The Sedimentology and Petrology of the Upper Cambrian Mt. Simon, Eau Claire, and Galesville Formations in Southeastern Minnesota

Abstract

The Mt. Simon Sandstone, Eau Claire Formation, and the Galesville Sandstone comprise the Upper Cambrian Dresbachian Stage, and are found as subsurface strata in southeastern Minnesota with scattered small outcrops along the Mississippi and St. Croix Rivers. The formations are relatively flat-lying units located in a structural lowland termed the Hollandale Embayment. The Mt. Simon has an unconformable contact with the underlying Precambrian basement. The Eau Claire is conformable with the underlying Mt. Simon and the overlying Galesville. The Galesville has an unconformable contact with the overlying Ironton Sandstone of the Franconian Stage. The Dresbachian formations thicken from northwest to southeast direction across the state, cross-bedding has a south-southwest direction of dip. The Mt. Simon is primarily a medium- to coarse grained, moderately sorted, thick bedded orthoquartzitic sandstone with some feldspathic sandstone and shale laminae. The Eau Claire contains fine-grained, well-sorted, thin bedded orthoquartzites, arkoses, and quartzose arkoses, and laminated shales. The formation is very fossiliferous and glauconitic. The Galesville consists of medium-grained, well-sorted, thick bedded orthoquartzitic and feldspathic-quartzose sandstones. Petrology of the three formations is simple. The mineralogy is dominated by quartz with secondary feldspars. Mic, clay matrix, carbonate, pyrite, and feldspar cement, glauconite, and fossils are accessory components. Heavy minerals include zircon (dominant in most samples), tourmaline, garnet, rutile, apatite, and opaques, and are well-rounded for the most part. Fossils include trilobites, brachiopods, mollusca, and worm burrows. The Mt. Simon Sandstone, on the basis of lithology, grain size and sorting, bedding and cross-bedding, is a nearshore, highly turbulent, shallow water deposit. The Eau Claire Formation, on the basis of lithology, grain size and sorting, bedding and cross-bedding, and fossils and glauconite, is probably an offshore shelf deposit of quiet, relatively deep water. Characteristics, however, also resemble tidal flat deposits, which the formation may be in part. The Galesville Sandstone, on the basis of lithology, grain size and sorting, bedding and cross-bedding, is a nearshore deposit of turbulent, shallow waters.