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 (δ 18 O = −7 ‰), although modified marine phreatic environments (δ 18 O = −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 δ 18 O 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 δ 18 O 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.

The Manson Impact Structure (MIS) in north-central Iowa, with a diameter of 35 km (22 mi), is the largest impact structure known in the United States. 40 Ar/ 39 Ar isotope studies of microcline grains from the MIS yielded an age of 65.7 ± 1.0 Ma, an age that is indistinguishable from the age of the K-T boundary impact layer in western United States exposures (Kunk et al., 1989). Izett (1990) and Sharpton et al. (1990) compared rock and mineral grains recovered from the K-T impact layer in exposures around the world with rocks recovered from the Manson structure and concluded that the MIS is the probable source of these grains. Stratigraphic studies of rock samples from water wells in the structure led to the discovery of a terrane of down-dropped blocks within the ring-graben complex, immediately inside the crater’s perimeter. Within this complex, an extensive area of thick Upper Cretaceous marine rocks were identified, apparently structurally preserved during the formation of the crater. These include rocks tentatively identified as Graneros Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, and Pierre Shale, and, when combined with the Dakota Formation rocks also encountered, total a thickness of at least 189 m (630 ft) of Upper Cretaceous strata. The marine units are not present in the immediate region outside of the MIS, having been eroded back to their present-day outcrop limits, as much as 300 km (185 mi) west of the MIS. Studies of these structurally preserved rocks provide valuable information about the extent and nature of Upper Cretaceous marine units near their eastern depositional limits. Additionally, preliminary studies indicate that in some areas of the MIS these units are overlain by impact ejecta deposits, implying the “instantaneous” burial of the Late Cretaceous surface and the preservation of an environmental record that has been eroded elsewhere along the eastern margin areas of the Western Interior.