GEOCHRONOLOGY AND THERMOCHRONOLOGY OF UPPER CRETACEOUS AND PALEOCENE DEPOSITS IN THE CHARLESTON-NEBO SALIENT AND THEIR DISTAL FORELAND EQUIVALENTS

The time span between source exhumation and sedimentation (i.e. lag time) is measured with the cooling age of the source material through thermochronology and the depositional age of the foreland deposits. Measuring the lag times of distal coarse deposits in the foreland basin can help to understand whether or not these were deposited during active shortening, or during times of flexural rebound in the thrust belt. We sampled coarse-grained proximal units in the Sevier thrust belt in Utah and their distal equivalents up to 300 km east of the thrust front, and generated detrital apatite fission track (AFT) and zircon (U-Th)/He (ZHe) ages. We also further constrained the depositional age of the distal coarse sediments through detrital zircon ages (DZ). ZHe ages from the Price River and North Horn Conglomerates, as well as their distal equivalents, are discordant, indicating that the system was not fully reset. This suggests that these strata never experienced T> ~180 °C; or, it could be that α-damage has contributed to He retention. AFT ages from these samples appear to be fully reset and show a consistent younging up-section. AFT cooling ages for the upper Campanian Price River Formation are 79.8 ± 6.3 Ma in the lower part of the formation and 74.5 ± 6.4 Ma higher up in the section. DZ ages of the distal equivalent of the Price River Formation (the Sego Sandstone) show a maximum depositional age of circa 76 Ma. The Maastrichtian to Paleocene North Horn Formation, which is separated from the Price River Formation by an angular unconformity, has an AFT age of 66.1 ± 6.2 Ma. This suggests that Paleozoic strata within the Charleston-Nebo salient were exhumed from ~4-5 km depth during the late Cretaceous, recording the timing of active deformation. The depositional ages of these units are within error of the cooling ages, indicating very short (approximating to 0) lag times, rapid exhumation of the Sevier fold-thrust belt, and syntectonic deposition. Furthermore, our DZ age of circa 76 Ma in the Sego Sandstone constrains its deposition to be coeval with shortening in the Charleston-Nebo salient and growth in the Santaquin culmination. Ongoing work is focused on producing data from the distal equivalents of these proximal syntectonic sediments.