Calibrating geologic strata, dinosaurs, and other fossils at Dinosaur Provincial Park (Alberta, Canada) using a new CA-ID-TIMS U–Pb geochronology

Abstract

The 100 m thick stratigraphic section exposed at Dinosaur Provincial Park (DPP, southern Alberta) contains bentonites that have been used for more than 30 years to date DPP's rocks and fossils using the K-Ar decay scheme. Limited reproducibility among different vintages of K-Ar and 40Ar/39Ar ages inhibited the development of a high resolution chronostratigraphy. Here we employ and further test a recently completed CA-ID-TIMS U–Pb geochronology and associated age-stratigraphy model to update temporal constraints on the Park’s bentonites, formational contacts, and other markers. In turn, we document rock accumulation rates, and calibrate ages and durations of informal megaherbivore dinosaur assemblage zones and other biozones. Weighted mean 206Pb/238U ages from five bentonites range from 76.718 ± 0.020 Ma to 74.289 ± 0.014 Ma (2σ internal uncertainties) through an interval of 88.75 m, indicating a duration of ~2.43 Myr and an overall rock accumulation rate of 3.65 ± 0.04 cm/ka. An increase in rate above the Oldman-Dinosaur Park formational contact conforms to a regionally expressed pattern of increased accommodation at ~76.3 Ma across Alberta and Montana. Palynological biozone data suggest a condensed section/hiatus in the uppermost portion of the Oldman Formation. Dinosaur assemblage zones exhibit durations of ~600–700 kyr and are significantly shorter than those in the overlying Horseshoe Canyon Formation. A decreased rate in dinosaur-assemblage turnovers in the last eight million years of the Mesozoic in western Canada may be explained by withdrawal of the Western Interior Seaway and the expansion of ecologically homogenous lowlands in its wake.