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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.
