Cloverly Formation Research Articles

Tectonically induced climate and its control on the distribution of depositional systems in a continental foreland basin, Cloverly and Lakota Formations (Lower Cretaceous) of Wyoming, U.S.A.

Continental sediments of the Cloverly and Lakota Formations (Lower Cretaceous) in Wyoming are subdivided into three depositional systems: perennial to intermittent alluvial, intermittent to ephemeral alluvial, and playa. Chert-bearing sandstones, conglomerates, carbonaceous mudrocks, blocky mudrocks, and skeletal limestones were deposited by perennial to intermittent alluvial systems. Carbonaceous mudrocks contain abundant wood fragments, cuticle and cortical debris, and other vascular plant remains representing deposition in oxbow lakes, abandoned channels, and on floodplains under humid to seasonal conditions. Intraformational conglomerates, sandstones, bioturbated and blocky mudrocks with caliche nodules, and bioturbated limestones characterize deposition in intermittent to ephemeral alluvial systems. Bioturbated limestones are encased in bioturbated mudrocks with abundant pseudo-slickensides. The presence of caliche nodules in some of the blocky to bioturbated mudrocks is consistent with supersaturation and precipitation of calcium carbonate from groundwater under semi-arid conditions. Caliche nodules, pseudo-slickensides, and carbonate-rich floodplain sediments are interpreted to have been deposited by intermittent to ephemeral alluvial systems under seasonal to semi-arid climatic conditions. Laminated mudrocks, siltstones, vuggy carbonates, bedded to nodular evaporites, pebbly mudrocks, and diamictites were deposited in evaporative alkaline lakes or playas. Pebbly mudrocks and diamictites are interpreted to represent deposition from channelized and unchannelized hyperconcentrated flows on a playa, resulting from intense rain events within the basin. The areal abundance and distribution of these depositional systems change systematically across the overfilled portion of the Early Cretaceous Cordilleran foreland basin in Wyoming. The lower part (A-interval) of the Cloverly and Lakota Formations is characterized by deposits of perennial to intermittent rivers that existed 300 to 1000 km east of the Sevier fold-and-thrust belt. Proximal to the Sevier fold-and-thrust belt, the A-interval of the Cloverly Formation and upper Ephraim Formation of the Gannett Group are typified by deposits of intermittent to ephemeral rivers and their associated floodplains. In the middle part (B-interval) of the Cloverly Formation, intermittent to ephemeral alluvial systems expand to 600 km into the basin. The upper part (C-interval) of the Cloverly Formation is characterized by playa deposits in the Bighorn and Wind River Basins and intermittent to ephemeral alluvial deposits along the front of the ancestral Sevier Mountains. Deposits of perennial to intermittent alluvial systems in the C-interval of the Cloverly and Lakota Formations are restricted to the Black Hills region, almost 900 km to the east of the Sevier Mountains. The change in the areal distribution of depositional systems through time within this continental foreland basin may be attributed to the development of a rain shadow associated with the uplift of the Sevier Mountains in the Early Cretaceous.

Sevier orogenesis and nonmarine basin filling: Implications of new stratigraphic correlations of Lower Cretaceous strata throughout Wyoming, USA

Lower Cretaceous nonmarine rocks throughout Wyoming have been used to date movements on major thrusts during the Sevier orogeny, evaluate the sedimentary response in the adjacent foreland basin, reconstruct subsidence histories, evaluate the driving mechanisms of that subsidence, and have been applied to basin modeling studies. However, detailed correlation and dating of these strata have been problematic for a century, making most detailed interpretations equivocal. New age data and correlations presented here have important implications for the evolution of the Sevier orogen and controls on basin fi lling. Lower Cretaceous strata in Wyoming show the former existence of a foreland basin fi ll that displays a thick foredeep depozone to the west (the Gannett Group), a thin forebulge depozone throughout central Wyoming (the Cloverly Formation), and thickening into a backbulge depozone in the Black Hills area to the east (the Lakota Formation). Basin subsidence is attributable not only to orogenic loading, but to dynamic loading as well. New data and correlations indicate that a thrust load was in place during the Neocomian (attributable to movement on the Paris-Willard thrust system) and that sediment supply was high (as represented by the upper Ephraim Formation). Subsequent deposition of the Peterson Limestone in the foredeep during the latest Neocomian and Aptian, and the development of a regional unconformity elsewhere, indicate a reduction in sediment supply that may have been caused by (1) a change to a drier climate, possibly related to progressive mountain-belt uplift and development or intensifi cation of an orographic rain shadow, or (2) a decrease in mountain-belt uplift rate and a reduction in mountain-belt relief. Unconformity development on the forebulge is consistent with the fi rst scenario. Subsequent deposition of the Bechler, upper Cloverly, and upper Lakota Formations during the Early to Middle Albian indicates increased sediment supply, progressive unroofi ng of the mountain belt, and movement on the Meade-LaketownParis-Willard thrust system. These events suggest renewed or accelerated mountainbelt uplift during the early to middle Albian. The Draney Limestone, which overlies the Bechler, correlates with an unconformity at the top of the Cloverly and Lakota Formations and is Albian in age. The Draney Limestone and the unconformity signify a return to conditions similar to those that existed during deposition of the Peterson Limestone. The results presented here differ from those of some researchers who contend that a foredeep did not exist during initial deposition of these strata, and that deposition was not coincident with thrusting during early stages of the Sevier orogeny.

