A TEST OF U/TH/PB DETRITAL ZIRCON AGES FOR DETERMINING DEPOSITIONAL AGES WITHIN THE MOWRY AND FRONTIER FORMATIONS, BIGHORN BASIN, WYOMING

We have determined U/Th/Pb detrital zircon (DZ) age spectra from 13 samples of the Late Cretaceous Mowry and Frontier Formations in the Bighorn Basin of northwestern Wyoming, in a test of the use of detrital grain ages for determining depositional ages for these strata. These units were studied due to abundant ash layers interbedded with the sandstones, and relatively good biostratigraphic control. Results are also reported from two volcanic clasts in the Torchlight conglomerate at the top of the Frontier Formation. Individual youngest peak ages range from 100 to 88 Ma with common peaks between 98 and 95 Ma (i.e., Cenomanian). We interpret these to represent first-cycle zircons derived from the active Cordilleran magmatic arc to the west. In three of the four stratigraphic sections sampled, DZ results yield statistically significant young age peaks that are stratigraphically consistent, are consistent with available paleontologic control, and indicate potential age resolution of approximately one to three million years. This pattern is not observed at Alkali anticline where all three Frontier sandstones have indistinguishable 97 – 98 Ma young age peaks. Cobble-size volcanic clasts from the Torchlight member of the Frontier Formation yield U/Th/Pb ages of 96.5 +/- 1.4 Ma and 96.5 +/- 1.3 Ma, only 2-6 Ma older than the enclosing strata, indicating rapid erosion and transport of coarse material eastward from the Cordilleran magmatic arc into the Cretaceous foreland basin. This suggests a direct linkage between magmatic flux in the orogenic hinterland and sediment flux into the foreland system.