New Mexico Geology

We document calcareous paleosols from Upper Pennsylvanian (lower Virgilian) strata of the Burrego Member of the Atrasado Formation in the Cerros de Amado of Socorro County, New Mexico. The Burrego paleosols are an excellent example of a scarce, climate-sensitive lithology in the Pennsylvanian strata of New Mexico. These paleosols contain mostly stage II to III carbonate horizons, and their overall morphology suggests deposition and pedogenesis under subhumid, seasonally dry conditions. This conclusion is consistent with paleobotanical and other data that indicate such climate conditions were widespread on Late Pennsylvanian Pangea. The mean value of the oxygen-isotope ratios from Burrego paleosol carbonates compares well with the values from Virgilian paleosols of the San Juan, the eastern Midland and Chama basins of New Mexico-Texas, suggesting similar conditions of temperature and paleoprecipitation. Application of the diffusion-reaction model to the mean carbon-isotope composition of the carbonate suggests a paleo-pCO₂ of approximately 400 ppmV, which is also consistent with estimates from correlative carbonate deposits that formed farther east in Late Pennsylvanian Pangea.