Stable isotope paleohydrology of pedogenic carbonates in the Wayan Formation (Albian) from the wedge-top depozone in the North American Cretaceous Western Interior Basin

Abstract

Determining the presence and effects of orogenic rain shadows on paleoclimate is challenging, but critical to understanding overall moisture and heat transport. This thesis presents new data and interpretation for the climate system of the Cretaceous greenhouse world, which is a major keystone in Earth system science. Stable isotopic paleohydrologic data on mid-Cretaceous paleosols spanning from paleoequatorial sites in Colombia to paleoarctic latitudes in Alaska have been used to constrain the oxygen isotope mass balance of the Albian hydrologic cycle. At mid-latitudes (40-50°N paleolatitude), sideritic paleosols predominate, indicating paleoenvironments with positive precipitation – evaporation (P-E) balances, local exceptions with negative P-E balances occur on the immediate leeward side of the Sevier Orogen, where calcic paleosols in the wedge-top depozone record paleoenvironments with negative P-E balances in the orographic rain shadow. Stratigraphic sections in the Wayan Formation of Idaho (WF) were sampled from the wedge-top depozone. The units consist of stacked m-scale mudstone paleosols separated by m-scale sandstone-siltstone beds. Sections were sampled for organic carbon isotope profiles, and B-horizons from 6 well-developed paleosols were sampled for detrital zircons to determine maximum depositional ages. The first of these from the WF has produced a U-Pb concordia age of 101.0 ± 1.1 Ma, placing it in the uppermost Albian. This same WF section has produced a stratigraphic trend of upwardly decreasing δ13C values ranging from -24 upwards to -27‰ VPDB, suggesting correlation to the late Albian C15 C-isotope segment. Pedogenic carbonates from the WF principally consist of micritic calcite, with carbon-oxygen isotope values that array along meteoric calcite lines (MCLs) with δ18O values that range between -9.47 up to -8.39‰ VPDB. At approximately 42°N paleolatitude, these MCL values produce calculated paleoprecipitation values of -8.12 to -7.04‰ VSMOW, a range that is consistent with the estimates produced from other proxies at the same paleolatitudes across North America. These results indicate that despite the orographic rain shadow effect, the processes of meridional atmospheric moisture transport in this locale were similar to those in more humid mid-latitude paleoenvironments elsewhere in the continent.