Paper No. 13
Presentation Time: 5:00 PM
PRESERVATION OF THE MIDDLE ORDOVICIAN WINNESHIEK SHALE IN A PROBABLE IMPACT CRATER
The Winneshiek Shale, an 18 m to 27 m thick Ordovician Lagerstätte-bearing shale, is restricted to and fills the upper portion of a 5.5 km diameter circular basin/crater of probable meteorite impact origin at Decorah, Iowa. Primary data sources available for study are water well samples, two cores, driller logs, and two exposures. The predominantly subsurface structure truncates and disrupts Lower Ordovician and Cambrian strata to a depth of at least 200 m below the base of the widespread St. Peter Sandstone. Additional evidence supporting an impact origin for the structure includes: anomalous sub-shale stratigraphy including breccia containing clasts and matrix sourced from Lower Ordovician and Cambrian units; sparse exposures of deformed and brecciated target-rock (Shakopee Fm) adjacent to the projected crater rim; the presence of quartz grains in well cuttings and core that show planar microdeformation features (PMs) that resemble shock-produced planar fractures (PFs) or cleavage similar to those found at the Rock Elm impact structure, Wisconsin; and the presence of quartz grains displaying multiple intersecting sets of apparent planar deformation features (PDFs) like those typically associated with meteorite impact shock metamorphism. Preliminary measurement and documentation of PMs crystallographic rational indices has been accomplished and additional measurements are in progress.
The majority of the wells in the structure penetrate less than 80 m below the Winneshiek Shale, but sample study from a few deeper wells support the following interpretations. The “Decorah Fault” appears to be a normal fault along the crater margin displaying over 100 m of offset, suggesting the presence of a down-dropped block. A deep well near the crater center produced cuttings with quartz pebbles similar to those of the deepest Paleozoic unit, the Mt. Simon Sandstone, suggesting disruption and uplift of the Mt. Simon. However, a sample and natural gamma log suggest that the Mt. Simon and overlying Eau Claire and Wonewoc formations are largely in place and at normal structural elevation. Additionally, we have not documented Precambrian crystalline rock fragments from any sample. This is problematic since an impact crater of such diameter would be expected to have disrupted some of the underlying basement.