CORRELATION OF (MOHAWKIAN) K-BENTONITES BY USING APATITE CHEMISTRY: IMPLICATION FOR DECORAH FORMATION CARBONATE AND SHALE DEPOSITIONAL SEQUENCES

Several Middle-Late Ordovician K-bentonites are widely distributed across eastern North America extending to the northern Mississippi Valley. Three of these – Millbrig, Elkport, and Dickeyville – were chemically identified and differentiated using primary apatite phenocrysts. This work supports a new stratigraphic framework.

Samples were collected from four localities along a NW-SE cross section in WI, IA, and MN. A total of 115 apatite grains (between 60 and 200 microns) were handpicked and mounted for electron microprobe analysis with an average of five points analyzed per grain. Grains were probed for the following elements: Cl, Mg, Mn, F, Si, Y, La, Ce, Nd, Sm, Gd, Dy, and Fe with Cl, Fe, Mg, and Mn concentrations being the most distinct and most useful discriminators. The results show that the Millbrig and Elkport K-bentonite are clearly distinguishable. The Dickeyville K-bentonite has a bimodal chemical distribution, some distinguishable and some less so in their Cl, Fe, Mg, and Mn concentrations. The successful discrimination of these closely spaced bentonites demonstrates again that apatite chemistry is a useful, relatively easy, and reliable correlation tool.

The chemical identification of these bentonites supports a new interpretation of the depositional history of the Decorah Formation in the northern Mississippi Valley. Previous work shows lateral facies changes between shales and carbonates. The new correlation shows a reciprocal facies pattern, with a lower shale-dominated unit overlain by an upper carbonate–dominated unit. We interpret this pattern to result from a water depth gradient, deepening to the northwest into the Hollandale Embayment. During the lower shale facies unit, fine clastics from the Transcontinental Arch were deposited in deep water, some in dysoxic conditions. These shales pinch out to the southeast against the Wisconsin Arch. Flooding of the up-ramp allowed for deposition of the carbonate-dominated overlying unit and down-ramp progradation and thinning into the embayment.