Sedimentology of the Tesuque Formation and tectonics of the Embudo fault system Near Dixon, New Mexico
— S. B. Aby and D. J. Koning

The northeastern part of the Velarde 7.5-minute quadrangle, in the Dixon area, is underlain by Tertiary strata of the Tesuque Formation and an isolated outcrop of Proterozoic metasedimentary rocks of the Hondo and/or Vadito Groups. Faults associated with the transfer zone between the San Luis and Española Basins of the Rio Grande rift traverse the map area from southwest to northeast. We use the term Embudo fault system to refer to this zone of deformation and have assigned informal names to individual faults and fault zones within this system. In this area, the Embudo fault system is a ~5-km-wide zone of normal, oblique-normal, and local reverse faults. The Chama-El Rito Member (~12.5-17.5 Ma), the informally defined (Steinpress, 1980) Dixon member (~11.8-13 Ma), the Ojo Caliente Sandstone Member (~11.5-13 Ma), and the Cejita Member (~7.5-10 Ma) of the Tesuque Formation are present. No radiometric ages have been obtained from within the map area and estimates listed above are a combination of estimates based on fossil evidence and regional stratigraphic relations. We provisionally designate mixed eolian, fluvially-reworked eolian and fluvial deposits lying above the Ojo Caliente Sandtone Member as the ‘Cieneguilla member’ of Leininger (1982). Pliocene basalt of the Servilleta Formation (~3.39(?) –2.81 Ma near Rinconada) caps La Mesita and mesas north of the Rio Grande. Quaternary landslides are ubiquitous north of the Rio Grande and on the flanks of La Mesita. Proterozoic Pliocene units are offset along numerous faults within the Embudo fault system. Offset is mostly normal left-lateral and dominantly down–to-the-northwest although outcrop patterns suggest a complex history of deformation including tilting, extension, localized(?) compression, and shear. Compression in this area is most likely related to a restraining bend in the southeastern margin of the fault system. Tertiary unit contacts are laterally offset between ~1 and 2.75 km within and across a zone of intense deformation ~.5 km wide that makes up the southeastern margin of the fault system. Vertical offset measurements within/across this zone cannot be made with confidence due to interfingering relations between Tertiary units and the complications arising from left-lateral offset. Pliocene basalt flows are offset 60-70 m vertically and possibly up to 460 m laterally across the La Mesita fault. Tertiary units generally dip toward the southwest. This tilting preceded or accompanied offset within the Embudo fault system and much of it probably occurred between deposition of the Dixon and Ojo Caliente members and deposition of the Cejita Member, although some post-Cejita tilting is also apparent. Paleocurrent measurements and clast counts largely confirm earlier interpretations that: 1) The Chama-El Rito Member was derived largely from the Latir volcanic field with input from Proterozoic and Paleozoic rocks near Taos and at least episodic input from the the Picuris Mountains. 2) The Dixon member and the younger Cejita Member were derived largely from Paleozoic rocks to the east and southeast with input from older Tertiary and Proterozoic units. 3) The Ojo Caliente Sandstone is an eolian unit transported from the southwest. 4) The Cieneguilla member represents primary eolian deposits interbedded with reworked Ojo Caliente sand and sediment derived from older Tertiary units, Proterozoic rocks of the Picuris Mountains and probably the southern Taos Range.