DETRITAL MULTI-DATING OF CORDILLERAN FORELAND BASIN CLASTIC ROCKS: IMPLICATIONS FOR FORELAND BASIN STRATIGRAPHIC MODELS

As a means of assessing foreland basin stratigraphic models, we present preliminary geo- and thermochronological data from Cretaceous clastic units in the Wyoming-Colorado-Utah sector of the Cordilleran foreland basin. The data include apatite fission track (AFT) ages from detrital apatites in sandstone and conglomerate clasts; U-Pb ages from detrital apatites in sandstone (AUPb); (U-Th)/He ages from detrital zircons (ZHe) in sandstone and within clasts; and detrital zircon U-Pb ages (ZUPb). Youngest clusters of ZUPb ages corroborate published biostratigraphic and radiometric ages from throughout the Cretaceous section. ZUPb age spectra <200 Ma mirror ages of high flux events in the Cordilleran magmatic arc. ZHe and AFT ages show that AFT is the preferred thermochronometer for assessing source terrane exhumation ages; AFT data generally show young populations that are consistent with exhumation ages in the thrust belt and only local evidence for significant annealing during burial in the foreland basin. AFT ages from proximal foredeep sandstones in the lower Cretaceous record exhumation at ca. 80-59 Ma associated with the Absaroka and Hogsback thrusts. ZHe data show inherited Paleozoic ages that suggest only partial resetting before exhumation in the thrust belt. Together the ZHe and AFT data bracket the depth of exhumation at 5-8 km, assuming a ~20°C/km geotherm. AFT ages from proximal conglomerates are nearly equal to depositional ages, indicating near-zero lagtimes and minimal storage in transit to the foreland basin. AFT ages from detrital apatites in medial to distal sandstones exhibit 0-5 Myr lagtimes, also suggesting rapid flushing of detritus from the thrust belt into the foreland region soon after exhumation in the thrust belt. However, lagtime interpretations of near depo-age AFT ages in some distal sandstone samples are compromised by Cretaceous AUPb ages consistent with derivation from the magmatic arc. Apatite double-dating (U-Pb and FT) is necessary to distinguish between true source exhumation ages and magmatic ages. Nevertheless, preliminary results suggest increasing exhumation rate in the Cordilleran thrust belt throughout the Cretaceous, and continuous rapid transit of fluvial detritus from 5-8 km deep in the thrust belt into the distal foreland region.