Mary River Formation Research Articles

A busy time at the beach: multiple examples of gregarious dinosaur behaviour inferred from a set of trackways from the Late Cretaceous of Alberta, Canada

A diverse collection of dinosaur tracks and trackways preserved on a single bedding plane record activities of many animals over a very short time, possibly just a few days or less. The assemblage is exposed on a detached slab of siltstone from the Campanian–Maastrichtian St. Mary River Formation in southwestern Alberta, Canada. Members of four different groups of dinosaurs are identified as the makers of either isolated tracks, or trackways of varying lengths, and include inferred hatchling tyrannosaurids, medium and large ornithomimids, small and medium-sized ornithopods, and a small hadrosaurid. All but one of the seven trackways attributed to hatchling tyrannosaurids occur as codirectional, equally spaced pairs, but one pair walked in the opposite direction, albeit parallel, to the other two. The pair of ornithomimid trackways also appears to represent two animals walking at the same speed, in the same direction, and turning together. The trackway attributed to a medium-sized biped, possibly Parksosaurus warreni, is the first evidence for the presence of these dinosaurs in the environment recorded by the St. Mary River Formation.

Enriching our knowledge of Late Cretaceous wetland plant communities: Zlatkovia crenulata gen. et sp. nov., an amphibious angiosperm from the St. Mary River Formation, Alberta, Canada

Sixty-seven coalified compression specimens of leaves, one attached to a stem, have been found in the St. Mary River Formation (uppermost Campanian–lowermost Maastrichtian, Upper Cretaceous) near Cardston, Alberta, Canada. Leaves have long multi-veined petioles with clasping bases, and an ellipsoid to obovate lamina with a convex to almost flat apex, and a crenate margin with one order of convex-convex chloranthoid teeth with angular sinuses. Tooth apices often end in a small concavity interpreted as a hydathode. Several primary veins enter the lamina in a flabellate manner. The central primary vein branches pinnately while lateral primaries, dichotomize toward the margin producing a reticulodromous pattern of secondaries. Secondaries arising from the lateral primary veins are the result of consistent dichotomies and anastomoses with a concurrent loss of gauge. Tertiary veins are alternate percurrent, and quaternaries form an irregular reticulate fabric. Quaternary veins form polygonal areoles with freely ending veinlets. Some specimens show a polygonal pattern of sub-epidermal aerenchyma like that of many other aquatics. Plants are compared to other fossil aquatic taxa, including those from the St. Mary River Formation and extant members of Ranunculaceae, Menyanthaceae and Nymphaeaceae. The specimens are described as a new amphibious aquatic: Zlatkovia crenulata Rothwell & Stockey gen. et sp. nov., eudicot family incertae sedis. This assemblage meaningfully enriches our understanding of aquatic and adjacent wetland habitats near the close of the Cretaceous, and emphasizes that the structure of modern wetland communities was well established before the end of the Mesozoic.

A taphonomic analysis of a multitaxic bonebed from the St. Mary River Formation (uppermost Campanian to lowermost Maastrichtian) of Alberta, dominated by cf. Edmontosaurus regalis (Ornithischia: Hadrosauridae), with significant remains of Pachyrhinosaurus canadensis (Ornithischia: Ceratopsidae)

Scabby Butte is an isolated exposure of Upper Cretaceous (uppermost Campanian to lowermost Maastrichtian) strata of the St. Mary River Formation. These rocks have produced a diverse assemblage of both terrestrial and aquatic vertebrates, although hadrosaurid and ceratopsid dinosaurs represent the largest component by volume. Almost all of these dinosaur remains were collected from a single bonebed (Site 2) at Scabby Butte and have been referred to the hadrosaurid Edmontosaurus regalis Lambe, 1917 and the ceratopsid Pachyrhinosaurus canadensis Sternberg, 1950. This study presents a quantitative taphonomic reanalysis of the originally published data, providing new information previously overlooked and important information about the age-class structure of the dinosaur fauna preserved at the site. Site 2 is a lag deposit with a minimum number of 11 individuals (two ceratopsid, nine hadrosaurid), with three-quarters of the material being adult-sized based on size-class analysis. Most elements have undergone moderate to severe breakage and abrasion, and are completely disarticulated, suggesting that they were transported from where they died; post-mortem scavenging is also a possibility, as evidenced by the presence of tooth marks and trample marks on several elements. Burial took place soon after scavenging, as there is little evidence of subaerial weathering.

