Abstract

Results of a field and laboratory study, made of stratigraphic relations, lithologic characteristics, and sedimentary history of nonmarine Upper Jurassic and Lower Cretaceous rocks in the southern Big Horn Mountains, Wyoming, are enumerated as follows. 1) The nonmarine section between the marine Sundance Formation (Upper Jurassic) and the definitely marine Thermopolis Shale (Lower Cretaceous) is divisible into 4 natural mappable units. Unit 1, the lowest, is essentially a variegated mudstone. Unit 2 is a white to gray cross-bedded sandstone which is commonly resistant and at many places conglomeratic. Unit 3 is a variegated mudstone like unit 1. Unit 4 is a black shale, interbedded with thin lenticular tan cross-laminated siltstone and fine-grained sandstone. The first 3 units are entirely nonmarine; the fourth includes some tidal-flat and marginal marine deposits. The contact between units 1 and 2 is disconformable, and that between units 3 and 4 is unconformable. The lower mudstone (unit 1) is the Morrison Formation; the white sandstone (unit 2) and the overlying upper mudstone (unit 3) constitute the Cloverly Formation; and the upper black shale and siltstone (unit 4) are the basal Thermopolis Shale. 2) Because the white sandstone forming the lower part of the Cloverly Formation is distinct and persistent, this unit is here named the Otter Creek Sandstone Member of the Cloverly. The upper part of the Cloverly is not given a formal name because of poor exposures and discontinuity along the eastern side of the Big Horn Mountains, but is simply referred to as the Mudstone member of the Cloverly. Unit 4, long known informally as the "Rusty Beds," has been variously placed by previous investigators in the uppermost Cloverly Formation or in the lowermost Thermopolis Shale. It is here formally named the Rusty Beds Member of the Thermopolis Shale. 3) Orientation of cross-beds, and dominance of sandstone or conglomerate in exposures on the western side of the Big Horn Mountains, suggest a source for the Morrison, and Otter Creek deposits, in the Overthrust Belt of southeastern Idaho and western Wyoming. The heavy minerals (chiefly rounded zircon, tourmaline, and rutile, with magnetite-ilmenite and leucoxene), pebbles (quartzite and fossil-bearing chert), and mechanical analyses suggest older sediments as source rock. Bentonitic units show that some volcanic debris was incorporated in the deposits. 4) The Mudstone member of the Cloverly appears to have been derived from a northeastern or eastern source. 5) Contact between Morrison Formation and Otter Creek member of the Cloverly appears to represent a hiatus. This is suggested by a combination of differences in cross-bed orientation, and by the heavy mineral content, especially the abrupt increase in percentage of rounded zircon and the decrease in garnet in passing from the Morrison to the Cloverly. This break may or may not correspond with the Jurassic-Cretaceous boundary. 6) The contact between Otter Creek and Mudstone members of the Cloverly is poorly exposed and is not yet well understood. It appears even and conformable at some places, but elsewhere it shows channeling of the Mudstone member into the Otter Creek Member. At one locality on the eastern flank of the Big Horn Mountains, evidence of intertonguing suggests that the 2 members may be more nearly facies equivalents than successive deposits.

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