ANATOMY OF AN ANOMALY - THE DEVONIAN CATASTROPHIC ALAMO IMPACT BRECCIA OF SOUTHERN NEVADA

The Alamo Breccia is a carbonate rock breccia of Late Devonian age in southern Nevada. It is an anomalous sedimentary unit because it has th e properties of a massive debris-flow and turbidity-current deposit th at would be expected to occur in deep water, but is intercalated over much of its area with typical shallow-water carbonate-platform beds. T he Breccia was created by the catastrophic detachment and flow, over a nearly horizontal surface, of previously deposited platform carbonate s. It crops out in 14 or more mountain ranges that cover an area of si milar to 10,000 km(2), conservatively averages similar to 50 m in thic kness, and contains a volume of 500+ km(3). Along the base it contains trains of individual detached blocks as much as 500 m long and 90 m h igh. Clasts generally grade upward to gravel-, sand-, or mud-sized par ticles at the top.The Breccia was generated by forces unleashed during the impact of an extraterrestrial object with Earth. The impact produ ced shocked quartz grains, unique ejecta spherules, and an iridium ano maly-which are present within the Breccia but absent from confining be ds. Internally the Breccia is segmented vertically into as many as fiv e sequentially thinner graded units created by successive tsunamis. In one range, peculiar deformed dolostone, shocked quartz sandstone, and sedimentary dikes and sills occur under the Breccia and deep-water li mestones rest over it, indicating a near-crater location. Surrounding detached megablocks and tsunamites suggest an annular crater trough. T he Breccia formed within the span of a few hours or days, and falls en tirely within a single early Frasnian conodont zone, at similar to 367 Ma. The well-documented middle Late Devonian (Frasnian/Famennian) ext inctions are similar to 3 Ma later. An impact scenario explains the kn own features of the Alamo Breccia: impact occurred on the Late Devonia n outer platform or slope; seismic shock delaminated the upper similar to 50 to 100 m of the platform, loosening carbonate-platform bedrock and creating trains of large blocks that may rest in an annular trough ; successive tsunamis reworked the loosened material, which was augmen ted by unknown proportions of ejecta containing shocked quartz from th e crater, carbonate spherules from the vapor cloud, and iridium from t he projectile. Mass flows west of the platform likely represent tsunam i backwash, shock-induced failure along the platform margin, and slump s from offshore topographic highs.