Geology of the northern Jemez Mountains, north-central New Mexico
— Kirt A. Kempter, Shari A. Kelley, and John R. Lawrence

Recent geologic mapping of eight 7.5-minute quadrangles in the northern Jemez Mountains has yielded numerous insights into the late Oligocene to Pleistocene sedimentary and volcanic history of the region. Important observations include: (1) identification of variations in the lateral and vertical distribution of the Pedernal Chert Member of the Abiquiu Formation across the region; (2) documentation of westward migration of volcanism between 9 and 8 Ma, followed by eastward migration between 7 and 5 Ma across the northern Jemez volcanic field; (3) mapping of several sedimentary deposits that preceded, interceded, and postdated the Toledo and Valles caldera eruptions; and (4) recognition of significant variations in thickness and degree of welding for the Otowi Member of the Bandelier Tuff. Several new Ar⁴⁰/Ar³⁹ ages for northern Jemez volcanic units are presented, together with a compilation of previous dates collected over the past four decades. These ages suggest that Lobato volcanic activity in the northeastern Jemez occurred primarily between 10.5 to 9 Ma, and was particularly intense between 10 to 9.5 Ma. The next major phase of volcanic activity occurred between ~8.5 to 7 Ma along the La Grulla Plateau, where andesitic and dacitic lavas cap a more mafic sequence of basaltic andesites to andesites. A relatively small volume of rhyolite was emplaced just northeast of the Valles caldera rim at ~7.1 Ma that we correlate to the Bearhead Rhyolite of the southern Jemez Mountains. This was followed by major Tschicoma Formation volcanism in the northeastern Jemez Mountains that was mostly emplaced between ~5.3 to 3 Ma. Along the western margin of Mesa El Alto, the El Alto basalt eruption occurred at ~2.9 Ma, followed by three El Rechuelos Rhyolite domes emplaced west of Polvadera Peak at ~2.1 Ma. Lastly, the Toledo and Valles caldera eruptions at ~1.61 Ma and ~1.25 Ma, respectively, deposited the Bandelier Tuff along two lowland corridors in the northern and northwestern Jemez Mountains.