New Mexico Geological Society Annual Spring Meeting — Abstracts

The Pajarito Plateau is an east-dipping ignimbrite terrain between the Jemez Mountains on the west and the Espanola valley on the east. Voluminous deposits of the Bandelier Tuff fonn the uppennost bedrock units. Older rock units including Miocene Santa Fe Group sedimentary rocks and interbedded volcanic rocks, Pliocene volcanic fanglomerates and conglomerates of the Puye Formation, and Pliocene Cerros del Rio and Pajarito Plateau volcanic rocks are exposed in east-trending deep canyons that merge with White Rock Canyon of the Rio Grande. This study uses borehole infonnation to determine the distribution and structural setting of pre-Bandelier units in the Pajarito Plateau area. Geochronology and geochemistry are used to correlate mafic flows intercalated in the Santa Fe Group and the Puye fonnation.

Selected basaltic samples dated by the ⁴⁰Ar/³⁹Ar method indicate that volcanism in the Pajarito Plateau was episodic. The volcanic rocks erupted during the Miocene (11.7-12.9 Ma and 8.4-10.9 Ma) and late Pliocene (2.3 ≥ 2.75 Ma) periods. Major and trace element analyses of the drillhole cuttings yielded variable geochemical results typical of tholeiite, hawaiite, and mugearite compositions. The oldest Miocene and most of the late Pliocene flows are tholeiitic in composition, whereas the late Miocene flows are dominated by evolved rocks of mugearite affinity. Some of the late Pliocene rocks are hawaiite in compositon. The Miocene and late Pliocene tholeiites, hawaiites, and mugearites in the subsurface of the Pajarito Plateau are temporally and geochemically similar to flows exposed in White Rock Canyon and may be genetically related. The thickness of the subsurface volcanic rocks of the Pajarito Plateau are variable and appear to be confined to a marginal graben east and parallel to the present-day Pajarito fault zone.

The Miocene basaltic rocks of the Pajarito Plateau erupted contemporaneous with early magmatic and tectonic activities in the adjacent Jemez volcanic field (13-7 Ma). The Pliocene mafic volcanism was rift-bound and overlaps with the late culminating stage of silicic eruptions in the Jemez volcanic field. The volcanic rocks of the Pajarito Plateau are interbedded with fluvial and lacustrine sedimentary rocks suggesting that episodes of intense tectonic activity, subsidence, and sedimentation overlapped during evolution of the Espanola basin of the Rio Grande rift.