Dinosaur eggshells from the lower Maastrichtian St. Mary River Formation of southern Alberta, Canada

North America is known for its rich uppermost Cretaceous record of dinosaur egg remains, although a notable fossil gap exists during the lower Maastrichtian. Here we describe a diverse dinosaur eggshell assemblage from the St. Mary River Formation of southern Alberta that, in conjunction with recently described eggs from the same formation in Montana, helps fill this gap and sheds light on the dinosaur diversity in this poorly fossiliferous formation. Three theropod eggshell types (Continuoolithus cf. C. canadensis, Montanoolithus cf. M. strongorum, and Prismatoolithus cf. P. levis) and one ornithopod (Spheroolithus cf. S. albertensis), are reported from Albertan exposures of the St. Mary River Formation, increasing the ootaxonomic diversity of the formation from two to five ootaxa. The taxonomic composition of the eggshell assemblage is consistent with the dinosaurian fauna known from the St. Mary River Formation based on skeletal remains. Spheroolithus eggshells constitute the majority of identifiable eggshells in our assemblage, a trend also observed in several other Upper Cretaceous formations from North America. Continuoolithus is shown to be synonymous with Spongioolithus, thus expanding the Maastrichtian geographic range of the ootaxon to include Utah. The St. Mary River eggshell assemblage supports a general trend of increase in eggshell thickness among theropod ootaxa from the uppermost Santonian through the Maastrichtian, which is inferred to reflect an increase in body size among some clades of small theropods through the Upper Cretaceous. Eggshell preservation in the St. Mary River Formation may be related to the semiarid climatic and environmental conditions that prevailed.

Paleoecological implications of two closely associated egg types from the Upper Cretaceous St. Mary River Formation, Montana

Two closely associated egg types occur at the same locality in the Upper Cretaceous (Maastrichtian) St. Mary River Formation in north central Montana. These specimens represent the first fossil eggs described from this formation. At least fifteen small ovoid eggs or egg portions are scattered through a 25 cm interval of rock. Five significantly larger, round eggs overlie these smaller eggs and are in close proximity to one another on a single bedding plane. The best preserved egg of the smaller size measures 36 mm × 62 mm and exhibits the prismatic, two-layered eggshell structure of a theropod egg. The dispersed distribution and inconsistent angles of these small eggs likely resulted from disturbance by subsequent nesting activity and/or possibly nest predation. At least twelve additional small prismatic eggs also occur at this site. We assign the small eggs as a new oogenus and oospecies, Tetonoolithus nelsoni, within the Prismatoolithidae. The large round eggs measure 130 mm in diameter and the eggshell displays substantial diagenetic alteration. These eggs likely belonged to a hadrosaur due to their similarity in egg size, shape, and eggshell thickness to Maiasaura eggs from the stratigraphically lower Two Medicine Formation. Eggs at different stratigraphic levels at this site indicate that conditions favorable to both dinosaur species persisted for an extended period of time. However, determining whether these dinosaurs occupied the nesting site at the same or different years remains beyond the resolution of the rock record.

