Characterization of Pedogenic and Diagenetic Carbonates from the Paleoequatorial Permo-Carboniferous Lodeve Basin, French Massif Central

Results of petrological analyses from paleosol and groundwater carbonates in Upper Pennsylvanian through Upper Permian strata of the Lodeve basin, French Massif Central, will be presented. The strata range from Kasimovian (Stephanian B) to Lopingian (Zechstein) with short-lived regional unconformities in the Asselian (Lower Rotliegend) and Upper Sakmarian/ Lower Artinskian (Upper Rotliegend 1). The Lodeve and surrounding basins record evidence of high-temperature, geo-pressured, fluid migration events related to continental rifting during Late Permian – Jurassic, which likely contributed toward diagenetic alteration of the pre-existing strata. This study attempts to delineate primary pedogenic, from deep-burial diagenetic, phases.

In order to assess paragenetic pathways for carbonate in the basin, we have analyzed (1) carbonate nodules in paleosol profiles with well-developed pedogenic structure (paleosol; n = 29), (2) lenticular carbonate nodules that preserve sedimentary structure (groundwater; n = 11), and vein-filling calcite-spar that cross-cut sedimentary strata (late diagenetic; n=3). X-ray diffraction analysis indicates spar-filled veins (n=3) are entirely composed of calcite, paleosol carbonate nodules exist as pure calcite (n = 2), mixed calcite and dolomite (n = 3), and pure dolomite (n = 24), and groundwater carbonates exist as pure calcite (n = 4), mixed calcite and dolomite (n = 3), and pure dolomite (n = 4). Dolomite is considered a later diagenetic phase in the Lodeve carbonates. Petrographic analysis of carbonates show three main textures: micrite, microspar and spar. In modern soils the most common pedogenic texture is micrite, and therefore the most conservative texture for reconstruction of paleoenvironment. These results, in conjunction with cathodoluminescence and staining, indicate that few of the paleosol or groundwater carbonate samples are pristine. Most are diagenetically altered. The results of this work have implications for the interpretation of carbonate stable carbon and oxygen isotope values and resultant paleoclimate reconstructions.