Marine Impact-Generated Tsunami Beds in Inner Carbonate-Platform Settings, Central Great
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
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.