A New Horned Dinosaur Reveals Convergent Evolution in Cranial Ornamentation in Ceratopsidae

Ceratopsid (horned) dinosaurs are an iconic group of large-bodied, quadrupedal, herbivorous dinosaurs that evolved in the Late Cretaceous and were largely restricted to western North America [1-5]. Ceratopsids are easily recognized by their cranial ornamentation in the form of nasal and postorbital horns and frill (capped by epiossifications); these structures show high morphological disparity and also represent the largest cranial display structures known to have evolved [2, 4]. Despite their restricted occurrence in time and space, this group has one of the best fossil records within Dinosauria, showing a rapid diversification in horn and frill morphology [1]. Here a new genus and species of chasmosaurine ceratopsid is described based on a nearly complete and three-dimensionally preserved cranium recovered from the uppermost St. Mary River Formation (Maastrichtian) of southwestern Alberta. Regaliceratops peterhewsi gen. et sp. nov. exhibits many unique characters of the frill and is characterized by a large nasal horncore, small postorbital horncores, and massive parietal epiossifications. Cranial morphology, particularly the epiossifications, suggests close affinity with the late Campanian/early Maastrichian taxon Anchiceratops, as well as with the late Maastrichtian taxon Triceratops. A median epiparietal necessitates a reassessment of epiossification homology and results in a more resolved phylogeny. Most surprisingly, Regaliceratops exhibits a suite of cranial ornamentations that are superficially similar to Campanian centrosaurines, indicating both exploration of novel display morphospace in Chasmosaurinae, especially Maastrichtian forms, and convergent evolution in hornmorphology with the recently extinct Centrosaurinae. This marks the first time that evolutionary convergence in horn-like display structureshas been demonstrated between dinosaur clades, similar to those seen in fossil and extant mammals [6].

Mammals from the St. Mary River Formation (Upper Cretaceous), Montana

ABSTRACT The St. Mary River Formation (Fm) crops out in restricted parts of southwestern Alberta and northwestern Montana. Mammals are poorly known from the formation, but material collected from Alberta has played a role in recognizing an ‘Edmontonian’ Land Mammal Age between the better sampled Judithian (∼79–74 Ma) and Lancian (∼67–65 Ma) land mammal ages. New and well-preserved multituberculate and metatherian material collected from a single locality in the lower third of the St. Mary River Fm of Montana adds complexity to the interpretation of a discrete ‘Edmontonian’ Land Mammal Age. We report here three taxa of multituberculates (Paracimexomys propriscus, sp. nov., Nidimys occultus, gen. et sp. nov., and a primitive ptilodontoid of comparable size to the smallest species of MesodmaJepsen, 1940, and CimexomysSloan and Van Valen, 1965) and two metatherian taxa (Leptalestes toevsi, sp. nov., and Turgidodon russelli). Paracimexomys propriscus and Turgidodon russelli are probably conspecific with Judithian forms that did not survive to the Lancian. Leptalestes toevsi and Nidimys occultus are taxa unique to the ‘Edmontonian’ but may represent lineages that separated from their closest relatives prior to the Judithian. Lack of Lancian-aspect mammals contrasts sharply with other ‘Edmontonian’ faunas, including an older fauna recovered from the Williams Fork Fm of Colorado, suggesting that the replacement of Judithian mammals by Lancian forms was a complex transition. We tentatively suggest that the Lancian fauna may have expanded into the northern part of the Western Interior in response to the appearance of new terrestrial habitats as sea level fell.

Cardstonia tolmanii gen. et sp. nov. (Limnocharitaceae) from the Upper Cretaceous of Alberta, Canada

Several new broad‐leaved monocots were identified in gray siltstones and fine‐grained sandstones from the Upper Cretaceous (Campanian‐Maastrichtian) St. Mary River Formation near Cardston, Alberta, Canada. Specimens are compression/impressions of long‐petiolate aquatic plants that were probably buried in situ. Leaf blades are entire, ovate to elliptic, with deeply cordate bases; leaf blades are 5–12 cm long and 3.5–8.5 cm wide. Petioles are at least 4 mm wide with five to seven primary veins that enter the leaf blade. Venation is campylodromous, resulting in 23–27 primary veins with three medial veins that remain unbranched to the apex. Major and minor secondary veins (ABAB pattern) diverge at angles of 45°–65° near the midveins and 90° near the leaf margin. Tertiary veins are usually unbranched, but occasional dichotomies and anastomoses occur. No freely ending veinlets are visible. The Cardston specimens are compared with extant leaves of Alismatales and show closest similarities to those of Limnocharis L., Hydrocleys Rich., and Butomopsis Kunth. These leaves are also similar to fossil leaves of Haemanthophyllum Budantsev, in particular H. cordatum Golovneva from the Maastrichtian‐Danian deposits of the Koryak Highlands, Russia. A reexamination of the genus Haemanthophyllum, the generitype (H. kamtschaticum Budantsev), the holotype for H. cordatum, and the Cardston specimens results in the description of a new genus, Cardstonia tolmanii gen. et sp. nov. (Limnocharitaceae). This study points to the need for reexamination of the remaining species in the genus Haemanthophyllum, which appear to represent a diverse assemblage of leaves of varying morphology that are probably not a natural group.

