New Mexico Geological Society Annual Spring Meeting — Abstracts

The Abiquiu Formation has traditionally been informally divided into three units: lower, Pedernal chert, and upper. During the course of recent 1:24,000 scale mapping in the Jemez Mountains, we came to recognize that two lithologically-distinct, geographically-restricted successions of conglomerate comprise the "lower Abiquiu" and that the provenance of these coarse sediments is quite different from the provenance of strata assigned to the "upper Abiquiu." Maldonado and Kelley (in prep.) have suggested restricting the name Abiquiu Fm. to "upper Abiquiu" strata and applying the name Ritito Conglomerate to "lower Abiquiu"conglomerate exposed near the village of Abiquiu. Here, we propose the name Gilman Conglomerate for conglomerate exposed near the village of Gilman in the SW Jemez Mountains. The basementderived Ritito Conglomerate is exposed along the SE flank of the Tusas Mountains, beneath the NW Jemez Mountains as far south as the NW corner of the Valles caldera, and as far west as San Pedro Parks in the Sierra Nacimiento. The volcaniclastic Gilman Conglomerate is present near the villages of San Ysidro, Jemez Springs, and Gilman along the Jemez fault zone. The restricted Abiquiu Fm. covers a broad region of north-central New Mexico, with exposures as far south as the SW Jemez Mountains. Clasts from the Latir volcanic field are common in the Abiquiu Fm. in the northern Jemez Mountains, but are absent in the SW Jemez Mountains. Localized basalt flows and volcaniclastic sedimentation that are temporally equivalent to late Abiquiu deposition elsewhere in the Jemez Mountains are preserved in the SE Jemez Mountains on St. Peters Dome. The Pedernal chert is 1-2 m thick on Cerro Pedernal and Encino Point and occurs in the uppermost part of the Ritito Conglomerate. The chert is absent east of the Cañones fault zone. Multiple layers of chert occur in the restricted Abiquiu Fm. on the west side of the Coyote fault zone and thin stringers of chert are present in the Abiquiu Fm. in the SW Jemez Mountains.

The distribution of the conglomeratic units indicates the presence of two distinct subbasins in the vicinity of the Jemez Mountains early in Oligocene time. Later widespread volcaniclastic sedimentation sourced from the Latir volcanic field covered the region, while localized volcanism and sedimentation occurred in the SE Jemez Mountains. Finally, silicaenriched groundwaters deposited chert below the basal contact and within the Abiquiu Fm.