Redefinition and description of the Los Pinos Formation of north-central New Mexico KIM MANLEY U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, Colorado 80225 ABSTRACT EARLY WORK tion and Treasure Mountain Tuff, and underlie the Hinsdale Formation. The. Los Pinos Formation, consisting of Stratigraphy Butler (1946) subdivided the Los Pinos upper Oligocene to upper Miocene volca- Formation south of Broke Off Mountain niclastic rocks with interbedded ash-flow The Los Pinos Formation was named by (Fig. 2) into three clastic members (Biscara, tuffs, flow breccias, and basalt flows, is Atwood and Mather (1932) and described Esquibel, and Cordito) and a member of exposed in the Tusas Mountains of north- later by Larson and Cross (1956), who gave interbedded basalt flows (Jarita). North of central New Mexico. The Los Pinos Forma- it a Miocene and Pliocene(?) age. As Broke Off Mountain, Butler was unable to tion contains two penecontemporaneous defined, it includes sedimentary and vol- distinguish the three distinct clastic units members: the Esquibel and the Cordito. canic rocks that overlie the Conejos Forma- but assumed the conglomerate beds present The Esquibel Member is a detrital apron of intermediate volcanic clasts derived from 107° 105° the north, primarily the San Juan Moun- tains of Colorado. It thins southward. In the southern Tusas Mountains, it is locally absent or underlies the Cordito Member. The Cordito Member, composed of pre- dominantly rhyolitic clasts, is present only in the southern Tusas Mountains and was derived from source areas to the east (the volcanic center near Questa and another center possibly buried beneath the Pliocene Taos plateau basalts in the Rio Grande rift). The Abiquiu Tuff of Smith (1938) is consi- dered equivalent to the Cordito Member. INTRODUCTION The Los Pinos Formation comprises vol- caniclastic rocks of late Oligocene through late Miocene age that crop out in the Tusas Mountains of north-central New Mexico and the adjacent southern San Juan Moun- tains of Colorado. Exposures of this forma- tion are being studied and mapped over about 2,600 km2 in the Tusas Mountains (Fig. 1). The area is predominantly a high (2,550 to 3,150 m in elevation), rolling upland characterized by a few deep canyons, a steep escarpment on the west side, and few access roads. It lies east of the San Juan Basin, north of the Española basin, and west of the San Luis Valley (Fig. 2). This paper clarifies the .depositional history of the Los Pinos Formation and suggests revisions in stratigraphic nomen- clature. Figure 1. Index map of the Tusas Mountains, shown by hachured area. Geological Society of America Bulletin, Part I, v. 92, p. 984-989, 3 figs., December 1981. 984 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/12/984/3430165/i0016-7606-92-12-984.pdf by guest on 02 October 2021 106°301 loeoiö1 106° 36' 30 % 10 Km Abiquiu Figure 2. Index map of the Tusas Mountains showing areas covered by V/i minute topographic maps keyed to the following numbers: 1. Bighorn Peak; 2. Los Pinos; 3. Broke Off Mountain; 4. San Antonio Mountain; 5. Burned Mountain; 6. Mule Canyon; 7. Tres Piedras; 8. Canon Plaza; 9. Las Tablas; 10. Petaca Peak; 11. Valle Grande Peak; 12. La Madera. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/12/984/3430165/i0016-7606-92-12-984.pdf by guest on 02 October 2021 986 K. MANLEY to be correlative to the two lower members flows of the Hinsdale Formation inter- unit are isolated exposures of volcaniclastic (Biscara and Esquibel). Butler's terminology bedded with the Los Pinos Formation in conglomerates composed primarily of clasts was confined to his unpublished dissertation southern Colorado give a K-Ar age range of derived from the flow breccias. These until incorporated in a report published by 26 to 5 m.y., and flows from the Servilleta conglomerates contain other intermediate- the New Mexico Bureau of Mines and Min- Formation overlying the Los Pinos are 4.5 to mafic-composition clasts of unknown eral Resources (Barker, 1958). m.y. (Lipman and Mehnert, 1975, p. 130). source, but they never contain the latite or rhyolite tuff clasts typical of the Esquibel Description of Units Defined by Butler DELETIONS FROM THE and Cordito Members. LOS PINOS FORMATION As described by Butler (1946), the four LOS PINOS FORMATION members have the following characteristics: Jarita Member the Biscara Member consists of abundant The Los Pinos as redefined includes volcaniclastic conglomerates containing nu- The Jarita Basalt Member and all basalt the Esquibel and Cordito Members. These merous clasts of dark-colored quartz latites, flows interbedded in the Los Pinos Forma- members are largely contemporaneous and conglomerates dominated by Precambrian tion are now included within the Hinsdale represent alluvial deposits that spread from crystalline clasts, tuffaceous sandstones, Formation. This revision was suggested by different source areas into the basin that and intrusive rocks. The