Detrital zircon geochronology from Cenomanian-Coniacian strata in the Bighorn Basin, Wyoming, U.S.A.: Implications for stratigraphic correlation and paleogeography

Abstract

A high-flux, Late Cretaceous magmatic event in the western United States has been tested as a zircon source for high-resolution chronostratigraphic correlation in coeval sedimentary rocks in northwest Wyoming. Thirteen samples of Cenomanian–Coniacian sandstone in the Bighorn Basin yielded more than 1200 U/Th/Pb detrital zircon ages from the Mowry Shale, the Frontier Formation, and the Cody Shale. In addition, two individual clast ages were obtained from a conglomerate located near the top of the Frontier Formation. These formations are dominated by detrital zircon grains that yield paleontologically constrained depositional or near-depositional ages. Each sample has a minimum of 22 grains comprising the youngest age peak. Individual youngest peak ages range from 99.4 to 87.7 Ma, spanning Cenomanian through Middle Coniacian time (Gradstein et al., 2012). Three of four stratigraphic sections yield samples with minimum age peaks that young upward, are consistent with available paleontological control, and suggest an age resolution of one–two million years despite an estimated analytical error of 2 percent (+/− 2 Ma for 100 Ma samples). An age reversal at the top of the fourth section demonstrates that recycling of older sediments into younger beds can be an important control on the age of zircon populations, even during intervals of sediment accumulation dominated by first-cycle zircons from an active magmatic arc. The presence of nearly depositional age volcanic cobbles at the top of the Frontier Formation implies rapid erosion and transport of coarse material from a volcanic source eastward into the foreland basin. The new detrital zircon data, in conjunction with available paleontological constraints, provide a framework for detailed stratigraphic correlation.