A 15-MYR LATE CRETACEOUS CARBONATE CARBON ISOTOPE CHEMOSTRATIGRAPHY FROM THE WESTERN INTERIOR BASIN FOR GLOBAL CORRELATION

Global climatic and geochemical events (e.g. Oceanic Anoxic Events) preserved in Late Cretaceous marine sequences have proven challenging to investigate in coeval terrestrial environments such as the Songliao Basin. This is due to limited age control stemming from a lack of biostratigraphy and isolated horizons with datable ashes. Carbon isotope (δ¹³C) chemostratigraphic correlations from locations with strong time control, such as the N. American Western Interior Basin (WIB), may provide an alternative method for developing chronostratigraphic frameworks since marine and terrestrial carbon reservoirs are linked via the atmosphere. A relatively continuous 15-Myr (Mid-Cenomanian to Early Campanian) carbonate carbon isotope chemostratigraphy (δ¹³C_carb) from the Denver Basin’s Aristocrat Angus 12-8 Core (AA-core) is presented spanning the Greenhorn and Niobrara Cyclothems. Results are compared with prior bulk organic carbon isotope (δ¹³C_org) measurements to more accurately understand variation in δ¹³C of dissolved inorganic carbon (δ¹³C_DIC) in the WIB, as well as underlying sedimentological processes controlling the δ¹³C records.

Roughly 325 powdered samples from the AA-core, previously analyzed for δ¹³C_org, %TOC, and %carbonate, were analyzed for δ¹³C_carb and δ¹⁸O_carb. Excursions of δ¹³C_carb and δ¹³C_org occurring at the same horizons add confidence to an interpretation of changing δ¹³C_DIC within the seaway, while discrepancies in excursions are attributed to either variation in fractionation between DIC and photosynthate, diagenetic alteration of carbonates, or changing organic matter type. Absolute δ¹³C_carb values and covariation of δ¹³C_carb and δ¹⁸O_carb data tested among windowed depth intervals are used to probe for evidence and extent of diagenesis in the AA-core. When combined with δ¹³C_org records, δ¹³C_carb chemostratigraphies from the WIB have the potential to distinguish changes in marine δ¹³C_DIC from localized alteration of carbonate or changes in type of organic matter. This analysis better constrains the Late Cretaceous δ¹³C reference curve for the WIB permitting correlation to chemostratigraphies in disparate basins where chronostratigraphic frameworks are limited by lack of bentonites or biostratigraphy.