Detrital Thermochronologic Lag Times in the Cordilleran Foreland Basin: A New Approach to Assessing Syn-tectonic vs. Anti-tectonic Deposition in Foreland Basins

The interpretation of coarse-grained deposits in distal foreland basins has varied greatly. Some have interpreted distal coarse sediments as a result of increased tectonic shortening, exhumation, and the resultant increase in source material. In contrast, others attribute these distal coarse sediments to periods of tectonic quiescence, flexural rebound, and reworking of proximal deposits into the distal foreland. We address this question by measuring lag-times of these sediments in the central Cordilleran foreland basin. 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. Lag times of distal coarse deposits should be relatively long if the sediments accumulated during periods of tectonic quiescence, whereas lag times should be short in the case of syn-thrusting distal deposition. 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). AFT cooling ages for the proximal 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. Ongoing work is focused on measuring lag-times of distal course sediments in the Sevier foreland from the Upper Jurassic to Upper Cretaceous time.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California