Hyperconcentrated Flows and Gastroliths: Sedimentology of Diamictites and Wackes of the Upper Cloverly Formation, Lower Cretaceous, Wyoming, U.S.A.

Abstract Diamictites and matrix-supported wackes of the upper Cloverly Formation (Lower Cretaceous) represent rivers laden with volcaniclastic sediment (hyperconcentrated flows). The Cloverly Formation of central Wyoming is dominantly fluvial, lacustrine, and playa deposits. The sediments accumulated east of the Cordilleran foredeep during the early stages of the Sevier orogeny. The diamictites and wackes are decimeters thick and stacked vertically, forming rock bodies up to 11 m thick. Pebbles and cobbles within the diamictites and wackes occur as isolated clasts or in poorly defined layers. Many such clasts exhibit some degree of polish. Clast lithologies can be correlated to rocks that were exposed in the mountain belt to the west. Provenance data are consistent with Early Cretaceous movement on the Meade–Laketown–Paris–Willard thrust system. The clay fraction of the diamictites and wackes is dominantly illite–smectite and smectite characteristic of altered volcanic ash. Diamictites and matrix-supported wackes lack primary sedimentary structures (e.g., cross-stratification) but do exhibit soft-sediment deformation features indicative of dewatering. Erosional bases and bedding geometries are indicative of channelized flow. The diamictites and matrix-supported wackes resemble debris-flow deposits. However, maintaining debris flows that behave as Bingham plastics on low depositional slopes 200–400 km from the mountain front is improbable. Rather, the diamictites and wackes represent hyperconcentrated flows. Deposition, either en masse by a Newtonian (or nearly Newtonian) flow that was turbulent throughout, or by progressive sedimentation from a stratified flow with a basal, incipient, granular mass flow overlain by a turbulent suspension is more consistent with the field data. Paleohydraulic calculations indicate that grain sizes up to small pebbles could have been transported in suspension, with larger clasts transported as bedload, consistent with sedimentological evidence. Much of the finer-grained suspended sediment was remobilized volcanic ash likely derived from the Idaho batholith to the west-northwest. The polished extraformational pebbles and cobbles, long regarded as dinosaur gastroliths (or stomach stones), are simply clasts associated with the hyperconcentrated-flow deposits. The polish exhibited by many of the clasts is attributable to transport in ash-laden flows. The occurrence of polished, extraformational stones in Cloverly diamictites and wackes may have implications for presumed gastroliths in other fluvial rocks of the western interior (e.g., the Cedar Mountain and Morrison formations).

