PALEOHYDROLOGIC EVALUATIONS OF CENOZOIC CALCRETES FROM THE HIGH PLAINS OF WESTERN KANSAS

Bedded to nodular calcretes in the Cenozoic deposits of western Kansas accumulated in a variety of paleohydrologic settings, including both vadose pedogenic accumulations and palustrine facies that involved ponded environments with phreatic carbonate accumulations. Petrographic evaluation of hand samples, involving polarized light microscopy and cathodoluminescence (CL) imaging show that these fine-grained carbonates are geochemically heterogeneous at the millimeter to micron scale, an observation that dictates the appropriate scale of microsampling for carbonate stable isotopes for diagenetic investigations. Studied examples are from Plio-Pleistocene strata from the Meade Basin in Meade County, late Miocene strata of the Ogallala Formation from Ladder Creek Canyon in Scott County, and late Quaternary strata from the Ehmke Playa in Lane County. Most of the carbonates examined are luminescent under CL, indicating calcite crystals with lattice substitutions of transition metals, likely related to reducing conditions in anoxic phreatic environments. Bright orange CL luminescence, indicative of Mn²⁺ substitutions in calcite are observed, but duller purple and violet CL luminescence colors are even more commonly observed, and their chemical controls are currently under investigation. The most advanced microsampling campaigns to date have been carried out on units from the Meade Basin, where microsample target sites were mapped on perfectly-registered CL and reflected light images of micropolished thick sections. The CL components there have yielded multiple discrete Meteoric Calcite Line (MCL) patterns in carbon and oxygen isotope space, with MCL δ¹⁸O values that range from -9.6 to -9.4‰ VPDB. These results suggest subtle variations in either the δ¹⁸O values of shallow groundwaters or temperatures during authigenesis. The MCL patterns from the Meade Basin have δ¹³C values that range from +0.6 to -4.5‰ VPDB, indicating that multiple sources of dissolved inorganic carbon were present in the shallow groundwater systems. Other CL components from samples in the Meade Basin have yielded Positive Linear Covariant Trends (PLCT) patterns with δ¹⁸O values that range upward from their respective MCL δ¹⁸O values by up to 1.5 per mil, suggesting evaporative enrichment in shallow ponded environments.