tuffaceous beds Butler (1971, p. 296) and formalized by preceded development of the Tusas Moun- include rhyolitic tuffs and clasts of a variety Lipman and Mehnert (1975). tains. Although no outcrops showing inter- of rhyolites, including a gray, aphanitic fingering were observed, it is presumed that rhyolite. The intrusive rocks, including a Biscara Member the two members do interfinger in the dike in Cañada del Agua, are described as region of Broke Off Mountain. "probably closely related in age to the Bis- The term "Biscara Member" is herein cara member" (Butler, 1946, p. 59). The abandoned. The rocks included in this Esquibel Member Esquibel Member is a volcaniclastic unit member by Butler are divided and reas- that contains abundant clasts of porphyritic signed to the Cordito and Esquibel Mem- The Esquibel Member, as adopted here, quartz latite characterized by feldspar phe- bers, the El Rito Formation, the Conejos(?) is the basal member of the Los Pinos For- nocrysts as long as 8 mm. Locally occurring Formation, and younger intermediate-com- mation, although it is largely contempo- conglomerate beds consist predominantly position flow breccias. The rhyolitic tuf- raneous with the Cordito Member. Butler of Precambrian clasts. The Jarita Mem- faceous beds, rhyolite tuffs, and units (1946, p. 61) named the Esquibel Member ber comprises the numerous, discontinuous containing gray aphanitic rhyolite clasts are for Esquibel Canyon north of Tres Piedras flows that overlie the Esquibel Member. It assigned to the Cordito Member. The and described it as similar to the undivided is divided into three types of geographically Precambrian-clast conglomerates are divid- gravel in the northern Tusas Mountains. restricted basalt. The upper member of the ed between the lower Esquibel Member and Barker (1958) mapped the "Biscara-Esquibel" Los Pinos, the Cordito, is a volcaniclastic the El Rito Formation, depending on their Member as the unit present in the same area unit composed predominantly of rhyolitic stratigraphic position above or below Trea- as Butler's Esquibel Member. I prefer to welded tuff clasts. It also includes a rhyolitic sure Mountain or Masonic Park Tuffs. adopt Butler's terminology and extend the welded tuff and rhyolitic intrusive rocks. Where neither tuff is present, as in the Las name "Esquibel Member" northward to Tablas quadrangle, reassignment has been Colorado to include the undivided gravel AGE particularly difficult. that Butler (1946, p. 65) suggested is largely The intrusive rocks within Butler's Bis- correlative with the Esquibel. The Esquibel The Los Pinos Formation is late Oligo- cara Member, including the "dike" in Member is distributed throughout the area. cene through late Miocene in age, using an Cañada del Agua, are actually extrusive, It is especially prevalent in the northern age range of 38 to 24 m.y. for the Oligocerie intermediate-composition flow breccias. Tusas Mountains, where it is as thick as 270 and 25 to 5 m.y. for the Miocene. The Los These flow breccias are similar in lithology m; southward, it thins and is locally absent Pinos directly overlies the Treasure Moun- to rocks of the Conejos Formation in Colo- due to nondeposition. tain Tuff and the Masonic Park Tuff in rado (Lipman, 1975), and outcrops north of The Esquibel Member overlies the El northern New Mexico. These tuffs have the Las Tablas quadrangle were mapped as Rito Formation, the Conejos Formation, K-Ar age ranges from 29.8 to 28.2 m.y. such by Butler (1971). However, in the Las the Conejos(?) Formation and younger flow (Lipman, 1975). An intraformational. rhy- Tablas and La Macera Quadrangles, the breccias, the Treasure Mountain Tuff and olitic welded tuff, the tuff of Cañada del flow breccias are in part at least 4 m.y. the Masonic Park Tuff, and Precambrian Agua, has an age of 25.9 m.y. (Bingler, younger than the uppermost Conejos For- rocks. The contact is possibly locally con- 1968, p. 36). Additional K-Ar ages support- mation, whose age in Colorado and the nor- formable with the Conejos(?) Formation ing the age of the Los Pinos Formation thernmost Tusas Mountains is controlled and younger flow breccias but is elsewhere a include an interbedded 17.7-m.y.-old flow by the overlying Treasure Mountain Tuff. nonconformable contact of considerable from southern Colorado (Steven and oth- The flow breccias in this area are therefore relief. The Esquibel is interlayered with ers, 1967) and an interbedded 20.7-m.y.-old assigned to a combined unit of the Cone- basalts of the Hinsdale Formation through- flow 8 km north of El Rito in northern New jos(?) Formation and younger intermediate- out the Tusas Mountains. Many of the Mexico (Manley and Mehnert, 1981). Other composition flow rocks. Included in this Hinsdale flows occur near or at the top of Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/12/984/3430165/i0016-7606-92-12-984.pdf by guest on 02 October 2021 LOS PINOS FORMATION, NEW MEXICO 987 the member.