Lizards from the Lower Cretaceous (Aptian–Albian) Antlers and Cloverly Formations

ABSTRACT Recent discoveries from the Aptian–Albian Antlers (Oklahoma) and Cloverly (Montana, Wyoming) formations provide significant additions to the Early Cretaceous record of lizards in North America. The lizards from the Antlers Formation include two teiids (one named), an anguimorphan, and a series of fragmentary jaws with “paramacellodid”-like teeth. The lizards from the Cloverly Formation include a new species of Paramacellodus and many indeterminate jaw fragments. The apparent lack of shared lizard taxa between the two units calls into question their temporal equivalence, which is based on similarity of dinosaur taxa. Although apparently distinct, the lizard faunas of both units are very similar to that of the Late Jurassic Morrison Formation in being composed primarily of “para-macellodid” or “paramacellodid”-like taxa. It appears that there was a period of relative stasis in the evolution of lizards in North America between the Late Jurassic and Aptian–Albian, paralleling a similar trend in Europe between the Late Jurassic and the Barremian (or later). Reported lizards from the Aptian-Albian of Mongolia present a level of taxonomic diversity much different than that of the same time period in Oklahoma, Montana, and Wyoming, but more like that of the Late Cretaceous of North America, suggesting that taxonomic groups common to the Late Cretaceous and Tertiary occurred earlier in Asia than in North America.

Effects of Syndepositional Faulting and Folding on Early Cretaceous Rivers and Alluvial Architecture (Lakota and Cloverly Formations, Wyoming, U.S.A.)

ABSTRACT Quantitatively reconstructed paleochannel hydraulics (e.g., channel discharges, slopes, velocities), geometries (e.g., widths, depths, sinuosities, channel patterns), and alluvial architecture record the local effects of syndepositional faults and folds. These effects are evaluated from Lower Cretaceous rocks in three areas: (1) the northern Black Hills, where the axes of syndepositional folds trend roughly perpendicular to paleoflow, (2) the northwestern Black Hills, where the trend of a syndepositional fault is oblique to paleoflow, and (3) the Wind River basin, where the trends of syndepositional faults are nearly parallel to paleoflow. In each area, channel-belt deposits are laterally and vertically connected where local subsidence rates were high, producing thick, laterally extensive sandstones. Areas of lower subsidence rates are characterized by isolated or laterally connected channel-belt deposits, and thinner sandstones. Surface deformation controlled the positions of some rivers because of the development of antecedent drainages, or by directing rivers toward areas of maximum subsidence. Some reconstructed channel slopes, which range from 0.62 10-4 to 5.43 10-4, record surface deformation. The three-dimensional alluvial architecture was simulated using the model of Mackey and Bridge (1995), together with quantitatively reconstructed paleochannel parameters, in order to provide quantitative insight as to possible conditions and processes that produced the observed alluvial architecture. The effects of deformation on channel slopes, together with depositional topography, were important in controlling avulsion and resultant alluvial architecture. The most important condition that governed the alluvial architecture was comparable rates of local subsidence and sediment accumulation. Paleochannel reconstructions indicate that the rivers were 48-180 m wide and 4.4-13.6 m deep, and had discharges of 64-1073 m3s-1. Channel deposits all appear to represent point bars and associated channel fills of meandering rivers. Sinuosities were comparable, ranging from 1.1 to 1.4, and show no consistent trends with other channel parameters. Hence, with the exception of two paleochannels (out of the thirteen that were reconstructed) differences in hydraulic parameters such as slope and sediment transport rate had no discernible effect on channel pattern or sinuosity.

Gobiconodonts from the Early Cretaceous of Oshih (Ashile), Mongolia

We describe here the first discovered mammalian remains from the Mongolian Early Cretaceous locality Oshih (Ashile). Four fragmentary, tooth-bearing specimens, probably corresponding to three individuals, have been recovered. All the fossils can be assigned to the family Gobiconodontidae (Chow and Rich, 1984). The specimens include three lower jaw fragments and one upper jaw fragment, and represent at least two different taxa.Gobiconodon hopsoni n. sp., is described and diagnosed here. This new species is larger than G. ostromi (Early Cretaceous Cloverly Formation, USA); thus, it is the largest triconodont and one of the largest Mesozoic mammals known. Gobiconodon sp., found also at Oshih, is slightly larger than G. borissiaki from the Early Cretaceous of Khoobur, Mongolia, but smaller than G. ostromi The specimens of this second species are poorly preserved and provide insufficient data for a diagnosis.The status of the different species of Gobiconodon and the new gobiconodontid Hangjinia is reviewed. In gobiconodontids and Triconodontidae, the maxillae appear to make a significant contribution to the orbital rim, a condition unusual among basal mammals, in which the lacrimal and jugal are the main components. Other triconodonts such as Jeholodens likely an “amphilestid”, appear to show the primitive mammalian condition for this feature. We present a brief consideration of triconodont relationships and discuss alternative placements of Gobiconodon among Mammaliaformes.

