OFFSHORE DEPOSITS AND EFFECTS OF MARINE, LATE DEVONIAN ALAMO IMPACT EVENT, SOUTH-CENTRAL NEVADA

Evidence of the early Late Devonian, oceanic Alamo Impact Event is contained in megabreccias, tsunamites, and shock-altered subjacent rocks exposed in nearly 20 mountain ranges of southern and central Nevada. Although post-event geologic processes have dismembered, relocated, and buried the crater, several previously documented lines of evidence verify its subcritical-impact origin. The Alamo impactor penetrated a wet sedimentary target composed primarily of carbonate strata with minor interbedded quartz sandstone and orthoquartzite. Impact-related deposits were emplaced in roughly semicircular belts of shoreward-thinning, polymict-megabreccia and tsunamite beds across the adjacent upper-foreslope, carbonate-platform, and shoreline settings.

We have now documented lower-slope Alamo Breccia channel-fill deposits in a probable radial pattern well west of the carbonate-platform margin. These relatively fine-grained, deeper-water deposits, which are correlated by conodont biostratigraphy to the previously known Breccia deposits, are further identified by their unusual fabric, heterolithic clast composition, and shock-altered quartz grains.

The regional downcutting by the Alamo Breccia across the carbonate platform and upper foreslope is now shown to continue and increase seaward. Middle Devonian lower-slope deposits beneath the Breccia to the west are much older than Middle and Upper Devonian rocks beneath the Breccia on the carbonate platform.

At deeper-water localities in the southern Hot Creek Range, located ~65 km northwest of the tectonically unrestored, inferred impact site, the Breccia is >40 m thick and consists of at least two distinct, fining-up depositional units. Polymict, platform- and slope-derived Breccia clasts, which commonly are as long as 50 cm, display tabular-clast imbrication indicating a southeasterly directed paleocurrent.

Our new data suggest an offshore impact event chronology: (1) An initial phase of regionally widespread submarine erosion and localized channeling, possibly by radial, outward-directed hypercurrents, just after impact. (2) A closely following, high-energy, craterward-directed, multi-stage megatsunami that emplaced the Breccia during the earliest post-impact phase of transient crater modification.