Paleogeomorphic Record of the
Carroll, Alan R.1 (1)
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
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California