Bulletin 35 - Abstract

Stratigraphy and Paleontology of the Cloverly Formation (Lower Cretaceous) of the Bighorn Basin Area, Wyoming and Montana

By John H. Ostrom

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Abstract

The nonmarine strata of the Bighorn Basin, lying between the Late Jurassic Sundance Formation and the Early Cretaceous Thermopolis Shale, have been subdivided into three formal units: the Morrison, Cloverly and Sykes Mountain Formations. Application of these terms to actual outcropping rock units has been quite inconsistent, and considerable confusion and disagreement exist over formation boundary positions. Traverses around the periphery of the Bighorn Basin and across adjacent areas to the north, west and south established the existence of seven distinctive lithic units. These are: a lowermost, drab gray, calcareous claystone; a massive, brilliant white, quartz-chert sandstone (in the southern part of the Basin); a variegated gray-green or pink, calcareous claystone; a massive, black chert, coarse sandstone or conglomerate (in the northern part of the Basin) ; a drab gray to white or pastel-colored, bentonitic and noncalcareous claystone rich in chalcedonic and baritic concretions; a discontinuous series of coarse, yellow to ochre-colored, clay-rich, feldspathic channel sands; and a brightly variegated, noncalcareous, bentonitic claystone rich in polished, siliceous pebbles and cobbles (“gastroliths” of some authors). These seven units have been designated here by informal terms: Unit I to Unit VII. Units I through III are considered as the Morrison Formation. Units IV through VII correspond wholly or in part with the Pryor Conglomerate, Little Sheep Mudstone and Himes Mudstone Members of the Cloverly Formation, as that formation was defined by Moberly (1960). Units VI and VII and the lower sands (Unit VIII) of the overlying Sykes Mountain Formation correspond to the Cloverly Formation as it was defined by Darton in 1906. Some authors have applied the term Morrison to the entire nonmarine sequence and limited the Cloverly Formation to basal sands (Unit VIII) of Moberly’s Sykes Mountain Formation. In order to provide a meaningful stratigraphic foundation for placement of the paleontologic collections obtained, the stratigraphic sections and terminology usage of previous workers are compared in detail with the informal lithic units recognized in this study.

Extensive collections of fossil vertebrate remains were made from Units V, VI and VII. The fauna, as presently known, is considerably less diverse than that of the Morrison Formation of other regions. It includes a new species of Ceratodus (C. frazieri), an indeterminate amioid, two baenoid turtles (Naomichelys speciosa Hay, 1908 and Glyptops pervicax Hay, 1908), a possible testudinid, indeterminate mesosuchian crocodilians, and several theropods (Deinonychus antirrhopus Ostrom, 1969; Microvenator celer [new genus and species]; an undefinable species of Ornithomimus; and an undefinable, but distinctive, large theropod. Also, of greatest abundance, are remains of a titanosaurid sauropod, a new genus and species of iguanodontid ornithopod (Tenontosaurus tilletti) and a new genus and species of acanthopholid ankylosaur (Sauropelta edwardsi). Despite extensive washing and sieving, no mammalian or other microfaunal remains were recovered.

The Cloverly fauna is totally distinct from that of the classical Morrison Formation. With the possible exception of the crocodilian remains, not a single Cloverly specimen is referable to a taxon presently known from the Morrison Formation. On the other hand, the few fragmentary vertebrate fossils that have been recovered from Units I, II and III in the study area are referable to well-established Morrison taxa. In addition, all specimens now known from the Cloverly Formation (Units V, VI and VII) appear to have closer affinities with Late Cretaceous taxa than with Morrison species. This may be explained by either a major environmental change or a significant time hiatus between Morrison and Cloverly time, or both. Comparison with faunas from the Arundel Formation of Maryland, the Glen Rose Formation (Trinity Group) of Texas and Oklahoma, and the Wealden beds of northwestern Europe indicate at least partial contemporaneity of the Cloverly and Glen Rose Formations and a somewhat greater age for the Arundel and Wealden beds. The age of the Cloverly, on the bases of fossil invertebrate and paleobotanical evidence, as well as the fossil vertebrates, is probably Late Aptian and Early Albian.