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.