DETRITAL ZIRCON U-PB MDA AND AGE PATTERNS ASSOCIATED WITH THE UNROOFING OF THE WILLARD-PARIS-MEADE THRUST SHEETS, SEVIER FOLD-THRUST BELT

U-Pb geochronologic analysis of detrital zircon (DZ) grains from strata within the early Sevier foreland basin east of the Willard-Paris-Meade (WPM) thrust sheet has provided insight into the initial timing and development of the fold-thrust belt. Maximum depositional ages (MDA) of foreland basin strata were inferred from U-Pb ages of the youngest, limited euhedral, and likely volcanic grains incorporated into the basin. In addition, each stratigraphic interval within the thrust sheet has a distinctive DZ age pattern which can be used in provenance analysis of the foreland basin fill. 27 synorogenic samples were collected from two transects within fluvial systems that developed in response to topographic relief from thrusting over ramps, as well as from formation of a foreland basin system. MDA’s from the Gannet Group include an age of 150 ± 2.8 Ma (n=10) from the basal part of the Ephraim Formation and an age of 115.2 ± 1.8 Ma (n=13) from the upper part of the Ephraim. A well-defined temporal progression exists within the synorogenic foreland basin units, spanning 115.2 ± 1.8 Ma within the Gannet Group, to 95.68 ± 1.4 Ma (n=38) within the Upper Frontier Formation. DZ spectra within the foreland basin sediment are consistent with erosion of Mesozoic to upper Paleozoic to Neoproterozoic strata from the WPM thrust sheet, as demonstrated by K-S statistics showing the quantitative comparison of age-distribution curves. Calculated P-values greater than 0.05 have a greater than 95% confidence of similar provenance. There is a strong correlation between the Nugget Formation, suggested to be the uppermost unit of the WPM thrust sheet, and the associated foreland deposits of the Ephraim and Bechler formations within the Gannet Group (0.977, 0.877). Correlations of age-distribution curves between the remaining foreland basin units and the units within the WPM thrust sheet show that the Oquirrh, Geertsen, and Mutual formations were a significant source for stratigraphically higher sediments, such as the Cokeville, Bear River, and Frontier formations. Analysis of the unroofing sequence in the foreland basin shows that the sedimentological history is consistent with DZ patterns correlated directly with the WPM thrust sheet.