Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 6-3
Presentation Time: 9:00 AM-6:00 PM


MCDONALD, Scott F., 1119 Giles Ave. NW, Olympia, WA 985024817

The Alamo Member at the Devils Gate Limestone type section (AMDG) includes the most distally reported Alamo Breccia impact ejecta, ~200 km from the inferred bolide impact centroid in southern Nevada. Exceptional preservation may be related to shelf morphology and ejecta blanket characteristics consistent with a non-orthogonal approach by the bolide.

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.

  • Summary Descriptions Assumptions Argument 1 of 4.pdf (997.6 kB)
  • Devils Gate LS Type Section Alamo Member images descriptions etc 2 of 4.pdf (3.4 MB)
  • Solar system analogs for orthogonal and oblique impacts, Luna, Mars, Earth 3 of 4.pdf (899.3 kB)
  • Map deposfacies interpretations, etc, suggesting bolid approach obliquity 4 of 4.pdf (1.8 MB)