2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 6
Presentation Time: 9:45 AM


SANDBERG, Charles A., U.S. Geol. Survey, Box 25046, MS 939, Federal Center, Denver, CO 80225 and MORROW, Jared R., Geological Sciences, San Diego State University, 5500 Campanile Dr., 237 GMCS, San Diego, CA 92182, sandberg@usgs.gov

The marine Alamo Impact is documented by many lines of evidence including: a widespread megabreccia, shocked-quartz grains, an iridium anomaly, carbonate lapilli, ejecta bombs, meltrock, injected sandstone dikes and sills, fallback breccia, and megatsunami deposits. Ejected and reworked Late Devonian to Cambrian conodonts redeposited within breccia matrix and enclosed lapillistone blocks not only date the impact, but also show that the transient crater was at least 1.7 km deep. The original impact site in southern Nevada is unknown, however, because most of the >44-km-wide crater has been later tectonically dismembered and buried, most likely beneath the Roberts Mountains allochthon. Recent work by J. Pinto and J. Warme indicates that a possibly unique sliver of the Alamo crater rim is preserved at Tempiute Mountain, NV. A distinctive feature of the Alamo Impact is the studding of shocked-quartz grains by pyrite pyritohedrons, later replaced by hematite. These provide for field recognition of impact-related beds. A proximal Alamo Breccia channel deposit atop a Middle Devonian olistolith is recognized at Milk Spring in the southern Hot Creek Range, NV. A distal channel deposit, displaying an iridium anomaly and resulting from megatsunami uprush or backwash, is present at Devils Gate, 13 km west of Eureka, NV.

New evidence is the finding of Alamo-related deposits in the Confusion Range, Burbank Hills, and Needle Range, western Utah. These wildly bedded deposits, located as far as 400 km from the inferred impact site, contain besides shocked-quartz grains, rounded light and dark quartz sand grains and feldspar grains. The latter indicate derivation from a granitic source terrane, probably around Granite Mountain, north of the Dugway Range, UT. A puzzling feature of the deposits is that they comprise two thin beds, separated by 1–2 m of carbonate-platform rocks representing a significant time separation. If the lower bed is interpreted as channel-deposited megatsunami backwash, the upper bed might represent a later torrential rainfall in the hinterland, producing similar deposits within the same valleys. Their origin from a second Alamo-like impact is considered possible but less likely.