatchewan into Alberta (Lyatsky et al., 2005).

Figure 6 - Traced magnetic lineament structure (background pattern) in eastern Alberta, Saskatchewan, and western Manitoba (West_SK Map 29). The coloured background shows the band-pass filter reduced to the pole magnetic anomaly. Lines indicate the interpreted primary structural elements (solid black), tectonic domains (dashed), sub-domains (grey), and the edge of the exposed Canadian Shield (purple). HRS = Hearne-Reindeer-Sask.


Figure 7 - Second vertical derivative of gravity from the combined Western Saskatchewan and TGI-2 projects. A larger scale image can be viewed as West_SK Map 26. Lines are as in Figure 6.

6. Conclusions Regional Bouguer gravity and aeromagnetic anomaly mapping of easternmost Alberta and western Saskatchewan (49º to 58ºN; 105º to 111º30'W) outlines a continuous tectonic framework of the Precambrian basement rocks within this area. Several high-resolution imaging methods such as extraction of tilt derivative, magnetic source pseudo-gravity gradient, Euler deconvolution, and feature extraction resulted in new attribute maps that indicate the complex geological structure of the Precambrian basement beneath the cover of Phanerozoic strata.

The area can subdivided into nine primary structural elements with distinct internal structural patterns: 1) the Rae Craton; 2) the Hearne Craton; 3) the Hearne-Reindeer-Sask Boundary Zone; 4) the Sask Craton; 5) the Sask-Reindeer Boundary Zone; 6) the Glennie Zone; 7) the Wyoming Craton; 8) the Great Falls Tectonic Zone; and 9) an unnamed zone in the southwestern corner of the study area. Within four of these, domains and sub-domains are identified based on their internal geophysical trend patterns.

Finally, selected geophysical attribute maps of Saskatchewan, western Manitoba, and easternmost Alberta were produced. They reveal continuous structural patterns of the crystalline basement within the study areas in the three provinces.


Saskatchewan

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Figure 8 - Magnetic tilt derivative map for eastern Alberta, Saskatchewan, and western Manitoba (West_SK Map 28). Lines as in Figure 6.

7. Acknowledgments We thank the Petroleum Technology Research Centre for organizing and funding, through an agreement with the then-Saskatchewan Industry and Resources (now the Saskatchewan Ministry of Energy and Resources, SMER), the GFWS Project. We also thank SMER and Manitoba Industry, Economic Development and Mines for initiating and supporting the TGI-2 Project. Potential-field data were provided by W. Miles (Geological Survey of Canada). A. Marsh provided well interpretations during the PTRC and TGI-2 studies. We appreciate many productive discussions with: K. Kreis (formerly of SMER); Z. Hajnal and D. Gendzwill (University of Saskatchewan); and thank J. Merriam and S. Sule (University of Saskatchewan) for critically reviewing the paper. The Summary of Investigations editors, F. Haidl and C. Gilboy, made invaluable contributions to the presentation. GMT software (Wessel and Smith, 1995) was used in preparation of the maps.

8. References Brown, D.L. and Brown, D.L. (1987): Wrench-style deformation and paleostructural influence on sedimentation in

and around a cratonic basin; in Longman, M.W. (ed.), Williston Basin: Anatomy of a Cratonic Oil Province, Rky. Mtn. Assoc. Geol., p57-70.


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Burwash, R.A., McGregor, C.R., and Wilson, J.A. (1994): Precambrian basement beneath the Western Canada Sedimentary Basin; in Mossop, G.D. and Shetson, I. (comp.), Geological Atlas of the Western Canada Sedimentary Basin, Can. Soc. Petrol. Geol./Alta. Resear. Counc., Calgary, p49-56.

Chandler, V.W. (1985): Interpretation of Precambrian geology in Minnesota using low-altitude, high-resolution aeromagnetic data, the utility of regional gravity and magnetic anomaly maps; Soc. Expl. Geophys., p375-391.

