PATTERNS OF SYNOROGENIC SEDIMENTATION AND UNROOFING HISTORY OF THE WILLARD-PARIS-MEADE THRUST SHEET, SEVIER FOLD-THRUST BELT

The Willard-Paris-Meade thrust sheet and associated synorogenic deposits are well exposed from SE Idaho and northern Utah to western Wyoming. This thrust sheet comprises a 10- to 15-km-thick package of miogeoclinal rocks that was emplaced ~60 km eastward within the Sevier fold-thrust belt. The thrust sheet included Jurassic–Triassic strata (now mostly eroded), mixed siliciclastic-carbonate upper Paleozoic strata, carbonate-rich lower Paleozoic strata, and quartzite-rich basal Cambrian to Neoproterozoic strata, which each display distinctive detrital zircon (DZ) signatures. The thrust system had a long deformation history recorded by westward thickening and coarsening synorogenic strata, including the Aptian(?)–Albian Gannett Group, Bear River and Aspen formations, and Turonian Frontier Formation. To better understand unroofing history and nature of flexural loading, an integrated thermochronologic and DZ/petrographic study of source bedrock and synorogenic strata is underway. Zircon U-Pb geochronology of 10 samples of synorogenic strata collected from two transects reveals consistent stratigraphically upward changes in DZ patterns. DZ spectra for the Gannett Group show abundant Jurassic and Triassic grains, moderate amounts of Paleozoic, Neoproterozoic, and Mesoproterozoic grains, and lesser Paleoproterozoic grains, consistent with erosion of Mesozoic to upper Paleozoic bedrock in the thrust sheet. Limited volcanic grains in the upper part of the group indicate a maximum depositional age <110 Ma. DZ spectra for the Bear River and Aspen formations have fewer Jurassic and Triassic grains and a complex mix of Paleozoic to Archean grains, consistent with erosion of mostly Paleozoic bedrock. DZ spectra for the Frontier Formation reveal increasing amounts of Mesoproterozoic and Paleoproterozoic grains, consistent with increased erosion of basal Cambrian to Neoproterozoic quartzites. The Aspen and Frontier formations contains tuffaceous layers and sandstones with abundant ~103 to 95 Ma volcanic grains that closely constrain depositional age. Total thickness of the synorogenic strata increase from ~1000 to >3000 m westward. These observations indicate protracted emplacement and loading by a dominant thrust sheet that was unroofed during the Early Cretaceous.