--> Abstract: Complex High-Resolution Lacustrine Record of Laramide Tectonism: Green River Formation, Wyoming; #90063 (2007)

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Complex High-Resolution Lacustrine Record of Laramide Tectonism: Green River Formation, Wyoming

 

Smith, M. Elliot1, Alan R. Carroll2 (1) University of Montana, Missoula, Missoula, MT (2) University of Wisconsin-Madison, Madison, WI

 

Eocene synorogenic alluvial deposits shed from the northern Uinta Mountains interfinger northward with well-dated lacustrine strata of the Green River Formation in Wyoming, providing an exceptionally high-resolution record of Laramide tectonism. Clast compositions in synorogenic conglomerate show that unroofing of the Precambrian core of the Uinta Mountains occurred continuously, but diachronously during the Paleocene-Early Eocene. The east-central part of the range, near its intersection with the Rock Springs uplift, was unroofed first, followed by progressively younger unroofing westward. In contrast, southward thickening of the Wilkins Peak Member of the Green River Formation implies that a more discrete pulse of subsidence occurred in the southern Bridger basin between 51.3 to 49.6 Ma. These apparently conflicting observations can be reconciled by considering temporal changes that occurred in the style of lacustrine sedimentation. Within the overfilled to balanced-fill basin phases represented by the Luman Tongue and lower Tipton Member, differential basin subsidence is not readily apparent due to a lack of continuous stratigraphic markers. During the balanced-fill phase of the upper Tipton Member, a deep lake filled most the basin and concentrated sedimentation near its margins, while the deepest areas of the basin were sediment-starved. Southward-thickening of the underfilled Wilkins Peak Member reflects generally lower lake levels, with resultant focusing of alluvial sedimentation into the basin depocenter. The return to balanced-fill to overfilled conditions after Wilkins Peak time once again obscured any evidence for significant differential subsidence. Spatial patterns of conglomerate deposition also reflect these temporal changes in lake type; the greatest basinward advances of conglomerate coincided with evaporative facies of the Wilkins Peak member, whereas conglomerate units retreated southward toward the mountain front during wetter basin phases.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California