NEW CONSTRAINTS ON THE TIMING OF DEFORMATION AT THE GLOVER BLUFF STRUCTURE NEAR COLOMA, WI FROM A CALCITE TWINNING ANALYSIS

Paleozoic sedimentary rocks in the U.S. midcontinent are typically undeformed, with local exceptions like those found at Glover Bluff, WI, where the Cambrian Jordan Sandstone and the overlying Ordovician Oneota Formation are conspicuously folded and faulted in the three exposed hills of the Pockrandt quarry. Read (1983) reported shatter cones from the North hill of the quarry, and therefore interpreted the deformation to be due to a meteorite impact. Because the youngest rocks are Ordovician in age, Read (1985) inferred that the impact postdates the deformed Ordovician strata. To further refine the relative timing of deformation, we collected an oriented block sample of a healed fracture filled by calcite from the North hill. We conducted strain analyses on mechanically twinned calcite within the vein in order to compare the strain patterns in the deformed rocks at the quarry to strain patterns that typify flat lying sedimentary rocks in the midcontinent. The morphology, width, and intensity of twins in the samples suggest that twinning occurred at relatively low temperatures. The shortening axis of the three dimensional strain ellipsoid using all of the data (ALL) is 0.96%, and the ellipsoid has an X/Z ratio of 1.44. The cleaned data (20% of the largest magnitude deviations removed) show ≤30% negative expected values, consistent with a single deformation episode. The strain ellipsoid determined using the calcite strain gauge technique indicates a WNW subhorizontal shortening strain (~6 degree plunge) despite the appreciable dip (~56 degrees) of bedding at the sample site. The WNW shortening strain is ~parallel to the WNW shortening direction determined for flat lying carbonates in the south-central WI area (Craddock, 1989). The similarity of the strains found at Glover Bluff to midcontinent strains in flat lying rocks suggests the calcite twins in the vein from Glover Bluff formed during the same regional twinning episode. Thus, if the ~horizontal shortening strains recorded by calcite twins in the U.S. midcontinent are related to late Paleozoic mountain building, as suggested by Craddock and van der Pluijm (1989), then the deformation (meteorite impact?) that tilted and faulted strata at Glover Bluff can be bracketed to a time interval that predates, at least in part, the Late Paleozoic Alleghenian orogeny.