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JOURNAL OF SEDIMENTARY RESEARCH, V OL. 75, NO. 1, JANUARY, 2005, P. 4354Copyright 2005, SEPM (Society for Sedimentary Geology) 1527-1404/05/075-43/$03.00 DOI 10.2110/jsr.2005.005

HYPERCONCENTRATED FLOWS AND GASTROLITHS: SEDIMENTOLOGY OF DIAMICTITES ANDWACKES OF THE UPPER CLOVERLY FORMATION, LOWER CRETACEOUS, WYOMING, U.S.A.

MICHAEL J. ZALEHA1 AND SHAYNE A. WIESEMANN2*1Department of Geology, Wittenberg University, Springfield, Ohio 45501-0720, U.S.A.

2Department of Geological Sciences, Indiana University, 1001 East Tenth Street, Bloomington, Indiana 47405-1403, U.S.A.

email: [email protected]

ABSTRACT: Diamictites and matrix-supported wackes of the upperCloverly Formation (Lower Cretaceous) represent rivers laden withvolcaniclastic sediment (hyperconcentrated flows). The Cloverly For-mation of central Wyoming is dominantly fluvial, lacustrine, and playadeposits. The sediments accumulated east of the Cordilleran foredeepduring the early stages of the Sevier orogeny. The diamictites andwackes are decimeters thick and stacked vertically, forming rock bod-ies up to 11 m thick. Pebbles and cobbles within the diamictites andwackes occur as isolated clasts or in poorly defined layers. Many suchclasts exhibit some degree of polish. Clast lithologies can be correlatedto rocks that were exposed in the mountain belt to the west. Provenancedata are consistent with Early Cretaceous movement on the MeadeLaketownParisWillard thrust system. The clay fraction of the diam-

ictites and wackes is dominantly illitesmectite and smectite charac-teristic of altered volcanic ash. Diamictites and matrix-supportedwackes lack primary sedimentary structures (e.g., cross-stratification)but do exhibit soft-sediment deformation features indicative of dewa-tering. Erosional bases and bedding geometries are indicative of chan-nelized flow.

The diamictites and matrix-supported wackes resemble debris-flowdeposits. However, maintaining debris flows that behave as Binghamplastics on low depositional slopes 200400 km from the mountainfront is improbable. Rather, the diamictites and wackes represent hy-perconcentrated flows. Deposition, either en masse by a Newtonian (ornearly Newtonian) flow that was turbulent throughout, or by progres-sive sedimentation from a stratified flow with a basal, incipient, gran-ular mass flow overlain by a turbulent suspension is more consistentwith the field data. Paleohydraulic calculations indicate that grain sizes

up to small pebbles could have been transported in suspension, withlarger clasts transported as bedload, consistent with sedimentologicalevidence. Much of the finer-grained suspended sediment was remobi-lized volcanic ash likely derived from the Idaho batholith to the west-northwest. The polished extraformational pebbles and cobbles, longregarded as dinosaur gastroliths (or stomach stones), are simplyclasts associated with the hyperconcentrated-flow deposits. The polishexhibited by many of the clasts is attributable to transport in ash-ladenflows. The occurrence of polished, extraformational stones in Cloverlydiamictites and wackes may have implications for presumed gastrolithsin other fluvial rocks of the western interior (e.g., the Cedar Mountainand Morrison formations).

INTRODUCTION

A hyperconcentrated flow is a sedimentfluid flow intermediate in naturebetween dilute, fully turbulent, stream flow and viscous, generally nontur-bulent, debris flow (Smith and Lowe 1991; Benvenuti and Martini 2002).Hyperconcentrated flows occur in a variety of depositional settings (in-cluding volcanic, alluvial fan, fluvial, and glaciofluvial) and, hence, theirdeposits should occur throughout much of the stratigraphic record. How-ever, most research has examined Paleogene and younger deposits in high-

* Present address: RMT, Inc., 1143 Highland Drive, Suite B, Ann Arbor, Mich-igan 48108-2237, U.S.A.

relief volcanic settings. To date, there have been few reports of hypercon-centrated flow deposits in rocks of pre-Paleogene age or representing morediverse environments (Sohn et al. 1999 is an exception). This paper doc-uments hyperconcentrated-flow deposits in the upper part of the LowerCretaceous Cloverly Formation in Wyoming. The deposits are preservedas diamictites and wackes associated with a nonmarine foreland-basin fill.The sedimentary environment was a low-relief alluvial plain located hun-dreds of kilometers from the mountain front.

The mechanics of hyperconcentrated flows and the character of theirdeposits are quite variable (Smith and Lowe 1991; Best 1992; Benvenutiand Martini 2002; Manville and White 2003). The process-based approachto the analysis of the Cloverly deposits complements previous studies byproviding additional insight as to the fluid and sediment dynamics of hy-

perconcentrated flows. An additional aspect of the Cloverly diamictites andwackes that is of particular importance is the presence of polished pebblesand cobbles in a finer-grained matrix. The mechanics governing the move-ment of such outsized clasts by hyperconcentrated flows has been some-what problematic (Manville and White 2003). We present a quantitativepaleohydraulic estimate for the entrainment and transport of these clasts.Additionally, the compositions of Cloverly extraformational clasts yieldimportant information regarding events in the Sevier thrust belt to the westfrom which the clasts were derived.

Our interpretation of the polished pebbles and cobbles as components ofhyperconcentrated-flow deposits has additional significance. For nearly acentury these clasts have been regarded as dinosaur gastroliths, or stom-ach stones (Wieland 1906; Brown 1907; Hares 1917; for a historicalaccount, see Stokes 1987; note that in some of the early literature, some

strata that were assigned to the underlying Morrison Formation have sincebeen reassigned to the Cloverly Formation). Researchers cite the polish,the local association with dinosaur-bone-bearing strata, and the presence inmudstones as some of the evidence indicating a dinosaurian origin. Al-though some researchers questioned the dinosaurian origin (Stokes 1942,1944; Moberly 1960; Mirsky 1962; Ostrom 1970), none offered a suitablesedimentological interpretation and, today, the clasts are generally regardedas gastroliths (e.g., Stokes 1987; Whittle and Onorato 2000). The persis-tence of the gastrolith view is, in large part, due to the lack of detailedsedimentological study of the rocks that contain the stones. The stones arepresent in what most researchers have regarded as fine-grained rocks. Be-cause the diamictites and wackes occur in a succession dominated by fluvialand lacustrine deposits, many researches have misinterpreted them as flood-plain deposits, primarily on the basis of the grain size of the dominant

lithology. Hence, the gastrolith interpretation of outsized clasts in flood-plain deposits associated with dinosaur-bone-bearing strata.

