EXTINCTION OF DINOSAURIAN MEGAFAUNA MAY EXPLAIN CONSISTENT FACIES CHANGES IN CONTINENTAL CRETACEOUS-PALEOGENE BOUNDARY SECTIONS

Cretaceous-Paleogene boundary (KPB) sections have been globally identified via direct geochemical and mineralogical evidence of the Chicxulub bolide impact, including iridium anomalies, glassy spherules, and shocked quartz. North American continental sections in which a KPB has been identified invariably exhibit a marked shift in sedimentary facies at or near the reported boundary level: uppermost Cretaceous strata are often characterized by hydromorphic paleosols and small, single-storied fluvial sandstones, whereas lowermost Paleogene strata are often characterized by lignites, large, multi-storied fluvial sandstones, and associated lateral accretion deposits. In sections where the KPB has been directly identified via Chicxulub ejecta, that facies shift occurs immediately above the KPB. In locations where the KPB has been placed via indirect data such as palynostratigraphy or carbon-isotope geochemistry, however, the KPB is sometimes reported a few meters below or above that facies shift. We propose that those bio- and chemostratigraphic tools do not always precisely identify the KPB in terrestrial sections due to taphonomic, lithologic, or other biases inherent in those records, and the precise KPB more likely coincides directly with the facies change. Recognizing that organic-rich lithologies occur in Cretaceous strata, and other factors (e.g., high fluvial energy) may overprint this pattern, we contend that this geographically widespread phenomenon cannot be explained by abiotic forces alone. We hypothesize that the facies shift itself was driven by the extinction of dinosaurian megafauna at the KPB. Large-bodied dinosaurs likely promoted an open vegetation structure that permitted frequent fluvial avulsion and clastic sediment input to distal floodplains; after the Cretaceous-Paleogene mass extinction dense, closed-canopy forests could take root, stabilizing meander belts and starving the distal floodplain of clastic sediment, which promoted the accumulation of organic-rich strata. Facies change in continental KPB sections thus suggests dinosaurian megafauna were ecosystem engineers that promoted habitat openness in the Upper Cretaceous (and perhaps earlier), and their extinction may have led to a dramatic reorganization of landscape structure in the earliest Paleogene.