Depositional Trends In Upper Paleozoic Terrigenous Clastic Rocks, Sacramento Mountains, New Mexico

Field analysis of terrigenous elastic units lying within the upper Pennsylvanian-lower Permian cyclic elastic/ carbonate sequence (Holder and Labore/ta Formations) of the northern Sacramento Mountains, New Mexico, illustrates a marine to non-marine environmental progression as infilling of the Orogrande basin occurred. The oldest units studied, flat-based sandstone bodies A, B and C (Virgilian), coarsen upward texturally and dis­ play large-scale (up to 5m thick) foresets, hummocky stratification, bimodal or polymodal cross-stratification trends, and abundant burrows. These characteristics indicate that siliciclastic shelf bars were present on a storm­ dominated and tide-influenced shelf. Higher in the section, units D (upper Virgilian) and E (lower Wolfcampian) exhibit fluvially influenced depositional features such as concave-up erosional bases, upward-fining textural sequences and unimodal or asymmetric, bimodal paleocurrent distributions. Units D and E are interpreted as estuarine and fan deltaic deposits, respectively Several upward-coarsening fan-deltaic sequences were recognized in the study area suggesting that delta shifting , rather than sea level fluctuations, was a significant local cause of cyclicity. The sequence of units A-E may represent the relatively continuous progradation of terrigenous elastic sediments from the Pedernal uplift into the Orogrande basin during late Paleozoic time.