Sauroposeidon proteles, a new sauropod from the early Cretaceous of Oklahoma

ABSTRACT Sauroposeidon proteles, a new brachiosaurid sauropod, is represented by an articulated series of four mid-cervical vertebrae recovered from the Antlers Formation (Aptian–Albian) of southeastern Oklahoma. Most Early Cretaceous North American sauropod material has been referred to Pleurocoelus, a genus which is largely represented by juvenile material and is not well understood. Regardless of the status and affinities of Pleurocoelus, the new taxon is morphologically and proportionally distinct. Among well-known sauropod taxa, Sauroposeidon is most similar to Brachiosaurus; particularly noteworthy are the neural spines, which are set forward on the centra and are not bifurcate, and the extremely elongate cervical ribs. Sauroposeidon and Brachiosaurus also share a derived pattern of pneumatic vertebral ultrastructure and a mid-cervical transition point, at which neural spine morphology changes from very low (anteriorly) to very high (posteriorly). Autapomorphies of Sauroposeidon include posterior placement of the di apophyses, hypertrophied pneumatic fossae in the lateral faces of the neural spines and centra, and an extraordinary degree of vertebral elongation (e.g., C8 = 1.25 m; 25% longer than Brachiosaurus). Additional sauropod material from the Early Cretaceous Cloverly Formation may be referrable to the new Oklahoma sauropod, which appears to be the last of the giant North American sauropods and represents the culmination of brachiosaurid trends towards lengthening and lightening of the neck.

Erratum: correction: Tribosphenic mammal from the North American Early Cretaceous

Nature 401, 363–366 (1999) The binomen Montanalestes keebleri was established for a tribosphenic mammal of probable eutherian affinities from the Lower Cretaceous (Aptian–Albian) Cloverly Formation, Montana, USA. The trivial name was designated for the Keebler family, of Billings, Montana.

Abstract: Origin and Distribution of Organic Matter in Mudstones of the Lower Cretaceous Cloverly Formation of Wyoming

Abstract: Origin and Distribution of Organic Matter in Mudstones of the Lower Cretaceous Cloverly Formation of Wyoming

Triconodont mammals from the medial Cretaceous of Utah

ABSTRACT Herein we describe triconodontid mammals from the upper part of the Cedar Mountain Formation (placed approximately at the Albian–Cenomanian, or Lower–Upper Cretaceous, boundary), Emery County, Utah. Three new species, based on dentulous jaw fragments and isolated teeth, are included; one is referred to Astroconodon, one to Corviconodon (both previously recorded from the Aptian–Albian), and one to a new genus. This diversity is comparable to that seen in the Aptian–Albian Cloverly Formation, and differs from succeeding faunas of the North American Cretaceous, in which triconodontids are rare or lacking altogether. The species differ greatly in size; the largest, at an estimated body weight of 750 g, is the largest known triconodontid and presumably incorporated vertebrate prey into its diet. Triconodont relationships are problematic. Craniodental evidence supports a monophyletic Triconodontidae, and there is weak support for a monophyletic grouping of the North American Cretaceous taxa within the family. However, an exclusive common ancestry for North American Cretaceous triconodontids cannot be readily identified among the morphological diversity known for Late Jurassic taxa. Instead, limited evidence at hand suggests that Late Jurassic Triconodontidae (Triconodon, Priacodon, Trioracodon) form a monophyletic group.

Neonate dinosaurian remains and dinosaurian eggshell from the Cloverly Formation, Montana

Neonate dinosaurian remains and dinosaurian eggshell from the Cloverly Formation, Montana

Non‐marine sedimentation in the overfilled part of the Jurassic‐Cretaceous Cordilleran foreland basin: Morrison and Cloverly Formations, central Wyoming, USA

