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.