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