KENTLAND IMPACT STRUCTURE
by: Charles O’Dale
- Type: Complex
- Age (ma): >97 – estimated as between Post-Early-Pennsylvanian and Pre-Pleistocene.
- Diameter: ~13 km
- Location: Indiana, U.S.A. N 40° 45′ W 87° 24′
- Shock Metamorphism: (a) Shatter cones, (b) deformation lamellae, cleavage and microfaults in quartz grains, (c) monomict and polymict breccias.
Geochemical Study of Rocks from the Kentland, Indiana, Impact Structure: Progress Report
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 . 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. (Koeberl, C.; Sharpton, V. L. – 1993)
Nestled below the flat, placid farmlands of northwestern Indiana lies one of Indiana’s strangest geologic features. An elliptical area (about 7 miles across), near Kentland in Newton County, contains highly deformed rocks that bespeak of incredible forces so great that layers of rock normally in a horizontal position 1,800 feet below the surface are folded and fractured and are now exposed at the surface in nearly vertical positions.
Since the 1880s, geologists have known about the Kentland structure. Many attempts have been made to explain its origin. One theory proposes that a meteorite hit the Earth and caused a large crater like those on the Moon; another is that trapped gases from deeply buried volcanism caused the faulting. Geologic evidence, such as shatter cones and minerals formed under very high pressure, imply an origin by meteorite impact. (Indiana Geological Survey)
The area of disturbed rock known as the Kentland, Indiana structure is the fourth largest known impact site in the United States (1978).
More than 700 m of Lower Ordovician through Pennsylvanian strata are known to be involved in the 12.5 km diameter Kentland disturbance. Under the structure (crater), a 1645 m thick succession of Paleozoic marine sandstone, carbonate and shale formations unconformably overly a terrain of granite and rhyolite.
R.S. Dietz, one of the early proponents of impact geology, studied Kentland’s impact structures in place, especially shatter cones, and found the orientation of the cones were invariably normal (perpendicular) to the bedding:
“The orientation of the shattercones suggests that, assuming that the beds were essentially horizontal prior to deformation, the shock force resulted from some type of explosion directly above the beds rather than from a crypto-volcanic explosion below the beds.” (Science 10 January 1947: 42-43.)
Laney, R. T. & van Schmus, W. R. A structural study of the Kentland, Indiana impact site Lunar and Planetary Science Conference March 13-17, 1978, Proceedings. Volume 2
Kentland Dome: Geologic Enigma Indiana Geological Survey