Petrographic, petrologic, and isotopic study of paleosol carbonates from the Permo-Pennsylvanian Lodeve basin, France.

摘要

Petrologic and geochemical results from paleosol and groundwater carbonates will be presented from the Upper Pennsylvanian through Permian strata of the Lodeve basin, French Massif Central. The terrestrial Lodeve basin sat in equatorial central Pangea and preserves evidence of extensive diagenetic overprinting, probably as a result of regional orogenesis and break up of the supercontinent. This study considers field, petrologic, mineralogic and geochemical data from paleosol clay mineralogy and carbonates as a proxy for pedogenesis, diagenesis, and regional tectonic events with implications for paleoclimate reconstruction from paleosols.Oriented aggregates were made and analyzed by X-ray diffraction to determine the clay mineralogy of the soil samples. Associated diagenetic overprinting of the paleosols includes conversion of 2:1 expansible clays (smectites) to illite in Vertisols.In order to delineate the paragenesis of the carbonates, samples were thin sectioned and analyzed by XRD to determine bulk mineralogy. Plane-polarized petrography and cathodoluminescene was employed to develop a cement stratigraphy, and different cements were analyzed for elemental chemistry and stable oxygen and carbon isotope composition. Most samples preserve multiple cements. From oldest to youngest, paleosol carbonate cements preserve: (1) micritic calcite interpreted to be unaltered Permian material, (2) anhydrite (3) mimetic, planar and nonplanar dolomite, (4) chert and chalcedony, (5) barite, (6) two separate generations of Fe-rich calcite spar and (7) a final generation of Fe-poor calcite spar. Cement 1 is likely unaltered Permian soil material, Cements 2-5 represent burial diagenesis, probably during the Permian, Cement 6 represents deep burial and high temperature diagenesis in the presence of reducing fluids, probably during the Triassic and Jurassic break-up of Pangea and again in the Cretaceous. Cement 7 represents uplift of the basin probably associated with Neogene volcanism. The results of this study have important implications for paleoclimate and paleoaltimetry studies that are based on isotopic values of paleosol carbonate nodules.