PETROGRAPHY AND PROVENANCE OF UPPER SANTA FE GROUP DEPOSITS IN THE ALBUQUERQUE BASIN, NEW MEXICO: PRELIMINARY RESULTS

Integration of geologic mapping and paleocurrent data with studies of sand and gravel petrography in the northern Albuquerque basin, near Albuquerque, Rio Rancho, and San Felipe Pueblo, New Mexico, suggest that lithofacies of the upper Santa Fe Group are locally differentiable. Stratigraphic sections were measured in Pliocene and early Pleistocene deposits, and sand and gravel samples were taken at different locations in the basin in order to understand the lithologic character of the Arroyo Ojito and Sierra Ladrones Formations. Deposits of the upper Arroyo Ojito Formation constitute the fluvial facies of the ancestral Rio Jemez/Guadalupe, Rio Puerco, and Rio San Jose, which drain the northwestern and western margins of the basin. Deposits of the Sierra Ladrones Formation are divided into the axial-fluvial ancestral Rio Grande (ARG), and piedmont detritus derived from the adjacent Sandia Mountains.

Upper Arroyo Ojito Formation pebbles are generally poorly sorted and primarily composed of volcanic tuff and red granite, with subordinate Pedernal chert, basalt, and sandstone. ARG pebbles are primarily moderately to well sorted and composed of well-rounded quartzite and a diversity of igneous clasts, including tan porphyritic intrusive and volcanic rocks; granite is relatively sparse. Sandia piedmont deposits contain abundant sedimentary clasts and pink granite. Preliminary results of sand petrography indicate crude facies-related trends. Sand composition is generally quartz rich (~50-80%) and ranges from arkose to feldspathic litharenite, suggesting derivation from diverse source terrains. ARG deposits are typically more varied and contain greater proportions of volcanic feldspar and volcanic fragments than in the Arroyo Ojito Formation. Volcanic detritus is very rare to nonexistent in the peidmont facies. Paleocurrent data show that ARG deposits typically have a southerly transport direction, whereas Arroyo Ojito paleoflow generally trended to the south-southeast. Piedmont paleoflow was typically to the west. The results of this research should allow for more confident differentiation of basin-fill sediments in the subsurface and in areas where gravel content is low.