RI-40 Sedimentary Rocks of Dresbachian Age (Late Cambrian), Hollandale Embayment, Southeastern Minnesota

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RI-40 Sedimentary Rocks of Dresbachian Age (Late Cambrian), Hollandale Embayment, Southeastern Minnesota

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Minnesota Geological Survey




The Dresbachian Mt. Simon Sandstone, Eau Claire Formation, and Galesville Sandstone of the Hollandale embayment of southern Minnesota are divisible into eleven major lithofacies and several subfacies. Because the formations are almost exclusively in the subsurface in Minnesota, lithofacies descriptions are based on cores, well cuttings, and geophysical logs. Along the eastern side of the Hollandale embayment, the lower Mt. Simon Sandstone consists of a thin basal conglomerate lithofacies overlain by medium- and large-scale, crossstratified and planar-stratified sandstone. Middle Mt. Simon is principally interbedded, coarsely interlayered sandstone, siltstone, and shale; and thin- to medium-bedded, structureless or crossstratified sandstone. The upper Mt. Simon is structureless sandstone with Skolithos and shelly (coquinoid) sandstone. These lithofacies resemble those from outcrops in western Wisconsin described by Driese and others (1981). Toward the west and south in south-central Minnesota, at the embayment center, mediumand large-scale, cross-stratified sandstone dominates in the Mt. Simon. Along the western side of the embayment, structureless sandstone dominates. There are fewer thin shale and siltstone beds in the Mt. Simon near the embayment center than along the eastern side of the embayment. Near the embayment center, the uppermost Mt. Simon Sandstone and basal Eau Claire Formation contain ferroan oolites and coated grains that are scattered in some beds and are the principal sand-sized particles in others. Ferroan oolites and coated grains are not observed in outcrop. Along the eastern side of the Hollandale embayment, the Eau Claire is composed principally of mixed sandstone and shale lithofacies and greensand lithofacies resembling Eau Claire lithofacies that crop out in western Wisconsin (Huber, 1975), especially in cores along structural strike with the Wisconsin outcrops. Red sandstone and shale lithofacies and dolostone lithofacies are at the base of the Eau Claire in south-central Minnesota. These are overlain by a ripple-cross-stratified or troughcross- stratified subfacies of the greensand lithofacies that is much thicker than laterally equivalent beds of greensand lithofacies to the north and east. The Galesville Sandstone, mostly structureless, planar-stratified, or trough-cross-stratified sandstone, appears to be conformable and interbedded with the Eau Claire Formation. The upper part of the Eau Claire and Galesville appear to be part of an upward coarsening sequence. There is evidence of slight disconformity between the Galesville and overlying Ironton Sandstone. The basal conglomerate of the Mt. Simon is interpreted as a braided fluvial deposit. Medium- to coarse-grained sandstone lithofacies of the lower Mt. Simon are interpreted as braid plain, braid delta, and littoral deposits. Fine- to medium-grained sandstone beds and shale beds in the middle Mt. Simon are interpreted as distal braid delta deposits. Sandstone beds in the upper Mt Simon are interpreted as sand shoals and tidal flat deposits. Beds of interbedded, finegrained sandstone and shale in the basal Eau Claire that are tidal flat deposits culminate this initial prograding sequence. Toward the end of the sequence deposition, ferroan oolites formed nearshore, where some were reworked by shifting tidal channels. Red sedimentary rocks were deposited in high tidal flat, channel, and deltaic environments. Carbonate rock was deposited in the southwestern Hollandale embayment as detrital sedimentation ended. The greensand lithofacies of the medial Eau Claire Formation, which records marine transgression at the base of the next prograding sediment sequence, is succeeded by shaly lagoonal deposits, sandy or shaly tidal flat deposits (upper Eau Claire Formation), and sandy foreshore or shoreface deposits (Galesville Sandstone). Variation of sandstone composition reflects selective mechanical reduction of contained potassium feldspar grains as observed by Odom (1975). Medium- to coarse-grained sandstone is quartzose, and very fine to fine-grained sandstone is highly feldspathic. This variation, reflected on gamma logs, helps to distinguish lithofacies. . Sandstone in core from southwestern Minnesota contains accessory minerals, including diaspore, that indicate the contribution of sediment from the Proterozoic Sioux Quartzite to the Mt. Simon Sandstone.



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Mossler, J.H., 1992, Sedimentary Rocks of Dresbachian Age (Late Cambrian), Hollandale Embayment, Southeastern Minnesota, Minnesota: Minnesota Geological Survey Report of Investigations 40, 71 p.

Suggested citation

Mossler, John H.. (1992). RI-40 Sedimentary Rocks of Dresbachian Age (Late Cambrian), Hollandale Embayment, Southeastern Minnesota. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/60785.

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