NEW CA-TIMS U-PB GEOCHRONOLOGY FROM THE BELLY RIVER GROUP (UPPER CRETACEOUS) AT DINOSAUR PROVINCIAL PARK, ALBERTA, CANADA, AND IMPLICATIONS FOR DINOSAUR BIOSTRATIGRAPHY OF THE WESTERN INTERIOR BASIN

Dinosaur Provincial Park (DPP) is a small area (80 km²) in southern Alberta that yields a uniquely-rich, well-preserved, late Campanian dinosaur assemblage from high latitudes of the Western Interior Basin. Stratigraphically-limited exposures (<90 m thick) comprise, in ascending order, the Oldman (partial), Dinosaur Park, and Bearpaw (partial) formations, all of which contain interbedded volcanic ash beds (bentonites). Radioisotopic dating of DPP's bentonites provides constraints on the age of the section and its fossils, rates of sediment accumulation and faunal turnovers, and chronostratigraphic correlations with other fossil rich units in the Western Interior. Four bentonites have been used consistently for radioisotopic dating during the past 26 years. These are semi-evenly spaced through a total section of 88.5 m. K-Ar analyses of biotites (1980s) yielded promising results, but error was relatively large (±~1.0 Ma, 1σ). ⁴⁰Ar/³⁹Ar geochronology (1990s–2000s) improved precision but yielded inconsistent results (>1% variability) due mostly to adjustments of monitor mineral ages, decay constants, and ongoing improvements in laboratory techniques and equipment. Zircon U-Pb analyses by LA-ICP-MS (2013) resulted in suites of ²⁰⁶Pb/²³⁸U dates with errors in the 0.51–0.75 Myr (2σ) range. Modern U-Pb zircon analyses by the CA-TIMS method from this section (and elsewhere in the Western Interior) have significantly improved precision and accuracy, and now provide a high-resolution temporal context for this and correlative sections and faunas. New weighted mean ²⁰⁶Pb/²³⁸U dates from DPP range from ca. 76.69 to 74.26 Ma (2σ internal errors < ±30 kyr) indicating that the section has a younger age but a much longer time span (2.43 Myr) than previously understood. Accordingly, the four dinosaur fossil assemblage zones known from DPP have considerably longer durations than previously understood, ranging from 400 kyr to 800 kyr. In line with previous results, CA-TIMS data confirm that rates of sediment accumulation are lower in the uppermost 22 meters of the section, reflecting transition to the marine Bearpaw Formation. The new geochronology now allows high-resolution correlation with distant fossiliferous exposures of the Judith River Formation in north-central Montana and the Kaiparowits Formation in Utah.