Anatomy of an Anomaly: The Devonian Catastrophic Alamo Impact Breccia of Southern Nevada

Abstract

The Alamo Breccia is a carbonate rock breccia of Late Devonian age in southern Nevada. It is an anomalous sedimentary unit because it has the properties of a massive debris-flow and turbidity-current deposit that would be expected to occur in deep water, but is intercalated over much of its area with typical shallow-water carbonate-platform beds. The Breccia was created by the catastrophic detachment and flow, over a nearly horizontal surface, of previously deposited platform carbonates. It crops out in 14 or more mountain ranges that cover an area of ∼10,000 km2, conservatively averages ∼50 m in thickness, and contains a volume of 500+ km3. Along the base it contains trains of individual detached blocks as much as 500 m long and 90 m high. Clasts generally grade upward to gravel-, sand-, or mud-sized particles at the top. The Breccia was generated by forces unleashed during the impact of an extraterrestrial object with Earth. The impact produced shocked quartz grains, unique ejecta spherules, and an iridium anomaly–which are present within the Breccia but absent from confining beds. Internally the Breccia is segmented vertically into as many as five 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 limestones rest over it, indicating a near-crater location. Surrounding detached megablocks and tsunamites suggest an annular crater trough. The Breccia formed within the span of a few hours or days, and falls entirely within a single early Frasnian conodont zone, at ∼367 Ma. The welldocumented middle Late Devonian (Frasnian/Famennian) extinctions are ∼3 Ma later. An impact scenario explains the known features of the Alamo Breccia: impact occurred on the Late Devonian outer platform or slope; seismic shock delaminated the upper ∼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 augmented by unknown proportions of ejecta containing shocked quartz from the crater, carbonate spherules from the vapor cloud, and iridium from the projectile. Mass flows west of the platform likely represent tsunami backwash, shock-induced failure along the platform margin, and slumps from offshore topographic highs.