Date of Award
Master of Science (MS)
A suite of 26 drill cores recovered from the Red River Valley Drilling Project has provided new information on the Precambrian basement of eastern North Dakota and northwestern Minnesota. Basement rocks in this area consist of intermediate to felsic coarse-grained massive or gneissic rocks, and intermediate to mafic metavolcanic and metasedimentary schists. The Precambrian rocks of the region are interpreted to be a buried extension of the Superior Province of the Canadian Shield, and are divided into terranes of granitic rock and mafic schist, on the basis of drill-hole samples, patterns seen in Superior Province rocks which crop out to the east, and regional geophysical trends.
In the southern part of the Red River Valley, a thick (up to 200+ ft. (75m)) weathering residuum is developed on the upper surface of the Precambrian, generally, but not exclusively, where the Precambrian is directly overlain by Cretaceous rocks. Where the deepest, least weathered rocks are foliated, ghost-like traces of the structures can be seen in slightly to moderately altered weathering products. Scanning electron microscope/microprobe studies show that micas and feldspars are altered to kaolin-group minerals. Regardless of original rock type, the end product of weathering is generally a white, greenish, or light reddish brown kaolinitic clay containing suspended angular quartz grains. Trends in major element chemistry are similar to those reported in studies of modern, exposed weathering profiles.
Evidence from this study points to intermittent or episodic kaolinite producing chemical weathering, beginning prior to Ordovician sedimentation, and lasting until deposition of Cretaceous sediments. The weathering took place under humid subtropical conditions.
The results of this study also show that under identical weathering conditions, different rock types alter to very similar weathering products.
Kelley, Lynne Irvine, "Kaolinitic weathering zone on Precambrian basement rocks, Red River Valley, eastern North Dakota and northwestern Minnesota" (1980). Theses and Dissertations. 157.