Paper No. 4
Presentation Time: 9:05 AM


KASS, M. Andy1, BEDROSIAN, Paul A.2, DRENTH, Benjamin J.3, BLOSS, Benjamin R.2, MCKAY, Robert M.4, LIU, Huaibao5, FRENCH, Bevan M.6 and WITZKE, Brian J.7, (1)Crustal Geophysics and Geochemistry Science Center, US Geological Survey, Denver Federal Center, MS 964, Denver, CO 80225, (2)US Geological Survey, Denver Federal Center, Bldg 20, MS 964, Denver, CO 80225, (3)U.S. Geological Survey, MS 964 Denver Federal Center, Denver, CO 80225, (4)Iowa Department of Natural Resources, Iowa Geological and Water Survey, 109 Trowbridge Hall, Iowa City, IA 52242, (5)Iowa Geological & Water Survey, Iowa Department of Natural Resources, 109 Trowbridge Hall, Iowa City, IA 52242, (6)Meteorite Impact Crater Studies, Dept. of Paleobiology, Smithsonian Institution, PO Box 37012, NMNH, Room E-310, MRC 0121, Washington, DC 20013-7012, (7)Department of Earth and Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242,

The Decorah Impact Structure is a probable buried impact crater of Middle Ordovician age located in Northeast Iowa, USA. Originally hypothesized by the Iowa Geological and Water Survey though identification of a unique shale layer and shocked quartz from borehole samples, the 5.5 km diameter structure is nearly completely concealed beneath the town of Decorah, Iowa and the surrounding area.

In late 2012 and early 2013, the US Geological Survey conducted airborne geophysical studies in the area to investigate structures and potential mineral resources associated with the 1.1 Ga Midcontinent Rift system. Full-tensor gravity gradiometry and airborne transient electromagnetic surveys were flown to investigate basement geometry and composition, as well as to map out the thick package of Phanerozoic sediments blanketing the region. Multiple survey lines intersected the impact structure, which was clearly visible in both the electromagnetic and gravity datasets.

The transient electromagnetic data identified and mapped the post-impact Winneshiek Shale, which is present only in the crater (having been completely eroded elsewhere in the survey area). The resulting 5.5 km diameter circular conductor aligned nearly perfectly with the structure inferred by the Iowa Geological and Water Survey. The airborne full-tensor gravity gradient data clearly demarcates a density low in each component consistent with the center of the impact structure.

The conductivity and density of some of the stratigraphic units both within as well as outside the impact structure were measured from core samples, and used to inform the modeling and inversion approaches. Both the electromagnetic data and the gravity gradiometry data underwent an extensive modeling and inversion procedure to investigate the geometry of the impact structure in three dimensions. From these results, we present a three dimensional model of the proposed Decorah Impact Structure and surrounding area. Not only will this model improve an understanding of the geology and hydrology of the region, but also will allow for more precise estimations of the energy and size of the impacting body.

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