A Diverse Dinosaur-Bird Footprint Assemblage from the Lance Formation, Upper Cretaceous, Eastern Wyoming: Implications for Ichnotaxonomy

A diverse assemblage of dinosaur and bird tracks from Niobrara County, Wyoming, represents the first vertebrate ichnofauna reported from the bone-rich Lance Formation (Maastrichtian, Upper Cretaceous). The ichnofauna includes a hadrosaur track with skin impressions; three theropod track types, including the tetradactyl track Saurexallopus zerbsti (ichnosp. nov.); a tridactyl dinosaur footprint with a fusiform digit III; possible Tyrannosaurus tracks; four distinctive avian ichnites; and invertebrate traces. The footprints are generally well-preserved and so offer a unique insight into the ecology of a small river valley during the Maastrichtian. Saurexallopus zerbsti ichnosp. nov. from the Lance is similar to Saurexallopus lovei recently reported from the Maastrichtian, Harebell Formation, of northwestern Wyoming, but is represented by much better material, facilitating amendment of the ichnogenus. Skeletal equivalents for Saurexallopus are not currently known. Similarly, the tridactyl track with fusiform digit III is similar to footprints reported from the coeval Laramie Formation of Colorado and may also be similar to ichnogenus Ornithomimipus from the Edmonton Group of Alberta (though not necessarily of ornithomimid affinity). The hadrosaurian track with the skin impression is reminiscent of a similar ichnite reported from the Maastrichtian, St. Mary River Formation in Alberta, which is herein named Hadrosauropodus langstoni as part of a reassessment of Cretaceous ornithopod track ichnotaxonomy. Such correlations demonstrate the utility of tracks for local or regional biostratigraphy (palichnostratigraphy) in western North America. It is also clear that tracks add to our knowledge of the composition and distribution of dinosaurian and avian components of Maastrichtian faunas. In particular the bird tracks indicate a diversity of at least four species, one of which was a semi-palmate form, hitherto unknown in the ichnological record and named Sarjeantichnus semipalmatus.

A new specimen of Plioplatecarpus (Mosasauridae) from the lower Maastrichtian of Alberta: comments on allometry, functional morphology, and paleoecology

An incomplete specimen of the plioplatecarpine mosasaur Plioplatecarpus from the lower Maastrichtian locality of Scabby Butte (Saint Mary River Formation) closely resembles the slightly older Plioplatecarpus primaevus from Saskatchewan (Bearpaw Formation). However, quantitative and qualitative differences are observed: the vertebrae of the Scabby Butte specimen are more gracile and, although of similar shape and proportions, are relatively smaller. Since these differences cannot be readily explained by allometry or sexual dimorphism, the hypothesis that this specimen represents a new species is provisionally accepted. However, in the absence of diagnostic cranial features, a new species is not named. The morphology of the well-preserved forelimb indicates that the animal cannot be reconstructed as a subaqueous flyer, but probably used the forelimbs for paddling. The associated matrix indicates preservation in overbank deposits in a deltaic system exhibiting no evidence of open marine environments. This suggests that this mosasaur was able to exploit estuarine and freshwater environments.

