Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..382k&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 382
Other
2
Crater, Kentland, Impact Craters, Indiana
Scientific paper
The Kentland impact structure is situated about 4 km east of the town of Kentland, Newton County, Indiana, at 40 degrees x 45'N and 87 degrees x 24'W. With a diameter of about 12.5 km, it is one of the larger impact structures preserved in the U.S. Quarry operations have exposed various Ordovician and Silurian rocks near the center of the structure. Almost all bedrocks are covered by Pleistocene glacial till. The crater shows a central structural uplift of about 4 km diameter with an intensely disturbed area of about 1 km diameter at the center. The uplift involves rocks from a preimpact stratigraphic depth of at least 600 m [1]. The uplift is surrounded by a circular depression 1.5 to 2 km wide, followed by a structural high, about 6.2 km from the center, that is assumed to be the present-day outer limit of the structure. Geophysical studies indicate a gravity high in the center (coinciding with the structural uplift) with an encircling gravity low marking the ring depression. For the origin of the Kentland structure, various endogenic hypotheses were proposed earlier, but over the past 2-3 decades meteorite impact has been established as the most likely origin. Various forms of shock metamorphism have been documented from the structure (mainly well-preserved shatter cones, shock deformed mineral grains, and monomict as well as polymict breccias). Monomict breccias often occur mainly in carbonate rocks, but some are also found in sandy units. Polymict breccias are more abundant and exposed in quarries. They contain mainly dolomite, calcite, chert, and shale clasts (size range millimeters to 0.5 m) with quartz grains and occasional shatter-cone fragments. The matrix of the breccias is hard and comprises light gray, fine- grained carbonate material. We have started a combined geochemical and petrographical study of breccias from the Kentland structure in order to define geochemical relationships between breccias and source rocks and search for a possible extraterrestrial component. We measured the bulk composition of two breccias, one glassy (pumice-like) rock from a dyke-invading limestone with silicate materials and another from the highly shocked central uplift containing sandstone (probably St. Peter), mixed with younger dolomites and limestone. Two more clasts were separated from the latter breccia and also analyzed. All breccias are relatively low in trace elements. The REE patterns are basically crustal and typical for those observed in carbonates or dolomites. The glassy impact breccia may have a minor cosmic component on the order of 0.02% as preliminary concentrations of Ir are about 0.15 ppb. Reference: [1] Laney R. T. and Van Schmus W. R. (1978) Proc. LPSC 9th, 2609- 2632.
Koeberl Christian
Sharpton Virgil L.
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