OBLIQUE ALAMO BOLIDE IMPACT? EXCEPTIONAL ALAMO BRECCIA PRESERVATION AT DEVILS GATE LS TYPE SECTION, EAST CENTRAL NEVADA
The Late Devonian, early Frasnian (ca. 378 Ma) AMDG crops out as two submembers. Basal “Unit B” is a slope-forming, ~0.25 to 1.5 M thick, dark grey, limestone diamictite containing subround clasts up to 15 CM long, the larger clasts in the thicker, wedge or channelized section. Unit B is unconformably overlain, in places along wavy bedding cleavage, by "Unit A", a 29 CM, graded, granule calcarenite bed or beds with low-level Iridium element anomalies several times those of impact-proximal breccias. Such regional, low-level anomalies support a cometary impact hypothesis. (Koeberl and others, 2003; McDonald, 2018; Morrow and Sandberg, 2006)
The AMDG depoenvironment was in a low-diversity, variable salinity, possibly lagoonal, shallow water outer shelf setting. Unit B lithology is similar to the crater 'Ring' zone at least 125 KM closer to the bolide impact. Unit A, on the other hand, is a thin, regional tsunamite sourced from the Alamo Breccia, part of the 'Runoff Realm', distal from the complex crater. (Casier and others, 2006; Morrow and others, 2008; Tapanila and others 2014)
Unit B’s exceptional preservation seems inconsistent with an orthogonal impact and isotropic, starburst pattern ejecta blanket model. The hypervelocity bolide may have struck at an low angle, producing a thick “downrange plume” or ejecta ray, that fell on the Devils Gate shelf, sliding into place as a mass-flow unit. Megatsunamis that followed lost much erosive energy while longitudinally traversing ~125 KM of the shallow shelf, scouring but leaving some of the Alamo diamictite preserved.