The Aquatic Angiosperm Trapago angulata from the Upper Cretaceous (Maastrichtian) St. Mary River Formation of Southern Alberta

A floating aquatic dicot with leaves assignable to Trapago angulata has been characterized from Upper Cretaceous (Maastrichtian) deposits of the St. Mary River Formation in southern Alberta, Canada. Reconstruction of the plant is based on nearly 500 specimens of various isolated and attached organs. Prominent floating rosettes of leaves were interconnected by submerged horizontal rhizomes with a distinct, crenulate surface and formed dense mats at the surfaces of small freshwater ponds. At least eight different leaf morphologies were produced. Rosettes bore opposite pairs of simple leaves and compound leaves with three, five, seven, nine, and possibly 11 leaflets. Highly dissected, submerged leaves of differing morphologies occur on rosette-bearing axes and rhizomes. Paired bud scales at the nodes of the crenulate rhizomes subtend both rhizome leaves and elongate, branched roots. Solitary flowers on long pedicels are attached in the axils of compound leaves. These remains are compared to those described a...

The Genesis and Recognition of Anastomosed Fluvial Deposits: Data from the St. Mary River Formation, Southwestern Alberta, Canada

ABSTRACT Data from modern fluvial systems show that the term anastomosed is best reserved for suspended-load systems with multiple channels and very low slope that flow in regions where the hydrograph has a seasonal peak. The interchannel areas are topographic lows relative to the channel hanks and are places where crevasse-splay, marsh, and lacustrine sedimentation occurs. Channel geometry is intrinsically a function of the rate of sediment partitioning; the coarsest sediment is deposited in the channels, on the levees, and in proximal splay complexes, whereas the silts and clays are deposited on the rest of the floodplain. Extrinsic controls on the formation of these systems are geological and climatic. They include a source of suspended sediment and a highly seasonal water budget. Preservat on of anastomosed fluvial deposits requires a mechanism for maintaining very low depositional slopes over geologic time scales. Data from the St. Mary River Formation in southwestern Alberta show that the stratigraphic equivalents of these rivers are ribbon sandstone bodies encased in thin sandstones, shales, and siltstones. The total distribution of facies, rather than proof of simultaneous flow in multiple channels, can be used to interpret correctly a deposit as anastomosed fluvial. Although anastomosed fluvial deposits form in a variety of tectonic settings, they are apparently most common in foreland basins because these basins provide suitable source rocks and thrusting as a mechanism that can maintain the low depositional slopes over millions of years. The mutual interaction among source rock, climate, and subsidence, all of which play critical roles in forming and preserving these distinctive fluvial deposits, allows inferences to be drawn regarding paleoclimate and paleoslope that may help constrain models of basin evolution.

The Association of Anastomosed Fluvial Deposits and Dinosaur Tracks, Eggs, and Nests: Implications for the Interpretation of Floodplain Environments and a Possible Survival Strategy for Ornithopods

The St. Mary River Formation (Maastrichtian) consists of anastomosed fluvial deposits containing several hundred track-bearing beds. Paleontologic and sedimentologic analyses of these beds indicate that large herbivores, ornithopods, inhabited a seasonal wetland dominated by marshes and lakes. Shallow tracks in fine-grained sediments, formed as the sediments dewatered to the point of stiffness, display the highest resolution of detail. The preservation potential of tracks in anastomosed fluvial deposits is large because of the abundance of soft substrates to record the tracks and the occurrence of annual flooding to rapidly bury the footprints. Comparison of the St. Mary River Formation to other anastomosed fluvial deposits as old as the Early Jurassic confirms that tracks are common in this type of deposit. The variation in preservation of track types and depth of penetration raises the possibility that ornithopods employed a survival strategy involving seasonal wetlands. The wetlands provided an abundant food source and at the same time the combination of a soft substrate and flooded conditions would have effectively countered the superior speed and agility of large carnivores. The relatively common occurrence of ornithopod eggshells from anastomosed fluvial deposits suggests that the abundant food supply accompanying the wet season also made the wetlandsmore » an ideal location to rear young. These data can be used to refine the interpretations of depositional environment derived from the sediments by allowing estimates to be made regarding the early post-depositional conditions of the sediments. 95 refs., 7 figs., 1 tab.« less

