Stratigraphy of the Tanos and Blackshare Formations (lower Santa Fe Group), Hagan embayment, Rio Grande rift, New Mexico Sean D. Connell, New Mexico Bureau of Geology and Mineral Resources–Albuquerque Office, New Mexico Institute of Mining and Technology, 2808 Central Ave. SE, Albuquerque, New Mexico 87106, [email protected], Steven M. Cather, Nelia W. Dunbar, William C. McIntosh, and Lisa Peters, New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801 Abstract Introduction by the Budaghers fault. The Hagan embay- ment lies just east of the Santo Domingo Over a kilometer of sedimentary and vol- The Santa Fe Group (upper Oligo- sub-basin, a deep structural sub-basin canic rocks of the Santa Fe Group are cene–Pleistocene) comprises the sedimen- within the northern part of the Albu- exposed in the Hagan embayment, a struc- tary and volcanic fill of the Rio Grande rift querque Basin (Grauch et al., 1999). This tural re-entrant between the Albuquerque (Chapin and Cather, 1994). Geologic map- and Española Basins. We identify two new tilted and fault-bounded embayment ping and stratigraphic studies of the Santa lithostratigraphic units, the Tanos and Black- exposes one of the most complete Phanero- share Formations, that resulted from late Fe Group exposed along Arroyo de la Vega zoic stratigraphic sections in central New Oligocene to late Miocene sedimentation de los Tanos in the Hagan embayment, Mexico (Pazzaglia et al., 1999). Paleogene along the northeastern margin of the Albu- northeast of Espinaso Ridge (Fig. 1), con- nonvolcanic deposits exposed in the querque Basin and Hagan embayment. We strain the initial development of the Rio Hagan embayment include arkosic sand- designate exposures in Arroyo de la Vega de Grande rift in north-central New Mexico. stone, conglomerate, and mudstone of the los Tanos as the type section and the San According to a K–Ar age reported for a Diamond Tail and Galisteo Formations Felipe Pueblo NE quadrangle as the type basalt flow near the base of the Santa Fe (Fig. 2; Stearns, 1953b; Gorham and Inger- area for these two formations. The Tanos Group (Kautz et al., 1981), these deposits Formation is 279 m (915 ft) thick at the type soll, 1979; Lucas et al., 1997). Conglomerate represent some of the oldest exposed rocks locality where it disconformably overlies the beds of the Diamond Tail and Galisteo For- Espinaso Formation. The Tanos Formation of the Albuquerque Basin area (cf. Bach- mations contain rounded metaquartzite, consists of a basal conglomerate overlain by man and Mehnert, 1978). These exposures chert, and petrified wood clasts (Stearns, mudstone and sandstone, representing dep- provide an opportunity to study early rift 1953b; Gorham and Ingersoll, 1979). Con- osition in a closed basin. The overlying sedimentation and to characterize lower glomerate of the upper Eocene–Oligocene Blackshare Formation is over 1,000 m (3,281 Santa Fe Group deposits that are buried by Espinaso Formation containing volcanic ft) thick and contains dominantly sandstone younger rift-basin fill in the Santo Domin- rocks and sandstone conformably overlies with interbeds of lenticular conglomerate, go sub-basin. The presence of upper the Galisteo Formation (Stearns, 1953a, b; conglomeratic sandstone, and minor mud- Oligocene deposits supports conclusions stone, commonly arranged in fining-upward Erskine and Smith, 1993). from other studies of rift-related extension sequences. Paleocurrent measurements, Volcanic products of the Espinaso For- gravel composition, and field relationships elsewhere in New Mexico (e.g., Chapin mation, which have been studied exten- indicate derivation from the neighboring and Cather, 1994; Smith, 2000; Mack et al., sively (Kautz et al., 1981; Erskine and Ortiz Mountains. The Blackshare Formation 1994; Smith et al., in press). Smith, 1993), are associated with the conformably overlies and interfingers with This paper summarizes results of geo- emplacement of the Ortiz porphyry belt in the Tanos Formation and represents a gener- logic mapping and stratigraphic studies of the Ortiz Mountains and Cerrillos Hills al westward progradation of the Ortiz the San Felipe Pueblo NE quadrangle (Stearns, 1953a; Kautz et al., 1981; Erskine Mountains piedmont during Miocene time. (Cather et al., 2000) and defines two new and Smith, 1993). The Ortiz Mountains are Stratal tilts of the Tanos–Blackshare succes- formation-rank lithostratigraphic units of sion decrease upsection, indicating that sub- a 26–36 Ma belt of Eocene–Oligocene por- sidence and deposition occurred concurrent- upper Oligocene and Miocene deposits of phyritic, hypabyssal-intrusive (subvol- ly. Subhorizontally bedded conglomerate the lower Santa Fe Group. In accordance canic) rocks exposed on the footwall of the and sandstone of the Pliocene–Pleistocene with the North American