THE HIDDEN FALLS MEMBER: LITHOSTRATIGRAPHY AND DEPOSITIONAL INSIGHTS INTO THE PLATTEVILLE FORMATION, SOUTHEASTERN MINNESOTA

The Middle Ordovician Platteville Formation of Southeastern Minnesota has historically been understudied since its initial identification. The Hidden Falls Member of the formation is a dolomitic bed with a high insoluble content and a distinct recessive notch in outcrops. It displays unique physical and hydrostratigraphic properties in its conchoidal fracture, and the way it acts as an aquitard within the regional carbonate aquifer. This study of the Hidden Falls Member aims to fill a void in the geologic history of Southeastern Minnesota, as no explanation has been provided for its deposition and the irregular behavior of the member. I use petrographic and microscopy techniques to elucidate the post-depositional history of the bed in order to determine what alterations have occurred in the region. Petrographic analysis of the member has revealed a large (>25%) content of K-feldspars. These K-feldspars are of mixed detrital and authigenic origin; in conjunction with the secondary dolomite of the bed they tell the story of a unique sequence of multiple enrichment and/or recrystallization events. Primary sediments were initially altered and re-precipitated as dolomite before secondary enrichment. I propose a Mississippi Valley-type (MVT) deposit which introduced authigenic K-feldspar as rims on in situ detrital grains and as interstitial microcrystalline grains. This MVT deposit also precipitated pyrite as a secondary mineral to the member, forming euhedral crystals and replacing calcite in fossil fragments. Further isotope analysis of secondary K-feldspar crystals is necessary in order to confirm the presence of a MVT deposit and link it to known fluid enrichments.

Keywords: Platteville Formation, Middle Ordovician, Petrography, Dolostone, Authigenic minerals, K-feldspar, Mississippi Valley-type deposits