PERVASIVE PALEOSEISMITES TRACK LARAMIDE TECTONISM IN FOSSILIFEROUS UPPER CRETACEOUS – PALEOCENE FORMATIONS OF THE BIGHORN BASIN, USA

Tectonism in the Late Cretaceous transformed terrestrial ecosystems in western North America, yet those tectonic processes can often only be inferred via changes in sediment provenance or geochemistry, or shifts in depositional environments. Here we report pervasive soft sediment deformation in the fossiliferous largely conformable sequence of the “Mesaverde”, Meeteetse, Lance (Upper Cretaceous), Fort Union, and Willwood (Paleogene) formations of the Bighorn Basin, Wyoming, that we attribute to liquefaction of unconsolidated sediment driven by seismicity related to Laramide orogenic events. We observe various convolute bedding morphologies, including highly folded, sometimes recumbent, internal layering, ball-and-pillow structures, and water escape features. Deformation occurs at scales ranging from 5 cm to 2 m, and is largely observed within sandstone-dominated lithologies. These features are developed within cross-bedded/laminated fine-grained sandstones, and they are more common near the top of sandstone strata, consistent with the liquefied condition persisting longer at the top of a layer during shaking.

Liquefaction is thought to occur only at shallow burial depths (<10 m), so the stratigraphically pervasive nature of these features implies that seismicity was largely persistent from at least the deposition of the Mesaverde “Group” through the Willwood Formation. Our field observations of potential liquefaction features in the upper Frontier Formation (Torchlight Member; ?Cenomanian–Turonian) may also represent the earliest evidence of its kind for Laramide tectonism, better matching earlier dates of the onset of Laramide tectonism from thermochronology. At least, our observations of soft sediment deformation throughout the Meeteetse, Lance, and lower Fort Union formations fill a stratigraphic gap between previous reports of Laramide-driven soft sediment deformation in the Mesaverde “Group” and the upper Fort Union through Willwood formations. These pervasive paleoseismites in the fossiliferous strata of the Bighorn Basin provide the opportunity to explore the role that tectonic prevalence and magnitude played in shaping landscape morphology and biodiversity leading up to and across the Cretaceous-Paleogene boundary in western North America.