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Upper Devonian, Marine Impact-Generated Tsunami Beds in Inner Carbonate-Platform Settings, Central Great Basin

 

Morrow, Jared R.1, Charles A. Sandberg2 (1) San Diego State University, San Diego, CA (2) Geologist Emeritus, U.S. Geological Survey, Denver, CO

 

The early Late Devonian (~382 Ma) Alamo bolide impact targeted a marine site offshore from a carbonate platform bordering western North America, including parts of eastern Nevada, western Utah, and southeastern California. The >40-km-diameter Alamo crater, which penetrated to the Cambrian or deeper, is now buried. However, evidence of the event includes megabreccias (impactites), impact-ejected fallout, tsunami beds (tsunamites), and seismically disrupted underlying rocks (seismites) exposed in >20 mountain ranges regionally.

 

Within the inner carbonate-platform setting, thin tsunamites, impact ejecta, and seismites provide the distal, onshore impact evidence. The occurrence of probable Alamo channels in west-central Utah suggests that an impact-generated mega-tsunami, which possibly reached 300 m in height near the crater, may have swept as far as 350 km onshore from the impact site. The tsunamites, which are tied by conodont microfossil biostratigraphy and stratigraphic position to thicker Alamo Breccia deposits to the southwest, are characterized by an erosive base and a complex internal fabric. This fabric comprises coarsely or inversely graded breccia and microbreccia, mixed clast composition, locally derived lithic rip-ups, convoluted laminations, dewatering structures, and probable shocked-quartz sand grains containing planar microstructures and hematite inclusions that also characterize shocked quartz in crater-proximal deposits.

 

Petrographic and geochemical analyses of the Alamo tsunamites indicate a distinct provenance, with a probable component of landward-derived material entrained by returning runoff. Our study of this ancient record could help predict onshore effects of a modern major catastrophe, such as an offshore bolide impact or a strong submarine earthquake.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California