Commission on La Bajada fault, just east of the study area Tuerto formation* overlie the Blackshare For- Stratigraphic Nomenclature (NACSN, (Kelley, 1977; Kautz et al., 1981; Maynard mation in angular unconformity. A basaltic 1983), we describe three informal members et al., 1990). The Espinaso Formation is flow near the base of the Tanos Formation of the Tanos Formation and four informal divided into a lower calc-alkaline assem- yielded an 40Ar/39Ar age of 25.41 ± 0 .32 Ma. members of the Blackshare Formation. We blage and an upper alkaline assemblage A volcanic ash within the Blackshare Forma- conclude with some implications that this 40 39 (Erskine and Smith, 1993). Rocks of the tion yielded an Ar/ Ar age of 11.65 ± 0.38 study has on models of the tectonic devel- lower assemblage were erupted between Ma and geochemically resembles one of the opment of the Española and northern middle Miocene Trapper Creek ashes from 36 and 34 Ma, followed by eruption of the the northeastern Basin and Range province. Albuquerque Basins (e.g., Ingersoll, 2001). upper assemblage between 30 and 26 Ma The base of the Tanos Formation is older (Maynard et al., 1990). than the basal Zia Formation, which is The stratigraphic nomenclature of rift- exposed along the northwestern margin of Geologic setting and basin fill (i.e., Santa Fe Group) in the study the Albuquerque Basin, indicating that dep- previous studies area has undergone a number of changes osition of the Santa Fe Group began earlier to over the years (Fig. 2). Stearns (1953a, b) the east. The Hagan embayment is a northeast-tilt- first described the study area in detail and ed structural re-entrant between the La defined the subjacent Eocene Galisteo and *Editor’s note: It is the lead author’s belief Bajada and San Francisco fault zones. Eocene–Oligocene Espinaso Formations. that the Tuerto gravels of Stearns (1953b) is a These fault zones define the eastern mar- Lucas et al. (1997) replaced the lower part mappable and lithologically well devined unit gin of the Rio Grande rift along different of Galisteo Formation with the Diamond that should be given formation-rank status. Ele- parts of their traces (Kelley, 1977). The Tail Formation. Stearns (1953b) tentatively vation of the Tuerto gravels to formation rank Hagan embayment is bounded to the north assigned deposits overlying the Espinaso will be proposed in a future publication. November 2002, Volume 24, Number 4 NEW MEXICO GEOLOGY 107 o 106 o 30' 105 45' 35 o Jemez t e 55' l u d a n volcanic f Los a r o s t i t field r Alamos G a j o M i a Pajarito R P o Plateau Española t s iisto Mts Basin r C e d e r g Cerros n a del Rio Santa Fe SangreS de Cr volcanic r Tent ive e R Rocks field ta F L an a S Santo B a j a Domingo d Fe a ta an nt sub-basin e S me s ay c mb SDP a Ga e liste r o Ck. p Santa m Cerrillos e Fig. 3 e d n Hills Ana n a t Mesa r Bf G AVT SFP T on HE G q EER a ue lis A R te y o o C . re io ek R AdT f Ortiz f B F L S Mts. New Placitas o Sandia Mexico 35 15' Mts. San Pedro Mts. SFP = San Felipe Pueblo LBf = La Bajada fault 0 10 20 km SDP = Santo Domingo Pueblo ER = Espinaso Ridge HE = Hagan embayment AVT = Arroyo de la Vega de los Tanos SFf = San Francisco fault AdT = Arroyo del Tuerto 0 10 mi Bf = Budaghers fault FIGURE 1—Location of the Santo Domingo sub-basin (Albuquerque Basin), Hagan embayment, Ortiz Mountains, San Felipe Pueblo 7.5-minute quadrangle study area (Fig. 3), and other major features of the northern Albuquerque and southern Española Basins. The Hagan embayment is bounded by the San Francisco, Budaghers, and La Bajada faults. The Tijeras fault (not shown) trends to the northeast through the Ortiz Mountains. Drainages of Arroyo de la Vega de los Tanos and Arroyo del Tuerto, San Felipe Pueblo, Santo Domingo Pueblo, Santa Fe, and Los Alamos are shown for reference. Formation to the Abiquiu(?) Formation, southern Española Basin. This group usage perennial streams and rivers associated primarily on stratigraphic position and the was extended throughout the rift (e.g., with the ancestral Rio Grande flowed presence of tuffaceous rocks exposed on Hawley, 1978; Chapin and Cather, 1994). toward southern New Mexico (Hawley, the footwall of the La Bajada escarpment The lower Santa Fe Group represents dep- 1978). Deposition of the upper Santa Fe (Fig. 1). Overlying deposits were assigned osition within internally drained basins Group ceased during Pleistocene time, to the Santa Fe Formation and Tuerto grav- (bolsons) that contain broad alluvial plains when the Rio Grande began to incise into els by Stearns (1953b). Kelley (1977) and ephemeral or intermittent playa lakes the earlier aggradational phase of the assigned the Abiquiu(?) Formation (sensu fed by streams draining emerging basin- Santa Fe Group basin fill (Spiegel and Stearns, 1953b) to the Zia Formation.