Dinosaur footprints with skin impressions from the Cretaceous of Alberta and Colorado

Cretaceous dinosaur footprints discovered in the J Sandstone of the South Platte Formation (Dakota Group) in Colorado and the St. Mary River Formation of southwestern Alberta exhibit the first reported foot pad skin impressions of large ornithopods. The Canadian tracks occur as sandstone casts preserved in mudstones, whereas the Colorado tracks are natural impressions in a sandstone bed overlain by shale. The South Platte Formation tracks occur as impressions in a widespread "dinoturbated" sandstone bed representing low-gradient, delta plain – coastal plain facies assemblages associated with the upper member of the group, the J Sandstone. Only one of the many iguanodontid trackways exhibits good skin impressions.The St. Mary River Formation palaeoenvironment is interpreted as an anastomosed fluvial system that flowed northeast over a low-gradient floodplain from Montana. Footprints, often preserved in trackways, were left as dinosaurs walked across lake and marsh sediments that were relatively well drained or in various stages of dewatering. The quality of preservation is variable, depending on the properties of the substrate, and only one hadrosaur footprint includes clear casts of skin patches on the bottom of the footprint. Similar track-rich facies assemblages, representing lowland coastal plain and deltaic environments, are found in both the Lower (Gething Formation, British Columbia) and Upper Cretaceous (Mesa Verde, Colorado) successions of western North America. Few substrates of these depositional environments were suitable for the preservation of skeletal remains, so the information derived from tracks is palaeontologically significant.

Thrust Controlled Foreland Basin Fluvial Architecture, St. Mary River Formation, Southwestern Alberta, Canada

Thrust Controlled Foreland Basin Fluvial Architecture, St. Mary River Formation, Southwestern Alberta, Canada

Stratigraphy, Sedimentology, and Petrology of Upper Cretaceous Horsethief and St. Mary River Formations, Western Montana: ABSTRACT

The Horsethief and St. Mary River formations were deposited along the Late Cretaceous epicontinental seaway, which then covered much of the western interior. Where these formations presently crop out along the disturbed belt in western Montana, the sedimentary units form a volcaniclastic sequence deposited during the final regression of the Montana Group and contemporaneously with early stage development of the Laramide orogeny. The facies preserved within these volcaniclastic sequences represent barrier island, high-destructive wave-dominated delta, and fluvial plain environments of deposition. The Horsethief, lower of the two formations, is divided into two facies sequences. Facies sequence A consists of coarsening-upward sequences of sandstones and interbedded shales. The predominant sedimentary structures include multidirectional planar and trough cross-bed sets, micro-cross-beds, and ripples. Ostrea sp. and Ophiomorphia are present and most abundant near the top of this sequence. These facies comprise a barrier island system consisting of shoreface, dune, tidal channel, and lagoonal environments. Facies sequence B, deposited along the depositional strike, consists of a coarsening-upward sequence of vertically stacked distributary channels that thicken and become more abundant upsection. Delta fringe and prodelta facies are absent; marine processes reworked the detritus i to barrier island systems. The St. Mary River Formation is divided into a lower and upper member. The lower member consists of shales, sandstones, limestones, and coals deposited in a lagoon landward of the barrier island system. The upper member contains trough cross-bedded, channel sandstones, overbank sandstones, shales, and carbonate-nodule horizons indicative of fluvial plain sedimentation. These genetically related sequences were deposited along a basin margin characterized by temperate-continental climates, moderate to high rainfalls, rugged relief, and marine processes characterized by microtidal to mesotidal ranges and dominated by longshore currents. Petrographic analysis indicates the detritus of these formations was derived from a magmatic arc provenance. The texture and composition of these sediments are dependent on the relative distance of sediment transport. Statistically significant correlations document a decrease in grain size as the distance of sediment transport increases within the entire section and within distinct environments, including middle shoreface, upper shoreface, and dune facies. The high percentage of volcanic constituents decreases as the distance of sediment transport increases and the grain size decreases. Additionally, higher energy environments result in deposition of coarser grained detritus and higher percentages of volcanic components. The interpretation of the depositional environments and petrographic characteristics as observed within these formations can be useful in analyzing facies sequences that have similar source lithologies and were deposited in basins with a similar evolution. The recognition of these facies is significant because of the potentially important application associated with hydrocarbon source and reservoir conditions, as well as heavy mineral assemblages. End_of_Article - Last_Page 459------------