AbstractLithostratigraphic, chronostratigraphic, sedimentological and penological data from the Upper Jurassic‐Lower Cretaceous Morrison and Cloverly Formations in central Wyoming allow detailed characterization of the early history of the central part of the Cordilleran foreland basin. The Morrison is divisible into three informal members: (1) a lower sandstone, deposited by a complex coastal dune‐foreshore–fluvial system during retreat of the Sundance sea; (2) a middle mudstone, deposited by muddy fluvial and ephemeral lacustrine systems during a period of regional, seasonal aridity; and (3) an upper sandstone, deposited by a sandy fluvial system of variable sinuosity. The overlying Cloverly Formation is divisible into two informal members: (1) a lower mudstone (previously considered as part of the Morrison Formation), deposited by muddy fluvial and lacustrine systems; and (2) an upper chert‐pebble conglomerate and sandstone, deposited primarily by gravel‐dominant braided rivers. Palynological data and a single fission‐track date indicate that the lower part of the middle Morrison mudstone is early to middle Oxfordian and the upper part of the lower Cloverly mudstone is Valanginian.Morrison sandstones are subarkosic, with average %QFL = 91,6, 3 and %QmFLt = 83, 6, 11. Cloverly sandstones are cherty litharenites and sublitharenites, with average %QFL = 99.6, 0,0.4 and %QmFLt = 82,0,18 (Gazzi‐Dickinson point‐counting method). Palaeocurrent data and sandstone compositions indicate a complex provenance including exirrabasinal sources in lower Mesozoic and upper Palaeozoic sedimentary and volcanic rocks of the Cordillera and intrabasinal sources of Proterozoic clasts in south‐central Wyoming. Cloverly sandstone compositions in the eastern part of the study area were influenced by short‐term fluvial reworking within the basin.The thickness of the composite Morrison‐Cloverly succession is practically constant over a distance of several hundred km east of the Idaho‐Wyoming thrust belt, and its internal chronostratigraphic zones are subparallel. On the other hand, equivalent strata in the Gannett Group of the thrust belt are at least three times thicker. This indicates that the Morrison and Cloverly in central Wyoming were deposited within the overfilled part of the foreland basin. Preliminary regional correlation indicates that coarse‐grained lithofacies in these rocks are significantly time‐transgressive, generally becoming younger toward the E and NE. Overfilling of the early Cordilleran foreland basin in central Wyoming was accomplished by progradation from the W and S. In spite of their three‐dimensional (3D) complexity, the Morrison and Cloverly Formations generally confirm theoretical model predictions for overfilled foreland basins.

Intrabasinal tectonic control on fluvial sandstone bodies in the Cloverly Formation (Early Cretaceous), west‐central Wyoming, USA

AbstractTemporal and spatial changes in alluvial sand‐body geometry and palaeodispersal patterns of the lower part of the Cloverly Formation (Early Cretaceous) in west‐central Wyoming suggest differential uplift within the developing medial to distal foreland basin and existence of subtle structurally controlled topography nearly 60 Myr before classic Laramide uplift in the Wyoming foreland province. This intraforeland structural topography exerted a significant control on dispersal and deposition of widespread gravels of Neocomian‐Aptian age.Surface studies have established the presence of two conglomeratic sandstones in the Cloverly Formation in the western Wind River Basin. Both conglomeratic sandstones are chert arenites, but the lower conglomeratic sandstone is characterized by dark chert pebbles derived from western and southwestern extrabasinal sources and was deposited by braided rivers, whereas the upper conglomeratic sandstone is characterized by intraformational clasts derived locally from adjacent floodplains and deposited by NE‐flowing rivers of moderate sinuosity which carried a higher proportion of suspended load in stable bank‐confined channels.Surface and subsurface mapping of sandstone‐body geometry in the lower dark chert‐pebble conglomeratic sandstone, together with palaeocurrent analysis of trough cross‐bedding, reveal a major NE‐flowing trunk river system in the area. Lithological correlation of surface sections to adjacent well logs, together with regional format correlation of fine‐grained intervals in well logs up to 120 km E of the outcrop belt, enable detailed mapping of the trunk system. The trunk system was approximately 5–10 km wide, and may have been confined by subtle structural topography developed by recurrent differential uplift along NE‐trending fractures in Archaean basement rocks. The fractures are along strike with diabase dike swarms and faults mapped in Archaean basement rocks of the Laramide Wind River uplift. The lower chert‐pebble sandstone is absent for a lateral distance of at least 30 km between these lineaments, including an area of at least 1000 km2, indicating the existence of a low NE‐SW‐trending interfluve perpendicular to the axis of the modern Wind River Range.Early Cretaceous development of structural topography and partitioning of the medial to distal foreland basin of west‐central Wyoming were most likely controlled by tectonic reactivation and differential uplift along fractures in Archaean basement rocks in response to early thrust loading and intraplate stresses during initial subsidence of the foreland basin.

