New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologic mapping by the U.S. Geological Survey and New Mexico Bureau of Mines and Mineral Resources depicts the geology, hydrology, and aeromagnetometry of parts of the Pueblo of Isleta. Our mapping is part of the Middle Rio Grande Basin Study and shows the western three-quarters of the reservation from just west of the foothills of the Manzano Mountains on the east to the Rio Puerco on the west. Major geologic features within the area are: (1) five basaltic volcanic fields; (2) deposits of the Santa Fe Group; (3) post-Santa Fe terrace deposits; (4) widespread calcic soils; and (5) high-angle normal faults.

The basaltic fields in the western half of the area include the following, listed from youngest to the oldest; (1) Cat Hills; (2) Black Mesa; (3) Isleta, (4) Cat Mesa; and (5) Wind Mesa. The Cat Hills field is composed of seven lava flows and 21 cinder cones. Samples from the oldest flow have ⁴⁰Ar/³⁹Ar wholerock ages of 0.11±0.03 and 0.098±0.02 Ma. A previous K-Ar age for the oldest flow is 0.14±0.04 Ma (Kudo and others, 1977). Samples from the youngest flow have discordant ⁴⁰Ar/³⁹Ar whole-rock ages of 0.49±0.16 Ma and 0.25±0.08 Ma. A dike exposed in the northernmost cinder cone has an ⁴⁰Ar/³⁹Ar wholerock age of 0.18±0.08 Ma. This and other cinder cones are aligned on a north-northeast striking fissure zone.

The Black Mesa basaltic lava flow is exposed northeast of the Isleta field and is interbedded with Rio Puerco sand and gravel facies of the upper part of the Santa Fe Group. No volcanic source for this flow 39 has been identified. A sample from this field has an ⁴⁰Ar/³⁹Ar age of 2.68±0.04 Ma.

The Isleta field is composed of five lava flows, two cinder cones, and one tuff ring (Kelley and Kudo, 1978). The lowest flow has an ⁴⁰Ar/³⁹Ar age of 2.75± 0.12 Ma and the overlying flow an ⁴⁰Ar/³⁹Ar age of 2.68±0.08 Ma and a previous K/Ar age of 2.78±0.12 Ma (Kudo and others, 1977).

The Cat Mesa field consists of a thick lava flow found locally at the base of the clay and silt facies of the Santa Fe Group (discussed below). A sample from this field indicates an ⁴⁰Ar/³⁹Ar whole rock age of 3.00±0.10 Ma.

The Wind Mesa field consists of three lava flows with minor cinder deposits. The flows are faulted and appear to be part of an exhumed volcano. Lag pebbles of the ancestral Rio Puerco lie on the lava flow 5 m above the present Santa Fe-lava flow contact supports this interpretation. A sample of the oldest flow has an ⁴⁰Ar/³⁹Ar whole-rock age of 4.01 ±0.16 Ma.

The exposed Santa Fe Group is considered to be part of the Sierra Ladrones Formation of Machette (1978) and composed of piedmont deposits from the Manzano-Manzanita Mountains and fluvial deposits of an ancestral Rio Puerco and ancestral Rio Grande. We have divided the ancestral Rio Puerco deposits in the westernmost part of the map area into three main lithofacies that are based on coarseness and clast content. From youngest to oldest, the facies are the following: (1) sand and gravel; (2) clay and silt; and (3) coarse sand and gravel. The fluvial deposits of the ancestral Rio Grande, exposed east of the Rio Grande valley, are divided into mapable lithofacies consisting of a lower fine-grained facies and an upper coarse-grained facies. The upper facies contains pumice and reworked ash and large clasts of the 1.22 Ma-Tshirge Member of the Bandelier Tuff.

Post Santa Fe terrace deposits are found along the present Rio Grande valley inset against the Santa Fe Group. The most extensive terrace deposit is called the Los Duranes Formation. Its age is restricted by the oldest Cat Hills flow (0.14, 0.11, and .098 Ma) that overlies it.

West of the Rio Grande, widespread calcic soils are divided into two mapable types (1) calcic soils of the Llano de Alburquerque and (2) younger calcic soils. The calcic soils of the Llano de Alburquerque overlie the sand and gravel facies of the Rio Puerco and are overlain by the basaltic flow of the Cat Hills. Therefore the Llano surface is older than the discordant range of ages (0.098 Ma to 0.49 Ma) of the Cat Hills basalt flows and considerably younger than the basalt of Cat Mesa (3 Ma). The younger calcic soils are found interbedded with and below eolian deposits and are conunonly found on the flanks of the Cat Hills cinder cones.

Calcic soils on the llanos east of the Rio Grande developed at the top of the ancestral Rio Grande deposits and on top of piedmont deposits that overlie the ancestral Rio Grande deposits. The piedmont-capped llano is 10 to 15 m lower than the ancestral Rio Grande upper surface.

A northerly striking high-angle normal fault zone forms a complex of grabens, half grabens, and horsts in the map area. From west to east, seven major fault zones include the: (1) Garcia (new name), (2) Cat Mesa, (3) Cedar Wash (new name), (4) Palace-Pipeline (new name), (5) McCormick Ranch, (6) Hubbell Spring, and (7) Sanchez faults.

The signatures on the aeromagnetic map reflect the major faults and volcanic fields that have been identified on the surface and shown on the geologic map. Both the hydrologically important Cat-Mesa and Hubbell-Spring faults are expressed on the aeromagnetic map. In addition, the map suggests that some segments considered part of the Hubbell Spring fault actually may not be connected.

The ground-water level contour map shows steep gradients in the western and eastern part of the map separated by a trough (Titus, 1963). The western (Cat Mesa) gradient on the west side of the Cat Mesa fault may reflect the juxtaposition of fractured basaltic rocks and coarser-grained Santa Fe Group finer-grained facies of the Santa Fe Group. Another possible model is the presence of a northerly-striking fissure system that may be a steep ground-water barrier that parallels the Cat Mesa fault. The wide ground-water trough, which appears to plunge to the south, separates the Cat Mesa gradient from the eastern steep gradient. The eastern gradient, located along the Hubbell Spring fault and referred to here as the Hubbell Spring gradient, may represent juxtaposition of basin-fill material with bedrock.