Analysis of Cross-Stratification Orientation in the St. Peter Sandstone in Southwestern Wisconsin

Abstract

The orientation of cross-bedding in the St. Peter Sandstone in southwestern Wisconsin has been known to be very complex and possibly even random. Special attention to cross-set trough-axis plunge azimuths, together with the recent reinterpretation of large-scale cross-stratification in the St. Peter as relict internal structures of large submarine sand dunes, waves, and ridges, provides added insight for a refined analysis of cross-set orientations and, thus, for inference of paleocurrent directions. In the hierarchical array of cross-sets west of Madison, Wisconsin, smaller scale sets show no significant orientation by vectorial and variance analyses. The large sets, however, have a significant preferred orientation toward the southwest (260° ± 85° standard deviation), and trough axes show a significant plunge toward the northwest (281° ± 82° standard deviation). Only by recognition of the complexity of the cross-stratification and by careful selection of particular geometric features to be measured can any meaningful index of paleo-currents be discerned. At least for the limited area of study (700 sq mi), the net sand transport direction was toward the west rather than south-southwest as previously postulated from limited data for the upper Mississippi Valley region.