File:Shattercone (quarry at Kentland Dome Impact Structure, Indiana, USA) 2.jpg
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English: The striated, cone-shaped feature in this rock is an impact shattercone. These structures form during impact events - powerful shock waves / pressure fronts moved through rocks, resulting in shock metamorphism. Shattercones have a three-dimensional cone-like structure, with the points of the cones directed toward the shock wave origination site. Undisturbed shatterconed rocks will have the apex of the cones pointing toward the direction of the incoming object (i.e., upward - toward space).
This sample comes from a quarry at the Kentland Dome Impact Structure in northwestern Indiana, USA. That area's landscape consists of flat plains of soil-capped, Wisconsinan-aged glacial drift, with few bedrock outcrops. Below the glacial cover, sedimentary rocks are flat-lying, with about 1 to 2 degree regional dips. At Kentland Dome, country rocks poke upward through the Pleistocene glacial sediments and dip outward in a dome-like structure. Mapping shows a concentric pattern of stratigraphic units - they are not flat-lying. The central uplift is considerable - estimated at about 2000 feet. The oldest rocks at the center of the structure are Middle Ordovician - this is the only site in this entire region where rocks of this age are at the surface. Beds are almost vertical and arranged in complex fault blocks. The Ordovician stratigraphic section includes the St. Peter Sandstone, Platteville Formation, Galena Formation, and Maquoketa Formation. Silurian carbonates occur above that. The section is not continuous - it's broken up into blocks. Outside the quarry at Kentland Dome, some Devonian, Mississippian, and Pennsylvanian outliers occur. Mapping is a challenge. That Kentland Dome itself is an impact structure is demonstrated by the presence of shattercones at the quarry, which are locally abundant - such structures are only known to form at impact sites. Coesite has been reported from Kentland Dome - it's a quartz-like, high-pressure, impact mineral. The age of the Kentland Dome Impact is undetermined, but may be Late Cretaceous. Locality: quarry at Kentland Dome Impact Structure, southern side of Route 24 & east of the town of Kentland, southern Newton County, northwestern Indiana, USA (40° 45' 55.00" North latitude, 87° 23' 14.10" West longitude) See info. at: en.wikipedia.org/wiki/Kentland_crater |
| Date | |
| Source | https://www.flickr.com/photos/47445767@N05/52802007274/ |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/52802007274. It was reviewed on 13 April 2023 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 19:53, 13 April 2023 | | 3,079 × 2,587 (6.5 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/52802007274/ with UploadWizard |
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