GSA Annual Meeting, November 5-8, 2001

Paper No. 0
Presentation Time: 4:30 PM


MORROW, Jared R., Department of Earth Sciences, Univ. of Northern Colorado, Greeley, CO 80639 and SANDBERG, Charles A., Geologist Emeritus, US Geol. Survey, Box 25046, MS 939, Federal Center, Denver, CO 80225-0046,

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.