Although dinosaur fossils are known from the Cloverly Formation, wefound no dinosaur fossils associated with any of the pebbles and cobbles.Additionally, we found no pebbles or cobbles associated with deposits thatwould be expected to house dinosaur fossils and, hence, gastroliths (e.g.,paleosols, floodplain deposits). Similar pebbles and cobbles do occur withintypical fluvial channel deposits of the middle Cloverly Formation. How-ever, we found no dinosaur fossils associated with those clasts, and suchpebbles and cobbles certainly are not unusual in fluvial channel depositsof the Mesozoic western interior. The data presented in this paper dem-


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44 M.J. ZALEHA AND S.A. WIESEMANN

FIG. 1.Regional outcrop map of the Morrison and Cloverly formations, andlithostratigraphic equivalents. Dots show locations of pebbles, cobbles, diamictites,and wackes identified in this study. BC, Baker Cabin Road; C, Cody; DD, DouglasDraw; FW, Fort Washakie; L, Lander; MS, Maverick Springs; My, Mayoworth;SM, Sheep Mountain; Th, Thermopolis; TS, Ten Sleep. The boxed area to the lowerleft is the approximate area shown in Figure 4.

FIG. 2.Generalized stratigraphy of Upper Jurassic and Lower Cretaceous rocksin central Wyoming (after Zaleha 2003). The rusty beds and subdivisions of theCloverly Formation (A, B, and C) are informal. The Cloverly A interval is boundedby unconformities. The Cloverly AB unconformity represents the missing Aptian.

onstrate that the polished pebbles and cobbles are a product of transportby hyperconcentrated flows and are not dinosaur gastroliths.

LOCATION AND GENERAL GEOLOGY

The Cloverly Formation is Early Cretaceous in age and crops out incentral Wyoming. Fieldwork for this study included outcrops in the WindRiver, Bighorn, and westernmost Powder River Basins (Fig. 1). The Clov-erly Formation is typically 24 m to 79 m thick and composed of fluvial,lacustrine, and playa deposits (Meyers et al. 1992; Zaleha et al. 2001;

Elliott 2002; Elliott et al. 2000; Elliott et al. 2002) which accumulated eastof the Cordilleran foredeep during the early stages of the Sevier orogeny.The Cloverly Formation unconformably overlies the nonmarine JurassicMorrison Formation and is, in turn, overlain by the Sykes Mountain For-mation in the Bighorn Basin and by the rusty beds (an informal memberof the Thermopolis Formation) elsewhere (Fig. 2). Both the Sykes Moun-tain Formation and the rusty beds represent transitional marine and ma-rine transgressive deposits.

The Cloverly Formation has been informally subdivided by earlier work-ers into three intervals, designated A, B, and C (Fig. 2; Meyers et al. 1992;May et al. 1995). The lower A and B intervals are dominated by sandstonesand conglomerates that represent channel-bar deposits of predominantlymeandering rivers (some with associated channel fills). Paleoflow wasmainly toward the northeast (Meyers et al. 1992; Zaleha et al. 2001). Lessabundant mudstones, locally interbedded with relatively thin sandstones,represent floodplain, lacustrine, and, to a lesser extent, fine-grained channel-fill deposits (Nolan 2000; Elliott 2002). Some mudstones contain root andburrow traces, ped structures, and other features indicative of paleosols(Elliott 2002; Elliott et al. 2000).

The C interval is characterized by mudstones, diamictites, and wackes(Elliott 2002; Elliott et al. 2000; Elliott et al. 2002; Wiesemann et al. 2000;Wiesemann 2001). In the Bighorn Basin, the C interval roughly corre-sponds to the upper part of the Little Sheep Mudstone and the lower HimesMembers of the Cloverly Formation (Moberly 1960). Elsewhere, the Cinterval has been informally referred to as the lavender beds (Love1948). The C interval is generally 20 m thick, but may be over 40 m thick

locally. Many mudstones of the C interval exhibit planar lamination andwave-ripple cross-lamination and have been interpreted as lacustrine andplaya deposits (Elliott 2002; Elliott et al. 2002). Other, less abundant mud-

stones, exhibit features indicative of pedogenesis, such as root and burrowtraces, ped structure, slickensides, and clay cutans, and are interpreted aspaleosols (Elliott 2002).

The age of the C interval is largely constrained by the ages of the ad-jacent strata. Fission-track dates of zircons from the C interval have largeerrors and indicate that the C interval may be Neocomian, Aptian, or Albian(Chen and Lubin 1997). However, one sample suggests that the uppermostC interval is Albian (Chen and Lubin 1997). Palynomorphs recovered froma mudstone of the underlying B interval near Manderson, Wyoming, yield-ed a date of Albian (Furer et al. 1997). Two samples from the Fall RiverFormation in the Black Hills, which is correlative to the rusty beds incentral Wyoming, yielded palynological dates of Albian (this study; Way1997). A bentonite in the basal Skull Creek Shale, which overlies the FallRiver Formation in the Black Hills, yielded an 40Ar/39Ar date on sanidinecrystals of Albian (104.4 0.5 Ma; Cobban et al. 1994). Collectively, thedata indicate that the age of the upper Cloverly C interval is Albian.

There are compositional differences between A-interval rocks and thoseof the B and C intervals. A-interval sandstones and conglomerates are dom-inated by gray and black chert grains (Meyers et al. 1992). In contrast, B-interval sandstones and conglomerates are dominated by intraformationallimestone and mudstone, and extraformational clasts of pink and whitequartzite, gray and red chert, silicified limestone (some with body fossils),crystalline quartz, chert conglomerate, and some undifferentiated litholo-gies (May 1992; Wiesemann and Suttner 1999; Wiesemann 2001). Thecompositions of these extraformational clasts are significant because theyare very similar to those of the polished pebbles and cobbles in C-intervaldiamictites and wackes. Gray and black chert, which dominate A-intervalsandstones and conglomerates, constitute less than 10% of B-interval sand-stones and conglomerates (Meyers et al. 1992). All extraformational pebble

and cobble lithologies can be matched with rocks in the thrust belt to thewest (see Provenance section below).

PROVENANCE OF EXTRAFORMATIONAL CLASTS

The compositions of the extraformational pebbles and cobbles provideconstraints on sediment sources and establish that those sources were notlocal. Provenance data also yield information regarding events in the Sevierthrust belt. We examined 433 randomly selected clasts from eight locationsin the Wind River, Bighorn, and westernmost Powder River Basins (Fig.1). No significant variations were evident between the different locations,hence, collective results are discussed below.


