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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 7. (July)

First Page: 933

Last Page: 933

Title: Paleoenvironmental Interpretation Based on Foraminifera of Coal-Bearing Almond Formation, Little Snake River Coalfield, Wyoming: ABSTRACT

Author(s): W. Anthony Bryant


The Upper Cretaceous Almond Formation (Mesaverde Group) in south-central Wyoming represents deposition in a variety of marginal marine environments. Foraminiferal assemblages recovered from cores and outcrops of the Almond in the Cow Creek area reflect this environmental diversity.

The Almond Formation is about 450 ft (135 m) thick and is divided into 2 informal members, both of which contain coal. Coals in the upper 100 ft (30 m) of the upper member are thin, but the lower member contains several thick beds. The coal-bearing parts of both members are characterized by repetitive coarsening-upward bay-fill deposits of mudstone and sandstone, commonly overlain by coal. A major coarsening-upward sequence in the lower part of the upper member is capped by sandstone interpreted to be a marine shoreface deposit. Fine-grained rocks in both members contain foraminifera.

Three foraminiferal assemblages are defined on the basis of faunal density, diversity, dominance, and taxonomic composition. A low-diversity agglutinated benthic assemblage interpreted as a hyposaline salt-marsh fauna occurs in the fine-grained rocks of the lower member. A high-diversity mixed agglutinated and calcareous benthic assemblage interpreted as a hyposaline bay to lagoon fauna occurs in shales in the lower part of the upper member. A moderate-diversity agglutinated benthic assemblage that occurs in fine-grained rocks in the upper part of the upper member is interpreted as an intermediate hyposaline salt marsh to interdistributary bay fauna.

These variations in benthic foraminifera populations provide significant insight into water characteristics in otherwise homogeneous sediments. The combination of lithologic and faunal studies provides improved paleoenvironmental interpretation over either method used independently.

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