ABSTRACT

Trough cross stratification is dominant in Upper Cambrian Sandstones in western Wisconsin. Paleocurrent analysis of such stratification is difficult because of the geometric complexity, but trough-axis orientations have been shown to provide less dispersion than cross-set orientations. Heretofore, trough-axis orientations have been used only sparingly as an index of sediment transport direction, so a study was undertaken in a region where extensive cross-set data already existed in order to further test the superiority of the trough-axis approach and to reevaluate regional Late Cambrian sediment dispersal for three formations.

Regional vector analysis of trough-axis orientations in western Wisconsin yielded sediment dispersal directions toward the south-southwest with much local variation (toward 232° for the Galesville Sandstone, 245° for the Tunnel City Group, and toward 180° for the Jordan Sandstone). The orientation data presented here are comparable with earlier-published results, respective formation mean directions for trough axes falling within at most a 90° arc of means for cross sets. But, with one exception, the trough-axis data is much less dispersed than previously reported cross-set data. The data collected from the Tunnel City Group sandstones are statistically nonsignificant probably due to poor exposures and the variety of environments represented within this unit in west-ce tral Wisconsin.

A southerly average sediment transport direction inferred from cross stratification is independently confirmed by a "train" of fine pebbles of red quartzite in Cambrian sandstones, which extends south from its source in the Baraboo district. An island complex 30 km long by 12 km wide existed in the Late Cambrian sea in south-central Wisconsin, where Precambrian Baraboo Quartzite hills projected above sea level. Cambrian sandstones within 5 or 6 km of the old islands show apparent longshore drift and lee-side circulation eddy influences. These anomalies produce greater dispersion for mean trough-axis orientations in the immediate Baraboo area than for a somewhat larger surrounding region, but other influences produced about the same amount of regional dispersion for the much larger wes ern Wisconsin region. Therefore, the presence of unknown islands could not be demonstrated from cross bedding analysis alone. The inferred regional dispersal direction is an index of the bottom-paleocurrent direction, which, in shallow seas, is influenced primarily by winds, tides, and topographic obstructions. The persistent regional pattern for the Wisconsin Cambrian sandstones is fully consistent with previously-postulated tradewind dominated marine circulation at a low paleolatitude interrupted by occasional brief tropical storms, whose effects are clearly recorded in coarse conglomerates adjacent to the Baraboo islands.