The Connection between Impact Craters and Shales

Two studies of fine grained crater fill sediments are currently in progress:

1) A study of the Flynn Creek Crater in Tennessee, and
2) A study of Meteor Crater in Arizona

Flynn Creek Study

The Flynn Creek Study was conducted in collaboration with Dave Roddy (Scientist Emeritus, USGS, Flagstaff), and is largely completed.  Initial results will be presented at the 2000 GSA meeting in Reno/Nevada.

Location of Flynn Creek Crater and crater cross section (from Roddy, 1968)

The Flynn Creek crater in north-central Tennessee was produced when a cometary body struck a flat lying succession of Ordovician carbonates. Conodont data from Devonian (Upper Givetian to Lower Frasnian) lag deposits that overly the Ordovician succession in the region, indicate flooding by the Devonian sea during the upper Givetian. A lack of shale accumulation suggests that these initial Devonian sediments were deposited in frequently winnowed shallow water. In the context of storm deposits found in the overlying Chattanooga Shale, the water depth during lag accumulation was probably on the order of 10 meters or less.

Originally flat lying Ordovician carbonates were deformed and tilted in the central portion of the crater.

Chaotic basal breccia overlain by crudely bedded breccia deposits.  The latter mark the onset of post-impact crater sedimentation.

Close-up photo of basal breccia. Lens cap for scale.

Within the crater, fractured and deformed bedrock is overlain by very coarse and chaotic breccia. The top portion of the breccia systematically fines upwards. Depending on location, one or more additional units of fine breccia and/or sand to silt size carbonate debris with weakly developed bedding are encountered above the graded breccia unit. Layers of bedded breccia are separated from the latter, as well as from each other, by thin, soft partings that consist of a mixture of clays, quartz sand and silt, carbonate debris, phosphatic debris, and conodonts. The conodonts are lower Frasnian in age and overlap with those found in lag deposits outside of the crater. These beds are in turn overlain by Devonian black shales.
Detailed study of crater stratigraphy and sedimentary features suggest the following sequence of events: (1) impact in shallow water during the lower Frasnian (381-382m.y.); (2) formation of the basal chaotic breccia as a fall-back deposit; (3) deposition of graded breccia as displaced water rushed back into the crater; (4) while the sea was still shallow, ejected material was washed back into the crater by storm-induced waves and currents; (5) with rising sea level, black shales were able to accumulate, first in the crater, and later also outside the crater.

Above picture shows an outcrop of Devonian shales (Chattanooga Shale) in the crater vicinity.  The two regional subdivisions of the Chattanooga Shale, the Dowelltown and Gassaway member, are clearly visible.  Whereas outside of the crater Devonian black shales only measure approximately 7-8 meters in thickness, within the crater there is a total thickness of up to 55 meters of Devonian black shales.  Most of this thickness represents an early Frasnian black shale succession that was deposited prior to deposition of the Chattanooga Shale as shown above, and attests to a pre-Chattanooga episode of Devonian black shale accumulation.  Yet, because of widespread erosion prior to Chattanooga deposition, this black shale unit has only been preserved in localized depressions, such as the Flynn Creek crater.


Meteor Crater Study

Meteor Crater near Winslow Arizona is the world's best preserved Meteor crater (Barringer Meteor Crater), and the first one that was correctly interpreted as the result of a Meteor impact.  The geologic relationships and processes recognized at Meteor Crater have been highly influential for the study of impact sites all over the world.

As at Flynn Creek, the Barringer Meteor Crater contains a post-impact sediment fill that so far has gone largely unstudied.  A study of the crater fill is underway in collaboration with Dave Roddy (Scientist Emeritus, USGS, Flagstaff).  The crater fill is exposed in old mine shafts that were constructed early in the 20th century while Daniel Moreau Barringer searched for the fortune in meteoritic iron which he was convinced lay at the bottom of the crater.   Initial examination of crater sediments was carried out in August 2000, and a comprehensive study of the crater succession is planned for 2001.  With erosion prevalent elsewhere in the region for most of the Quaternary, the Meteor Crater fill is important because the crater depression allowed preservation of a 50,000 year sedimentary record that may reveal new insights into the quaternary climatic history of the region, as well as into post-impact depositional processes.

The shaft opening at bottom of the floor of Meteor Crater, the "Gates to Hell" are open. Dave Roddy returning from the depth of the shaft.  It's dusty down there.

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© Jürgen Schieber, UTA Department of Geology
Last updated: August 22, 2000 .