Date of Award
Doctor of Philosophy (PhD)
The goal of this study was the determination of a similar depositional sequence for sediments from a number of shallow basins in southwestern North Dakota and the comparison of this sequence with other work either in the area or in similar geographic environments.
Samples were obtained by augering, coring, and trenching selected basins. They were analyzed by means of: core description; detailed texture analyses on selected cores by sieve, pipette, and SediGraph techniques; and mineralogy by x-ray diffraction. Computer analyses were used to characterize grain-size distributions and to correlate texture and mineralogy by cross-association.
The results of the study verified a similar depositional sequence in the basin sediments. The sediments exhibited two dominant colors, olive and brown. The reduced olive colors were lower in the basin sediments and were associated with silt and clay-size material. Overlying these sediments was the zone of brown, oxidized sediments, which in basin margin or near-shore locations was composed of sand-size particles. In central basin locations the near-surface sediments were oxidized silt and clay particles. The sand layer, therefore, occurred as a wedge on the upwind sides of the basins. This sequence with minor variations was repeated from basin to basin.
The mineralogy of the sediments was simple, with quartz the most commonly occurring non-clay, and montmorillonite the most common clay mineral. The mineralogic association of the clay minerals suggests their presence is largely sedimentologic and not pedologic.
The analyses provided three major conclusions. First, the sediments in the basins have a related geologic history as verified by the depositional sequence. This sequence fits the post-glacial climatic sequence worked out for this area as well as Texas and New Mexico. Briefly, that sequence consisted of a period of moist climatic conditions, followed by a period of extremely dry climatic conditions. This dry period was followed by a return to more moist climatic conditions, which have continued to the present. Second, the quartz/feldspar ratios and oxidized zones at the bedrock contact indicate the basins are significantly older than the sediments in them, which are primarily Holocene in age. Third, the bedrock contact determination so critical to the study can be made on the basis of the quartz/feldspar ratios, oxidized zones, physical character of the bedrock and the presence of in situ lignite.
Seidel, Robert E., "Deflation basin stratigraphy: southwestern North Dakota" (1986). Theses and Dissertations. 272.