TEMPORAL CONSTRAINTS ON THE TERMINAL CRETACEOUS DINOSAUR FAUNA IN COLORADO AND NORTH DAKOTA

The terminal Cretaceous dinosaur fauna in the Western Interior of North America is dominated by Triceratops, Tyrannosaurus, and Edmontosaurus that are known from Alberta to New Mexico. The highest occurrence of these genera is within the uppermost few meters of Cretaceous strata. In several basins, these “high dinosaurs” occur in the same local sections that contain the physical indicators of an asteroid impact correlated to the Chicxulub impact structure. Continued discovery of new dinosaur specimens and associated volcanic ash beds, in concert with U-Pb geochronology, magnetostratigraphy, and palynostratigraphy analyses in the Denver Basin in Colorado and the Williston Basin in southwestern North Dakota is contributing to an increasingly robust chronostratigraphic framework for correlating dinosaur occurrence relative to the K-Pg boundary. In the Denver Basin, where outcrops are poor the framework relies mostly on two cored wells and a basin-wide model based on well logs. In the North Dakota field area which is outcrop rich, the framework is constrained by a high-resolution GPS survey used to create digital surface coincident with the K-Pg datum. While the last occurrence of the Triceratops fauna continues to converge with the K-Pg horizon, its first occurrence remains poorly constrained. The age of the base of the Hell Creek Formation in Montana and the Dakotas and the Lance Formation in Wyoming is poorly understood and is presently unresolved by geochronology (no dated ash beds), magnetostratigraphy (occurring in the long normal polarity of subchrons C30n/C31n), or palynostratigraphy (all within one palynofloral zone). An 40Ar-39Ar date for the uppermost ash beds in the Cretaceous marine Pierre Shale provides a maximum age for the base of the Triceratops Zone and a maximum duration of the zone of ca. 4.6 Myr. In North Dakota, extrapolation from the K-Pg using a sediment accumulation rate based on the thickness and duration of the Cretaceous portion of subchron C29r yielded a minimum duration of ca. 1.4 Myr. New U-Pb geochronology from Colorado now suggests a duration of ca. 2.6 Myr for the Triceratops fauna but it is not clear how much of this time is represented by sections in Montana and the Dakotas. The earliest Triceratops in the USA is a specimen from Weld County, Colorado that occurs at approximately 68.5 Ma.