THE GLOVERS BLUFF STRUCTURE, A POSSIBLE IMPACT FEATURE EXPOSED IN THE MID-CONTINENT CAMBRO-ORDOVICIAN STRATA OF CENTRAL WISCONSIN

The Glovers Bluff structure is an enigmatic deformed structure located in an area of relatively undeformed Cambrian sandstones in southcentral Wisconsin. The structure lies in a cluster of three hills exposed in Pockrandt Quarry. Read, 1983, reported shatter cones from dolostones in the north wall of the quarry. Since then more shatter cones have been found. Read described the structure as a crater 10 km in diameter and inferred that the impact postdates the deformed Early Ordovician strata. At the quarry there are three major rock units. The Cambrian sandstone of the Jordan Fm. is overlain by a layer of conspicuous red breccia-conglomerate, which is overlain by Ordovician dolostone of the Oneota Fm. All are intensely deformed with several folds and faults.Read claimed that the red breccia-congomerate layer has clasts derived from both the overlying an underlying strata which he interpreted as a product of friction between a sliding slab and less well indurated material beneath from the impact. Other authors have speculated that the red breccia-conglomerate is a fall back breccia from the impact.

Since starting our research, problems with previous interpretations arose as we examined the rocks more closely. 1) Although shatter cones have been reported from the quarry, no shocked quartz has been found. 2) If the hills are the crater itself, then where is the center of the crater? 3) Where is the fallback debris? Cuttings from the Coloma well contain what Read described as peculiar quartz pellets with diameters up to 5 mm although no samples are available for confirmation. 4) Read claimed that the red breccia-conglomerate layer is a product of the meteor impact. Our observations indicate that the breccia-conglomerate layer is part of a consistent stratigraphy throughout the quarry and is involved in the deformation. Thus, this layer had to have formed prior to impact.

We will evaluate the origin of the red breccia-conglomerate and other peculiar breccia fabrics found in dolostones in the quarry. We will map structures in the quarry and integrate these results with gravity maps from Dr. Jeff Plescia of Johns Hopkins to evaluate what part of an impact structure is represented. Finally, we will examine the Jordon-Oneota contact outside the deformed area to evaluate depositional environments represented by the red breccia-conglomerate.