New Mexico Geological Society Annual Spring Meeting — Abstracts

The Thurman Formation Was sampled in two localities along the Rio Grande rift for paleomagnetic and single-crystal ⁴⁰Ar/³⁹Ar geochronology to determine the timing and duration of deposition and sanidine provenance of the synrift basin fill. More than 300 paleomagnetic specimens (32 sites) were drilled in 25-m intervals from two approximately 4OO-m-thick sections of sandstones, conglomeratic sandstones, and debris flows near Johnson Spring in the Rincon Hills and Apache Canyon in the Caballo Mountains. About 60 sanidines were analyzed by ⁴⁰Ar/³⁹Ar single-crystal laser fusion from three stratigraphic horizons at both localities to determine the age and provenance of individual grains. Provenance ages span 34.9 to 27.4 Ma but cluster at ages of 34.7, 33.3-33.5, 27.6-27.8, and 27.4 Ma.

Petrographic examinations show that the tuffaceous sandstones have detrital I grains of quartz, plagioclase, and sanidine and detrital volcanic rock fragments of rhyolitic tuff, pumice, and deuterically altered or oxidized andesite. Opaque minerals in the rock fragments include magnetite, titanomagnetite, pyrite, ilmenite, and hematite. Authigenic cement of clay, zeolites, and hydroxides coat rims of grains and rock fragments. Clay and zeolites delicately line relict textures of pumice and tuff fragments and partially infill dissolved plagioclase grains.

Specimens collected for paleomagnetic studies were examined by reflected light microscopy and by alternating field and thermal demagnetization methods to assess the magnetic mineralogy, components of remanent magnetism, and the nature of the variations in remanence. Within the Apache Canyon section of the Thurman Formation, two magnetic polarity zones are identified and, based on tbe position of sanidine provenance ages within pumice and matrix of debris flows, the zones may span the time near the reversal at 27.4 Ma.

The sanidine provenance ages in the Thurman Formation can be correlated with high-precision ⁴⁰Ar/³⁹Ar sanidine geochronology of rhyolitic ash-flow tuffs in the Mogollon-Datil volcanic field. Sanidines dated at ~27.6 Ma may represent reworked fall material from the tuff of Walking X Ranch (27.6 ± .10 M) and sanidines dated at 27.4 ±.05 Ma from pumice in a debris flow may represent fall material from South Canyon Tuff (27.4 ± .07 Ma). These sanidines were derived from an area more than 20 km to the west and north, where the caldera explosions of the Mogollon-Datil volcanic field and areal extent of the ash-flow tuffs are located.

Correlation of the sanidine dates with the regional extent of dated tuffs, independently established paleocurrent directions, and the; magnetic polarity stratigraphy show that the rhyolitic tuffaceous sands of the Thurman were largely derived by the erosion of Box Canyon Tuff (33.5 Ma) and then Kneeling Nun Tuff (34.9 Ma) during or soon after the eruption of South Canyon Tuff (27.4 Ma) from the Mount Withington caldera. The Thurman Formation is a syn-to posteruptive alluvial fan sequence of stacked debris flows that rapidly accumulated in an early rift basin of the Rio Grande rift soon after the catastrophic eruption of South Canyon Tufflat 27.4 Ma. The Thurman Formation records the erosion and unroofing of theandesitic and rhyolitic volcanic rocks of the Oligocene Mogollon-Datilvolcanic field which were disrupted during the explosion of Mount Withington caldera at 27.4 Ma. Throughgoing late Oligocene paleodrainage in central New Mexico was southward.