PALEOENVIRONMENTAL INTERPRETATION OF ALASKA’S LARGEST DINOSAUR MEGATRACKSITE FROM THE UPPER CRETACEOUS CANTWELL FORMATION, DENALI NATIONAL PARK AND PRESERVE

The Upper Cretaceous Cantwell Formation in Denali National Park and Preserve (DENA) preserves one of the most prolific trace fossil records of high latitude dinosaurs known. However, considerable work remains to be conducted on the paleoenvironmental settings associated with these extensive track-bearing strata. In part, this is due to difficulties in correlating between outcrops because much of the Cantwell Formation is heavily faulted and folded, requiring geochronology to clarify temporal and spatial relationships. In the summer of 2015, a new megatracksite was discovered in DENA that exposes multiple horizons of near-vertically inclined track-bearing surfaces covering an area the size of a football field. This site, known as The Coliseum, has over 65 meters of vertical section composed of alternating horizons of mudstone and fine to medium grained sandstone and sparsely deposited bentonite. The site preserves a diverse assemblage of ornithopod, ceratopsid and avian and non-avian theropods preserved as true tracks, underprints and natural casts.

A facies analysis was conducted to better understand the depositional environments associated with the track-bearing horizons. A detailed measured section indicates the tracks are primarily preserved on the laterally continuous, fine to medium grained sandstone horizons commonly co-occurring with asymmetrical ripple marks, plant remains, and invertebrate traces. We interpret these units to represent shallow water crevasse splay sheet sands within an alluvial floodplain depositional environment. Zircons collected from a tephra near the base of the section provided a U-Pb date of 69.4±0.9 Ma, which falls within analytical error of radiometric dates for the Prince Creek Formation (PCF) in northern Alaska, from which an abundant body fossil record of polar dinosaurs is known. Paleobotanical, trace fossil and sedimentary evidence in The Coliseum indicates that dinosaurs inhabited alluvial plains near forested areas at high paleolatitudes (>70° north). Trace and body fossils of the Cantwell and Prince Creek Formations, respectively, provide complimentary data that expand our understanding of high latitude, Late Cretaceous terrestrial ecosystems on a greenhouse Earth.