The Golden Valley Formation of North Dakota crops out in isolated remnants over a large area of southwestern North Dakota. The formation has two members, the lower Bear Den Member, which is characterized by kaolin and bright colors and the upper Camels Butte Member, which is characterized by micaceous sand and clay.
Two different theories, either in place weathering or detrital deposition, have developed as to the origin of the kaolinite in the lower member. Early workers suggest that the kaolinite of the member is detrital in origin for the following reasons:
1. No unconformity is present at the top of the Bear Den Member.
2. The unit is too uniform and widespread, and has a stratiform nature that cannot be explained by weathering,
3. Plagioclase, amphibole, pyrite, and siderite are present but would not have been able to withstand weathering.
4. Kaolinite and muscovite show parallel alignment.
This study reviews these ideas and more recent work that suggests that the kaolinite of the Bear Den Member was formed in place by weathering. ln addition, analyses made with the scanning electron microscope, microprobe, and X-ray diffractometer give the following evidence that this member has been formed by a weathering process:
1. The mineralogy and mineral profiles are consistent with those of a weathered horizon.
2. Favorable climatic conditions for weathering were present during the Eocene.
3. The topography and thickness relationships of the Bear Den are similar to that of a soil.
4. The kaolinite horizon transgresses Sentinel Butte strata.
5. Scanning electron photomicrographs illustrate a varying degree of crystallinity in the kaolinite.
6. Chemical data show that leaching of the Bear Den sediment has occurred.
7. The Bear Den Member compares favorably with similar units that have been interpreted as weathered horizons.
A review of the evidence for and against the theory of weathering indicates that the kaolinite of the Bear Den Member formed in place by weathering processes
