New Mexico Geological Society Annual Spring Meeting — Abstracts

Across much of northern and central New Mexico, the Sandia Formation forms the base of the Pennsylvanian succession and represents early synorogenic deposits associated with the initiation of the Ancestral Rocky Mountains (ARM) orogeny. The Sandia Formation was first named in 1900 and rapidly became part of the New Mexico lithostratigraphic chart. In northern and central New Mexico the Sandia Formation is a mixed siliciclastic and carbonate succession, particularly characterized by thick beds of quartz-rich sandstone, conglomeratic sandstone and conglomerate. The high amount of siliciclastic sediment was derived from tectonically active local ARM uplifts composed of Proterozoic basement rocks.Thickness of the Sandia Formation in central New Mexico ranges from 7 to162 m, with the thinner sections close to the ARM highlands. At most places the Sandia Formation rests on Proterozoic basement with an angular unconformity. Locally it overlies Mississippian or Lower Pennsylvanian sedimentary rocks. At most places the dominantly or entirely siliciclastic sediments of the Sandia Formation are conformably overlain by thicker limestone/cherty limestone units of the Gray Mesa Formation. In northern New Mexico sandstone of the Sandia Formation is commonly subarkose, whereas to the south most sandstones are quartzarenites. Towards southern New Mexico (Fra Cristobal, Caballo Mountains) the Sandia Formation interfingers with the Red House Formation.Limestones of the Sandia Formation are composed of various microfacies types including phylloid algal wackestone, bioclastic mudstone, wackestone, floatstone and grainstone to rudstone containing a diverse fossil assemblage. The depositional environment of siliciclastic sediments ranges from fluvial to fluvio-deltaic, coastal swamp and brackish, to coarse-grained high-energy nearshore (storm dominated shelf) and fine-grained middle-outer shelf deposits. Limestone containing a low-diverse fossil assemblage indicates deposition in a restricted shallow marine shelf environment. Limestone types with a diverse fossil assemblage accumulated in open, normal marine shallow shelf settings of low to high energy.The Sandia Formation yields abundant fossils of nonmarine (plants and amphibians) to fully marine (especially algae, foraminifers including fusulinids, brachiopods, crinoids, bryozoans and sharks teeth) organisms. Fusulinids and limited conodont records indicate that the Sandia Formation is mostly of Atokan age, but locally the lower part of the formation is late Morrowan.The Sandia Formation was deposited as part of the extensive Absarokan marine transgression and is characterized by distinct lateral changes in thickness and facies as a result of the ARM deformation. Cycles are present within the dominantly marine successions of the Sandia Formation, but cannot be traced laterally over long distances, so we assume that the formation of the cycles was caused mainly by tectonic movements of the ARM, although some glacio-eustatic influence cannot be ruled out.