Abstract

Exposures of Upper Cretaceous and lower Paleocene strata at the Moscow Landing site in western Alabama, U.S. Gulf Coastal Plain, include a series of isolated, thin coarse-grained clastic bodies referred to as the Clayton sands. Positioned at the K–Pg transition and below a regional transgressive surface, these sand bodies previously have been interpreted as lowstand incised valley fills or as catastrophic megawave deposits associated with the end-Cretaceous bolide impact on the Yucatan peninsula. The sedimentology and ichnology of the Moscow Landing exposure were revisited in attempt to elucidate the mechanisms responsible for Clayton sand deposition. Stratigraphic relations and character of sand bodies strongly suggest that impact-generated faulting and deformation of the underlying Maastrichtian Prairie Bluff Chalk created Clayton sand depocenters, that the bulk of Clayton sands was emplaced, possibly from the south, by runup and backwash of oscillating impact-generated megawaves, and that transgressive ravinement followed shortly after catastrophic deposition. Several Clayton sand bodies contain in their upper parts internal bioturbated horizons characterized by Thalassinoides, for which there are two viable explanations. These horizons may record short-term colonization by allochthonous crustaceans that were transported and transplanted by waning megawave currents. Alternatively, associated sand bodies may be composites; Thalassinoides horizons may reflect colonization of sands that accumulated under normal marine conditions in remaining accommodation space atop megawave deposits in response to ensuing transgression. Careful systematic biostratigraphic sampling and analyses will be necessary to test the relative virtues of these two scenarios.

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