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Abstract: Planar-Horizontal Bedded Sandstones Filling Fluvial Channels: High-Energy, Shallow-Water Deposition of the Molina Member of the Wasatch Formation


Although crossbedding constitutes a high percentage of the sedimentary structures in most fluvial sandstones, many of the broadly lenticular sandbodies comprising the nonmarine Molina Member of the Wasatch Formation (Piceance basin, northwestern Colorado) are composed almost entirely of planar-horizontally bedded sandstones. The depositional environment that produced these sandstones is interpreted to have been fluvial, wherein episodic flooding of a sand-choked system produced rapid deposition that kept pace with rising flood waters. This resulted in shallow-water, high-energy, sheeted deposition. This interpretation is supported by abundant parting lineation, and local crossbedding showing alternate building of topsets and foresets (rising-water structures). Rapid deposition is also suggested by post/syndepositional compaction shears, and common soft-sediment deformation due to post-depositional dewatering. Scattered pebbles up to 4 cm diameter were moved in this generally northward-flowing system. Local lateral accretion structures suggest that a meandering system existed between floods, but that the bedding units and sedimentary structures produced by flooding had the highest preservation potential.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado