⁴⁰Ar/³⁹Ar geochronology of tephra layers in the Santa Fe Group, Espanola Basin, New Mexico
— William C. McIntosh and Jay Quade

Laser-fusion ⁴⁰Ar/³⁹Ar analyses yield high-precision, stratigraphically consistent ages for a sequence of tephra layers within Santa Fe Group strata in the Española Basin, northern New Mexico. Ages determined for crystals from individual tephra layers are tightly grouped within samples and agree closely with stratigraphic order:

Formation          Member                          Tephra                  Mineral                    Age(Ma)
Chamita Fm      upper tuffaceous member    upper ashes       sanidine                 6.78 ±0.03
Chamita Fm      upper tuffaceous member    lower ash             sanidine                6.93 ±0.05
Chamita Fm      lower tuffaceous member     lower ash             hornblende           7.7 ±0.3
Tesuque Fm     Skull Ridge Member             White Ash #4       sanidine               15.45 ±0.06

These ⁴⁰Ar/³⁹Ar results help refine age estimates of the Tesuque and Chamita Formations, which have been subjects of previous controversy. The 15.45 Ma age for White Ash #4 confirms the general scheme of paleomagnetic correlations for the Skull Ridge Member proposed by Barghoorn (1981) and Tedford and Barghoorn (1993), but contradicts previous younger zircon fission-track ages. Early Barstovian fossils found in the White Operation Quarry just below White Ash #4 date to ~15.4 Ma. The results from the lower (7.7 Ma) and upper (6.95-6.75 Ma) tuffaceous zones of the Chamita Formation conflict with previous interpretations of Chamita Formation age based on magnetostratigraphy and too-young fission track dates (5.3 and 5.6 Ma). However, a recalibration of magnetostratigraphy with ⁴⁰Ar /³⁹Ar results matches well with recent revisions of the geomagnetic polarity time scale (Cande and Kent, 1992). The important late Hemphillian fossils found in the San Juan and Rak quarries in the upper tuffaceous zone are now dated to between 6.95 and 6.75 Ma. Based on the age and K/Ca ratios of sanidine, tephra layers in the upper tuffaceous zone represent distal fall facies of the Peralta Tuff, a rhyolite-dome-related pyroclastic sequence erupted 40-50 km to the southwest.