CHEMOSTRATIGRAPHIC PROFILES OF THE CRETACEOUS OAE 1D, MID-CENOMANIAN EVENT, AND OAE2 FROM LONG-LINE DRILLCORES, CRATONIC MARGIN OF THE NORTH AMERICAN WESTERN INTERIOR BASIN

High-resolution organic δ¹³C profiles of Cretaceous units from three long-line cores in northern and central Kansas (KGS Gaydusek #1, KGS Kenyon #1, and KGS Jones #1) reveal records of the Albian-Cenomanian OAE 1d, Mid-Cenomanian Event (MCE), and the Cenomanian-Turonian OAE2 in strata of the Dakota, Graneros, and Greenhorn formations. The most expanded records of the OAE 1d and MCE occur in the more basinward KGS Jones #1 core. The pronounced negative carbon isotope excursion (CIE) of the OAE 1d (Gröcke et al., 2006, Geology 34:193-196; Richey et al., 2018, EPSL 491:172-182) occurs in all three cores, but the overlying positive CIE is truncated in the KGS Kenyon #1 core, as is the case in the Rose Creek Pit exposed along the Kansas-Nebraska border. The MCE (~ 97 Ma) is preserved in the marine Graneros Shale, just below the geographically-widespread X-bentonite (95.53 ± 0.36 Ma). Organic carbon δ¹³C profiles of the MCE in the cores are characterized by abrupt transitions from baseline δ¹³C values ranging from -26‰ to -25‰ VPDB below the MCE, to baseline δ¹³C values ranging from -28‰ to -27‰ VPDB above the MCE. The positive CIE of the MCE rises to peak δ¹³C values of -22‰ to -20‰ VPDB. The abrupt shift in baseline δ¹³C values from below to above the MCE is a unique feature of the cratonic margin of the WIS, related to abrupt landward shift of the eastern shoreline during eustatic sea-level rise. Rock Eval pyrolysis data show that the δ¹³C shift coincides with abrupt increase in Hydrogen Indices of sedimentary organic matter in the Graneros Shale. This shift in δ¹³C values reflects changes in the balance of organic matter burial in the Graneros Shale from terrestrially-dominated below, to marine-dominated above. The MCE flooding horizon is characterized by cone-in-cone (CIC) limestones with an associated benthic molluscan fauna—in both core and outcrop. We interpret the CIC limestones to be constructive products of cold fresh-water seeps on the sea floor based on stable isotope compositions. CIC calcites have δ¹³C values that range from -22 to 0‰ VPDB, and δ¹⁸O values that range from -8 to -4‰ VPDB. These data are arrayed in discrete meteoric calcite line patterns related to successive increments of fibrous calcite growth resulting from the discharge of regional groundwater flow systems that were recharged on adjacent land areas.