Skip to Main Content
Book Chapter

Surficial polymict impact breccia unit, Wetumpka impact structure, Alabama: Shock levels and emplacement mechanism

By
David T. King, Jr.
David T. King, Jr.
Geology Office, Petrie Hall, Auburn University, Auburn, Alabama 36849-5305, USA
Search for other works by this author on:
Jared R. Morrow
Jared R. Morrow
Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA
Search for other works by this author on:
Lucille W. Petruny
Lucille W. Petruny
Geology Office, Petrie Hall, Auburn University, Auburn, Alabama 36849-5305, USA
Search for other works by this author on:
Jens Ormö
Jens Ormö
Centro de Astrobiología, Instituto Nacional de Técnica Aeroespacial, 28850 Torrejon de Ardoz, Spain
Search for other works by this author on:
Published:
October 01, 2015

At Wetumpka impact structure, the surficial polymict impact breccia crops out discontinuously over a relatively small area within the intrastructure terrain. This polymict breccia unit contains a significant number of shocked quartz grains that represent a slightly lower shock pressure regime than the previously documented shocked quartz population obtained from a separate subsurface impact breccia unit. Specifically, the shocked quartz grains found in the polymict breccia unit display two distinct types of planar microstructures (named herein P1 and P2), and the host grains range in size from fine sand to pebbles and cobbles. P1 elements closely resemble planar fractures (PFs) or planar cleavage in quartz, which occur in multiple sets of open, parallel, fl at to curviplanar planes aligned with distinct crystallographic orientations. P2 elements are much shorter, much thinner, and more closely spaced than P1 planes, and they resemble in part closed, partly decorated to nondecorated, classic, longer planar deformation features (PDFs). However, these P2 elements are not PDFs. Sets of P2 elements are commonly developed off of, or are crosscut by, through-going P1 planes, which form “feather features” (FFs). P1- and P2-type planar microstructures are associated with low shock pressures (~7–10 GPa), whereas more typical planar deformation features previously described in Wetumpka's subsurface impact breccias are associated with shock pressures of 10–16 GPa. We attribute the difference in Wetumpka's shock levels (i.e., between quartz grains in the polymict impact breccia and previously described quartz grains in the deeper, subsurface impact breccias) to differences in provenance of these grains from within the impact structure's transient crater. Proximal ejecta deposits, which were derived from more shallow reaches of the target materials, are the most likely candidate for sources for the P1- and P2-bearing grains in the surficial polymict impact breccia unit. We interpret the present distribution and occurrence of the surficial impact breccia unit to be best explained by late-modification-stage slumping of proximal ejecta from the impact structure's rim.

You do not currently have access to this article.
Don't already have an account? Register

Figures & Tables

Contents

GSA Special Papers

Large Meteorite Impacts and Planetary Evolution V

Gordon R. Osinski
Gordon R. Osinski
Centre for Planetary Science and Exploration, Departments of Earth Sciences and Physics and Astronomy, University of Western Ontario, 1151 Richmond St., London, ON N6A 3K7, Canada
Search for other works by this author on:
David A. Kring
David A. Kring
Center for Lunar Science and Exploration, Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, USA, and National Aeronautics and Space Administration (NASA) Lunar Science Institute, and NASA Solar System Exploration Research Virtual Institute
Search for other works by this author on:
Geological Society of America
Volume
518
ISBN print:
9780813725185
Publication date:
October 01, 2015

References

This Feature Is Available To Subscribers Only

Sign In or Create an Account

This PDF is available to Subscribers Only

View Article Abstract & Purchase Options

For full access to this pdf, sign in to an existing account, or purchase an annual subscription.

Subscribe Now