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The Geology of the New Richmond Sandstone

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The Geology of the New Richmond Sandstone,Prairie du Chien Group,Southeastern MinnesotaCristina RobinsSenior Integrative ExerciseMarch 9, 2005Submitted in partial fulfillment of the requirements for a Bachelor of Arts degree,Carleton College, Northfield, MN, USAThe Geology of the New Richmond Sandstone,Prairie du Chien Group, Southeastern MinnesotaCristina RobinsCarleton CollegeSenior Integrative ExerciseMarch 9, 2004Advisor:Dr. Clint Cowan, Carleton CollegeAbstract:This study examines the New Richmond Sandstone, a relatively unknown heterolithicassemblage of sandstone, carbonate, and shale that is commonly considered part of theShakopee formation of the Prairie du Chien group. Stratigraphic columns constructedfrom seven outcrops of the New Richmond in southeastern Minnesota and northeasternIowa show that it varies between 7 m and 20 m in thickness within the study area andconsists of two facies: the Prairie Island and the Root Valley. Previous studies foundthat the New Richmond was deposited in part through eolian action. SEM studies ofindividual grains did not confirm this.Keywords: New Richmond, sandstone, Shakopee, Prairie du Chien, Ordovician,stratigraphy, SEM data, MinnesotaiTable of ContentsIntroduction..................................................................................................................... 1Literature Review .................................................................................................. 1The Prairie du Chien Group .................................................................................. 4Surrounding Lithologic Units ................................................................................ 4Methods ........................................................................................................................... 5Field...................................................................................................................... 5SEM ...................................................................................................................... 5Results ............................................................................................................................. 7Stratigraphy .......................................................................................................... 7Lithofacies............................................................................................................. 10SEM ...................................................................................................................... 14Discussion ........................................................................................................................ 14Stratigraphy .......................................................................................................... 14Lithofacies............................................................................................................. 16SEM ...................................................................................................................... 18Conclusions...................................................................................................................... 19Acknowledgements.......................................................................................................... 19Works Cited .................................................................................................................... 21Appendices: ..................................................................................................................... 23Stratigraphic Columns........................................................................................... 23Outcrop Locations ................................................................................................. 24iiIntroductionThe Prairie du Chien (PDC) Group, of Early Ordovician age, crops out insouthern Minnesota, northeastern Iowa, and southern Wisconsin, and extends into partsof the subsurface of Michigan and Illinois, ranging from ~60 – 600m in thickness (Smithet al., 1996). It is composed of three main members: the Oneota Dolostone, the NewRichmond Sandstone, and the Shakopee Dolostone (Figure 1). Extensive study has beencompleted on the dolostone members of the PDC (Johnson and Simo, 2002; Smith et al.,1996; Smith and Simo, 1997). There is comparably little information available on theNew Richmond, and of the few studies available, their results are somewhat conflicting.This study adds to the pool of information and clarifies some points about the NewRichmond, providing a broad overview of the member by using stratigraphy andscanning electron microscopy [SEM] data to determine details of deposition andcomposition.Literature ReviewIn 1882 L. C. Wooster proposed that the New Richmond beds be considered anindividual member of the what was at the time called the Lower Magnesian Group (Shea,1960; Squillace, 1979; Ulrich, 1924). H.F. Bain, in 1906, changed the name of the LowerMagnesian Group to the Prairie du Chien Formation, later evolving into the Prairie duChien Group (Needham, 1932; Shea, 1960; Squillace, 1979; Ulrich, 1924). For a moredetailed history of the nomenclature of the New Richmond, see Ulrich (1924), Needham(1932), Shea (1960), or Squillace (1979).11Group Formation LithologyMaquoketaSt. LawrJordanOneotaShakopeeSt. PeterGlenwoodPlattevilleDecorahGalenaSilurianShale, dolomiteQuartz areniteDolomite to sandy limestoneSandy to silty dolomiteQuartz areniteSiltstone, shale, dolomiteDolomite, limestone, shaleLimestone, shaleDolomite, limestone, shaleDolomitic siltstoneU. Cam.SinnipeeMiddle OrdovicianLower OrdovicianTremp. Prairie du Chien AncellUpper OrdovicianDolomite50 mFigure 1: Generalized stratigraphic column of the Prairie du Chien and surrounding units. Modified from (Smith and Simo, 1997).New RichmondQuartz arenite23The New Richmond Member consists of a thin (maximum ~20 m) heterolithicassemblage of sandstone, carbonate, and shale, and is often considered part of theShakopee Formation of the PDC Group (Davis, 1966; Johnson and Simo, 2002; Runkel,2002; Squillace, 1979). Defining characteristics of the New Richmond include eoliancross-bedding and sandy stromatolites (Austin, 1974; Davis, 1968; Shea, 1960; Squillace,1979). Where present, it is occurs between the two dominantly dolomitic units of thePDC, the Oneota and upper Shakopee. The New Richmond is thickest in southeasternMinnesota, in the


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