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45HYPERCONCENTRATED FLOWS AND GASTROLITHS

Compositions and Sources

The total population of extraformational clasts has a composition of 38%quartzite, 33% silicified limestone, 10% chert, 5% quartz arenite, 5% con-glomerate and litharenite, 4% crystalline quartz, and 5% unidentified li-thologies.

Quartzite and Crystalline Quartz.Quartzite clasts include maroon,coarse-grained, cross-stratified quartzite; pink, submature, pebbly quartzite;

and gray, supermature, vitreous quartzite (Fig. 3A). These clasts likely werederived from the upper Proterozoiclower Cambrian Brigham Group,which contains more than 3000 m of quartzites in southeastern Idaho andnorth-central Utah (Christie-Blick 1982, 1997). The maroon quartzite ap-pears to have been derived from the Proterozoic Mutual Formation. Thepink, pebbly quartzite may have been derived from the Lower CambrianCamelback Mountain Quartzite (or equivalent Tintic or Geertsen Canyonquartzites). The gray quartzite may have come from multiple sources, in-cluding, for example, the Proterozoic Caddy Canyon Formation. Clasts ofwhite and pink crystalline (vein) quartz (Fig. 3A) may have been de-rived from basement rocks.

Silicified Limestone.Clasts of silicified limestones are typically gray,tan, and orange-red. Many clasts contain fossils, including foraminifera(abundant fusilinids), brachiopods, gastropods, echinoderms, bryozoans,and sponge spicules (Fig. 3B, C). Such fossils are common to many of the

Mississippian through Permian carbonates in the thrust belt, such as theMadison, Amsden, Wells, and Phosphoria formations (cf. Moberly 1960;Oberlindacher and Roberts-Tobey 1986). Moberly (1960) noted Permianfusilinids in a jasper clast. Stokes (1944) identified Carboniferous andPermian fusilinids in several clasts. Some silicified limestones exhibit brec-ciated textures.

Chert.Chert clasts are typically gray, black, and red, and mottled com-binations thereof (Fig. 3D). Most chert is different in appearance from thePermian Phosphoria-derived chert common to Lower Cretaceous conglom-erates in the region (e.g., Cloverly A interval, Kootenai Formation; Suttner1969; Furer 1970; May 1992, 1993). Likely sources of the gray and blackchert are chert layers and nodules in Mississippian and Pennsylvanian car-bonates, such as the Mississippian Mission Canyon Formation and the Per-mo-Pennsylvanian Wells Formation (cf. Oberlindacher and Roberts-Tobey

1986). The red chert, as well as other gray and black chert clasts, may bethe result of the disaggregation of conglomerate and pebbly lithareniteclasts (see below). Some red chert also may be unfossiliferous parts of thered silicified limestone discussed above.

Quartz Arenite.Clasts of red, fine-grained, quartz arenite (Fig. 3E)appear to have been derived from Triassic red beds, such as the AnkarehFormation (cf. Lawton 1994). Such units are laterally extensive throughoutsouthern Idaho and northern Utah, and are common to all of the majorthrust sheets in the mountain belt.

Conglomerate and Litharenite.Clasts of conglomerate and pebblylitharenite are of two types. Both types contain abundant grains of grayand black chert, but only one type contains red chert. Those with only grayand black chert are quite distinctive and were derived from conglomeratesand sandstones of the Lower Cretaceous Ephraim Formation (lower Gan-nett Group) in the thrust belt (Fig. 3F; cf. Eyer 1969; Furer 1970; DeCelles

et al. 1993). Clasts with red chert are of unknown origin. Disaggregationof both types of clasts may have supplied some of the gray, black, and redchert grains mentioned above.

Implications

The Sevier orogenic belt developed in association with an Andean-typeplate boundary located at the western margin of North America. Sevierthrusting commenced during the Late Jurassic and continued into the earlyCenozoic (Lageson and Schmitt 1994; Taylor et al. 2000). The major west-ward-dipping thrusts that developed in Idaho, Utah, and Wyoming areshown in Figure 4.

Most of the Cloverly clasts can be correlated to lithologies that werelikely exposed on the Willard and Meade thrust sheets during the EarlyCretaceous, with the exception of the Proterozoic quartzites, basement crys-talline quartz, and the Lower Cretaceous Ephraim clasts. The only possiblesource for the Proterozoic quartzites and crystalline quartz was the Paristhrust sheet because these lithologies were not yet exposed on the otherthrust sheets (Craddock 1992; Yonkee 1992; DeCelles et al. 1993; DeCelles1994; DeCelles and Mitra 1995). The source for the Ephraim clasts was

most likely the Meade thrust sheet. DeCelles et al. (1993) present con-vincing evidence for movement on the Meade thrust, erosion of the Ephra-im Formation, and incorporation of Ephraim clasts into the overlying andadjacent Bechler conglomerate (also part of the Gannett Group). The firstoccurrence of Ephraim clasts in both the Bechler Formation and the Clov-erly B/C interval suggests that these units are stratigraphically equivalent(Zaleha 2003). Our interpretations of provenance and Early Cretaceousmovement on the MeadeLaketownParisWillard thrust system is consis-tent with previous data and interpretations (Wiltschko and Dorr 1983; Crad-dock 1992; DeCelles et al. 1993; DeCelles 1994).

DIAMICTITES AND WACKES

The diamictites and wackes (sensu Pettijohn et al. 1987 and Boggs 1992)

are the most problematic lithologies in the study area. A diamictite is amatrix-supported conglomerate, a conglomerate being any rock with 30% grains 2 mm in size (Boggs 1992, 2001). A wacke is a sandstonewith 15% to 75% matrix of grains 0.03 mm in size (medium silt; Pet-tijohn et al. 1987). Neither term is intended to carry any genetic connota-tion.

