The mineralogy and petrology of the Eau Claire formation west-central Wisconsin
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The Eau Claire Formation (Upper Cambrian) in west-central Wisconsin is primarily a very fine-grained, feldspathic sandstone interbedded with green-grey shale. The Eau Claire Formation can be subdivided into four lithic units recognizable on the basis of bedding style and shale content. These are, from the base upward: a lower thin-bedded unit, a lower massive unit, an upper thin-bedded unit, and an upper massive unit. The fine- and very fine-grained, feldspathic sandstones of the Eau Claire Formation grade downward into the underlying medium- to coarse-grained Mount Simon Formation, and the contact is placed at the top of the uppermost medium- or coarse-grained sandstone bed. In the outcrop area the Eau Claire-Galesville contact is unconformable. The unconformable relationship is indicated by an abrupt change in lithology and the greatly reduced feldspar content of the Galesville Formation. The unconformable relationship is further indicated in some outcrops by a basal conglomerate and an erosional surface showing several feet of relief. The fine-grained sandstones of the Eau Claire Formation contain much higher concentrations of detrital and authigenic feldspar and glauconite than either the Mount Simon or Galesville Formations. The amount of K-feldspar present in these units is inversely related to the mean grain size of the detrital sediments. The authigenic feldspar overgrowths have a high potassium content, a monoclinic structure, and a structural state similar to that of orthoclase. The glauconite occurs as green pellets with a 1M mica structure. A partially reducing environment of formation is assumed for the glauconite based on the presence of ferrous iron. It is proposed that the diagenetic conditions which occurred in the Eau Claire Formation were essentially constant throughout the Upper Cambrian. Petrographic observations indicate that the glauconite formed penecontemporaneously with sedimentation, prior to feldspar authigenesis. Dolomitization of the carbonate present and formation of the authigenic, K-feldspar overgrowths are believed to be linked processes. Authigenesis of the feldspar and dolomitization occurred in the presence of solutions rich in potassium, alumina, silica, and magnesium.