LATE-MIOCENE TO EARLY-PLEISTOCENE GEOLOGIC HISTORY OF ISLETA AND HUBBELL SPRING QUADRANGLES BASED ON 40AR/39AR AGES AND GEOCHEMICAL CORRELATION OF VOLCANIC ROCKS
LOVE, D.W., DUNBAR, N., MCINTOSH, W.C., MCKEE, C., New Mexico Bureau of Mines
and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy
Pl., Socorro, NM 87801; CONNELL, S.D., JACKSON-PAUL, P.B., New Mexico Bureau
of Mines and Mineral Resources, New Mexico Institute of Mining and Technology,
2808 Central Ave. SE, Albuquerque, NM 87106; and SORRELL, J., Environment Department,
Pueblo of Isleta, P.O. Box 1270, Isleta, NM 87022
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 base-surge
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
and/or 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 B ash (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.