Resolving the history of early fault slip and foreland basin evolution along the Wyoming salient of the Sevier fold-and-thrust belt: Integrating detrital zircon geochronology, provenance modeling, and subsidence analysis
A. Gentry, W.A. Yonkee, M.L. Wells, E.A. Balgord, 2018. "Resolving the history of early fault slip and foreland basin evolution along the Wyoming salient of the Sevier fold-and-thrust belt: Integrating detrital zircon geochronology, provenance modeling, and subsidence analysis", Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson, Raymond V. Ingersoll, Timothy F. Lawton, Stephan A. Graham
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The Willard-Paris-Meade thrust sheet is the oldest and westernmost sheet to develop in the Wyoming salient of the Sevier fold-and-thrust belt. This dominant thrust sheet was transported ~60–70 km eastward, but despite its importance to the overall shortening budget of the Cordilleran orogenic system, the timing for onset and cessation of major fault slip is poorly constrained. Synorogenic strata associated with emplacement of the sheet thicken westward, reaching a total thickness >2 km near the thrust front, and are divided into five intervals: (1) lower and (2) upper parts of the Aptian(?) to Albian Gannett Group (Ephraim and Bechler Formations); (3) Upper Albian Smiths, Thomas Fork, and Cokeville Formations and correlative Bear River Formation; (4) uppermost Albian to Cenomanian Sage Junction Formation and partly correlative Aspen Shale; and (5) Upper Cenomanian to Turonian Frontier Formation. Integrated sedimentologic, detrital zircon provenance, and subsidence analyses of these foreland basin strata record a protracted history of thrust sheet emplacement. Detrital zircon U-Pb age spectra, conglomerate clast types, and sandstone petrography record an unroofing sequence, with progressive erosion of Lower Mesozoic to Upper Paleozoic to Lower Paleozoic bedrock levels, starting in the more western Paris sheet and continuing across the Meade and Willard sheets. Synorogenic strata also contain young grains derived from the Cordilleran magmatic arc that bracket maximum depositional ages of: 115.2 ± 1.8 Ma and 116.2 ± 2.3 Ma in the upper Ephraim Formation to 107.9 ± 2.4 Ma in the overlying Bechler Formation, which were deposited during early thrust slip; 101.6 ± 0.7 Ma, 101.4 ± 0.6 Ma, and 98.8 ± 0.4 Ma in the Cokeville, Sage Junction, and Aspen Formations, respectively, which were deposited during increased subsidence and thrust slip rates; and 95.6 ± 1.4 Ma in the Frontier Formation, which was deposited during late slip, eastward thrust propagation, and early uplift of the Wasatch anticlinorium. The lower Ephraim Formation was deposited during the Early Cretaceous Cordilleran magmatic lull and lacks young zircon grains, such that timing for thrust initiation is not well constrained, ranging from ca. 130 to 120 Ma. Regional variations in stratigraphic thickness and sedimentation rates, combined with flexural modeling, indicate that a foredeep began developing in front of the Paris thrust and then migrated eastward as the Meade and Willard thrusts propagated and overrode their own synorogenic strata, with increasing rates of thrust loading and subsidence from ca. 105 Ma to 95 Ma.