HR: 09:30h
AN: T51D-07    [Abstracts]
TI: $^{40}$Ar/$^{39}$Ar Temporal Constraints on Eocene Uplift, Subsidence, and Paleohydrology in the Laramide Foreland, Western U. S.
AU: * Smith, M E
EM: msmith@geology.wisc.edu
AF: Department of Geology and Geophysics University of Wisconsin-Madison, 1215 W Dayton st, Madison, WI 53706 United States
AU: Carroll, A R
EM: carroll@geology.wisc.edu
AF: Department of Geology and Geophysics University of Wisconsin-Madison, 1215 W Dayton st, Madison, WI 53706 United States
AU: Singer, B S
EM: bsinger@geology.wisc.edu
AF: Department of Geology and Geophysics University of Wisconsin-Madison, 1215 W Dayton st, Madison, WI 53706 United States
AB: Due to their sensitivity to relatively subtle changes in regional drainage patterns, Eocene lake deposits of the Green River Formation offer a unique and richly detailed record of landscape modification caused by orogenic processes in the broken foreland of the western U. S. Recently obtained $^{40}$Ar/$^{39}$Ar age determinations for 22 interbedded tephras provide excellent temporal resolution of this record, and enable inter-basin correlations at an unprecedented level of precision (approaching 2$\sigma$ uncertainties of $\pm$ k.y.). Green River Formation strata span an interval of ~8 m.y., beginning and ending with freshwater fluvial-lacustrine deposits. Two episodes of regional basin closure and evaporite deposition, each lasting $\sim$1-2 m.y., coincide with evidence for active Laramide faulting at basin margins and increased rates of sediment accumulation. Evaporite deposition therefore appears to have been principally caused by enhanced uplift of basin sills rather than increased aridity. Regional stratigraphic relations, facies types, and $^{40}$Ar/$^{39}$Ar geochronology permit deduction of the following paleodrainage history: 1) $>$$\sim$51.3 Ma: Fluvial-lacustrine deposition occurred in greater Green River, Piceance Creek and Uinta basins. The onset of lacustrine deposition is not well-dated due to a paucity of tephras. 2) $\sim$51.3-49.7 Ma: The greater Green River and Piceance Creek basins both became terminal sinks that received overflow from neighboring freshwater basins. Coarse clastic basin-marginal alluvial strata, cross-cutting fault relations, and pronounced differential subsidence in both basins indicate active uplift of the Uinta Mountains and surrounding ranges. 3) $\sim$49.7-49.1 Ma: Lake Gosiute expanded in extent, coincident with an influx of water and sediment derived from volcanic centers to the north. Episodic overflow over the eastern Uinta uplift flushed dissolved solutes southward, freshening Lake Gosiute while evaporite deposition continued in Lake Uinta. 4) $\sim$49.1-48.4 Ma: Fresh water spilled consistently from Lake Gosiute into an expanding Lake Uinta. The saline, organic-rich Mahoghany zone of the Parachute Creek Member was deposited over an interval of 0.6 $\pm$ 0.3 m.y. coincident with deposition of the freshwater upper LaClede bed of the Laney Member and alluvial Bridger Formation in the greater Green River Basin. 5) $\sim$48.4-46.3: Fluvial volcaniclastic sediments progressively filled the greater Green River and Piceance Creek basins. Alluvial and freshwater lacustrine deposition dominated both basins. Saline lake deposition continued unabated in the Uinta Basin. 6) $\sim$46.3-45.0 Ma: Lake Uinta, limited to the western Uinta Basin, became hydrologic closed as evidenced by bedded evaporite deposition. The change to evaporite deposition coincided with an increase in differential subsidence, reflecting a renewal of tectonic deformation. The up-section disappearance of west-directed volcaniclastic input into the eastern Uinta Basin at $\sim$46.3 Ma suggests that drainage diversion may also have contributed to hydrologic closure. 7) Following $\sim$45.0 Ma, Lake Uinta returned to fluvial-lacustrine deposition and was subsequently filled with alluvial deposits.
DE: 8102 Continental contractional orogenic belts
DE: 4239 Limnology
DE: 1815 Erosion and sedimentation
DE: 1035 Geochronology
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting