EVOLUTION OF THE ALAMO IMPACT EVENT: FROM FIVE SECONDS TO FIVE MINUTES

Superb stratigraphic exposures in numerous mountain ranges in southern Nevada provide detailed scenarios for the evolution of the Late Devonian marine Alamo Impact breccias during early phases of the Alamo Event. Only one range, at Tempiute Mountain, lies within the Crater Rim Realm. It exhibits a 500-m-thick sequence of deformed bedrock, fallback breccias, and resurge units created within a few minutes to probably only a few days after impact. A contrasting profile formed nearby to the east, in 12 ranges of the Ring Realm. The undeformed carbonate platform is overlain by 50 to 100 m of detached and restructured strata, mixed with added ejecta, to become the Alamo Breccia. This interval exhibits: 1. A basal, thin, monomict detachment breccia, overlain by platform rocks that were partitioned into megaclasts up to 500 m long; these clasts were created, lifted, and partially destroyed by vibrations from Love and Rayleigh surface seismic waves. 2. The ejecta curtain added allochthonous material and destructively bombarded the loosened clasts, creating a complex polymict breccia. 3. Base surge and plume collapse created storms of carbonate accretionary lapilli that hardened quickly via hydration into lapillistone beds. 4. Tsunami uprush further mixed the debris. 5. Backwash plus voluminous runoff from impact precipitation sorted the debris into a capping stack of upward-thinning and -fining graded beds. These events coincided with development of the transient crater and proposed surrounding rings. During the first few hours after impact, ring adjustment caused debris flows to slide outward across down-tilted terraces between rings and toward ring footwall cliffs. Such debris flows account for the haphazard 50 to 100 m distribution of Breccia thicknesses across the Ring Realm, for movement of megaclasts upward within the Breccia so that some protrude from the top, for complete dismemberment of the lapillistone beds , and for the centrifugal imbrication of some large clasts. The stark difference in impact stratigraphy over the comparatively short ~10 km distance between the Crater Rim at Tempiute Mountain and the ranges of the Ring Realm, compared to the similar impact stratigraphy across the vast area of the Ring Realm, implies significant post-Event tectonic shortening between the two Realms.