Stratigraphy of Upper Jurassic Morrison and Lower Cretaceous Cloverly Formations of Big Horn Basin, Northern Wyoming: ABSTRACT

Abstract

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 sediments 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 quartzarenites. The olive-green mudstones in the upper portion of the formation alternate with red-brown calcareous mudstones or shales producing red banding. A lenticular bed of siliceous accretionary lapilli is present in the upper portion of the formation along the west flank of Sheep Mountain anticline, north of Greybull, suggesting a closer proximity to volcanic vents than previously hypothesized. The Cloverly Formation (Lower Cretaceous) consists of three members: the Pryor Conglomerate, the Little Sheep Mudstone, and the Himes; it overlies the Morrison Formation both conformably and unconformably. Its basal contact is sometimes marked by a lenticular, cross-bedded, conglomeratic quartzarenite or a pebble conglomerate consisting of clasts of black chert (Pryor Conglomerate) derived from uplifted Paleozoic deposits to the west and deposited as channel lag. The Little Sheep Mudstone Member is composed of variegated mudstones which are generally noncalcareous bentonitic, and laced with abundant chalcedony and barite concretions and veinlets. The Himes Member also contains a stacked sequence of devitrified tuffs interbedded with bentonitic mudstones, which locally may be several m ters thick. The individual tuffs, however, are generally less than 1 m (3 ft) thick and are very fine grained, buff to white, and massive. Also, relatively thin lenticular lithic wackes occur in the Himes Member. These are anomalous to the quartzarenites of the rest of the Morrison and Cloverly. A thick lenticular, trough-cross-bedded, quartzarenite caps the formation. 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 wind-blown volcanic material from vents to the west. However, this ratio could also be ob ained in a rapidly subsiding basin whose river systems are characterized by long periods between avulsion, which is common in an arid to semiarid environment. End_of_Article - Last_Page 1346------------