Description

Diamictites (Fig. 5) are typically light gray, and individual beds aretypically 0.3 m to 1.0 m thick. Diamictites consist of granules, pebbles,and cobbles surrounded by a matrix of sand-, silt-, and clay-size grains.The composition of the clay-size fraction of both diamictites and wackes,as determined by X-ray diffraction, is dominantly mixed-layer illitesmec-tite and smectite. The cement is silica. Many granules, pebbles, and cobbles

exhibit some degree of polish, but unpolished stones also are common.Mean grain size of the diamictites (as determined from hand samples) istypically granules (23 mm). The amount of matrix generally ranges from30 to 50%. Most diamictites exhibit concentrations of pebbles and cobbles(35 cm, up to 78.5 cm, b axis) in poorly defined, discontinuous layersin the lower 1030 cm of beds. Granules and small pebbles (0.51.5 cm),and more rarely large pebbles and cobbles, occur sparsely distributedthroughout the beds. Aside from these trends, diamictites exhibit no grain-size segregation (Fig. 5A). Locally, gravel-size clasts exhibit a poorly ex-pressed imbricate fabric, particularly in the lower parts of beds (Fig. 5B).Diamictites lack primary sedimentary structures such as cross-stratification,ripple cross-lamination, and planar stratification. Evidence of pedogenesis,such as root and burrow traces, blocky structure, slickensides, clay cutans,and mineral segregations, also is absent. Some diamictites, however, doexhibit soft-sediment deformation features, such as flow structures.

Wackes are typically light gray, and individual beds are typically 0.20.5 m thick. There are two types of wackes: (1) those that are matrix-supported, typically consisting of 50% sand, and (2) those that are grainsupported, typically consisting of 50% sand. Mean grain size (as deter-mined from hand samples and thin sections) typically ranges from veryfine to very coarse sand. The matrix is a mixture of silt- and clay-sizeparticles whose abundance varies from 40 to 70% (typically 4060%).Granules, pebbles, and/or cobbles are sparsely distributed throughout manywackes but typically constitute 1% of the population that is of sand sizeand greater.

Matrix-supported wackes (Fig. 6) are similar to the diamictites; the only


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FIG. 3.Extraformational pebbles and cobbles. Some clasts are reflective because of their high degree of polish. A) Crystalline (vein) quartz, left; quartzite on theright. B) Silicified limestones with fossils. C) Silicified limestone with fusilinids. D) Red and black chert. E) Red quartz arenite. F) Ephraim conglomerate with gray andblack chert. Scales are in centimeters.


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FIG. 4.Tectonic map of the IdahoWyomingUtah salient showing the majorwestward-dipping thrusts (barbed lines) of the Sevier fold-thrust belt (modified fromDeCelles and Cavazza 1999). See Figure 1 for location of this map.

notable difference is that of grain size. Matrix-supported wackes lack pri-mary sedimentary structures and lack evidence of pedogenesis. Granules,pebbles, and cobbles are sparsely distributed but are concentrated locallyin the lower parts of beds, forming poorly defined, discontinuous layers(Fig. 6A, B). Matrix-supported wackes also exhibit soft-sediment defor-mation structures, such as compaction features and flow structures (Fig.6A, C). Load features, such as ball-and-pillow and flame structures, areevident along some basal surfaces.

Grain-supported wackes are markedly different from matrix-supportedwackes and diamictites. Grain-supported wackes exhibit planar laminationand cross-lamination (Fig. 7A, B). Current-ripple cross-lamination is mostcommon. In some cases, it is difficult to determine the specific type ofcross-lamination, but some might be wave-ripple cross-lamination. Soft-sediment deformation features such as distorted and convolute laminae arecommon, particularly in what appear to have been planar-laminated wackes(Fig. 7B). Load features, such as ball-and-pillow and flame structures, arepresent along some basal surfaces. Grain-supported wackes are typicallyinterbedded with decimeter-scale mudstone beds that exhibit planar lami-nation, cross-lamination, and rare climbing-ripple cross-lamination. Burrowtraces, millimeters in diameter and mainly oriented oblique to laminae, arecommon in some mudstone beds (Fig. 7C). Locally, burrowing appears tohave obliterated laminae. Mudcracks are apparently absent.

Diamictites and both types of wackes are typically stacked vertically,forming larger-scale rock bodies 211 m thick (Fig. 8), herein referred toas diamictite/wacke bodies. Lateral extents of these bodies in outcrop areon the order of hundreds of meters to a few kilometers. Diamictite/wackebodies typically overlie laminated mudstones. Basal surfaces are erosionalwith local relief generally 2 m or less. Basal surfaces either parallel theunderlying bedding or are concave upward with observable large-scale re-lief up to 5 m. Hence, some diamictite/wacke bodies exhibit channel mor-

phologies, whereas others are tabular. The lowermost bed of a diamictite/wacke body is typically a diamictite (decimeters to 1 m thick) with intra-formational and extraformational clasts. Intraformational clasts are typicallymudstone comparable to the underlying bed. The tops of diamictite/wackebodies are generally flat and overlain by laminated mudstones with burrowtraces.

Where diamictite/wacke bodies are differentially cemented by silica,bedding is readily evident. In other areas diamictite/wacke bodies are poor-ly to moderately cemented and highly weathered, exhibiting a crumblyappearance. In such cases, internal bedding, if present, is difficult to dis-cern. Where evident, bedding is either parallel to that of subjacent andsuperjacent laminated mudstones, inclined in one direction (typically 10), or convex upward. The outcrop at Douglas Draw is a particularlygood example of bedding geometry (Fig. 8; see Fig. 1 for location). The

outcrop consists of two faces that are roughly perpendicular to each other.In one face, the diamictite/wacke body exhibits a concave-upward basalerosion surface. This surface is overlain by a succession of convex-upwardbeds in which the inclinations decrease upward (Fig. 8A). In the side view,these same beds are inclined in one direction and decrease in inclinationupward (Fig. 8B). Beds throughout this diamictite/wacke body are arrangedinto couplets, or bedsets, that typically exhibit erosional bases. Each bedsetconsists of either a diamictite overlain by a wacke, or a matrix-supportedwacke overlain by a grain-supported wacke. Some bedsets are capped bylaminated mudstones. The Douglas Draw outcrop is typical of diamictite/wacke bodies; however, some lack interbedded grain-supported wackes andmudstones. Still other relatively thin diamictite/wacke bodies lack bedding.

Interpretation

The geometries, bedding, and sedimentary structures of the diamictite/wacke bodies, as well as their association with nonmarine facies, indicatedeposition by some type of subaerial flow. The large amount of illitesmectite and smectite suggests that the clay-size fraction represents alteredvolcaniclastic material, consistent with the findings of Tabbutt and Barreiro(1990), Elliott (2002), and Elliott et al. (2002). Because of this alteration,it is not possible to determine the original grain size of the current clay-size fraction (i.e., originally it may have been coarser than clay size). Theerosional basal surfaces and abundant intraformational mudstone clasts in-dicate that the diamictites and matrix-supported wackes were deposited byflows that were turbulent at least part of the time. Concave-upward basalerosion surfaces indicate channelized flow. The tabular diamictite/wackebodies are interpreted as laterally connected channel deposits comparableto fluvial sandstones and conglomerates of the underlying A and B intervals(Yokoyama 1999; Nolan 2000; Zaleha et al. 2001), as well as fluvial sand-stones elsewhere (e.g., Willis 1993; Zaleha 1997). The lack of primarysedimentary structures indicative of bedload transport, such as planar andcross-stratification, suggest that grain sizes up to small pebbles were trans-ported in suspension. Larger pebbles and cobbles associated with erosionsurfaces likely were transported as bedload, but they were too few to formbedforms.

