Impact Processes and Products of the Marine Alamo Event, Southeastern Nevada and Western Utah

Recent work partitions the Alamo Breccia into six genetic Realms, providing a working model for the early Late Devonian (~382 Ma), marine Alamo Impact Event. Each Realm exhibits discrete processes and stratigraphic products preserving a well-exposed record of both proximal and distal impact phenomena. The first five Realms (Rim, Ring, Runup, Seismite, and Runoff) form roughly concentric semicircular bands onshore across the Devonian shallow-water carbonate platform. The sixth, the Runout/Resurge Realm, consists of off-platform Breccia channel deposits and crater-proximal olistoliths.

Processes and products include: (1) Shock Waves – Evidence of passing impact shock waves is recorded in the platform Realms by large, tabular carbonate megaclasts, fluidized bedding and clasts, liquefied clastic injection dikes and sills, and seismites; (2) Other Shock Deformation – Includes macroscopic shatter cone-like structures documented in the Rim Realm and widespread planar microstructure-bearing shocked quartz from the Breccia in all Realms except the Seismite; (3) Shock Melt – Probable melt is evidenced by plastically deformed clasts and microscopic carbonate quench fabrics within interpreted fallback breccias in the Rim Realm and by small, globular silicate grains disseminated within Breccia matrix; (4) Ejecta – Proximal and distal ejecta include weak iridium enrichment, shocked quartz grains, carbonate accretionary lapilli, rare accretionary “bombs”, target-rock lithic clasts, and fallback breccias in the Rim Realm; (5) Crater Modification – Probably more widespread than currently recognized, this includes listric faults interpreted in the outer Ring Realm, probable slump blocks in the Rim Realm, and probable olistoliths in the Runout/Resurge Realm; and (6) Marine-target Effects – In the offshore Runout/Resurge Realm, these consist of seafloor erosion and thick, Breccia channel fill probably deposited during crater-directed resurge flow; in the Ring and Runup Realms, these include normally graded tsunamiites; and, in the Runoff Realm, continent-sourced siliciclastic grains occur in Breccia channels.