Cretaceous Stratigraphy of the Glacier National Park Area, Northwestern Montana

ABSTRACT A sequence of Cretaceous formations as much as 7,000 ft (2,134 m) thick crops out in the eastern and southeastern parts of Glacier National Park and in the adjoining area to the east. Recognition of the many persistent lithologic units in this Cretaceous sequence should be of value in interpreting the complex structure of the region. The oldest rocks, the Kootenai Formation of Early Cretaceous age, consist chiefly of variegated mudstone, siltstone and sandstone of nonmarine origin. In the Park area the thickness of the Kootenai is probably greater than 1,000 ft (305 m). A conspicuous black chert conglomerate, the Cut Bank Sandstone Member, forms the base of the formation and rests unconformably on Upper Jurassic rocks. The Blackleaf Formation, also of Early Cretaceous age, conformably overlies the Kootenai and is about 750 to 800 ft (229-244 m) thick. A marine unit consisting of dark-gray fissile shale and quartzose sandstone forms the basal part of the Blackleaf. Most of the formation is alternating beds of light-coloured, nonmarine clastics. The marine Marias River Shale of Late Cretaceous age unconformably overlies the Blackleaf and is about 1,200 to 1,300 ft (366-396 m) thick. Most of the Marias River is dark-gray shale that contains calcareous and ferruginous concretions and is locally sandy and calcareous. Conformably overlying the Marias River Shale is a marine transitional unit consisting of interbedded shale and sandstone, herein referred to as the Telegraph Creek Formation. The formation ranges in thickness from 120 to 170 ft (36-52 m) on the Kevin-Sunburst dome east of Glacier National Park. The Virgelle Sandstone, which averages 160 ft (49 m) thick, is a massive light-brown sandstone conformably overlying the Telegraph Creek Formation. Resting conformably on the Virgelle Sandstone is the Two Medicine Formation, a nonmarine unit that consists mostly of mudstone with minor lenticular sandstone. The formation is about 2,000 ft (610 m) thick in the Park area. A dark-gray marine formation, the Bearpaw Shale, conformably overlies the Two Medicine Formation. The Bearpaw is nearly 400 ft (122 m) thick east of the Park, but thins to only 235 ft (72 m) south of the Park. The Bearpaw grades upward into a unit of alternating sandstone, siltstone and shale that is as much as 400 ft (122 m) thick. The Horsethief Sandstone, about 90 ft (27 m) thick, is a massive cliff-forming sandstone that was deposited in a regressive marine environment above the Bearpaw. Conformably overlying the Horsethief Sandstone is the St. Mary River Formation, a nonmarine sequence. The St. Mary River Formation is about 1,000 ft (305 m) thick near the eastern margin of the Disturbed belt. Another nonmarine unit, the Willow Creek Formation, overlies the St. Mary River Formation. The Willow Creek consists of about 800 ft (244 m) of variegated clayey rocks and sandstone. The top of the Cretaceous is somewhere within this formation.

