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Investigation of Unusual Breccias in the Mesoproterozoic Atar Group, Mauritania: Tsunami Deposits Related to Extraterrestrial Impact?

Geoffrey J. Gilleaudeau
University of Tennessee, Department of Earth and Planetary Sciences Knoxville, TN; [email protected]

An enigmatic 4-6 m thick breccia interval exists in carbonates of the Mesoproterozoic Atar Group, Mauritania. The deposit is traceable across >1,000 km of the West African craton, and has been linked chemostratigraphically to similar deposits in Algeria and Mali. Deposits consist of amalgamated, massive to normally-graded breccias (clasts up to 1 m in diameter), separated by irregular scour surfaces. Breccias are underlain by fluidization evidence (ball-and-pillow structures) and contain clasts dissimilar to underlying strata, suggesting significant transport distances. Combined with bidirectional clast imbrication, these features suggest deposition by a tsunami wave train. Most tsunamis are earthquake induced, yet even the largest earthquakes produce limited wave heights (<15 m). The extreme lateral extent (>1,000 km) of seafloor disruption and subsequent deposition suggests a possible non-seismic origin of tsunami energy. One possible hypothesis involves displacement of a large portion of the water column by a marine bolide impactor. This hypothesis will be tested through geochemical analyses for impact tracers (Platinum group elements, Os-isotopes), as well as impact tsunami waveform modeling. Regardless of tsunamigenic mechanism, detailed sedimentological and petrographic analysis of Atar breccias will extend our understanding of tsunami propagation in shallow seas. Establishing a set of criteria by which to distinguish tsunami deposits in the geologic record would be a crucial step toward understanding the extent and distribution of tsunamis in the geologic past.

AAPG Search and Discovery Article #90083 © 2008 AAPG Foundation Grants in Aid