T69 Impact Cratering on the Earth, Moon, and Planets: Remote, Field, and Lab Studies

Sunday, 9 October 2011
1:30 PM - 5:30 PM
Room L100H-J

THE HIKO SCARP: A PALEO-TOPOGRAPHIC BOUNDARY MARKING THE OUTER RIM FAULT OF THE ALAMO IMPACT CRATER, NEVADA

Paper number: T69-15
Program: Technical Sessions
Time: 5:15 PM – 5:30 PM

Authors

Abstract

The Late Devonian Alamo impact targeted an epicontinental carbonate platform to produce the Alamo Breccia, but did not involve basement crustal rocks. Assessing the morphology and dimensions of the Alamo crater is partly obfuscated by post-impact tectonics, but extensive outcrop exposures in southeast Nevada provide repeated profiles of the carbonate succession. The Alamo Impact Database now includes over 100 measured sections of the Guilmette Formation, which documents rocks below, within, and above the impact horizon, and these are compiled in a GIS framework to resolve geometric relations of the Alamo Breccia units within the surrounding strata. Results of these measured sections define a 10 km long paleo-scarp, oriented NNW near the town of Hiko, Nevada.

The Hiko Scarp is defined by thickness and facies relations demonstrating topographic relief at the time of Alamo event. Outcrops immediately west (seaward) of the Hiko Scarp have thick, 30-100 m, deposits of Alamo Breccia, and include autobreccia and polymict breccias overlain by subtidal post-impact deposits, whereas east of the Scarp, Alamo Breccia is <10 m, contains no autobreccia, and is overlain by dolomitized shallow water to emergent facies. Topographic relief along the Hiko Scarp persisted for at least three parasequences after the impact, as evidenced by a quartz-rich silty and sandy dolostone barform that runs parallel to the position of the Hiko Scarp – an otherwise anomalous facies at this time or region of the carbonate platform.

We interpret the Hiko Scarp as the surface expression of the outer rim fault formed during the modification stage of the Alamo crater, which coincides with the Ring-Runup transition posited by the Realm model. If the Ring-Runup transition is used as a proxy for the concentric outer rim fault scarp, this feature must have a minimum diameter greater than 90 km based on the N-S distance between outcrops in the Delamar and Golden Gate ranges.