Wetumpka Meteor Crater

    The following day, Saturday, November 8, was a bracing, sunny, late-fall day -- just perfect for exploring a meteor crater. Gathering in Wetumpka for our field trip were: David King, Larry Owsley, Robert Rock, Scott Thompson, Ricky Wood, Ron & Jeanetta Hatherley, Allen & Christy Screws, Jason Schein, David Newton, Russell Whigham, and five of Dr. King's students; Jewel Benson, Jaime Demick, Michael Skotnicki, and Janet and Fred Leemhuis. We met at the Hardee's in Wetumpka, piled into three vehicles, and headed out on what turned out to be a most enjoyable and informative tour.

    Here is synopsis of our trip led by Dr. King:

	At 10:00 AM on the day of the trip, we converged from across the region in Wetumpka.  Here we are in the parking lot of Hardee's restaurant, as Dr. King goes over the day's itenerary and giving a brief description of the stops along the tour.
Our first stop was at this out-cropping on the outer slope of the crater wall, just a few yards off of US highway 231.  Dr. King is shown describing the processes that heaved the previously level layers of rock up to over 50°.
	Close-up image of the up-lifted rock in the crater's western wall.

Our second stop was atop "Bald Knob", the highest point on the crater rim. Here, Dr. King explains the crater's shape and rebound feature, seen halfway between where we were, and the crater's eastern rim, visible on the horizon.  From the Bald Knob site, we visited the borrow pit and megaconglomerate road cut stops (shown in Figures 3 & 5 below).

The final stop of the field trip was at the summit of the central peak of the crater. A gas pipeline right-of-way provided a clear path to the crater rim on the distant horizon. 

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Thanks to Dr. King for providing the following photos and captions.

Dark line with dot-shading indicates limits of outcrop of units msg and mag, crystalline metamorpic basement rocks (mica schist) that form what is left of the deeply eroded rim of the impact crater. The unit inside the crater (Kwcm) is the Wetumpka crater melange, a mixture of slump blocks from the crater rim and megaconglomerate deposits. Rocks in the crater melange range in age from 95 to 81.5 million years. The age of the crater is thought to be about 81.5 million years because of the age of youngest rock in the melange. Unit K outside the crater rim is undeformed by the impact and ranges in age from 95 to 83 million years.

FIGURE 1. GEOLOGY AND TOPOGRAPHY OF WETUMPKA IMPACT CRATER

FIGURE 2. VIEW FROM NORTHERN RIM, WETUMPKA IMPACT CRATER

Labeled photomosaic panorama from Mr. Roy Lemmon's backyard (Bald Knob Road). Tapering end of south rim is indicated. Photo taken from north rim viewpoint. Wetumpka urban area on US 231 is indicated.

Labeled photomosaic panorama taken from eastern side of an abandonded sand borrow pit in crater's interior floor, near the northern rim. Darkest (nearly vertical) lines indicate boundaries between large blocks, 3 to 9 m across, that probably fell off the northern rim shortly after impact. From left, the first 4 blocks are Tuscaloosa Formation, the block on the far right is Eutaw Formation. Both formations are Late Cretaceous age, but the Eutaw is much younger than the Tuscaloosa. From left (or south), the layering in each block dips as follows: first, southeast; second, vertical; third, south; fourth, northwest; and fifth, northeast.

FIGURE 4. FOLIATION IN NORTHERN RIM	Layering (metmorphic foliation or mineralogic alignment) in crystalline bedrock that is uplifted in the base of the northern rim. Exposure is located on US280 on the north end of the business district, Wetumpka. Overlay labeling shows attitude of foliation.  Compass diagram shows attitude of foliation (short arrow is dip direction). Solid arrow is post-impact; open arrow is pre-impact.
Impact-generated deposit near central rebound peak (Buck Ridge Road). Overlay labeling shows large rounded clasts in a matrix of sand (megaconglomerate). Rounding indicates transport of clasts and abrasion during impact event. Clasts are from crystalline bedrock and overlying sedimentary layers (Tuscaloosa and Eutaw Formations). Scale is 30 cm.	FIGURE 5. MEGACONGLOMERATE AT CRATER CENTER

FIGURE 6. IMPACT ANGLE AND VECTOR

The remaining crater rim is a horseshoe shape, with the south rim missing.  Originally, it was thought that the asteroid came in from the northeast at a low angle and blew the south wall out.  More recent discoveries made when the  core well samples taken in the summer of 2009, strongly suggest that the southern wall of the crater was first pulled deeper into the sea that covered the site, then returned as the associated tsunami washed the separated crater wall section back into the newly formed crater, filling it in with remnants of the southern wall.   This is a very simplified version of Dr. King’s conclusions.  Be sure to attend his next lecture for a thorough explanation.

The asteroid impact at Wetumpka produced Alabama's greatest natural disaster in the last 81.5 million years. Based on formulae developed to study the effects of nuclear weapon detonations, the area of total devastation (atmospheric shock wave with peak overpressure exceeding 14 kPa) would be within a radius of 40 km. Similarly, the maximum clearday infrared flashburn radius would have been 47 km. These areas are indicated. Continuous ejecta (rock particles from the impact) would have fallen over an area within 7.5 km, and random rock falls would have covered a much larger area. A Richter-magnitude 8.5 to 9 earthquake would have occurred at crater center.

FIGURE 7. AREA OF DESTRUCTION

King, David T., Jr., 1997, The Wetumpka impact crater and the Late Cretaceous impact record: Alabama Geological Society Guidebook 34B, p. 25-56. [Source: Ala. Geol. Soc., P.O. Box 866184, Tuscaloosa, AL 35486]

Neathery, Thornton L., et al., 1976, Cryptoexplosive structure near Wetumpka, Alabama: Geological Society of America Bulletin, v. 87, p. 567-573.

Hall, John C., 1996, Ground zero - The Wetumpka astrobleme: Alabama Heritage, Fall issue, p. 7-13.