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Cenomanian to Turonian marine strata exposed in the Big Sioux River Valley of Iowa and South Dakota contain numerous concretion horizons. Concretion-bearing strata from the upper Dakota (type), Graneros, Greenhorn, and Carlile Formations were studied near their erosional limits along the eastern margin of the Western Interior Seaway. Concretion horizons formed in beds with concentrations of fossiliferous debris, including fish-scale siltstones with starved megaripple bedforms, inoceramid and inoceramid-ostreid packstones, and dense bioturbated accumulations of ammonoids and inoceramid, ostreid, and other bivalves. Concretions in the Carlile contain paleontologic and sedimentologic indicators for a general upward shallowing sequence, with nonmarine environments represented...

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