Stratigraphic Positioning of the Lower Cretaceous Conglomerates, Wind River Basin, Wyoming and Implications for Possible Hydrocarbon Traps

Most previous outcrop and subsurfaces studies of the Lower Cretaceous conglomerates and conglomeratic sandstones in Wyoming have assumed a time equivalency for these deposits. The conglomerates have been utilized to identify the Jurassic-Cretaceous boundary and interpret tectonic conditions within the Sevier foreland basin. However, the authors integrated subsurface-outcrop correlations show that the conglomerates occur at distinctly different stratigraphic levels, thus invalidating their use in marking the Jurassic-Cretaceous boundary and complicating interpretations of their tectonic significance. A chert-bearing conglomerate occurs at the base of the Cloverly Formation over the entire western flank of the Wind River basin except within a 30 km Cloverly outcrop belt near Lander. The zero edge of this unit lies just east of Muskrat field where it may be a facies equivalent with what has previously been interpreted as the Upper Jurassic Morrison Formation. In contrast, in the eastern quarter of the Wind River basin, a thick chert-bearing conglomerate occurs in the upper part of the Cloverly Formation. This conglomerate may be the time-stratigraphic equivalent to the transitional marine 'Rusty beds' present in the western margin of the basin. In both areas, the conglomerates and conglomeratic sandstones are encased in thick mudstones. Paleocurrent data suggest different sourcemore » areas for the eastern and western conglomerates. The basal conglomerate was derived from the southwest, whereas the younger, eastern conglomerate was derived from the south. Their areal distributions have been useful in suggesting areas of potential structural-stratigraphic hydrocarbon plays.« less

The postcranial skeleton of the ornithopod dinosaur Tenontosaurus tilletti

Tenontosaurus tilletti is a moderate-sized ornithopod dinosaur from the Lower Cretaceous (Aptian–Albian) Cloverly Formation in Montana and Wyoming. Tenontosaurus remains are abundant. The vertebral...

The Early Cretaceous mammalGobiconodon(Mammalia, Triconodonta) from the Cloverly Formation in Montana

ABSTRACT Gobiconodon ostromi, sp. nov., described from two partial skeletons collected from the Cloverly Formation in southcentral Montana, is closely related to Gobiconodon borissiaki Trofimov (1978) from the Early Cretaceous of Mongolia. The unusual dental characters of this new form are enlarged caniniform incisors, reduced canines, and replacement of the molariform teeth that lie distal to the presumptive premolars. Despite the fact that the molariform occlusal pattern is the same as in Amphilestes of the Middle Jurassic and the overall structure of the molariform teeth is comparable to that in amphilestid triconodonts, the dentition is otherwise so anomalous that Gobiconodon is assigned to a new family. The scapula possesses a supraspinous fossa and an apparently large coracoid. The humerus exhibits large deltopectoral and medial crests, and protuberant epicondyles; the planes of the proximal and distal halves of this bone are substantially offset. The pelvis conforms to a generalized mammalian patte...

Evaluation of unconformities in upper jurassic and lower Cretaceous nonmarine deposits, Bighorn Basin, Wyoming and Montana, U.S.A.

Although unconformities are common in ancient fluvial sequences, their age significance is often difficult to determine. Where absolute age control is lacking, various physical criteria can be used to evaluate the relative duration and significance of major unconformities. In an area which encompasses over 10 6 km 2 of the west-central United States, a major unconformity is inferred to separate the Morrison Formation of Late Jurassic age from overlying Early Cretaceous conglomeratic sandstones. Previous studies have suggested that the disconformity at this contact may represent as much as 40 Ma. However, physical sedimentologic observations of the Morrison Formation and the overlying Cloverly Formation in the Bighorn Basin of Wyoming and Montana do not support this interpretation. Sandstone composition, clay mineralogy and fluvial architecture show gradational and intraformational changes upsection, rather than an abrupt change at the Morrison/Cloverly contact. In addition, scour surfaces are common within each formation and do not necessarily occur at the formation boundary. The results of this study complement recent absolute and relative age dating which suggests that the Morrison Formation is partially of Early Cretaceous (Neocomian) age, and that the overlying Lower Cretaceous conglomeratic sequence (which includes the lower part of the Cloverly Formation) may be older than Aptian and possibly of Late Neocomian age. The upper part of the Morrison Formation and the lower part of the Cloverly Formation may therefore represent continuous, although episodic, deposition of a fluvial system as it evolved through time.

