LANDSCAPE EVOLUTION ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY IN SOUTHWESTERN NORTH DAKOTA

The conformable succession of the Hell Creek Formation (Upper Cretaceous) and the overlying Fort Union Formation (Paleocene) provides a record of the terrestrial response to the K/Pg event. In addition to preserving an extraordinary record of flora and fauna across the extinction interval, these successions record a geologically rapid base-level rise that was approximately contemporaneous with the extinction and deposition of Chicxulub-impact indicators. An outstanding question that remains is whether this base-level rise is attributable to: 1) a multi-thousand-year transgression of the Western Interior Seaway adjacent to a relatively low-relief coastal plain, causing a rise in the water table; or 2) catastrophic flooding of lowlands due to a upland denudation, rapid channel accretion, and hydrologic reorganization resulting from wildfires/deforestation consequential of the impact. Herein we aim to address which of these hypotheses is most consistent with a paleolandscape reconstruction at Mud Buttes, a laterally extensive exposure where the boundary clay (BC) is unambiguously coincident with the Hell Creek-Fort Union contact. A total of 127 excavated trenches were examined laterally across ~2 km transect of K/Pg boundary. The uppermost Hell Creek is comprised of three pedotypes: Pale Silt Inceptisol (PSI), Olive Clay Vertisol (OCV), and Chocolate Clay Vertisol (CCV), which range from moderately well-drained, to variable to poorly drained, to very poorly drained, respectively. All of the BC occurs superjacent to the very poorly-drained CCV, despite this pedotype accounting for only ~50% of the pre-event landscape. The BC above the CCV pedotype is minimally disturbed with rare plant and insect traces, therefore, the edaphic features preserved in the CCV pedotype are not attributable to post-impact overprinting. This indicates that hydromorphic conditions began 10²-10³ years prior to deposition of the BC. Furthermore, 46% of BC is overlain by lignite, despite lignite occurrence on only 17% of the earliest Paleogene landscape. This reveals that the poorest-drained landscape positions of the latest Cretaceous persisted through the aftermath of the event and into the recovery interval. These relationships are most consistent with a non-catastrophic base-level rise that began prior to the impact event.