The diamictites and matrix-supported wackes have affinities for the con-tinuum of processes from fluvial to debris flow. However, the sedimentarycharacteristics of the diamictites and matrix-supported wackes are incom-patible with sediments transported and deposited by typical rivers. Suchdeposits display ubiquitous cross-stratification, planar stratification, and/or


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FIG. 5.Polished slabs of diamictites (in both, up is to the top of the page). A) Poorly sorted diamictite lacking primary sedimentary structures and pedogenic features.B) Diamictite with a poorly expressed imbricate fabric (long axes of grains trending lower left to upper right). Scales are in centimeters.

current-ripple cross-lamination (e.g., Jackson 1976; Ashley 1990). Rather,the diamictites and matrix-supported wackes more closely resemble thedeposits of debris flows. However, debris-flow mechanics and the paleoen-vironmental setting argue against a debris-flow interpretation.

The sediment source for the clasts in the diamictites and wackes was theSevier orogen, located 200 km to 400 km to the west. Reconstructed chan-

nel slopes of B-interval rivers are on the order of 104 (see Zaleha et al.2001 for methods and comparable data). The depositional slopes associatedwith the C interval are assumed to be comparable because the B and Cintervals are in conformable contact and there are numerous associated lakeand playa deposits. The elevation of the Sevier mountain belt during theEarly Cretaceous was on the order of 1.5 km to a maximum of 4 km, withtopographic relief on the order of 12 km (Jordan 1981; Yonkee et al.1989; DeCelles 1994). Given that runout distances for most debris flowsare typically 25 times their descent height (Iverson 1997), debris flowsoriginating in the Sevier mountain belt would be expected to have hadmaximum runout distances on the order of 2550 km. Hence, it appearsphysically impossible that debris flows behaving as non-Newtonian Bing-ham plastics could be maintained on such low slopes 200400 km fromthe mountain front. An alternative explanation is deposition from hyper-concentrated flows. There are both qualitative and quantitative criteria forinterpreting the Cloverly diamictites and matrix-supported wackes as theresult of deposition from either fully turbulent or stratified hyperconcen-trated flows.

Hyperconcentrated Flows.The evidence suggests that the flows re-sponsible for deposition of the diamictites and matrix-supported wackesbehaved largely as Newtonian fluids; either as true, purely Newtonian flu-ids, or as non-Newtonian fluids with negligible shear strength and not be-having as Bingham plastics. Transport and deposition by hyperconcentratedflow can account for these hydraulic characteristics, as well as the sedi-mentological characteristics of the diamictites and matrix-supported wack-es.

A hyperconcentrated flow generally is regarded as a sedimentwatermixture containing 2047 volume percent sediment (4070 weight percent;Beverage and Culbertson 1964; Costa 1988). However, the rheology ofsuch flows is dependent not only on sediment concentration but also onparameters such as sediment composition and size, shear rate, and bedroughness (Hampton 1972; Metzner 1985; Wan and Wang 1994; Wang

and Larsen 1994; Coussot 1995; Wang et al. 1998; Baas and Best 2002).For example, flows may acquire a shear strength (non-Newtonian character)and exhibit laminar flow (or stratified turbulent and laminar flow) at clay-mineral concentrations as low as 313 vol.% (Hampton 1972; Wan andWang 1994; Baas and Best 2002). The acquired shear strength apparentlyresults from electrostatic forces acting between clay-mineral particles.Flows with purely noncohesive particles may remain Newtonian and tur-bulent throughout up to sediment concentrations of 47 vol.% (Rodine andJohnson 1976; Metzner 1985; Wan and Wang 1994). This is relevant tothe Cloverly deposits because it is not possible to determine the originalamount of clay, if any, because of diagenetic alteration of the volcanic ash.Flows with 47 vol.% sediment, regardless of grain composition, exhibitsignificant shear strength and are regarded as debris flows. However, rhe-ology also is dependent upon shear rate. For example, an increase in shearrate may cause either an increase or a decrease in apparent viscosity (de-pending on the details of the flow) such that a given flow may transformfrom Newtonian to non-Newtonian or vice versa (Metzner 1985; Coussot1995). The grain size of bed material and bed roughness also can affectturbulence, drag, and fluid velocity through a variety of processes, someof which may include a series of feedback mechanisms (Wan and Wang1994; Wang and Larsen 1994; Wang et al. 1998; Baas and Best 2002).

The fact that flows with similar sediment concentrations can exhibit dif-ferent rheologies, and vice versa, has led to confusion regarding the use ofthe term hyperconcentrated flow (see Manville and White 2003 for adiscussion). As originally used by Beverage and Culbertson (1964), theterm pertains only to sediment concentration (4080 wt.%), not rheology


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FIG. 6.Matrix-supported wackes. A) Polished slab with dispersed granules anda pebble (up is to the top of the page). The soft-sediment deformation feature as-sociated with the pebble (left) likely resulted from compaction that accompanieddewatering. B) Outcrop of the lower part of a matrix-supported wacke bed with alarge pebble and a cobble. C) Polished slab exhibiting soft-sediment deformationstructures attributable sediment flowage during dewatering (up is to the top of thepage). Scales are in centimeters.

FIG. 7.Polished slabs of grain-supported wackes and an associated mudstone (inall photographs, up is to the top of the page). A) Grain-supported wacke exhibitingan end-on view of current-ripple cross-lamination. B) Grain-supported wacke exhib-iting distorted and convolute laminae. C) Mudstone with burrow traces (severalhighlighted with arrows) and planar laminae. Bioturbation increases vertically.Scales are in centimeters.

(although their research focused on flows with substantial clay-mineral con-centrations). Later, Pierson and Costa (1987) and Costa (1988) applied theterm to flows with 4070 wt.% (2047 vol.%) sediment, but did implicaterheology. To simplify our discussion, we follow the usage of Beverage andCulbertson (1964) and apply the term only with regard to sediment con-centration, but using the values of Costa (1988; i.e., 2047 vol.%). We do,however, acknowledge the rheologic complexities of sediment-laden flowsand their nomenclature.

