TESTING EXTINCTION AS A MECHANISM FOR ENVIRONMENTAL DISRUPTION AT THE CRETACEOUS-PALEOGENE BOUNDARY (MT, USA)

The Cretaceous-Paleogene (K-Pg) boundary represents one of the most dramatic mass extinctions in earth history. However, the landscape evolution associated with this mass extinction is poorly understood. A unique and abrupt change in sedimentary facies accompanies the extinction boundary in the Upper Great Plains of North America. In the past, this change in facies has been interpreted as a shift from well-drained fluvial basins to an abruptly flooded landscape, and the proposed explanatory scenarios (marine transgression, Laramide tectonism) have been considered independent of the K-Pg extinction event. A new explanatory scenario (Sheehan et al., 2004; Fastovsky et al., 2008) suggests that the extinction event itself represents a possible extraordinary mechanism for the environmental change. Specifically, the new scenario proposes that several months of darkness following the bolide impact caused a rapid loss of vegetative cover. This, in turn, resulted in increased erosion across drainage systems, choking rivers and producing a flooded landscape, which could explain the sudden appearance of “variegated” siltstones and coal seams that characterize the lowermost Paleogene deposits. The purpose of this study was to test the feasibility of this explanatory scenario by studying K-Pg deposits preserved in Makoshika State Park, near Glendive, Montana. A reevaluation of the lowermost Paleogene facies indicates that the ubiquitous “variegated” siltstones might represent fluvial splay complexes rather than acquiescent settling from suspension as previously reported. The presence of numerous scour surfaces within the “variegated” siltstones indicates an increase in runoff and erosion following the extinction event, which supports the new explanatory scenario. However, the presence of incised valleys and widespread gypsum in the coal seams also suggests that the lowermost Paleogene deposits were at least partially influenced by sea level.