An investigation of the clay mineral suite in a type section of the Jackfork Group, Ouachita Mountains, Arkansas

Includes bibliographical references.Includes illustrations and maps.Correlation of the Carboniferous Jackfork Group is difficult because of similar lithology and a lack of fossil evidence. Variations in the clay mineralogy of this group were noted in the course of this study and may prove to be useful for correlation, providing they persist laterally. The study area was located in the Frontal Ouachita Mountains, Arkansas, in a type section defined by Morris (1964). Three clay mineral groups and mixed-layer material were present in most of the samples. In order of importance these were: illite, mixed-layer material, chlorite, and kaolinite. The relative percentages of these components varied widely, so that a general statement describing the clay mineral suite of the Jackfork Group is not feasible. One significant trend within the clay mineral suite was the gradual but persistent increase of illite up the section. This trend, coupled with the disappearance of kaolinite in the uppermost Jack- fork, may indicate a deepening basin and a more-distant shoreline. Salient percentages of kaolinite occurred in several thin zones in the type section. Since these were usually located near sandstone beds, post-depositional alteration was suspected. The mineral lepidocrocite was present in one well-defined zone and may be a useful stratigraphic marker. Paleosalinity studies were largely inconclusive, but indicated a semi-marine depositional environment.M.S. (Master of Science