EXPRESSION OF THE MIDDLE CENOMANIAN EVENT ALONG THE CRATONIC MARGIN OF THE CRETACEOUS WESTERN INTERIOR SEAWAY (WIS)

The Middle Cenomanian Event (MCE; ~ 97 Ma) is the first positive Carbon Isotope Excursion (CIE) that occurs stratigraphically below the positive CIE of the well-known OAE2 Cenomanian-Turonian boundary event (~ 94.5 Ma). In the mid-continent USA, the stratigraphic record of the MCE is preserved in the marine Graneros Shale, just below the geographically-widespread X-bentonite (95.53 ± 0.36 Ma). The interval of the MCE is penetrated by three long cores in Kansas, the KGS Gaydusek #1 (Washington Co.), KGS Kenyon #1 (Republic Co.) and KGS Jones #1 (Lincoln Co.). Organic carbon δ¹³C profiles of the MCE in these 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 peak of the MCE in the Gaydusek #1 core rises up to a δ¹³C value of -22‰ VPDB. The abrupt shifts in baseline δ¹³C values from below to above the MCE may be a unique feature of the eastern margin of the WIS, probably related to an abrupt landward shift of the gently westward-sloping cratonic shoreline during an abrupt eustatic sea-level rise. We interpret the shift in baseline δ¹³C values to reflect a change in the balance of organic matter burial in the Graneros Shale from terrestrially-dominated below, to marine-dominated above. The abrupt MCE flooding event is associated with unusual paleohydrologic phenomena. The MCE horizon is characterized by cone-in-cone limestones associated with a benthic molluscan fauna—in both core and outcrop. We interpret the fibrous calcites of the cone-in-cone limestones to be products of cold fresh-water seeps on the sea floor. These calcites have δ¹³C values that range from -22 to 0‰ VPDB, and δ¹⁸O values that range from -8 to -4‰ VPDB. Data from these calcites are arrayed in discrete meteoric calcite line (MCL) patterns that are suggestive of successive increments of fibrous calcite growth resulting from unique groundwater δ¹⁸O compositions. The MCL δ¹⁸O values reported here are closely similar to a group of MCLs reported by Phillips et al. (2007; Palaeo-3 246:367-389) from the Upper Albian D₂ sequence boundary exposed along the Rose Creek Escarpment on the Nebraska-Kansas border, probably related to regional Cretaceous groundwater flow systems discharging from the Laurentian landmass.