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Paleogeomorphic Record of the Green River Formation: A New Perspective on an Old Problem

 

Carroll, Alan R.1 (1) University of Wisconsin-Madison, Madison, WI

 

Recent studies of the Green River Formation have revealed that geomorphic evolution of the surrounding orogenic and volcanic landscape exerted a profound and previously under-appreciated influence on lacustrine sedimentation. At the broadest scale, the birth of large lakes corresponded with regional exposure of erosion-resistant basement lithologies in adjacent Laramide uplifts. The resultant sharp decrease in sediment supply caused basin starvation, permitting the expansion of slakes dominated by carbonate sedimentation. At finer scales, the sudden lake-type changes that occurred during Green River Formation deposition appear to primarily record drainage reorganizations in the surrounding landscape. For example, the transition from the underfilled Wilkins Peak Member to the balanced-fill lower LaClede Bed coincided with both the reappearance of fish, and with a dramatic shift to higher lakewater 87Sr/86Sr ratios (from ~0.711 to ~0.714). This change required only a few thousand years, and likely reflects geomorphic capture of a river system that drained highly radiogenic, basement-derived detritus to the east. A later transition between balanced-fill and overfilled facies of the LaClede Bed corresponds with a ~5‰ negative shift in lakewater 18O. This change required ~100 k.y., and is interpreted to record capture of high-elevation precipitation that fell in the Challis volcanic province in central Idaho. Expansion and southward migration of the Challis and Absaroka volcanic provinces initially forced the establishment of regional north to south drainage through the Green River lake system, and ultimately led to the demise of the Green River lakes by choking them with volcaniclastic sediments.

 

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