The physical processes by which sediment is transported by, and depos-ited from, hyperconcentrated flows are unclear. Below, we present twopossible scenarios for the formation of the diamictites and wackes by de-position from hyperconcentrated flows: (1) fully turbulent flow with enmasse deposition, and (2) stratified flow with deposition from a basal, in-

cipient granular mass flow. Both interpretations are consistent with ob-served characteristics and similar in their implications.

Fully Turbulent Flow, en masse Deposition.Under this scenario,each hyperconcentrated flow represents a large-discharge event. The flowwas turbulent throughout and exhibited Newtonian or largely Newtoniancharacteristics. Grains up to granules or small pebbles were transported insuspension, supported by turbulence and the buoyancy of the sedimentwater mixture. Here, the sedimentwater mixture is treated as a single-phase fluid (see Wan and Wang 1994 for a discussion; also Rodine andJohnson 1976). Much of the finer-grained suspended sediment was remo-bilized volcanic ash from mountain slopes upstream, floodplains, and/or


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FIG. 8.Outcrop of the diamictite/wacke bodyat Douglas Draw (see Fig. 1 for location). A)

The deposit displays a concave-upward basalsurface (highlighted by the resistant layerindicated with arrows) that is overlain by asequence of convex-upward beds (mostprominent in the central part of the deposit)whose inclinations decrease upward. Theorientation of current-ripple cross-laminationindicates that paleoflow was out of the page andto the right. The photograph in Figure 8B wastaken perpendicular to this photograph aroundthe left side of the hill. Maximum thickness ofthe deposit is 11.5 m. B) In this view, the bedsare inclined to the right, and their inclinationsdecrease upward. Paleoflow was to the right andinto the page. People at right for scale.

river beds. One probable source for the ash was the Idaho Batholith to thewest-northwest (Tabbutt and Barreiro 1990). Larger pebbles and cobbleswere transported as bedload. Transport of the clasts in ash-laden flows canaccount for the polish exhibited by many of the stones (Elliott 2003).

Deposition results from rapid flow deceleration accompanying passageof the flood wave. Under this scenario, deposition occurred very rapidly,en masse or nearly so (cf. Wan and Wang 1994; Vrolijk and Southard1997; Dinehart 1998). En masse deposition would result in poor grain-sizesegregation and an absence of primary sedimentary structures (Figs. 5A,6A). Upon deposition, the sediment dewatered and compacted, producingthe various soft-sediment deformation features (Fig. 6A, C). Rare imbri-cation (Fig. 5B) may have resulted from compaction, shearing of the de-formable bed by the overriding fluid, or erosion of the bed around thegrains (cf. Allen 1982).

Sediment deposition and dewatering caused a decrease in the sediment

concentration of the flow until it became a more typical two-phase streamflow, i.e., sediment and water treated separately (cf. Sohn et al. 1999). Suchflow transformations have been noted during the waning stages of modernhyperconcentrated flows and inferred from recent deposits (Scott 1988;Best 1992). The grain-supported wackes were deposited during this dilutestream flow. Planar stratification, cross-stratification, and cross-lamination(Fig. 7A) were produced by low-relief bedwaves (Best and Bridge 1992),dunes, and current ripples, respectively (Ashley 1990). Flame structuresassociated with the basal surfaces of some grain-supported wackes attestto the fluidized nature of the underlying deposits. Local climbing-ripplecross-lamination is consistent with flow deceleration, high suspended-sed-iment concentrations, and rapid grain fallout (Ashley et al. 1982). Planar-laminated wackes and mudstones represent the final stages of depositionas suspended sediment settled out of slow-moving or stagnant water (Fig.7B, C). Subsequently, as the substrate stabilized, burrowing organisms dis-rupted the sediment (Fig. 7C). The absence of mudcracks, if not a samplingor preservational bias, indicates that the river was probably perennial.

The above describes the sequence of events leading to the deposition ofa single bedset, or couplet (cf. Sohn et al. 1999). Hence, each bedset rep-resents deposition from a single hyperconcentrated flood. Bedding in diam-ictite/wacke bodies, as is evident at Douglas Draw (Fig. 8), requires suc-cessive hyperconcentrated flows. Such bedding is morphologically similarto that associated with point bars and braid bars in more typical fluvialchannels. However, bedding associated with diamictite/wacke bodies wouldhave been produced by a different process under this scenario because therewas minimal bedload transport. The bedding may reflect areas of prefer-

ential deposition associated with local variations in turbulent shear stressand flow velocity related to channel morphology. The decrease in incli-nation of successive beds vertically through a diamictite/wacke body re-flects progressive channel filling.

In some diamictite/wacke bodies (and unusually thick beds), bedding ispoorly expressed or absent, even in fresh exposures. The uniform appear-ance is attributed to successively stacked diamictites and/or matrix-sup-ported wackes. Because of the similarity of these deposits, bedding surfaceswould be difficult to discern (cf. Major 1997). Similarly, the large pebblesand cobbles that appear to float in the matrix and that could be inter-preted as having been part of the suspended load, may represent bedloadassociated with indiscernible bedding surfaces.

Equations that describe fluid flow and sediment dynamics can be usedto estimate the conditions under which the diamictite and matrix-supportedwacke sediment was transported and deposited. The equations presented in

the Appendix were used to estimate the maximum grain sizes that couldbe transported in suspension by the hyperconcentrated flows, and also toevaluate the conditions necessary to transport the largest pebbles and cob-bles as bedload. The following exercise quantitatively evaluates the feasi-bility of the qualitative interpretations above, but it is not intended as arigorous paleohydraulic reconstruction.

Equation 5 (Appendix) can be solved for volumetric sediment concen-trations in the range for hyperconcentrated flows, i.e., 2047 vol.%. Valuesfor most parameters necessary to apply Equation 5 are given in the Ap-pendix. The remaining parameters that need to be estimated are slope, S,and flow depth, d. Channel slopes were taken to be comparable to thosereconstructed for B-interval rivers in this part of the basin (see Zaleha etal. 2001 for methods and comparable data). As noted previously, this is areasonable assumption given that the B and C intervals are in conformable

contact, and considering the aggrading, nonmarine foreland basin settingand abundant lakes and playas. Channel slopes reconstructed from threeB-interval channel deposits were 1.83 104, 2.00 104, and 2.04 104. In this study we used a channel slope of 2.00 104.

