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Tsunamis in South Dakota; two asteroid impacts inferred from the Cretaceous/ Tertiary (K/ T) boundary interval in Badlands National Park


, : Tsunamis in South Dakota; two asteroid impacts inferred from the Cretaceous/ Tertiary (K/ T) boundary interval in Badlands National Park. Abstracts with Programs - Geological Society of America 31(7): 473

A marine K/T boundary section is exposed in several locations in the North Unit of Badlands National Park. Correlation along an 8 kilometer east-west transect shows the following sequence from bottom to top: 1) fossil-bearing marl (belemnites, mollusks, arthropod fragments, and heavy bioturbation) of the upper Late Maastrichtian Jeletzkeytes nebrascensis Zone (sample geomagnetic polarity chron C30n); 2) a 0-8 cm thick glauconitic spherule (?) layer; 3) a 15-20 cm thick, finely laminated red clay (C29r) bearing a depauperate fauna consisting only of tiny clams (Anomia sp. ); 4) a 0.5-3. 0 m interval of soft sediment deformation with abundant east-west oriented rip-up and roll-up structures; and 5) alternating, 10-25 cm thick sand and shale beds (C29n-C28r). This overlying sand and shale unit reaches a maximum thickness of 16 meters below the basal Eocene unconformity. The glauconite spherule layer (2), which may represent gradual air fall of ejecta from a distant impact, defines a K/T extinction event in the Western Interior Seaway (WIS). The 20 cm thick red clay (3) demonstrates that slow marine deposition continued for possibly thousands of years before a second, closer event occurred, preserved as the zone of disruption (4) The deformation of this zone is consistent with the idea of impact-generated seismic energy or tsunamis advancing through the WIS. The overlying sand and shale sequence (5) possibly represents a change in sedimentation conditions influenced by climatic cycling in the WIS following the K/T. Correlation of the two possible K/T impacts with the Chicxulub or other proposed K/T impacts are unresolved at this time, although the tsunami interpretation suggests that the second impact occurred closer to the study area.

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