New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent geologic mapping (1:24,000) of the Cerro del Grant 7.5 minute quadrangle (CdGQ) was completed under the New State Map Program. CdGQ encompasses an area north of the Valles caldera and includes two geologic terranes: (1) a western zone of Late-Triassic to Pliocene sedimentary rocks and (2) a zone of volcanic rocks ( ?8-1.2 Ma) exposed in the higher elevations on La Grulla Plateau. Uniformly east-dipping sedimentary strata include shale, sandstone, conglomerate, evaporite deposits, and chert. These are locally overlain by volcanic rocks of the Polvadera and Tewa Groups. Volcanic acivity began with earliest basalt flows (8.67 Ma) that appear interbedded with Ojo Caliente Sandstone of the Santa Fe Group on Canones Canyon. Effusive eruptions, preserved a series of Lobato Formation (locally 7.85-7.90 Ma) andesite flows exposed in the escarpment at Encino Lookout, issued onto an east-sloping surfaced beveled on Santa Fe Group. Remnants of a discovered shield cone occur west of the escarpment. The flow series is locally intruded by a mass of Lobato dacite. Deposition of the Tschicoma Formation, informally subdivided here into two subunits, closely followed Lobato volcanism. Voluminous lower-member two-pyroxene andesites cover much of La Grulla Plateau. Upper-member hornblende±biotite dacites (7.63-7.72 Ma) occur as discontinuous, stratigraphically overlying domes and limited flows conspicuously aligned along the east plateau margin, bordering Canones Creek. Structural control of Tschicoma dacite eruptive centers is apparent. Rhyolitic ignimbrites of the Bandelier Tuff (1.62-1.22 Ma) occur in level remnant outcrops following extensive Pleistocene erosion. Post- and syn-volcanic normal fault movement is evident.