Two different approaches were used to estimate channel depth, d. Thefirst was to assume that the channels were comparable in depth to those ofthe B interval: 4.5 m, 6.8 m, and 6.8 m, mean of 6.0 m. The secondapproach is comparable to the methods applied to fluvial deposits. Thethickness of a diamictite/wacke body was taken as the channel depth, pro-vided that bedding was clearly discernible to ensure that the body consistedof a single channel deposit (i.e., a single story, sensu Bridge and Diemer


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TABLE 1.Paleohydraulic parameters and maximum grain sizes capable of beingtransported in suspension by hyperconcentrated flows of various concentrations,

with channel depth (d) of 6.0 m, and slope (S) of 2.00 104.

VolumetricSediment

Concentrationf

(kg m3)

(N m2)u*

(m s1)D

(mm)

0.000.200.30

0.400.47

100013301495

16601776

11.815.617.6

19.520.9

0.110.110.11

0.110.11

0.28 (med sand)1.36 (vc sand)3.32 (granules)

9.01 (pebbles)19.79 (pebbles)

For convenience, grain size has been converted from meters to millimeters. med: medium; vc: very coarse.

TABLE 2.Paleohydraulic parameters and maximum grain sizes capable of beingtransported in suspension by hyperconcentrated flows of various concentrations,with channel depth (d) estimated from outcrops, and slope (S) of 2.00 104.

VolumetricSediment

Concentrationf

(kg m3)

(N m2)u*

(m s1)D

(mm)

Douglas Draw, lower diamictite/wacke body: d 11.5 m

0.000.200.300.400.47

10001330149516601776

22.630.033.737.440.1

0.150.150.150.150.15

0.52 (c sand)2.53 (granules)61.7 (pebbles)

16.74 (pebbles)36.81 (pebbles)

Douglas Draw, upper diamictite/wacke body: d 2.3 m

0.000.200.300.400.47

10001330149516601776

4.56.06.77.58.0

0.070.070.070.070.07

0.11 (vf sand)0.55 (c sand)1.34 (vc sand)3.65 (granules)8.02 (granules)

Maverick Springs: d 5.5 m

0.000.200.300.400.47

10001330149516601776

10.814.416.117.919.2

0.100.100.100.100.10

0.23 (f sand)1.12 (vc sand)2.74 (granules)7.44 (pebbles)

16.34 (pebbles)

For convenience, grain size has been converted from meters to millimeters. vf: very fine; f: fine; med:medium; c: coarse; vc: very coarse.

1983; Willis 1993; Zaleha 1997) and not vertically stacked channel deposits(i.e., multi-story).

The values above were used in Equation 5 to estimate the maximumgrain size capable of being transported in suspension by the hyperconcen-trated flows. To show the effect of increasing sediment concentration andfluid density, grain sizes for clear water flows (0.00 vol.%) also were cal-culated. Results are presented in Tables 1 and 2. Observations suggest thatfor most diamictites and matrix-supported wackes the maximum grain sizethat appears to have been transported in suspension is 5 mm (i.e., the largest

grains dispersed throughout the deposit). Rarely, this maximum grain sizeis 1015 mm. Results presented in Tables 1 and 2 indicate that it wasphysically possible for the hyperconcentrated flows to transport these grainsin suspension. Additionally, the onset of suspension for grains with sizesgreater than the thickness of the viscous sublayer may occur at Vgs/ u*values greater than 1 (where Vgs is a grains settling velocity in a sedimentwater mixture and u* is the shear velocity), possibly as high as 2.5 (Ninoet al. 2003; also see discussion in Komar 1988). This would effectivelyincrease the grain-size results in Tables 1 and 2. Hence, our estimates ofthe maximum grain size capable of being transported in suspension areconservative.

The calculated bed shear stresses (according to Equation 7 in the Ap-pendix) permit an evaluation of the feasibility of transporting the largerpebbles and cobbles as bedload. The maximum grain size that appears tohave been transported as bedload in most diamictites and wackes is 35

cm (b axis). The largest cobbles that we observed, although rare, are onthe order of 78.5 cm (b axis). Relationships between critical shear stressand grain size are imperfect (see discussions in Komar 1988 and Manvilleand White 2003). Studies that have attempted to relate the two often treatthe threshold of entrainment on a bed of uniform grain size with a fluiddensity that of water. Such was clearly not the case for the diamictites andwackes because the large pebbles and cobbles are sparsely distributed. Adecrease in the mean grain size of bed material results in a decrease of thecritical shear stress (Middleton and Southard 1984). Further, an increase influid density also produces a decrease in the critical shear stress (althoughan increase in suspended-sediment concentration also tends to damp tur-bulence). With these considerations in mind, comparisons of the bed shearstresses in Tables 1 and 2 with standard relationships between critical shearstress versus grain size (e.g., Komar 1988; Bridge 2003) indicate that thecalculated shear stresses are of the order necessary to transport the largerpebbles and cobbles as bedload.

Stratified Flow.This interpretation of the diamictites and matrix-sup-ported wackes invokes essentially the same processes as the model pro-posed by Manville and White (2003) for sediment-laden flows associatedwith the Taupo breakout flood deposits (although the settings are different).Many aspects of their model are similar to those presented by Postma etal. (1988), Best (1992), Sohn (1997), and others. The reader is referred tothese four papers, and references therein, for details of the sediment dy-namics. Below we present the fundamentals of the model as they pertainto the Cloverly deposits, with some minor modifications to fit our obser-vations.

In this scenario, the flow is turbulent throughout during rising stage,eroding the channel and entraining intraformational mudstone clasts. Aspeak discharge is approached, progressively more sediment is added to theflow by bed erosion and sediment delivery from upstream. The clay fractionof the total sediment load is negligible. As mentioned previously, much ofthe clay-size fraction in the diamictites and wackes originally may havebeen granular, noncohesive volcaniclastics. Sediment concentration contin-ues to increase until vertical variations in sediment concentration cause theflow to become stratified (gravity transformation of Fisher 1983) into abasal incipient granular mass flow, a transition zone, and a superjacentturbulent suspension (cf. Vrolijk and Southard 1997). This type of strati-fication for granular, sediment-laden flows is opposite that observed byBaas and Best (2002) for clay-rich hyperconcentrated flows where the basal

layer is turbulent and the superjacent zone is laminar plug flow. Sedimentconcentrations decrease vertically throughout the entire flow. The basallayer possesses the highest sediment concentration; the transition zone isan area of rapid vertical decrease in sediment concentration; and the tur-bulent suspension is an area of gradual vertical decrease in sediment con-centration.

