PEDOGENIC FEATURES OF THE LOWER PERMIAN SANGRE DE CRISTO FORMATION, NORTHERN NEW MEXICO: PALEOCLIMATIC INTERPRETATION

The Lower Permian (Wolfcampian) Sangre de Cristo Formation (= Abo Formation) of northeastern New Mexico consists of silty mudstones and laterally discontinuous sandstones deposited on an aggrading alluvial plain. At this time, northern New Mexico was situated in a near equatorial position (~4^oN). Locally, mudstones display a variety of pedogenic features. Common mudstone fabrics vary from platey to prismatic, although crumb fabrics occur rarely; some beds display prominent pedogenic slickensides. Drab root traces are common, extending to a maximum vertical length of 40 cm. Less common are calcareous nodules, which vary from small (1 to 2 cm) bodies with diffuse boundaries to vertically stacked discrete cm-scale nodules (rhizocretions). Most of the mudstone beds in the ­ca. 90-m measured section contain calcretes that are immature (Stage I to II), but the middle portion of the section contains mature calcrete beds (Stage III to IV), with stacked nodules forming coalescing upper boundaries. The overall character of the paleosols suggests a persistent semi-humid, seasonal climate during most of the interval of deposition, but with increased aridity during formation of the more mature calcretes. Intercalated micritic limestone beds contain root traces, are of laterally variable thickness and grade to nodular calcretes. These are interpreted as floodplain pond carbonates that have undergone pedogenic alteration (palustrine limestones), indicating long periods of soil formation under semi-arid conditions.

XRD analyses indicate that all of the pedogenic carbonate is low-Mg calcite. The isotopic composition of the pedogenic carbonate displays a substantial range of values. δ¹³C ranges from -4.7 to -7.1 to ^o/_oo (VPDB); mean δ¹³C = -5.9 ^o/_oo (VPDB). δ ¹⁸O ranges from -3.6 to -6.23 ^o/_oo (VPDB); mean δ ¹⁸O = -4.9 ^o/_oo. Most of the range of variation in isotopic composition is accounted for by isotopically heavier carbonate (both carbon and oxygen) precipitated in shallow ponds subject to intense pedogenic reworking (palustrine carbonate). These data are consistent with the interpretation of an overall semi-arid, seasonal climate during Wolfcampian time, but with some fluctuations. This is supported by the presence of estivation burrowers in the tetrapod fauna, including the amphibians Dissorophus and Diplocaulus.