New Mexico Geological Society Annual Spring Meeting — Abstracts

Local basalt flows and tephra, as well as fluvially recycled pumice, tuff, and ash from silicic eruptions in the Jemez Mountains are buried within deposits of the upper Santa Fe Group near Isleta Pueblo, NM. Geochemical comparisons and ^40/39Ar-ages of many of these volcanic rocks help correlate deposits exposed on different fault blocks and in different depositional/erosional settings, leading to a more complete geologic understanding of this part of the Albuquerque Basin. The oldest exposed basaltic tephra is an undated hawaiite found near the base of exposures on the highest uplifted block beneath Mesa del Sol. Isleta Volcano and at least three smaller eruptive centers are tilted to the south-southeast west of and beneath the Rio Grande valley. Dates for Isleta volcano tightly constrain the eruptive history: 2.79 ±0.04 Ma (large block within basesurge in the tuff-cone), 2.75± 0.03 Ma (flow 1 of Kelley and Kudo, 1978), 2.78±0.06 Ma (flow 2 of Kelley and Kudo, 1978),2.73 ±0.04 Ma (flow exposed along Highway 85), 2.68±0.04 Ma (Black Mesa flow northeast of Isleta Volcano). Base surge deposits and cinders from four localities on at least three fault blocks on the east side of the Rio Grande valley have similar chemistry to these dated units and all appear to be from the same eruptive sequence. These similar tephras crop out in fault blocks at elevations of 1497, 1539, 1548, and 1585 m, indicating significant uplift to the east.

At least six geochemically distinct, fluvially reworked pumice units crop out up-section from the basaltic tephras in southeast-tilted fault blocks. Overlying those units are >100 m of ancestral Rio Grande deposits containing dated andlor geochemically identified volcanic ejecta: San Diego Canyon pumice (1.71±0.04 Ma); lower Bandelier pumice (1.61 Ma); reworked Cerro Toledo pumice, obsidian, and ashes (1.3-1.5 Ma); upper Bandelier Tuff boulders (1.22 Ma); and Valles Dome ash (1.05 Ma). Some tephra layers consist of individual glass shards up to several hundred microns in diameter, consistent with derivation from fall deposits. Others consist of rounded pumice that range in size from a few mm to several cm. The compositions within individual deposits are consistent and distinct from pumice above and below, suggesting little mixing of primary pumice in the aggrading fluvial system. A Pleistocene terrace deposit inset against Pliocene units contains cross-bedded Lava-Creek Bash (0.66 Ma). This ash consists of delicate glass shards up to 300 microns in size that appear to have undergone little, if any, transport following primary deposition.