An investigation of unusual shale intervals within a echinoderm-stromatoporoid carbonate buildup from the Kimmswick Formation, Jefferson County, Missouri

The Kimmswick Formation (Middle Ordovician) is a fossiliferous unit representing a shallow-marine platform composed of extensive cross-bedded crinoid grainstones. The Kimmswick has previously been described as a monotonous crystalline limestone with little variation in its sedimentology or faunal assemblage. However, this study documents that the depositional history of the Kimmswick is more complex with the appearance of minor shale lenses associated with a echinoderm-stromatoporoid carbonate buildup that show reef-like characteristics. There are three distinct major shale lenses identified at three separate outcrops at one locality near House Springs, Missouri. Two of the shale lenses appear similar in nature; small lenses of red, waxy, laminated shale. The third shale unit is gray, siltier, consolidated, and includes large carbonate clasts. Based on these occurrences within the Kimmswick, three hypotheses were made on their origin; (1) Primary shale with secondary sub-aerial exposure, (2) secondary infill through karstic deposition, and (3) alteration of the House Springs K-bentonite into three different shale units. Understanding which of these hypotheses is correct is essential in determining the aquifer quality of the Kimmswick. Regionally, the House Springs K-Bentonite, a nearly isochronous rock unit used for precise correlation, is not well-preserved in the Kimmswick with only a few documented occurrences. The goal of this project is to determine which of the three hypotheses is correct. The study was conducted in two parts. Fieldwork yielded stratigraphic columns for each of the outcrops yielding 10 bulk samples. The samples were collected for analysis using X-Ray Fluorescence (XRF) for geochemical quantification of the rock and X-Ray Diffraction (XRD) to understand the clay mineralogy. Based on XRD analysis, the red and gray shale units have different origins. The XRF analysis provided insight into the geochemical composition of the shales. This combined data will aid in the determination of which hypothesis is correct. Further determination of the origin of the shales will aid in the understanding of the Kimmswick as an aquifer for groundwater flow.