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Department of Earth Sciences,,Montana State University, Bozeman, Montana

Abstract: Carbonate Sedimentation in a Foreland Basin Lake System, Upper Cretaceous Beaverhead Group, Southwest Montana

The Upper Cretaceous Beaverhead Group of southwest Montana comprises a sequence of synorogenic coarse clastic sediments shed from the Sevier fold and thrust belt and Laramide foreland uplifts into adjacent basins. At the McKnight Canyon type section of the Beaverhead Group near Dell, Montana, the sequence contains three units: 1) a lower conglomerate, 2) a middle limestone/siltstone, and 3) an upper conglomerate. The 566 m thick Campanian to Maastrichtian middle limestone/siltstone member of the Beaverhead Group represents the deposits of a perennial freshwater, carbonate producing lake that developed adjacent to the Sevier fold and thrust belt in a partitioned foreland basin.

Lithologies encountered in this limestone component indicate a shallow, freshwater environment of deposition with episode fluctuations in wave energy and clastic input. Periods of high wave energy are indicated by oncoidal biomicrite, oncoidal sandstone, and cyanobacterial encrusted skeletal fragment biosparite/micrite. Low energy conditions are suggested by the fine-grained bioturbated micrite. Clastic influx into the lake system is recorded by the sandstone and oncoidal sandstone lithologies. Lithologies containing well developed oncoids, oncoidal sandstone and oncoidal biomicrite, contain concentrically layered, round to ellipsoid, oncoids ranging from 0.5 to 4.5 cm. Visible nuclei of many of the oncoids are composed of rip-up clasts, broken skeletal fragments, and broken oncoid fragments. Nuclei composition, the well developed concentric nature of the oncoids, and the influence of cyanobacteria all indicate a shallow water, high energy environment of development. Cyanobacterial biosparite and biomicrite contain densely packed framework grains of, invertebrate skeletal fragments encrusted by concentric layers of cyanobacterial growth in a possible early stage of oncoid development. The concentric nature of the cyanobacterial encrustation as well as the broken condition of the skeletal fragments indicates an environment with wave energy suitable for the rolling, flipping, and breaking of skeletal fragments while the prominence of cyanobacteria in these samples indicates an environment within the photic zone. Bioturbated micrites are dense, fine-grained carbonate mud lacking internal structure or lamination; many contain minor invertebrate skeletal fragments. The high degree of bioturbation and presence of invertebrate skeletal fragments indicate a shallow water environment of deposition. Sandstones are quartz arenite and lithic arenite and reflect episodic influx of clastic detritus into the lake system. Vertical stratigraphic relations between these facies reflect fluctuations between high and low wave energy conditions and episodic periods of clastic influx into the lake system.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana