C STRATIGRAPHY FROM WELL CUTTINGS TO LINK LATE ORDOVICIAN SEDIMENTS IN MISSOURI AND NEW YORK ACROSS LITHOLOGICAL GRADIENTS

The ratio of ¹³C/¹²C in carbonate carbon (δ¹³C_carb) has been an essential tool for regional as well as global correlation of Late Ordovician strata. The presence of a positive d¹³C_carb excursion in the Late Ordovician (Mohawkian), located meters above a widely traceable K-bentonite bed, has enabled the correlation of spatially disparate strata deposited in vastly different environments. Precise knowledge of the chronostratigraphic nature of flooding and subaerial exposure surfaces and formation boundaries is key to constructing a sequence stratigraphic history, particularly for cratonic deposits which often lack diagnostic features for the identification of systems tracts. We present δ¹³C_carb data from two outcrops in Missouri as well as cuttings and core samples from numerous wells in New York to demonstrate the efficacy of δ¹³C_carb chemostratigraphy in determining the precise chronostratigraphic relationship between the slowly subsiding cratonic interior and a rapidly subsiding onshore-offshore transect >1,000km away. δ¹³C_carb allows higher-resolution correlation between the Trenton and Utica Formations across a lithological gradient from shaley carbonates to black shales in NY where lithostratigraphic correlations were ambiguous. Further, δ¹³C_carb chemostratigraphy shows a gradational eastward thinning of the Black River Formation away from the regional depocenter in central NY, indicating that the unconformity with the overlying Trenton Formation was sub-aerial in the eastern part of the state. Finally, the time-varying nature of δ¹³C_carb allows us to confidently compare coincident subsidence rates across Laurentia for sections not bounded by ash beds. We find differential subsidence rates greater than an order of magnitude between MO and NY. Our results highlight the time-transgressive nature of the Black River-Trenton unconformity, supporting previous work based on K-bentonite stratigraphy. The agreement between our results using K-bentonites and δ¹³C_carb supports the use of δ¹³C_carb chemostratigraphy with well cuttings to identify the nature and stratigraphic relationship of unconformities that may not be obvious from geophysical logs alone. Our results add to the body of knowledge essential for constructing and refining principles of cratonic sequence stratigraphy.