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 within the Codell Sandstone Member. Petrographic and stable isotopic data on diagenetic carbonates in the Carlile provide evidence for the early establishment of meteoric phreatic environments (δ¹⁸O = −7 ‰), although modified marine phreatic environments (δ¹⁸O = −2.5 ‰) are also represented. Variable degrees of fluid-rock interaction resulted in highly variable carbon isotopic compositions of diagenetic carbonates from both environments. Skeletal calcites from inoceramid and ostreid bivalves that inhabited dysaerobic benthic environments have anomalously low δ¹⁸O values and do not provide an accurate record of ambient seawater chemistry because of vital effects, possibly related to the activity of chemosynthetic symbiotic bacteria. Data from diagenetic carbonates in the Carlile are consistent with a proposed long-term secular trend of increasing oxygen isotopic ratios in Turonian (−2.5 ‰) to Maastrichtian (−0.5 ‰) marine carbonates from the Western Interior Seaway, probably related to global cooling. Petrographic and geochemical evidence for deep infiltration of dilute meteoric fluids during late Turonian subaerial exposure of Carlile strata, and relatively small marine phreatic–meteoric phreatic δ¹⁸O shifts in carbonate cements, are suggestive of humid paleoclimates along the eastern margin of the seaway, and conflict with prior suggestions of semiarid to slightly evaporitic paleoclimates in the area.