PATTERNS OF SYNOROGENIC SEDIMENTATION ASSOCIATED WITH THE UNROOFING OF THE WILLARD-PARIS-MEADE THRUST SHEETS, SEVIER FOLD-THRUST BELT

The Willard-Paris-Meade thrust is the oldest and western-most sheet to develop in the Wyoming salient of the Sevier FTB. The 10-15 km thick thrust sheet was emplaced ~60 km eastward and included Jurassic-Triassic strata, mixed siliciclastic-carbonate upper Paleozoic strata, carbonate-rich lower Paleozoic strata, and quartzite-rich basal Cambrian to Neoproterozoic strata. Each stratigraphic interval has a distinctive detrital zircon (DZ) age signature, which can be used in provenance analysis of foreland basin fill. The thrust system had a long deformation history recorded by westward thickening and coarsening synorogenic strata. DZ U-Pb geochronology of 27 synorogenic samples collected from two transects reveals an unroofing sequence with stratigraphically consistent changes in DZ patterns. DZ spectra for the Gannett Group in the lower part of the basin fill are consistent with erosion of Mesozoic to upper Paleozoic strata. Maximum depositional ages (MDA) from limited euhedral and likely volcanic grains for the foreland basin sequence show the following for the northern transect: basal Ephraim Formation, 150 ± 2.8 Ma (n=10); upper Ephraim, 115.2 ± 1.8 Ma (n=13); lower Bechler, 116.2 ± 2.3 Ma (n=9); upper Bechler 107.3 ± 3.3 Ma (n=4). These data clarify that there is not a significant unconformity at the base of the Bechler, and that the Bechler conglomerate facies at Red Mountain spans the depositional interval including the ~112 Ma Drainey Limestone, which is absent at Red Mountain but present elsewhere. The southern transect yields the following MDAs: Upper Gannett, 109 ± 6.1 Ma (n=5) and 108.3 ± 3.4 Ma (n=10); Cokeville, 101.6 ± 0.7 Ma (n=32); Lower Sage Junction 101.55 ± 0.5 Ma (n=67); Upper Sage Junction, 101.3 ± 0.6 Ma (n=29); Aspen, 98.8 ± 0.4 Ma (n=51); lower Frontier, 99.93 ± 0.6 Ma (n=21); upper Frontier, 95.68 ± 1.4 Ma (n=38). DZ spectra for the Bear River and Aspen formations in the middle part of the basin fill are consistent with erosion of mostly Paleozoic bedrock. DZ spectra in the Frontier Formation in the upper part of the basin fill are consistent with increased erosion of basal Cambrian and Neoproterozoic quartzites. The early history of deposition is elusive due to the potential bias from the early Cretaceous Sierran magmatic lull and a lack of datable materials related to corollary studies.