STABLE ISOTOPE PALEOHYDROLOGY OF PEDOGENIC CARBONATES IN THE WAYAN AND BLACKLEAF FORMATIONS (MID-CRETACEOUS) OF IDAHO AND MONTANA

Stable isotopic paleohydrologic data have been published from mid-Cretaceous (Albian-Cenomanian) paleosol locales in the North American Western Interior Basin. From 40°N to 50°N paleolatitude, nearly all paleosols contain pedogenic siderite (FeCO₃), indicative of terrestrial paleoenvironments with strongly positive precipitation-evaporation (P-E) balances. Local exceptions occur in a north-south trending zone on the immediately leeward side of the Sevier Orogen, where calcic paleosols containing pedogenic calcite (CaCO₃), in the form of carbonate nodules, indicate terrestrial paleoenvironments with negative P-E balances; the record of an orographic rain shadow. Calcic paleosols in the Wayan and Blackleaf formations are being studied to produce carbonate δ¹³C and δ¹⁸O datasets to contrast with paleosol paleohydrology from elsewhere in the Cretaceous of North America.

Stratigraphic sections from the Wayan Formation (WF) were measured and sampled from the Caribou Basin in Bonneville County, Idaho and. stratigraphic sections from the Blackleaf Formation (BF) were measured and sampled near Lima in Beaverhead County, Montana. Both sections consist of 4 to 5 meter-thick intervals of stacked meter-scale mudstone paleosols, separated by meter-scale sandstone-siltstone beds. Intact measured sections from both units were sampled for organic stable carbon isotope profiles.

To constrain chronostratigraphic relationships among all sampled sections in both units, 11 paleosol B-horizons with well-developed soil structure were sampled for volcanogenic zircons, and are being analyzed for U/Pb dates at the Isotope Geochemistry Laboratory at KU, the first of which from the WF has produced an age of 101.24±0.96 Ma. This same WF section has a trend of δ¹³C values decreasing upward from -24 to -27‰ VPDB, suggesting correlation to the late Albian interval above the C15 carbon isotope feature of Bralower et al. (1999).

Pedogenic carbonates from the WF and BF principally consist of micritic calcites that yield meteoric calcite lines (MCLs) with δ¹⁸O values that range between -10.59 up to -8.39‰ VPDB. These MCL values at 42-43°N paleolatitude produce estimated paleoprecipitation values of -9.24 to -7.04‰ VSMOW, which is consistent with the estimates of Suarez et al. (2009) at the same paleolatitudes.