BIOSTRATIGRAPHIC IMPORTANCE OF THE NEW CA-TIMS U-PB GEOCHRONOLOGY FOR THE BELLY RIVER GROUP AT DINOSAUR PROVINCIAL PARK (CAMPANIAN, ALBERTA, CANADA)

Dinosaur Provincial Park (DPP; southern Alberta) yields a well-preserved and uniquely-diverse assemblage of late Campanian dinosaur fossils. Because the assemblage plays a central role in documenting Late Cretaceous peak-dinosaur-diversity, a chronostratigraphic framework for DPP’s fossils has evolutionary and paleoecological significance that extends beyond DPP’s boundaries. Here we employ high-precision CA-TIMS U-Pb zircon geochronology to constrain the ages of five bentonites that are semi-evenly spaced through the exposed bedrock section. Weighted mean ²⁰⁶Pb/²³⁸U dates for the bentonites range from 76.7 Ma to 74.3 Ma (with 2σ internal errors of ≤30 kyr), indicating that the section is younger than previously understood and that it encompasses a longer time span (2.4 Myr). Combined with biostratigraphic data, these results confirm a previous hypothesis that the Oldman–Dinosaur Park formation contact becomes younger toward southeastern Alberta. A significantly lower sediment accumulation rate near the Oldman-Dinosaur Park formation contact suggests the presence of a ~230 kyr hiatus (or stacked hiatuses) near the contact. Coincident first occurrences of five palynomorph taxa—that otherwise show stratigraphically staggered first occurrences in southeastern Alberta—support an interpretation that the hiatus is limited to the uppermost few meters of the Oldman Formation. Previously amassed sedimentological and stratigraphic data suggest that the hiatus may be a consequence of overall lower rates of sediment accumulation and interrupted sedimentation along the northern edge of the Oldman sedimentary lobe. Rates of sediment accumulation stay largely constant through the Dinosaur Park Formation, but may decline upwards into the Bearpaw Formation. Applying a currently accepted ammonite biochronology to DPP’s age-calibrated strata correlates the bottom of the section with the middle of the Baculites scotti Zone, and the top of the section with the approximate base of the Baculites compressus Zone. Four previously recognized dinosaur-assemblage zones at DPP are also calibrated and show durations of approximately 600–700 kyr, similar to those of ammonite biozones.