G. S. Austin

Weathering of the Sioux quartzite near New Ulm, Minnesota, as related to Cretaceous climates

ABSTRACT An exposure of Precambrian Sioux Quartzite near the city of New Ulm, Minnesota, shows evidence of intensive chemical weathering. The silica cement matrix material of the quartzite has been removed and the argillaceous part (probably sericite) of the cement matrix has been altered to kaolinite. The regolith is composed of loose pink sand with a kaolin clay matrix and is as much as 20 feet thick. The regolith probably is no younger than Early Cenomanian and in part may be correlated with the Van Bibber Shale Member and older members of the South Platte Formation of Albian age in Colorado. Upper Cretaceous sedimentary rocks consisting primarily of red and green shale, siltstone, and sandstone overlie the weathered quartzite. Illite and montmorillonite are the principal clay minerals in these rocks, with kaolinite present in minor amounts. The significant change in clay mineralogy, coupled with paleontological evidence, indicates a pronounced change in climatic conditions during Cenomanian time. The humid tropical climate with good drainage conditions which was conducive to kaolinite formation was replaced by a temperate climate with poor drainage. The principal clay minerals incorporated in the sedimentary rocks formed after the climatic change are illite and montmorillonite. The illite may have been derived from erosion of older illite-bearing rocks and/or from chemical weathering of the silicate minerals in these rocks. The montmorillonite may have been formed by the alteration of volcanic ash from sources lying to the west. It is therefore possible to distinguish rocks formed before and after the climatic change on he basis of clay mineralogy. The Sioux Quartzite stood as a highland during the formation of the kaolinitic regolith and shed its weathering products into topographically lower areas. The quartz sand produced from the quartzite was reworked by streams flowing to seas lying to the west and now makes up a large part of the Dakota Formation of Minnesota. Glacial erosion during Pleistocene time removed most of the weathered zone from exposures of the Sioux Quartzite, but bleached joints, or the roots of the weathered zone, remain as evidence of the past humid, tropical conditions.

Soil lead distribution at an abandoned smelter site in Socorro, New Mexico

Soil Pb concentrations from three 30.5-m× 30.5-m squares at an abandoned smelter site in Socorro, New Mexico, are reproducible only within several hundred ppm. There was no statistically significant difference in log-transformed sample means for two of the three squares tested, due to the large degree of variability among samples from each square. Even where sample means are not significantly different, attempts at replication using similar sample spacing and collection techniques yield radically different spatial distributions of Pb. This variability suggests that common assumptions about soil contamination at hazardous waste sites must be reconsidered and also that understanding the statistics of contaminant distribution may be the key to designing effective clean-up strategies.

Clay mineralogy of the Glenwood Formation, southeastern Minnesota and adjacent areas

ABSTRACT Variations in the relative abundance of kaolinite and illite in the clay-size fraction of the Ordovician Glenwood Formation of southeastern Minnesota, western Wisconsin, and northeastern Iowa suggest that the source area for this shaly unit was to the southwest. Kaolinite is most abundant in the southwesternmost samples; it decreases in amount to the northeast and is absent in the most northeasterly samples. Illite increases in abundance as kaolinite decreases. Limestone and dolomite beds in the shale are more abundant in the illite-rich region to the northeast.