Elongated sandstone concretions record river system evolution across the Cretaceous-Paleogene boundary, Montana, USA

The Cretaceous−Paleogene (K-Pg) extinction caused significant environmental upheaval, including the disappearance of non-avian dinosaurs and widespread disruption of ecosystems. The Hell Creek and Fort Union formations in the upper Great Plains of North America preserve a record of environmental change across the K-Pg boundary (KPB). Sedimentological studies suggest that the Hell Creek Formation represents a well-drained floodplain, with increased standing water leading to the formation of ponds and peat bogs close to the KPB. This shift has been attributed to the disruption of river systems by short-term extinction-driven flooding in the Western Interior. To test this, we analyze ∼2900 paleocurrent measurements from elongate concretions mapped over ∼5000 km2 in the Hell Creek area of eastern Montana, USA. These measurements, combined with previously published chronostratigraphic constraints, allow us to trace river system evolution over time. Paleocurrents shift from a uniform NW-SE direction to a chaotic and later bimodal pattern ∼400 k.y. after the KPB, but no major shifts occur at the KPB itself. Additionally, we develop a method to identify meandering rivers based on paleocurrent distributions and use it to assess river planform changes. Environmental crises have been suggested to cause transitions from meandering to braided rivers, but we find no evidence of such a change. This suggests that the K-Pg extinction did not significantly disrupt regional drainage or river systems in the Hell Creek area. The results demonstrate the potential of remote sensing for reconstructing paleoriver dynamics and suggest that some river systems may be stable across mass extinction events.