Marine Cretaceous-Tertiary boundary section in southwestern South Dakota

Abstract

A distinctive zone of disrupted strata, which we interpret as a distal manifestation of the end-Cretaceous Chicxulub impact event, occurs over 300 km 2 in southwestern South Dakota. This disrupted zone is within the Fox Hills Formation, ranges from 0.5 to 5 m in thickness, and contains large-scale slump-roll structures, clastic dikes, flame structures, and massive, homogenized beds. The zone is ∼0.5 m above a belemnite fauna Sr dated as 67.6 ± 0.5 Ma, contains scaphitid ammonites characteristic of the Jeletzkytes nebrascensis ammonite zone of the Fox Hills Formation, and is capped by a 0.5–4-cm-thick brownish-black mudstone that contains spherules. Pollen of the late Maastrichtian Wodehouseia spinata palynostratigraphic zone occurs immediately above and below the disrupted zone. The disrupted zone is overlain by an additional 25 m of marine Fox Hills Formation. These stratigraphic relationships suggest that the upper part of the Fox Hills Formation in this part of South Dakota is Paleocene; that the Western Interior Seaway was locally present well into the Paleocene; and that scaphitid ammonites may range the Cretaceous-Tertiary (K-T) boundary.