Flexure and Sedimentation in Sevier Foreland: Part 3, a Model for Distal Foreland Sedimentation, Big Horn Basin, Wyoming: ABSTRACT

Lower Cretaceous nonmarine deposits in the Big Horn basin consist of mudstones, limestones, and sandstones of the upper Cloverly Formation and conglomerates of the basal Cloverly Formation. These conglomerates are part of an extensive network of fluvial gravels spread throughout the Western Interior. Flexural and paleohydraulic modeling has shown that it is difficult to derive the basal Cloverly conglomerates in the Big Horn basin from the Idaho-Wyoming thrust belt to the west. Instead, the fine clastics of the upper Cloverly Formation document thrust-induced flexural subsidence during the Aptian to Albian. Evidence for a west-dipping slope along the distal margin of the foreland basin is found within the Greybull Sandstone member of the upper Cloverly Formation document thrust-induced flexural subsidence during the Aptian to Albian. Evidence for a west-dipping slope along the distal margin of the foreland basin is found within the Greybull Sandstone Member of the upper Cloverly Formation. The Greybull sandstone was deposited by streams flowing from the craton, across central Wyoming and into the foreland trough to the west. A model of foreland-basin sedimentation can be distilled, in which coarse-grained clastics are localized near the thrust front whereas fine-grained detritus fills the bulk of the basin. Flexural bendingmore » of the distal foreland region creates a thrustward-dipping slope along which fluvial sediments may be transported toward the thrust front. Hence, the initiation of Sevier foreland-basin sedimentation is not recorded by the first occurrence of western-derived conglomerate. Instead, nonmarine mudstones, limestones, and eastern-derived fluvial sandstones represent the earliest distal sedimentary response to the Sevier orogeny.« less

Stratigraphy and Depositional History of Lakota and Cloverly Formations, Powder River Basin, Wyoming: ABSTRACT

The partially equivalent Lakota and Cloverly Formations comprise the basal Cretaceous strata in outcrop exposures along the eastern and western margins, respectively, of the Powder River basin. They unconformably blanket a subaerial erosional surface that developed during Early Cretaceous time. Paleocurrent analysis of outcrops surrounding the Powder River basin, together with petrographic data, demonstrates that the formations had different source areas: sediments of the Cloverly were derived from the Sevier orogenic belt, whereas those of the Lakota were derived primarily from areas east and southeast of the present-day Powder River basin. The formations also display different facies: conglomeratic sandstones of the Cloverly appear to have been deposited by braided stre ms, whereas the well-sorted, fine to medium-grained sandstones of the Lakota were deposited by meandering fluvial systems. Although confirming biostratigraphic data are lacking, stratigraphic relationships strongly suggest that initiation of Lakota deposition preceded that of the Cloverly. The upper parts of both formations include red, green, and beige mudstones and appear to be correlative. They record a period of slow sedimentation immediately prior to transgression of the sea. They also record a change in paleoslope over much of the Powder River basin, the original northeastward-sloping surface having been tilted northwestward in response to subsidence within the foreland basin. Cloverly deposition was terminated by rapid transgression of the Thermopolis sea. Basal beds of the Thermo olis interfinger southeastward with deltaic deposits at the top of the Lakota. Lakota deposition was terminated somewhat later, the youngest Lakota strata being overlain by the Fuson tongue of the Thermopolis. End_of_Article - Last_Page 850------------

Skeletal reconstruction and life restoration of Sauropelta (Ankylosauria: Nodosauridae) from the Cretaceous of North America

A skeletal reconstruction and life restoration are presented for the Early Cretaceous nodosaurid ankylosaur Sauropelta edwardsi. This composite is based on several partial skeletons having armor preserved in situ from the Lower Cretaceous Cloverly Formation of Wyoming and Montana. Comparison of foot morphology with that of the ichnofossil Tetrapodosaurus borealis, from the Lower Cretaceous Gething Formation of British Columbia, suggests that the footprint is that of Sauropelta. A hypothesis is presented suggesting that most ankylosaurs are found on their backs because of bloating.