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Depositional environments, facies distribution, and provenance analyses of Upper Jurassic through lowermost Upper Cretaceous strata in western Wyoming, eastern Idaho, and northeastern Utah suggest that episodic tectonic activity along the Paris-Willard thrust system strongly influenced deposition during early development of the Wyoming-Idaho-Utah thrust belt and associated foreland basin. Synorogenic conglomerates present at various stratigraphic levels in these strata reveal periods of rapid uplift. In general, the synorogenic units contain proximal cobble-boulder conglomerates (braided stream) which grade downslope into distal pebble conglomerates and coarse-grained sandstones (meandering stream). Periods of relative tectonic quiescence and/or less rapid uplift and eros on are represented by interbedded finer grained fluvial, lacustrine, and marine deposits.
During the Late Jurassic, erosion of incipient highlands prior to thrusting resulted in eastward progradation of beach/barrier sandstones represented by the Stump Formation. Initial intensive thrusting followed in the latest Jurassic and earliest Cretaceous, with the newly formed highlands shedding proximal braided stream cobble-boulder conglomerates and more distal meandering stream pebble conglomerates and sandstones of the Ephraim Formation into the subsiding foreland basin. Continued subsidence coupled with a decrease in clastic input due to subdued uplift, resulted in establishment of extensive lacustrine systems and deposition of the Peterson Limestone. Renewed movement on the Paris-Willard thrust system then gave rise to the proximal conglomerates and distal sandstones and muds ones of the Bechler Formation. The overlying lacustrine Draney Limestone and marginal lacustrine Smoot Formation represent another period of continued basin subsidence with little or no uplift.
The Wayan and laterally equivalent Bear River Formations represent, respectively, near-source fluvial and shallow beach-marine deposition following Smoot/Draney accumulation. Wayan strata were deposited on a meandering stream flood plain and are indicative of slow uplift and erosion in the source area. Bear River strata consist of a beach sandstone unit underlain and overlain by transgressive marine shales. Alternate deposition of transgressive and regressive facies resulted from either eustatic changes in sea level or differential uplift and erosion of the Paris-Willard highlands.
Bear River marine shales grade upward into marine shales of the Aspen Formation, deposited when the Mowry sea transgressed across the thrust belt region. Lack of significant tectonic activity during this time is suggested by the paucity of sand in the Aspen Formation.
Aspen strata pass upward through marginal marine strata into a thick sequence of meandering stream deposits of the Frontier Formation that were derived from erosion of subdued Paris-Willard highlands. Subsequent transgression of the Greenhorn sea westward covered the entire region, producing extensive Frontier marginal marine sandstones and marine shales. Renewed intensive uplift in the source area caused rapid eastward progradation of the Greenhorn sea shoreline and concurrently deposited cobble conglomerates in northeastern Utah. Coarse detritus was deposited in eastward-flowing braided streams near the source area (northeastern Utah) and in meandering stream channels farther eastward (western Wyoming). The Niobrara sea subsequently covered much of the region, and synorogenic sedime tation related to the Paris-Willard thrust system was thus completed.
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