Cordell, L. and Grauch, V. (1985): Mapping basement magnetization zones from aeromagnetic data in the San Juan basin, New Mexico; in Hinze, William J. (ed.), The Utility of Regional Gravity and Magnetic Anomaly Maps, Soc. Expl. Geophys., p181-197.

Kreis, L.K., Ashton, K.E., and Maxeiner, R.O. (2004): Geology of the Precambrian Basement and Phanerozoic Strata, Saskatchewan; Lower Paleozoic Map Series – Saskatchewan, Sask. Industry Resources, Misc. Rep. 2004-8, CD-ROM, Sheet 1 of 8.

Li, J. and Morozov, I. (2005): Potential field investigation of Williston Basin basement, Case Histories II, 2005; CSEG Convention, Calgary, URL<www.cseg.ca/publications/2005>, accessed 22 March 2008.

__________ (2006): Regional geophysical mapping of the Sub-Phanerozoic basement, eastern southern Saskatchewan and southwestern Manitoba; in Saskatchewan and Northern Plains Oil & Gas Symposium, Sask. Geol. Surv., Spec. Publ. No. 19, p1-18.

__________ (2007a): Geophysical investigations of the Precambrian basement of the Williston Basin in south-eastern Saskatchewan and south-western Manitoba; URL <http://seisweb.usask.ca/Reports/TGI2/>, accessed 22 March 2008.

__________ (2007b): Geophysical investigations of the Precambrian basement of the Williston Basin in western Saskatchewan; URL<http://seisweb.usask.ca/Reports/West_SK/>, accessed 22 March 2008.

Lyatsky, H.V., Pană, D.I., and Grobe, M. (2005): Basement structure in central and southern Alberta: insights from gravity and magnetic maps; Alberta Energy and Utilities Board, EUBA/AGS Spec. Rep. 72, 76p.

Marsh, A. and Heinemann, K. (2005): Regional stratigraphic framework of western Saskatchewan – Phase I; Petroleum Technology Research Centre, URL<http://www.ptrc.ca/publications.php?f_action =news_detail&news_id=14022>, accessed 22 March 2008.

__________ (2006): Regional stratigraphic framework of western Saskatchewan – Phase II; Petroleum Technology Research Centre, URL http://www.ptrc.ca/publications.php?f_action=news_detail&news_id=15586, accessed 24 October 2008.

Miles, W., Stone, P.E., and Thomas, M.D. (1997): Magnetic and gravity maps with interpreted Precambrian basement, Saskatchewan; Geol Surv. Can., Open File 3488, 1:500 000-scale map.

Reid, A.B., Allsop, J.M., Granser, H., Millete, A.J., and Somerton I.W. (1990): Magnetic interpretation in three dimensions using Euler deconvolution; Geophys., v55, p80-91.

TGI-2 Williston Basin Project Working Group (2008): Williston Basin architecture and hydrocarbon potential in eastern Saskatchewan and western Manitoba, Targeted Geoscience Initiative II (TGI-2); URL<http://www.gov.mb.ca/stem/mrd/geo/willistontgi/index.html>, accessed 12 April 2008.

Thomas, G.E. (1974): Lineament-block tectonics, Williston–Blood Creek Basin; Amer. Assoc. Petrol. Geol. Bull., v58, p1305-1322.

Verduzco, B., Fairhead, J., Green, M., and MacKenzie, C. (2004): New insights into magnetic derivatives for structural mapping; The Leading Edge, v23, p116-119.

Wessel, P. and Smith, W.H.F. (1995): New version of the Generic Mapping Tools released; EOS Trans. Amer. Geophys. Union, v76, p329.

Williams, S., Fairhead, J.D., and Flanagan, G. (2005): Comparison of grid Euler deconvolution with and without 2D constraints using a realistic 3D magnetic basement model; Geophys., v70, no3, pL13-L21.

geophysical characterization of the precambrian …...magnetic properties of the sub-phanerozoic...

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