The Ceratopsian Dinosaurs and Associated Lower Vertebrates from the St. Mary River Formation (Maestrichtian) at Scabby Butte, Southern Alberta

Scabby Butte, a limited exposure of late Cretaceous sediments in southern Alberta, Canada, is an important source for the large ceratopsian dinosaur Pachyrhinosaurus Sternberg. New cranial material confirms this taxon's place among short-faced ceratopsids, and circumstantial evidence suggests that it possessed a spiked frill reminiscent of Styracosaurus. Postcranial bones associated with Pachyrhinosaurus skulls in a bonebed deposit probably belong to the genus and indicate a massive body approaching Triceratops dimensions. Another ceratopsian, Anchiceratops, is sparsely represented.All classes of gnathostomes, except birds, are represented by fragmentary material at Scabby Butte. Of 21 lower vertebrate taxa, fourteen are new to the St. Mary River Formation (Squatirhina, Squatina, Amia, Belonostomus, Paralbula, Platacodon, Opisthotriton, Boremys, Plioplatecarpus, a tyrannosaur, a coelurosaur, an ornithomimid, Troödon, Edmontosaurus, Edmontonia).

THE PROBLEM OF THE WILLOW CREEK FORMATION

The Willow Creek formation of south western Alberta is known to include both Upper Cretaceous and Paleocene deposits in the disturbed belt of the foothills. On the plains to the east, however, the Cretaceous portion may be absent, and the formation may rest unconformably on the underlying late but not latest Cretaceous St. Mary River formation.

Montana Group Stratigraphy, Lewis and Clark County, Montana

Geologic mapping in the Disturbed Belt of northern Lewis and Clark County, Montana, has revealed major northeast-southwest trending facies changes within the Upper Cretaceous Montana Group. Typically, from oldest to youngest, the Group comprises the Telegraph Creek, Virgelle, Two Medicine, Bearpaw, Horsethief, St. Mary River, and Willow Creek Formations, aggregating about 5,000 feet in thickness. The basal formations of the Montana Group, the Telegraph Creek and Virgelle, are regressive shallow-marine and lagoonal feldspathic sandstones and shales rising steeply across time lines north and east. Overlying them with gradational contact is a lagoonal facies of sandstone and shale of the Two Medicine Formation, about 550 feet thick and equivalent to the upper Eagle Sandstone. The herein-defined Big Skunk Formation, 2,100 feet of volcanic-rich, arkosic sedimentary rock, disconformably overlies the Eagle equivalents of the Two Medicine and fills the upper two-thirds of the Two Medicine stratigraphic interval along the western margin of the Disturbed Belt. The Big Skunk includes from bottom to top, members A through E, which are, respectively: a basal sequence of grayish red, volcanic-rich, sedimentary breccia and mudstone; an ash-flow tuff 121 feet thick; variegated, volcanic-rich mudstones, shales, and sandstones; gray to black mudstones and conglomeratic sandstones; and gray-green mudstones and sandstones interstratified with tuffs. Stream deposition in a piedmont environment predominated; the source vents were probably a few miles westward. Northeastward he Big Skunk thins to a tongue of volcanic-rich strata, underlain by 1,125 feet and overlain by 125 feet of floodplain-deposited Two Medicine shale and mudstone. These overlying Two Medicine strata pinch out southwestward. As the southern edge of the Bearpaw Shale lies north of Lewis and Clark County, the marine Horsethief Formation disconformably overlies Two Medicine along the eastern margin of the Disturbed Belt. Marine Horsethief sandstones grade southwestward to a brackish-water facies and then to lagoonal mudstone and shale composing the basal St. Mary River Formation. Reworking of a volcanic-conglomerate facies formed magnetite-rich sandstones capping the Horsethief. Undifferentiated strata of the St. Mary River and Willow Creek Formations overlie the Horsethief Formation along the eastern edge of the Disturbed Belt and the Big Skunk Formation along the western edge. This undifferentiated unit includes a basal lagoonal facies, a mudstone-shale floodplain facies, and an upper sandstone facies all aggregating at least 3,730 feet in thickness and all rising northeastward across time lines. The Montana Group in northern Lewis and Clark County, Montana, is essentially a large clastic wedge, 6,550 feet thick and regressive north and east. The sedimentary detritus was eroded largely from volcanic source terranes. The active period of vulcanism represented by the Big Skunk Formation is time-correlated with the Elkhorn Mountains vulcanism of central Montana, and should be distinguished from the nearby Adel Mountain vulcanism, which is post-Horsethief and pre-Eocene thrust faulting in age.