New Mexico Geological Society Annual Spring Meeting — Abstracts

The Bear Springs Peak (BSP) quadrangle in the southern Jemez Mountains records a significant portion of Keres-Group volcanism, the earliest phase of volcanic activity in the Jemez Mts. Volcanism in the BSP area occurred primarily along N-S and SW-NE structural features when this area represented the western margin of the Rio Grande rift. The oldest deposits include flows and tuffs of the Canovas Canyon Rhyolite (Tcc and Tcct) and basalts of the Paliza Canyon Fm (Tpb). New and previous age data suggest that this bimodal volcanic suite was emplaced primarily between 10 and 8.5 Ma, with most of the basalts erupted west of Bear Springs Peak and rhyolites erupted primarily from N-S-trending fractures traversing the central portion of the quadrangle.

The Paliza Canyon Fm is widespread, including intercalated basalt flows (Tpb) and volcaniclastic sediments (Tg) in the western portion of the quadrangle, a thick (>300 m) package of dominantly andesitic rocks (Tpa) exposed in the western wall of Peralta Canyon, and a younger set of dacitic lavas (Tpd) exposed along the quadrangle’s northern border and in the Ruiz Peak area. On the western half of the quadrangle, volcaniclastic sediments are intercalated with basaltic and andesitic lavas. In some locations, ponding of Paliza Canyon lavas occurred along fault margins, suggesting contemporaneous tectonic and eruptive activity. In general, thick andesite is sandwiched between the older mafic lavas and the younger dacitic lavas, with the entire sequence emplaced mostly between ~10 to 7 Ma.

Rhyolitic volcanic activity in the BSP area returned shortly after 7 Ma, as a wide variety of flows, domes (Tbr) and tuffs (Tbt) related to the Bearhead Rhyolite were emplaced between ~6.8 to 6.0 Ma. Although most of these rhyolites intruded as domes and plugs along earlier N-S structural features, rhyolite dikes along the western edge of Peralta canyon trend NE-SW. Rift-related structural activity continued after emplacement of the Peralta tuff member, as >60 m of displacement of this unit is observed along the Cerrito Yelo fault south of Cerrito Yelo. Following the Bearhead Rhyolite episode of volcanic activity, volcanism in the Jemez Mountains shifted northward while late Miocene to early Pliocene volcaniclastic sediments of the Cochiti Fm were deposited in the southeast portion of the BSP quadrangle. After ~5 Ma of volcanic quiescence in the BSP area, primary volcanic deposits reappeared in the Quaternary, as distal flows of the Bandelier Tuff (both Otowi and Tsirege members) partially filled paleocanyons where the modern Peralta, Paliza and San Juan canyons now exist. Capping the volcanic sequence is the 50-60 ka El Cajete tephra, erupted from the El Cajete crater ~9 km north of the quadrangle. The distribution and thickness of the deposit in the BSP quadrangle (>5 m thick in pumice mines north of Paliza Canyon), clearly indicates a SSE dispersal axis during this phase of the eruption.