29 Midcontinent Paleoclimatology

Tuesday, 26 March 2019
1:30 PM - 5:45 PM
Kings Room

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

Paper number: 29-6
Program: Theme Sessions
Time: 2:55 PM – 3:15 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 2

doi: 10.1130/abs/2019SC-326757

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