Mid-Cretaceous paleosols of the Western Interior Basin: Paleohydrologic and stratigraphic implications

Recent paleoclimatological research conducted by University of Iowa investigators has focused on reconstructing meteoric δ18O paleolatitudinal gradients from sphaerosiderite proxy records on the eastern margin of the Cretaceous Western Interior Basin (KWIB) (30–50° N paleolatitude). The goals of this investigation were to: (1) extend the meteoric δ 18O latitudinal transect along the western margin of the KWIB into high paleolatitudes (British Columbia [52°N] and North Slope, Alaska [75°N]); (2) develop a mass balance model to quantify changes in the mid-Cretaceous hydrologic cycle; and (3) assess problems with the sphaerosiderite paleoclimatic proxy.; Meteoric δ18O values compiled from sphaerosiderites throughout the KWIB (34°N to 75°N paleolatitude) exhibit a steeper, more depleted latitudinal trend than modern (predicted) values (3.0‰ [34°N latitude] to 9.7‰ [75°N] lighter). The δ 18O gradient is also steeper and more depleted (5.8‰ [34°N] to 13.8‰ [75°N] lighter) than a predicted mid-Cretaceous gradient. The steeper and more depleted (relative to the modern gradient) meteoric sphaerosiderite δ 18O latitudinal gradient resulted from increased air mass rainout effects in KWIB coastal areas. The sphaerosiderite δ18O values were used to constrain a mass balance model of the hydrologic cycle, and quantify KWIB precipitation rates.; Thirty sphaerosiderite-bearing paleosols of the Boulder Creek Formation (MD 80-08 core) of British Columbia typically have invariant δ 18O and variable δ13O values (meteoric sphaerosiderite lines; MSLs). The MSLs indicate meteoric groundwater compositions ranged from −11‰ to −16‰ (SMOW). The 69.75–68.6 meter depth interval has a +1.31‰ enrichment in the average sphaerosiderite δ18 O values. The micromorphology, δ18O excursion, siderite cation ratios, and pyrite abundance in this interval reflects marine influence on pedogenesis.; Siderite-bearing paleosols of the Nanushuk Group (N. Slope Alaska) provide a high paleolatitude (75°N) meteoric δ18O proxy record. Siderite δ18O values from four paleosol horizons in the NPR Grandstand #1 Core, range between −17.6‰ and −14.3‰ (PDB). The δ13C values range from −4.6‰ to +10.8‰ (PDB).; Examples of diagenetically-altered sphaerosiderites from the Dakota, Swan River, and the Success S2 Formations, were examined to assess preservation of early-diagenetic δ18O and δ13O values. Despite later meteoric diagenesis, early-diagenetic meteoric δ 18O values were recovered for all three samples.