SAGA OF THE ALAMO BRECCIA

The Alamo Breccia is an ~382Ma Late Devonian, 1 to 400-m-thick, carbonate complex exposed in ~25 Nevada Great Basin ranges. It was recognized in the Guilmette Formation in 1990 by John Warme and Alan Chamberlain at Mt Irish, where it is ~100m thick, then confirmed in adjacent ranges, researched by Colorado School of Mines graduate students, announced in 1991, and met by many geological skeptics. Hypotheses for its origin were volcanism, seismicity, tsunami, impact, and carbonate platform collapse westward into the Paleopacific Ocean. All these processes, except volcanism, contributed to the Breccia genesis.

A 1992 field trip included experienced Nevada stratigraphers, and Walter Alvarez, who all concluded that the Breccia was a widespread catastrophic event. By 1993 Charles Sandberg confirmed its exact age using conodonts. In 1995 imbedded shocked and studded quartz grains confirmed impact. For 20+ years Warme, Sandberg, the late Jared Morrow, and their students and colleagues led field trips and published a score of papers detailing the Breccia and formalizing it as a Guilmette member. In 2001 a GSA Wet Targets Field Forum gathered global impact experts to Nevada and the Breccia, which by then was divided into three Zones based on thickness and internal facies: Zone 1, a single location most proximal to the target; Zone 2, ring(s); Zone 3, tsunami surges. The Zones were arranged in an eastward expanding semicircle originating near the platform edge. This scheme, assumed to represent the eastern half crater, was adopted without question by all later researchers.

In 2008 Jésus Pinto and Warme summarized the Breccia into six genetic Realms, using Crater Rim, Ring, and Runup for the above Zones, plus platform Runoff, Seismite, and offshore Runout/Resurge. The latter includes kilometer-scale carbonate olistoliths and olistostromes discovered by Poole and Sandberg (2015) that were likely sourced from the platformn. Tectonically unreconstructed, the Realm complex stretches ~300 km north-south and ~100 km east-west. All published estimates of the missing target position and crater size are circumstantial; these elements may be buried by thrust sheets or volcanics, uplifted and eroded, or the assumed half crater may instead be a large-scale scar that was left when the platform margin collapsed upon impact and was sloughed away.