PALEOSOLS OF THE UPPER CARBONIFEROUS-LOWER PERMIAN CUTLER GROUP, NORTHERN NEW MEXICO: PALEOCLIMATIC INTERPRETATION

The Upper Pennsylvanian (Virgilian) to lower Permian (Wolfcampian) El Cobre Canyon and Arroyo del Agua formations in the Chama Basin of northern New Mexico are well exposed in a 250+ m section in Canyon del Cobre (Rio Arriba County), northern New Mexico. The section constitutes a red-bed succession consisting mainly of silty mudstones and sandstones with sheet-like to ribbon geometry and very minor limestones. These sediments represent channel-fill deposition by braided streams, and overbank muds (dominantly), sandy splays and (minor) floodplain ponds. Locally, mudstones display a variety of pedogenic features. Common mudstone fabrics vary from blocky to platey, and less commonly prismatic. Some beds display prominent sand-filled desiccation fractures, rhizoliths and drab root traces. Calcareous nodules are common in most mudstone beds, varying from small semi-spherical bodies to larger botryoidal masses. Less common are discrete layers of coalescing nodules. These typically have nodular upper surfaces and bases that are gradational, transitioning downward to vertically-stacked, discrete, cm-scale nodules (rhizocretions) and isolated nodules. Most of the calcretes in the studied section are immature (Stage I to II), but mature calcrete beds (Stage III to IV), with coalescing nodules forming laterally continuous (K) horizons, occur at multiple levels throughout the section. Minor discontinuous micritic limestone beds are of laterally variable thickness, have nodular upper surfaces and commonly contain root traces. These are interpreted as floodplain pond carbonates that have undergone pedogenic alteration (palustrine limestones), indicating long periods of soil formation under subhumid to semi-arid conditions. Results of isotopic analysis of the carbonates are pending. The character of the paleosols, combined with floral evidence, suggests an overall semi-humid, seasonal climate during most of the interval of deposition, with intervals of increased aridity during formation of the more mature calcretes. At this time, northern New Mexico was situated in a near equatorial position (~4^o N) and likely influenced by proximity to the ITCZ, but the position of the ITCZ may have varied episodically due to orbital forcing. No evidence of long-term climate change is presented by the 250+ m section studied.