Geology and structure along part of the northeast Jemez Mountain, New Mexico
— David P. Dethier and Blake A. Martin

The Miocene sedimentary fill and interlayered volcanic deposits of the central Rio Grande rift record tectonic and volcanic events in surrounding source areas with considerable clarity. Deformation of the fill and changes in sedimentary textures, lithologies, and paleocurrent directions give the most complete record of late Cenozoic tectonic evolution within the rift.

The central Rio Grande rift of Chapin (1979) initially evolved as a structural sag along Laramide structures in middle Oligocene time, after most arc volcanism in nearby areas had ceased (Baldridge and others, 1980). There is no evidence that the early rift was bounded by major faults (Baltz, 1978), but adjacent mountain areas provided sediment to the internally drained basin. Until at least mid-Miocene time, the western margin of the Espanola Basin was defined by faults along a part of the Nacimiento uplift presumably now buried by Jemez volcanic rocks (Golombek and others, 1983). Minor faulting marked the eastern margin along the flank of the Sangre de Cristo Range.

Basin-filling sediment oflate Oligocene to mid-Miocene age is mainly volcaniclastic debris and arkosic sandstone deposited on alluvial fans and exposed on the margins of the Espanola Basin as the Abiquiu, Los Pinos, and Picuris Formations (Manley, 1979). By about 11 my. ago, the western margin of the Espanola Basin had shifted eastward, volcanic activity was underway in the Jemez Mountains, and faulting disrupted the Oligocene and Miocene units. The Espanola Basin tilted westward in mid- to late Miocene time along faults presumably now covered by Jemez volcanic rocks (Golombek and others, 1983), and in latest Miocene or early Pliocene the Velarde graben formed in the central part of the basin (Manley, 1979). The graben is thought to be the most active part of the Espanola Basin at present and is bounded by the Pajarito fault on the west, an unnamed NE-trending fault on the north, and an ill-defined set of north-trending faults located east of the Rio Grande (Manley, 1979; Golombek and others, 1983).

Inferences about the nature and timing of the mid-Miocene tectonic event and subsequent development of the Velarde graben are based primarily on the stratigraphy and structure of the Santa Fe Group. Decades of paleontologic and stratigraphic studies on the Santa Fe sediment were summarized in a monumental work by Galusha and Blick (1971), who divided the sediment into the Tesuque and Chamita Formations. The Tesuque Formation was subdivided into the following members (ascending): Nambe, Skull Ridge, Pojoaque, Chama—El Rito, and Ojo Caliente Sandstone. With the exception of the Ojo Caliente, an eolian unit, all other members of the Tesuque are considered to be alluvial-fan deposits. Studies by May (1980, this guidebook) on the Chama—El Rito, by Steinpress (1981) on the Santa Fe near Dixon, and by Manley (1979; 1981) on other parts of the Espanola Basin have contributed greatly to our understanding of mid-Miocene stratigraphy. However, detailed stratigraphic information and paleocurrent measurements are sparse in many key areas of the basin, particularly in critical areas near the Jemez Mountains. Smith and others (1970), for instance, did not subdivide the Santa Fe Group, and Kelley (1978) mapped the area east of the NE Jemez Mountains incorrectly as Ojo Caliente Sandstone. We report results of a detailed study on the Santa Fe Group exposed in a narrow horst at the east edge of the northeast Jemez Mountains. The upthrown block is exposed a few kilometers north of the northwest corner of the Velarde graben, near the intersection of the Jemez lineament (Chapin, 1979) and the Parajito fault. We tentatively correlate the Santa Fe exposed in the horst and nearby areas with the upper part of the Chama—El Rito Member of the Tesuque Formation.