NEW EVIDENCE ON SIZE AND MARINE SITE OF LATE DEVONIAN ALAMO IMPACT, SOUTHERN NEVADA

The early Late Devonian (early Frasnian) Alamo Impact targeted an oceanic, off-platform site in southern Nevada. Because the original crater is now dismembered and buried beneath younger rocks, its size and site must be deduced through multiple converging lines of geological and paleontological evidence. Previous and new evidence includes the catastrophically emplaced Alamo Breccia, tsunamites, shock-metamorphosed quartz grains, carbonate accretionary lapilli, a small iridium anomaly, sub-Breccia clastic injection, deep-water Breccia channels, and ejecta material.

Four recent finds shed new light on the depth and location of the original crater. (1) Biostratigraphically and paleogeographically diagnostic Late Cambrian to early Late Devonian conodont microfossils, recovered from impact-ejected carbonate lithic fragments within lapillistone blocks in the upper part of the Breccia. (2) Abundant, altered lithic fragments of argillite, chert, and quartzite, probably ejected from lower Paleozoic or possibly upper Proterozoic target formations by the impact, found within lapillistone beds and scattered throughout the upper parts of both the onshore and offshore Breccia. (3) Abundant polycrystalline grains of finely crystalline quartzite or chert, possibly derived from the upper Proterozoic to Lower Cambrian Prospect Mountain Quartzite, occurring within shock-metamorphosed, quartz-sand populations in the upper Breccia. (4) Five deep-water, upper- and lower-slope Breccia channels, which were positioned radially around the impact site and filled by probable crater-directed resurge deposits.

Our newly acquired data demonstrate a minimum Alamo Impact excavation depth of 1.7–2.5 km beneath the Late Devonian seafloor. Scaling calculations, based on other previously documented marine impact craters, suggest that the Alamo crater had a minimum final diameter of 44–65 km. Paleogeographic and provenance information derived from ejected conodonts and diagnostic lithic fragments limits the original site of the Alamo Impact to a longitude at, or just east of, the present position of the Roberts Mountains thrust, and further constrains the likely present position of the tectonically transported upper part of the crater to an area between the Timpahute and Hot Creek Ranges, southern Nevada.