NEW EVIDENCE OF EARLY ORDOVICIAN TECTONISM IN THE UPPER MISSISSIPPI VALLEY
G.A. Ludvigson and M.P. McAdams
Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 9, 1980, 29 p.
Recent stratigraphic investigation in the Upper Mississippi Valley has reinstated Ulrich's controversial "Canadian-Ozarkian" unconformity at the base of the Shakopee Formation in the Prairie du Chien Group (Early Ordovician). The cause of the locally angular truncation of strata beneath the unconformity, however, has never been satisfactorily explained. The hypotheses of tectonic origin was tested by making strike and dip measurements from folds truncated by the Shakopee Formation, and by measuring a large number of fracture orientations from strata above and below the unconformity. The strike azimuths measured from folds beneath the unconformity align along an east-west trend (mean=89°), and have a standard deviation (33°) that compares favorably with that from data reported from the Precambrian rocks of the Baraboo synclinorium in Wisconsin. The fracture populations from rocks above and below the unconformity were compared by the Chi-Square test and are significantly different at the 95% confidence interval. A standardized residual technique was used to identify those fracture orientations in the pre-Shakopee units which may have been present before Shakopee sedimentation. The fracture sets obtained are arrayed in extension, conjugate shear, and tension fracture orientations that would result from a stress field with sigma 1 = north-south, sigma 2 = vertical, and sigma 3 = east-west. This stress field is consistent with the postulated east-west fold trend, strongly suggesting that the structures below the unconformity are tectonic in origin. The study area is characterized by east-west trending linear magnetic gradients, indicating the possibility of reactivation of basement structures during Early Ordovician tectonism. These long-ignored structures may be economically significant. They apparently have controlled the distribution of ore deposits in the Lower Ordovician rocks, and this suggests that a new vertical exploration model is appropriate for the Upper Mississippi Valley base metal district.