New Mexico Geological Society Annual Spring Meeting — Abstracts

Pumiceous sedimentary deposits containing cobble-sized clasts are exposed in bluffs located about 5 km east of San Antonio, NM. These deposits attain a maximum thickness of about 5 m and consist of, in order of ascending stratigraphic position, 1) basal ash-rich fluvial and hyperconcentrated flood-flow deposits; 2) coarse debris flow deposits (at least two flows are represented), 3) sandy hyperconcentrated flood-flow deposits; and 4) local, rafted blocks of pumice at the top. Due to effects of non-deposition, erosion, and recent mining activities, component facies are rarely all exposed together in a solitary outcrop.

Stratigraphy and styles of soft-sediment deformation suggest deposition occurred during a single flood event. Debris flows were presumably initiated by rhyolitic eruptive events in the Jemez area (1.1 to 1.5 Ma) although more precise correlations remain controversial. Probably because of their low density, debris flows represented by the pumice deposit near San Antonio are extraordinarily fartraveled; they are about 220 km downstream from their source. Deposition and preservation of pumice and underlying axial floodplain mudstones in the study area may be related to down-valley constriction of flow by the juxtaposed Pleistocene fans of San Pedro Arroyo and Walnut Creek. If these pumiceous deposits are temporally related to a Jemez eruption, then their inset nature (see figure) indicates an age of 1.1 Ma or older for the Tio Bartolo surface of Sanford et ale (1972).