PENNSYLVANIAN-PERMIAN PALEOENVIRONMENTAL TRENDS FROM LITHOSTRATIGRAPHY, CLAY MINERALOGY, AND STABLE ISOTOPE GEOCHEMISTRY OF THE MADERA AND SANGRE DE CRISTO FORMATIONS, MORA COUNTY, NEW MEXICO

This study presents lithostratigraphic, mineralogic, and stable isotope proxies of equatorial paleoclimate from Upper Pennsylvanian through Lower Permian strata of the Taos Trough, in north-central New Mexico. This Upper Paleozoic sequence includes mixed marine carbonate and terrestrial clastic rocks Upper Pennsylvanian Madera Fm strata which underlie terrestrial-dominated fluvio-lacustrine siliciclastic Lower Permian rocks of the Sangre de Cristo Fm. Paleosol morphologies include eutric Argillisols, Calcisols, and gypsic subgroups. XRD analyses of pedogenic clay minerals show a shift from kaolinite, smectite, and illite assemblages in the lower part of the stratigraphy to poorly-ordered 2:1 phyllosilicate and illite assemblages, in the uppermost part of the stratigraphy. Carbonate-associated organic matter samples from paleosols in the upper Madera and Sangre de Cristo formations have δ¹³C values ranging from -25.1 to -21.1‰. Paleosol calcite δ¹³C values are ~-5‰ in Virgilian strata, ~-8‰ in lower Wolfcampian strata, and ~-5‰ in topmost Wolfcampian strata. Missourian, Virgilian and Wolfcampian strata have pedogenic calcite δ¹⁸O values that range from ~-4 to -6.5‰. However, calcite δ¹⁸O values of topmost Wolfcampian strata range from +1 to +3 ‰. Calcite and co-existing carbonate-associated organic matter have Δ¹³C_(CC-OM) values ranging from 13.1 to 19.5‰.

The occurrence and distribution of climate-sensitive geochemical indicators preserved in paleosols of the Madera and Sangre de Cristo formations of the Taos Trough provide evidence of long-term aridification, punctuated by a period of increased seasonality, during deposition of the upper Pennsylvanian and lower Permian strata. Argillic pedotypes, containing high relative proportions of kaolinite and HIM in paleosol matrix samples, relative negative δ¹⁸O_(CC) values of paleosol carbonates, and MAP estimates calculated from Δ¹³C_(CC-OM) of paleosol carbonates and associated occluded organic matter all suggest seasonal, humid conditions of pedogenesis during the Pennsylvanian-Permian transition. The prevalence of paleosols characterized by calcic and gypsic features, high proportions of illite, paleosol carbonates significantly enriched with respect to ¹⁸O, and MAP estimates calculated from Δ¹³C_(CC-OM) of paleosol carbonates and carbonate-associated organic matter, ranging from 29.0 to 19.0 cm/year in the upper Sangre de Cristo Formation suggest that the Taos Trough was subject to a period of significant aridification during Early Permian time.