46th Lunar and Planetary Science Conference (2015)
WETUMPKA IMPACT STRUCTURE, ALABAMA. E. Heider1, D.T. King, Jr.1, and J. Ormӧ2, 1Geology Office, Auburn University, Auburn, Alabama 36849-5305; [email protected]; 2Centro de Astrobiología, Torrejon de
Ardoz, 28850, Spain.
Introduction: Wetumpka impact structure is a ~ 5
km diameter, Late Cretaceous, marine-target impact
crater [1, 2, 3]. Recent special permission for access to
land along utility lines and local roads and a review of
recent core drilling led to construction of a shallow
geological half-transect across the western part of the
crater (Figs. 1 and 2). From northwest to southeast, the
half-transect encompasses deformed crystalline rim
terrain, steeply inclined sedimentary target formations,
folded and faulted sedimentary target layers (parts of a
transported, overturned sedimentary flap), a central
impact breccia, and resurge chalk deposits. The formative events that generated the uppermost units in the
geological half-transect are all related to late stage
modification of the Wetumpka impact structure (very
large mass movements and aqueous resurge) and likely
represent the last few minutes in Wetumpka’s sequence
of events.
Geological half-transect methods: A LiDAR dataset with ~ 2 m resolution was acquired from the
Elmore County Revenue Commissioner’s office in Wetumpka, Alabama, and used to produce a digital elevation model (DEM) of the impact structure. Then, a
published geologic map made by Tony Neathery and
others [4] was digitized and placed over the LiDARbased DEM (Fig. 1). Using spatial analyst tools in
ArcGIS, a topographic profile was made for the half
transect. Finally, field observations along the half transect and related shallow core-drilling results [including
3, 5, 6] were synthesized to generate the shallow geological cross section (i.e., the geological half transect)
in Figure 2.
Interpretation: From the core-drilling results, we
know that there is an impactite sand unit overlying the
sedimentary megablock unit (and there is a crystalline
block unit below those layers; see Fig. 2). From field
studies, we have observed that above the impactite
sand unit, there is a highly deformed sequence of layers
that is comprised of sediments of the Tuscaloosa Group
(Kt) overlain by Eutaw Formation (Ke), and above
that, more Tuscaloosa Group (Kt). This sequence of
Tuscaloosa-Eutaw-Tuscaloosa or “Kt-Ke-Kt” is best
explained as a classic “overturned flap” stratigraphy,
which could have developed only at the southern and
southwestern quadrant of the early Wetumpka crater
rim [3]. This area is presently the site of a structurally
disturbed region lying just outside the present crater [2,
3]. We think that the overturned flap developed in that
area and rapidly moved into the crater interior as a re-
sult of a very large mass movement event similar to the
development of the “impact-modified sediments” comprising the “inverted sombrero” at the Chesapeake Bay
Impact structure [7]. The present structurally disturbed
area is therefore the material that was directly underlying the mobilized overturned flap. As the Kt-Ke-Kt
layer mobilized and spread across the structure’s interior region, it was deformed and subsequently covered
the crater fill, including the impactite sands penetrated
by shallow core drilling [3, 5, 9]. This hypothesis accounts for the phenomenon previously called “broken
formations” that has been reported within the structure’s interior for several years [1-3]. Proximal ejecta,
including large, shocked crystalline megablocks that
landed upon the overturned flap, were transported into
the impact structure’s interior as well, thus forming the
“central breccia unit” [3]. Resurge chalk deposition [3,
10] immediately followed the very large massmovement event.
References: [1] King Jr. D. T. et al. (2002) EPSL
202, 541-549. [2] King Jr. D. T. (2006) MAPS 41,
1625-1631. [3] King Jr. D. T. and Ormӧ J. (2012) GSA
Spec. Paper 483, 287-300. [4] Neathery T. L. et al.
(1976) GSA Bull. 87, 567-573. [5] Markin J. (2011)
Auburn University MS Thesis. [6] Johnson R.C.
(2007) Auburn University MS Thesis. [7] Horton Jr. J.
W. (2006) MAPS 41, 1613-1624. [8] Rodenas P.T.
(2012) Auburn University MS Thesis. [9] Rodesney
S.N. (2014) Auburn University MS Thesis. [10] King
Jr. D. T. et al. (2014) LPS XLV, Abstract #2139.
Acknowledgements: Work by Heider was supported by the Barringer Family Fund, Gulf Coast Association of Geological Societies, and the Alabama Geological Society. Work by King and Ormӧ was supported by NASA PGG; King by the city of Wetumpka and
the Poarch Creek Tribal Endowment Committee.
Figure 1. – Next page – LiDAR-based DEM with digitized geologic map from [4]. Geologic half-transect
line is marked (red). Also, locations of key shallow
core holes are marked.
Figure 2. – Next page – Geological cross section from
northwest to southeast along the geological half transect, the red line, in Figure 1. Drill core 09-03 is from
[5]; drill cores 09-01 and 02 from [3]; and drill core
98-01 from [6]. Steeply dipping sedimentary formations near the crater wall are modified from [5].
Crystalline rim terrain described by [8].
46th Lunar and Planetary Science Conference (2015)
Figure 1.
Figure 2.