New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologic mapping, paleocurrent data, and studies of gravel in the northern Albuquerque basin, near Albuquerque, Rio Rancho, and San Felipe Pueblo, New Mexico, indicate that lithofacies of the upper Santa Fe Group are locally differentiable. Stratigraphic sections were measured and sampled in Pliocene and lower Pleistocene deposits at different locations throughout the northern Albuquerque basin in order to understand the lithologic character of the Arroyo OJito and Sierra Ladrones Formations. Deposits of the 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. The Sierra Ladrones Formation is divided into axial-fluvial ancestral Rio Grande (ARG) deposits, and piedmont detritus derived from the adjacent Sandia Mountains.

Gravel of the upper Arroyo Ojito Formation is 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 Mountains piedmont deposits contain abundant sedimentary and pink granite clasts. 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.

Sand composition is generally quartz rich (~50-80%) and ranges from arkose to feldspathic litharenite, suggesting derivation from diverse source terrains. Preliminaryresults of sand petrography suggest that petrographic trends based only on sand composition are only weakly correlated among lithofacies; however, the sand petrography part of the study has not been completed yet. Ifthis preliminary finding for sand composition is correct, then, reliance on sand petrography alone for provenance and lithofacies determination may be misleading without other supporting data, such as comparisons to geologic mapping, gravel petrography, and paleocurrent data.