|During several very hot weeks in June, July,
and August, 1998, the authors were part of a team that completed the first
scientific core drilling inside the Wetumpka impact crater, located near
Wetumpka, a small city in central Alabama. The team consisted of
ourselves, two drillers, and a geologist from Vulcan Materials Company
of Birmingham, Alabama. The drilling was conducted by Vulcan
as an in-kind gift to Auburn University following a successful scientific
proposal submitted to them by David T. King, Jr. and Thornton L. Neathery.
Wetumpka impact crater is an unusual-looking geological feature that sits astride the boundary between Alabama's Gulf coastal plain (a low-relief terrain covering the southern half of the state) and Appalachian piedmont (a moderate-relief terrain covering a significant part of central Alabama). Wetumpka has an unusual origin in that the point of impact was located in shallow marine water. Further, Wetumpka target material consisted of unconsolidated soft sediments overlying crystalline basement rock.
Wetumpka is remarkable in that its rim, made of crystalline basement rock is semi-circular, a shape thought to result from an oblique or low-angle impact along a northeast-to-southwest trajectory. The crater floor has topography that has been heavily dissected by stream erosion, but a rebound peak, typical of complex-type craters, is discernable at crater center. Wetumpka's unusual geological features were first noted by reconnaissance mapping in 1891, but were not recognized as constituting an impact crater until completion during 1969-1972 of detailed geological mapping by a team headed by Thornton L. Neathery.
Wetumpka's crater floor displays a crater fill comprised of a mixture of blocks ranging in size from several acres across to fist-size and smaller. This chaotic mix of target rocks was created within a few tens of seconds after impact due to release of a vast amount of explosive energy. Energy released due to detonation of the incoming asteroid is estimated to have been 1,500 megatons (a megaton is equal in explosive power to one million tons of TNT), or 15% of explosive power of the world's Cold-War nuclear arsenal. Detonation at Wetumpka opened a crater that today is approximately 4 miles in diameter. Blast effect pulverized rock within a crater cavity that extends over 650 feet deep (beyond the limit of our drilling depth this past summer). Judging from crater diameter, we think that the Wetumpka asteroid diameter was approximately 1100 feet.
We have used relative age dating to determine an age of asteroid impact at Wetumpka. Specifically, by looking at age of the youngest rocks within Wetumpka's crater fill, a relative age estimate can be made. The youngest rocks known in the crater fill have fossils in them which are thought to be approximately 80 to 83 million years old. In the absence of a more precise radiometric age date, we are using this date as presumed age of impact. Thus, Wetumpka's age is probably 22 to 25 million years older than the Yucatan Penninsula's huge crater at Chicxulub, directly across the Gulf of Mexico, and 6 to 9 million years older than the major crater at Manson, Iowa.
Drilling commenced on the morning of June 24, 1998, at drill-site number 1, which was located, we thought, as near to the center of Wetumpka impact crater as we could reach. (We were limited in places to drill on Wetumpka's rebound peak because of property owner concerns, but outcrops in the selected area showed some impact breccia at the surface.) We soon discovered that drill-site number 1 was on the central rebound peak's eastern flank and not exactly at its center. Being on the central peak's flank caused us to have to drill through almost 327 feet of steeply inclined target strata comprising the flank before penetrating into several types of impactites, or crater-filling pulverized rock. Invaluable samples were recovered from our drilling operation in the form of continuous, 1.75-inch diameter cylindrical core.
The impactites in our drill cores displayed interbedded layers of (1) diamictites containing crystalline basement fragments brought up from great depth, (2) impact breccias (containing similar crystalline basement fragments), (3) huge blocks of target rock up to 35 feet thick, and (4) clay-rich sands. Diamictite is a rock that consists of broken pieces of target rocks in a matrix of finely pulverized rock. Impact breccia is a rock that consists of broken pieces of target rocks that generally lack finely pulverized matrix material. Both diamictite and impact breccia are common impactite rocks in the world's crater-filling units. Huge blocks encountered in drilling were both pieces of deep crystalline basement rocks and chunks overlying softer sedimentary target rocks. Clay-rich sands appear to be a slurry formed from pulverization of the softer sedimentary-rock target material within the impact crater.
We drilled at site number 1 until July 10, 1998, when total depth of 638.5 feet was reached. The drilling at this site was very difficult due to relative softness of impactites and their abrasive characteristics. We averaged less than 50 feet/day, even though we were using Vulcan's newest truck-mounted drilling equipment and their best drilling crew. Total depth was determined not by our reaching bottom of the crater fill, but by the fact that drilling progress became essentially nil at 638.5 feet due to poor down-hole conditions. After drilling ended, it took two days just to recover all drilling pipe from the ground.
We started drilling at drill-site number 2, located about 200 yards west of number 1, on July 15, 1998. Drilling continued at site number 2 until approximately August 4. A total depth of 589 feet was achieved, and drill-hole number 2 had to be abandoned prior to reaching bedrock beneath the crater for the same reasons as at drill-site number 1. The overlying steeply inclined sedimentary strata noted at drill-site number 1 were thinner at site number 2, being penetrated at a depth of 260.5 feet, and impactites were found below this depth. Diamictites, impact breccias, target-rock blocks, and clay-rich sands like those seen at drill-site number 1 were recovered in drill core from this drill hole. These drill cores display characteristics that have been described from drill cores taken at Manson crater (Iowa), Reis crater (Germany), and Ames crater (Oklahoma), and other craters.
Drilling at sites 1 and 2, both located upon the central rebound peak, was considered to be most significant from a scientific point of view because such a location would have experienced maximum possible impact pressure and other direct effects from asteroid impact. At present, we are conducting research to determine if shocked minerals were produced and are present at Wetumpka. Many craters have shocked quartz, but some notable craters do not, e.g., Upheaval Dome, Utah. It remains to be seen if we find shocked quartz**, the best and most definitive proof of impact, but even if we do not, it is abundantly clear that Wetumpka is an impact crater (and a relatively well preserved one!).
David T. King, Jr. is a Professor of Geology at Auburn University, Auburn, Alabama firstname.lastname@example.org. Lucille W. Petruny is an Instructor at Auburn University. Thornton L. (Tony) Neathery is a consulting geologist and president of Neathery and Associates, Tuscaloosa, Alabama.
*This article was originally published in VOYAGES! magazine (May/June 1999) and has been slightly modified here.
**In January 1999 after submission of this story,
we discovered shocked quartz in several microscopic thin sections from
drill hole number 1. In February 1999, at a news conference at Wetumpka
City Hall, we announced discovery of shocked quartz. Dr. Peter Schultz,
Brown University, participated in the news conference with us and reviewed
evidence confirming meteortic impact with the persons in attendance and
with the press. Presence of shocked quartz was also confirmed a few
weeks later by Dr. Christian Koeberl, Geochemistry Institute, University
of Vienna (Austria). Working with Dr. Willis E. Hames in the Department
of Geology, Auburn University, we have determined subsequently that shocked
quartz from Wetumpka shows two sets of shock deformation
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