Miocene-Oligocene Volcaniclastic Deposits in the Northern Abiquiu Embayment and Southern Tusas Mountains, New Mexico Daniel J

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Miocene-Oligocene volcaniclastic deposits in the northern Abiquiu embayment and southern Tusas Mountains, New Mexico Daniel J. Koning, Kirt Kempter, Lisa Peters, William C McIintosh, and S. Judson May, 2011, pp. 251-274 Supplemental data available: http://nmgs.nmt.edu/repository/index.cfm?rid=2011003 in: Geology of the Tusas Mountains and Ojo Caliente, Author Koning, Daniel J.; Karlstrom, Karl E.; Kelley, Shari A.; Lueth, Virgil W.; Aby, Scott B., New Mexico Geological Society 62nd Annual Fall Field Conference Guidebook, 418 p.

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DANIEL J. KONING1, KIRT A. KEMPTER2, LISA PETERS1, WILLIAM C. MCINTOSH1, AND S. JUDSON MAY3 1 New Mexico Bureau of Geology & Mineral Resources, New Mexico Institute of Mining & Technology, Socorro, NM 87801; [email protected] 2 2623 Via Caballero del Norte, Santa Fe, NM 87505 3 3508 Lynbrook Drive, Plano, TX 75075

ABSTRACT—- ment, and these strata extend northward into the Tusas Mountains. The Abiquiu embayment lies between the Embudo fault system and the Colorado Plateau, and is generally considered as a northwest extension of the Española Basin. This volcaniclastic sediment includes two general stratigraphic levels. The lower contains felsic-clast-dominated volcaniclastic conglomerate !"#$%$$$# Abiquiu Formation. At the top of the lower unit, conglomerate of the Cordito Member prograded southwestward over Abiquiu Formation sandstone. The upper volcaniclastic interval is composed of the Tesuque Formation. We propose applying the name Duranes Member (new name) to a purplish gray conglomerate, composed primarily of porphyritic dacite-andesite clasts, shed '$!'*'"+'/9:%- %$$$%#+<=9>?Z??[% %<='\9]$% between the lower and upper volcaniclastic intervals is diachronous: to the northeast it is ~23.5 Ma and to the southwest it is 20-21 Ma. A 40Ar/39%\$%{%|>9>|}9? this formation in the El Rito area. The top of the Duranes Member (Tesuque Formation) is 13.5-14.0 Ma and the top of the Chama-El Rito Member is ~13.5 Ma.

INTRODUCTION needed to distinguish the creek from the town) and Ojo Caliente and La Madera along the Rio Ojo Caliente (Fig. 1). The northern Recent mapping (see Methods section below) and geochro- %:- nology in the northern Abiquiu embayment and southern Tusas tains, which rise up to 300 m above the Abiquiu embayment (Fig. Mountains (40-60 km west of Taos, NM) have elucidated our 1). These mountains are underlain by older Cenozoic clastic units understanding of the sedimentology plus stratigraphic and age (i.e., El Rito Formation, Ritito Conglomerate, Abiquiu Forma- relations among volcaniclastic units of Oligocene-Miocene age. tion, and Cordito Member of the Los Pinos Formation) that onlap These units can be subdivided into two stratigraphic levels. The Proterozoic rocks (Fig. 2). Rift-related normal faults extend lower level includes the Cordito Member of the Los Pinos For- through these highlands and locally preserve the Tesuque Forma- mation and the Abiquiu Formation. The upper level consists of tion on the hanging walls of large normal faults (Figs. 1 and 3). conglomerate and sandstone of the Tesuque Formation. The Tes- In a structural sense, the northern Abiquiu embayment-south- uque Formation has previously been included as part of the Santa ern Tusas Mountains occupies a northward transition from the '- Española Basin to the San Luis Basin. Both basins occupy major nic rocks related to the Rio Grande rift (following Spiegel and half-grabens formed during Rio Grande rifting, with the Española Baldwin, 1963). Past workers have not placed the Abiquiu and half-graben tilted principally to the west and the San Luis Basin Los Pinos Formations into the Santa Fe Group, but we advocate tilted principally to the east (Kelley, 1978; Smith, 2004). The % structural boundary between the Española and San Luis Basins and Baldwin (1963). After summarizing the lithologic character- has been thought to coincide, at least in part, with the largely istics and stratigraphic relations of these volcaniclastic units, this buried basement ridge extending southeast of La Madera towards $% Cerro Azul (Fig. 1; Cordell, 1979; Muehlberger, 1979; Manley, aspects of their deposition in the northern Abiquiu embayment 1979 and 1984), although Dungan et al. (1984) noted that the and southern Tusas Mountains. east tilted structural style of the Abiquiu embayment shared The Abiquiu embayment is the topographically low area similarities with the San Luis Basin (note that this is not true between Black Mesa and the Colorado Plateau (Fig. 1). Gener- for the southwestern or central part of the embayment). In gen- ally weakly cemented (locally moderate to strong cementation) eral, the Abiquiu embayment is an extensively faulted structural Oligocene-Miocene