Paper No. 12
Presentation Time: 11:00 AM


WARME, John E., Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401 and PINTO, Jesus A., Residencias El Solar de La Quinta, 2C-43, Los Teques, 1201, Venezuela,

Seismic-wave damage from the Late Devonian (~382Ma) marine Alamo Impact Event is exposed in 25 or more mountain ranges across thousands of square kilometers in southeast Nevada and adjoining areas. Expansive and well-exposed outcrops of the target carbonate rocks provide probably the best three-dimensional views on Earth of the upper and lower boundaries and complex internal characteristics of the crater margin, ring region, debris fields, and far-field tsunami surge beds, runoff channels and bedrock seismites. The central crater has not been located due to post-impact tectonic disturbances, uplift and erosion, and/or volcanic cover.

A preserved section of the crater margin exhibits 300 m of sheared and shattered dolostone bedrock autobreccia with a variety of shattercone-like structures. It is permeated with a system of sedimentary dikes and sills that emanated from the central crater. The upper ~30 m lifted and collapsed nearly in place, and is covered by a heterolithic true fallback breccia of unusual clasts that had lost strength, perhaps from seismic shock, and became tightly welded together after they accumulated. This pile of damaged rocks is overlain by two separate resurge breccias, each 30-m-thick, deposited in the newly formed crater and covered by post-event anoxic marine limestone.

Radially outward, across the expansive ring zone, seismic waves detached the carbonate bedrock 50 to 100 m below the contemporaneous surface and partitioned it into megablocks as much as 500 m in length. They vibrated, partially disintegrated, and created a spectrum of clast sizes. This newly formed breccia was restructured by debris flows that cascaded down tilted terraces as ring faults evolved shortly after impact.

A 10-m-thick interval of complexly deformed carbonates, ~150 km south of the target zone, is interpreted as a distal surface seismite. It extends for 5+ km, parallel to bedding, as four dolostone beds that were fractured, detached, folded, broken, fluidized and pervasively injected. Seismites are also interpreted beyond the rings ~150 km north of the target zone.