STABLE ISOTOPIC PALEOHYDROLOGY OF PEDOGENIC CARBONATE NODULES FROM PLIOCENE PALEOSOLS IN THE MEADE BASIN, SOUTHWESTERN KANSAS

The preservation of numerous calcareous paleosols in Neogene strata of the Meade Basin provides a unique opportunity to investigate diagenetic heterogeneity and trends in pedogenic carbonate nodules. The spatial orientation of diagenetic trends in carbon and oxygen isotope space is particularly valuable for determining both the paleohydrologic processes recorded by the formation of pedogenic carbonate and for assessing the reliability of paleoenvironmental reconstructions based on the stable isotope values of diagenetically altered carbonates.

In this study, we utilize cathodoluminescence (CL) imagery to guide a micro-sampling campaign for stable isotopic analyses (δ¹³C and δ¹⁸O) of carbonate components that have discrete luminescence characteristics mapped at the thin section scale. Pedogenic carbonate nodules from five discrete superimposed paleosols were investigated from one Pliocene-aged lithostratigraphic section in the Meade Basin.

CL images indicate that all paleosol samples have undergone some degree of phreatic overprinting. Isotopic results reveal different diagenetic trends in C and O space that provide information about paleohydrologic processes. These trends include meteoric calcite lines (MCL) that indicate early meteoric diagenesis and positive linear covariant trends (PLCT) that reveal evaporative effects during carbonate precipitation.

In three of the samples, four discrete MCLs were identified with δ¹⁸O values of -9.36, -9.38, -9.46, and -9.64‰ VPDB, suggesting subtle variations in either the δ¹⁸O value of meteoric water or temperature during carbonate formation. Variations in δ¹³C values between the different MCLs (-4.48 to +0.55‰ total range) suggest different dissolved inorganic carbon sources in the groundwater systems. Four PLCT patterns were also apparent and differ from their respective MCLs by between 0.61 and 1.46‰, indicating slightly different degrees of evaporative enrichment. Two other samples are characterized by relatively invariant carbon and oxygen isotopic values. Nonetheless, clear differentiation is apparent between components with different CL characteristics.

Ongoing petrographic work will enable us to characterize the morphology of different carbonate components and further delineate and interpret diagenetic features.