Stable Isotope Paleohydrology of Pedogenic Carbonates in the Wayan and Blackleaf Formations (Mid-Cretaceous) of Idaho and Montana

Abstract

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 were measured and sampled from the Caribou Basin in Bonneville County, Idaho. The Wayan sections consist of 4 to 5 meter-thick intervals of stacked meter-scale mudstone paleosols, separated by meter-scale sandstone-siltstone beds. Steeply-dipping stratigraphic sections from the Blackleaf Formation were measured and sampled near Lima in Beaverhead County, Montana. In order 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. In addition, intact measured sections from both units were sampled for organic stable carbon isotope profiles. The organic carbon samples are being analyzed for δ13C values at KU. Carbon isotope profiles are used for correlating sections to the global Cretaceous C-isotope stratigraphy. Petrographic studies of pedogenic carbonate nodules from the Wayan & Blackleaf fms, and δ13C and δ18O analyses are being used to interpret stable isotope paleohydrology. Major scientific questions include: (1) Do calcites from these units produce estimates of groundwater δ18O values that are similar to those from pedogenic siderites from the same North American paleolatitudes?; and (2) Will diagenetic trends from pedogenic carbonates permit quantitative evaluation of the evaporation deficit in the orographic rain shadow of the proximal Cretaceous North American foreland basin?

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015