Marine impact craters on Earth: Field investigation of the Wetumpka impact structure, a well-preserved marine impact crater, and the K-T boundary in the Alabama Gulf Coastal Plain
8-11 March 2007 • Wetumpka, Alabama, USA
Forum Conveners and Field-Trip Leaders:
David T. King, Jr., Geology, Auburn University, Auburn, Alabama 36849, USA
Jens Ormö, Centro de Astrobiología, Instituto Nacional de Tecnica Aeroespacial, Ctra de Torrejon a Ajalvir, km 4, Torrejon de Ardoz, 28850 Madrid, Spain
Lucille W. Petruny, Astra-Terra Research, Auburn, Alabama 36831-3323, USA
Thornton L. Neathery, Neathery and Associates, 1212-H Veterans' Parkway, Tuscaloosa, Alabama 35404, USA
Ashley Taylor Gilomen, Jennifer Glidewell, Germari de Villiers, Reuben C. Johnson
Auburn University, Auburn, Alabama 36849, USA
This field forum, focused on the topic of marine impacts by cosmic objects, was held at Wetumpka, Alabama, USA, on 8-11 March 2007. The field forum objectives were to examine the well-exposed, 7-km-diameter, marine-target impact structure at Wetumpka and to examine distal ejecta from the marine-target Chicxulub impact structure in México (i.e., the Cretaceous-Tertiary [K-T] boundary1), which is exposed in western Alabama. The field forum accomplished these objectives, as recounted in this report.
In addition to the two conveners, the field forum was attended by 37 participants, two field-trip co-leaders, and four student assistants. To make it easy for international participants to attend the field forum, it was scheduled in the days immediately preceding the annual Lunar and Planetary Science Conference in Houston, Texas.
Day 1, 8 March:
Volunteered and invited posters were presented and participants enjoyed an evening meal at the civic center after two brief lectures by the conveners.
Field day, 9 March:
A brief oral presentation giving a geologic overview of the Wetumpka impact structure was given at breakfast in order to introduce the participants to the localities to be visited during the day as well as the issues and open questions to be discussed. The field day included eight stops within the Wetumpka impact structure. The stops began with an overview stop on the crystalline rim to view breccia dikes. Subsequent stops included a road cut through the western crystalline rim, a sedimentary slump deposit of mega-blocks inside the rim, an exposure of impactite slurry sands including mega-blocks, the center of the structure to see resurge deposits of polymict proximal ejecta, core samples (plus lunch and poster viewing), an example of deformation in the extra-structure disturbed terrain, and a final stop to see weathering and truncation in the crystalline rim. A brief poster presentation was made by the conveners at each stop to introduce the participants to the outcrop and to the special issues to be discussed. At selected stops on the outcrop, invited posters on related topics were also presented (including Jared Morrow, San Diego State University). Posters and discussions continued prior to dinner that evening, which ended with a gala dinner at the Wetumpka Civic Center arranged by the Wetumpka Impact Crater Commission, the City of Wetumpka, the Elmore County Commission, and some local businesses. At this dinner, proclamations from the city, county, and the Alabama governor's office were presented. The governor's proclamation established the week of May 4-11 as "Wetumpka Impact Crater Week" in Alabama.
Day 3, 10 March:
The group departed Wetumpka for an area in Wilcox County to view exposures of the K-T boundary that include distal ejecta with impact spherules from the Chicxulub impact. With special cooperation and help of local plantation owners, we visited one site near the Alabama River. We arrived in time for lunch, held at a small pavilion on the plantation and provided by the Crater Commission. The pavilion also served as an excellent locality for invited and volunteered poster presentations of topics related to the issue to be discussed at the outcrop. An introductory overview of the K-T boundary was given by Jan Smit, Vrije Universiteit, Netherlands, which was followed by brief comments on the formation of impact spherules from Bruce Simonson (Oberlin College) and others. The group spent several hours discussing the K-T boundary section and sampling the outcrop, which is well exposed in that area. Upon return to Wetumpka, we were seated at a farewell dinner in an historic venue, which was arranged by a local utility company. The following day was largely consumed by participant departures.
The field excursions and discussions at meals generated a plentiful exchange of ideas, and this was augmented by four informal poster sessions and some brief formal and informal talks. The "posters in the field" concept used during this field forum was particularly effective in communicating ideas and stimulating discussions. Central Alabama is well-suited for this type of field forum because within a short distance are a well-exposed marine impact crater (Wetumpka) and excellent outcrops of the K-T boundary (ejecta from the Chicxulub marine impact).
We want to thank the members of the Wetumpka Impact Crater Commission (Hazel Jones, chair, and Marilee Tankersley, chair for the 2007 Field Forum arrangements) for their essential support of this field forum and all the planning and coordination they provided for us. We also thank the City of Wetumpka (Jo Glenn, Mayor) and the Elmore County Commission (Joe Faulk, chair) for their help and support. Finally, we thank Auburn University (College of Sciences and Mathematics and Department of Geology and Geography) for their support, as well as the several local businesses that helped the Crater Commission conduct this field forum.
NOTE: This field forum would not have been possible without the kind cooperation and permission of private landowners in Elmore and Wilcox counties. These landowners have stated that geologists should not venture on their land without prior notice and without specific written permission. We are most grateful for their help.
For readers wanting to know more about the Wetumpka impact structure and the Cretaceous-Tertiary boundary in western Alabama, we provide the following references.
King, D.T., Jr., and L.W. Petruny, 2007, Impact spherule-bearing, Cretaceous-Tertiary sand body, Shell Creek stratigraphic section, Alabama, USA, in Evans, K., et al., eds., The Sedimentary Record of Impacts: Boulder, Colorado, Geological Society of America and the Society for Sedimentary Geology Special Paper (in press).
King, D.T., Jr., T.L. Neathery, L.W. Petruny, C. Koeberl, and W.E. Hames, 2002, Shallow marine-impact origin for the Wetumpka structure (Alabama, USA): Earth and Planetary Science Letters, v. 202, p. 541-549.
King, D.T., Jr., J. Ormö, L.W. Petruny, and T.L. Neathery, 2006, Role of sea water in the formation of the Late Cretaceous Wetumpka impact structure, inner Gulf Coastal Plain of Alabama, USA: Meteoritics and Planetary Science, v. 41, p. 1625-1631.
Smit, J., Roep, Th.B., Alvarez, W., Montanari, A., Claeys, P., Grajales-Nishimura, J.M., and Bermudez, J., 1996, Coarse-grained, clastic sandstone complex at the K-T boundary around the Gulf of Mexico: Deposition by tsunami waves induced by the Chicxulub impact?, in Ryder, G., Fastovsky, D., and Gartner, S., eds., The Cretaceous-Tertiary event and other catastrophes in Earth history: Geological Society of America Special Paper 307, p. 151-182.