2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 19
Presentation Time: 8:00 AM-12:00 PM


FELDL, Nicole, STEWART, Kevin G and BRALOWER, Timothy J., Department of Geological Sciences, Univ of North Carolina, Chapel Hill, NC 27599, nfeldl@lycos.com

Strata at Moscow Landing, Alabama provide a dramatic record of the earthquake and tsunami generated by the Cretaceous-Tertiary (K/T) bolide impact at Chicxulub, Mexico. The outcrop at Moscow Landing extends approximately 1.5 km along the west bank of the Tombigbee River and preserves uppermost Cretaceous Prairie Bluff Chalk overlain by coarse sand and conglomerate, which has previously been assigned to the base of the Danian Clayton Formation. Calcareous nannofossil biostratigraphy, however, particularly the distribution of Thoracosphaera and Braarudosphaera, show that the basal Clayton Sand was probably deposited at the K/T boundary. The basal Clayton sand is a discontinuous deposit that fills grabens and slump scars in the underlying Prairie Bluff Chalk and has been interpreted to be an impact-generated tsunami deposit. There are two generations of faults in the rocks: one set that is synchronous with the K/T boundary, and one set that is Tertiary. The K/T faults are easily recognized by the soft-sediment deformation associated with the faulting and the abrupt termination of the faults in the overlying Clayton Formation. The K/T faults may have formed during slumping of the continental margin associated with the magnitude 11-13 impact earthquake. Non-impact-related Tertiary faults can be traced for several meters into the overlying Clayton and Porters Creek Formations and in some cases may represent reactivation of the K/T structures. Other impact-related features in the Prairie Bluff Chalk include clastic dikes and sills derived from the basal Clayton sand. The presence of sills and shallowly inclined dikes indicate that the sand was forcefully injected into the underlying chalk as opposed to passively infilling fissures. Sand grains in the basal Clayton sand also show evidence of in situ fracturing. We interpret both the sand injection and the fracturing of the grains to be evidence of high pressure associated with rapid loading of the sediments by the water mass of an impact-generated tsunami.