The incipient granular mass flow (frictional zone of Sohn 1997) de-velops above the static bed. A granular mass flow (sensu Iverson and Vall-ance 2001; essentially a debris flow for the purposes here) exhibits a time-dependent rheology that is a function of mixture agitation, sediment con-centration, and fluid pressure. Manville and White (2003) include the wordincipient to indicate that propagation of the basal flow is due not onlyto inertial gravity forces (as in the case of a granular mass flow sensustricto) but also to tangential boundary shear stresses imposed by the su-perjacent turbulent suspension (cf. Vrolijk and Southard 1997). The incip-ient granular mass flow exhibits laminar flow and shearing.

The basal flow is coupled to the superjacent turbulent flow by turbulentshear and momentum transfer resulting from particle exchange between thetwo layers. As a result, the basal layer is capable of flowing on low slopes(Manville and White 2003; Rodine and Johnson 1976). This momentumtransfer occurs across the transition zone (the collisional zone of Sohn1997) which is dominated by grain-to-grain interactions.

The downward transport of particles by turbulence and rapid suspensionsettling results in the transfer of particles from the turbulent flow to theincipient granular mass flow. Particles with variable hydrodynamic prop-erties will inevitably intersect the incipient granular mass flow because of


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their random turbulent motions. High particle concentrations inhibit thetransfer of remobilized grains from lower to upper parts of the flow. Theseprocesses produce poor sorting in the granular mass flow and result in apoorly sorted deposit.

Deposition from the incipient granular mass flow occurs by incrementalfrictional freezing near the base. This accounts for the lack of primarysedimentary structures such as cross-stratification. Continuous sedimenta-tion at the base of the flow results in a steady rise of the depositional

surface (i.e., the static bed). The thickness of the granular mass flow ismaintained by the continuous transfer of sediment from the superjacentturbulent suspension. Progressive aggradation of the bed results in a depositwhose thickness is a function of the rate and duration of sedimentationrather than the overall depth of the flow.

Clasts too large to be transported in suspension travel along the staticbed. Clasts smaller than the thickness of the incipient granular mass floware entrained within it, transported by the rheologic strength of the flow,buoyancy, and grain-to-grain collisions. These clasts, as well as othersmaller pebbles and granules, experience shearing, which may result inimbrication or flow-parallel alignment. Clasts larger than the thickness ofthe basal layer project into the turbulent suspension, where they are exposedto strong viscous drag and Bernoulli lift forces. These larger clasts moveas bedload by rolling, which is possible because the basal layer in which

they are partially embedded is deformable. Because of this rolling action,larger clasts may exhibit imbrication. With progressive sedimentation,clasts of all sizes eventually become buried by the rising aggradationalsurface. Hence, the position of each clast represents the former position ofthe aggrading bed.

Clasts may form crude layers along the bed at an instant in time. Largerclasts that are dispersed throughout the diamictites and matrix-supportedwackes also reflect the former bed position. However, because sedimen-tation occurs by continuous frictional freezing, no sharp textural boundaryis produced. This would account for large floating clasts that are notassociated with a discernible bedding surface (Fig. 6B).

During waning flow stages, the rate at which sediment is transferred fromthe basal flow to the static bed exceeds the rate at which sediment is sup-plied from the superjacent turbulent suspension. This causes the incipientgranular mass flow to thin and eventually disintegrate. The entire flow is

now a fully turbulent, dilute stream flow (i.e., a typical river flow). Fromthis point, interpretations of the grain-supported wackes, interbedded mud-stones, and bedding geometries follow those discussed previously.

The calculations regarding suspension criteria are applicable to the abovescenario only during rising flow stages that precede the development of thebasal layer, and also to entrainment of grains from the surface of the in-cipient granular mass flow. Relating bed shear stress to the transport ofpebbles and cobbles as bedload is invalid. A possible exception is a casewhere most of the surface area of the largest cobbles protrude into theturbulent suspension.

CONCLUSIONS

Diamictites and matrix-supported wackes of the upper Cloverly Forma-tion are the deposits of channelized hyperconcentrated flows laden withvolcaniclastic sediment (ash). Deposition from the flows occurred either enmasse by a Newtonian (or nearly Newtonian) flow that was turbulentthroughout, or by progressive sedimentation from a stratified flow with abasal, incipient, granular mass flow overlain by a turbulent suspension.During individual depositional events, progressive sedimentation causedthe hyperconcentrated flows to transform into more typical two-phasestream flows from which the grain-supported wackes were deposited. Sub-sequent dewatering produced various soft-sediment deformation structures.Vertically stacked diamictite/grain-supported wacke couplets (and matrix-supported wacke/grain-supported wacke couplets) represent successiveflows and reflect progressive channel filling. Channels were a few meters

to 11 m deep. Paleohydraulic calculations indicate that clasts up to smallpebbles could have been transported in suspension, with larger clasts trans-ported as bedload. Much of the finer-grained suspended sediment was re-mobilized volcanic ash likely derived originally from the Idaho Batholithto the west-northwest.

The lithologies of extraformational pebbles and cobbles within the diam-ictites and wackes can be correlated to rocks that were exposed in theSevier mountain belt to the west. Provenance data is consistent with Early

Cretaceous movement on the MeadeLaketownParisWillard thrust sys-tem. The first occurrence of these clasts in fluvial sandstones and conglom-erates of the Cloverly B interval, and their first association with the diam-ictites and wackes within the C interval, suggest that these deposits can beused for stratigraphic correlation.

The extraformational pebbles and cobbles, long regarded as dinosaurgastroliths, are simply clasts deposited by the hyperconcentrated flows. Thepolish exhibited by many of the clasts is attributable to transport in theash-laden flows. We do not deny the existence of gastroliths. Those foundin the rib cages of, or in close association with, dinosaur fossils (e.g.,Gillette 1995; Mateus 1998; Sanders et al. 2001) are excellent candidates(although some of those are debated, as well; Lucas and Heckert 2000).However, the stones in diamictites and wackes of the upper Cloverly For-mation are not gastroliths. Researchers are cautioned to carefully examine

the sedimentology of nonmarine rocks housing similar stones, such as theCedar Mountain and Morrison formations.

ACKNOWLEDGMENTS

This research was funded, in part, by National Science Foundation grant EAR-9628367 to L.J. Suttner and Zaleha. Critical reviews for the Journal of SedimentaryResearch by G. Nadon, C. Paola, and M. Stokes, and technical editing by J. Sou-thard, G. Nadon, and M.J. Kraus, are appreciated and improved the manuscript. K.L.Milliken handled the final editorial phases. We thank L.J. Suttner for assistance inclast identification, and W.S. Elliott and E. Kvale for their assistance in the field.

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