Stratigraphy of Upper Jurassic Morrison and Lower Cretaceous Cloverly formations of Big Horn basin, northern Wyoming
The Morrison and Cloverly Formations in the Big Horn basin of northern Wyoming and southern Montana are part of a distal edge of a westward-thickening clastic wedge of sediment deposited in an elongate intracontinental basin in the western North American craton. These formations reflect orogenic and volcanic activity in the western Cordillera during Late Jurassic and the subsequent eastward migration of volcanic centers during Early Cretaceous. The Morrison Formation (Upper Jurassic) conformably overlies the Jurassic marine Sundance Formation and consists of light olive-green, lenticular, calcareous siltstones and mudstones interbedded with white to buff or yellowish green, massive and cross laminated, calcareous quartz-arenites. The Cloverly Formation (Lower Cretaceous) consists of three members: the Pryor Conglomerate, and Little Sheep Mudstone, and the Himes; it overlies the Morrison Formation both conformably and unconformably. Both the Morrison and Cloverly Formations are characterized by high ratios of overbank fines relative to coarse channel sands. It has been assumed, but not documented by detailed sedimentologic study, that the deposits were part of an aggrading alluvial flood plain complex dotted by seasonal lakes and swamps and crossed by braided rivers. This model deviates from most modern braided systems which are characterized by rapid lateral mobility and the lack of fine-grained overbank material. The large ratio of fine-grained siltstones and mudstones to coarser grained sandstones can be explained by a number of processes, the most probable being rapid overbank aggradation as a result of a large influx of windblown volcanic material from vents to the west.
- Research Organization:
- Iowa State Univ., Ames
- OSTI ID:
- 6432889
- Report Number(s):
- CONF-8309274-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Vol. 67:8; ISSN AAPGB
- Country of Publication:
- United States
- Language:
- English
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