New Mexico Geological Society Annual Spring Meeting — Abstracts

Pliocene volcanism in the Cerros del Rio volcanic field; southeast of the Jemez Mountains, included explosive hydromagmatic eruptions when magma encountered groundwater in high-transmissivity ancestral Rio Grande gravel facies in the upper Santa Fe Group. Eruptions at the margin of the volcanic field produced basalt lava flows and hydromagmatic tuff interbedded with basin-fill sediment between the towns of Cochiti Lake and Peria Blanca in the northern Santo Domingo Basin. The locations of source vents are difficult to reconstruct because of partial burial beneath Quaternary fill terraces and the large area disturbed and buried during the construction of Cochiti Dam.

Based on remaining outcrops and correlations of US Army Corps of Engineers (USACE) core logs, we interpret a line of contemporaneously active vents to underlie nearly the entire length of the dam. Hydromagmatic basaltic tuff, at least 90 m thick and containing abundant rounded cobbles indicating derivation from ancestral Rio Grande gravel, is exposed along the Rio Grande at the outlet works for the dam and near the base of the dam in Canada de Cochiti. Bedding dips and slump folds indicate that these outcrops are located along the inner rim of tuff-ring craters. Laminated lacustrine clay to fine sand crop out above crater-fill tuff in Canada de Cochiti. USACE core logs and photos taken during dam construction reveal similar lacustrine fill, approximately 15 m thick, within a 0.5-km wide crater now located under the damI adjacent to the outlet works. In all outcrops the tuff overlies ancestral Rio Grande gravel.

Two basalt lava flows (⁴⁰Ar/³⁹Ar age of 2.71 ±0.04 Ma by W. Mcintosh) form prominent faulted cuestas along the lower Santa Fe River and behind Pena Blanca and extend to 1 km south of the hydromagmatic-tuff outcrops in Canada de Cochiti. The flows rest on and are separated by southward-thinning layers of hydromagmatic tuff with surge cross-bedding indicating likely derivation from the above-described craters farther north. These volcanic rocks rest on piedmont facies basin-fill alluvium. There is no indication of weathering, reworking, or deposition of other sediment during the emplacement of the interbedded lava flows and tuff, suggesting eruption during a single eruptive episode. A probable source for the lava flows was a hill of basalt mapped by the USACE along the north side of the Santa Fe River on the east side of present Cochiti Dam. This hill, which projected >50 m above the eastward dipping, upper basalt-flow contact, was quarried during dam construction and the site is now buried under a fill apron.

The apparent contemporaneity of lava-flow and tuff eruptions from different vents suggests (a) control on eruptive style by the subsurface distribution of the voluminous. high-transmissivity gravel aquifer. and (b) eruption from a single dike, possibly along a pre-existing fault. We interpret the USACE core logs to indicate the presence of a down-to-the-east fault with N20°-35°W trend nearly coincident with the dam and passing through all three known eruptive centers. This fault exhibits a southward increase in displacement from ~15 m to 30 m. The fault has no surface expression and does not displace fill terraces of likely middle Pleistocene age.

Lacustrine clay and silt, as much as 25 m thick, are present in the subsurface over an area of 3 km² and in one small outcrop area NE of the line of volcanic vents. These probably accumulated in a lake formed by impoundment of the ancestral Rio Grande behind the volcanic deposits.