New Mexico Geological Society Annual Spring Meeting — Abstracts

Stratigraphic sections measured in the type area of the Sierra Ladrones Fm, 10-15 km northwest of San Acacia, NM, document the lithologic character of this widely mapped unit, which was originally defined without a type section. Stratigraphic sections in Arroyo Tio Lino and at Loma Blanca (LB) encompass tilted fluvial deposits that interfinger to the west with better cemented, piedmont sandstone and conglomerate derived from adjacent basin-bounding uplifts. Elsewhere, these fluvial deposits are faulted against older rocks. These fluvial deposits are typically lightgray to very pale-brown (10YR hues), moderately to well sorted, weakly to well cemented sandstone with trough cross stratification; mudstone is common as rip-up clasts. South-southeast trending paleocurrent observations, and sparse, fluvially recycled pumice pebbles at LB, which are geochemically correlative and age-equivalent to the Peralta Tuff, indicate post-late-Miocene south-flowing fluvial drainage. A measured section (U), 3 km southwest of La Joya, NM, is tentatively correlated to the Arroyo Ojito Fm, and contains Blancan (pliocene) fossils (Equus simplicidens, and E. scotti(?)), and light-brown (7.5YR) sandstone, conglomerate, and bedded mudstone deposited by S-SE flowing streams. The upper part of the LJ section is interpreted to have been deposited by an E-NE flowing ancestral Rio Salado. The Salas Arroyo section, east of the Rio Grande, contains trough cross-bedded, locally tephra-rich, fluvial sand that interfinger to the east with piedmont deposits derived from adjacent basin-bounding uplifts.

Field relations and geologic reconnaissance suggest that an ancestral Rio Grande deposited the Salas Arroyo section. The Tio Lino and Lorna Blanca sections, exposed in structurally higher fault blocks along the western margin of the basin are possibly correlative to an older ancestral Rio Grande; however, definitive correlation of the Tio Lino section to ancestral Rio Grande deposits in the Socorro basin has not yet been established. The LJ section represents deposition by streams of the ancestral Rio Puerco/San Jose/Salado fluvial system, not streams from the eastern basin-margin piedmont, as previously interpreted.