Stratigraphic Sequences and Paleoenvironmental Interpretation of Upper Cretaceous Almond Formation in Cow Creek Area, Little Snake River Coalfield, South-Central Wyoming: ABSTRACT

Abstract

The Almond Formation is a paralic deposit, underlain by continental, fluvial Pine Ridge Sandstone and overlain by marine Lewis Shale. In the Cow Creek area, the Almond is 440-490 ft (135-150 m) thick and consists of two unnamed members. The lower member, 170-240 ft (52-73 m) thick, is composed predominantly of 5-25 ft (1.5-7.5 m) thick, repetitive, coarsening-upward sequences of thin brown mudstone and bioturbated, ripple-laminated sandstone. These sequences are commonly overlain by carbonaceous siltstone-mudstone and coals as thick as 16 ft and locally intertongue with trough cross-bedded sandstone. The upper member is 230-290 ft (70-88 m) thick. The basal part consists of 40-60 ft (12-18 m) of gray clay shale, interbedded with sandstone toward the top, and 10-30 ft (3-9 m) of burrowed (Ophiomorpha) sandstone, hummocky cross-bedded in the lower part and trough cross-bedded in the upper part. The upper 160-220 ft (49-67 m) are primarily repetitive coarsening-upward sequences similar to those in the lower member, except the upper member contains brackish-water or shallow marine bivalves, more gray clay-shale than brown mudstone, and fewer, thinner ( The Almond in the Cow Creek area is a lower delta-plain deposit that intertongues with stacked shoreline deposits to the southeast. The coarsening-upward sequences of the lower member accumulated in the upper reaches of brackish-water bays. Sandy parts of these sequences are distributary-channel splays, which provided platforms on which freshwater peat swamps developed. The upper member was deposited in a more seaward location than the lower member. The basal part of the upper member is a shallow or restricted marine, offshore and shoreline deposit and the upper part accumulated in coastal bays and fringing swamps. These paleoenvironmental interpretations, which are supported by results of Foraminifera studies, account for the origin and stratigraphic position of economically important coals in the lower member but not in the upper member of the Almond. End_of_Article - Last_Page 931------------