Redefinition and description of the Los Pinos Formation of north-central

KIM MANLEY U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, 80225

ABSTRACT EARLY WORK tion and Treasure Mountain , and underlie the . 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 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 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

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36' 30

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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.

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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 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 , 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

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the member. The Esquibel is overlain Sandstones in the Esquibel Member are Agua which overlies the basalt in that locally by the Cordito Member. fine to coarse grained, moderately well canyon by about 10 m. This is a more map- The Esquibel Member is predominantly a sorted and laminated or cross-bedded; cross- pable contact because gravel clasts derived poorly lithified, poorly to moderately well beds 3 m in height were observed. Sand- from the tuff of Cañada del Agua are re- sorted, sandy conglomerate interbedded stone is composed of feldspar and quartz, cognizable in conglomerates throughout the with gravelly sandstone, mudflow deposits, abundant volcanic rock fragments, pumice Cordito, whereas the Jarita Basalt Member and shale. Sandstones and conglomerates fragments, volcanic glass, and heavy miner- is not everywhere present. The Cordito is are white to blue-gray; mudflows are gener- als, many of which appear to be phenocrysts overlain by flows of the Servilleta Forma- ally light brown. weathered from pumice fragments. tion, and in part, by the sedimentary rocks Volcaniclastic conglomerates are most Mudflow deposits and claystone units of the Santa Fe Group on the east; it also common in the Esquibel. Gravel clasts as occur locally. The former are typically non- underlies and interfingers with the Santa Fe large as 2 m are present and generally sub- sorted conglomeratic sandstones containing Group to the south near Ojo Caliente (May, rounded, although rounding is highly vari- clasts common to conglomerates. The rare 1980). The Cordito Member interfingers able. The largest clasts observed are in the claystones are either pale pink or white clay with basalt flows of the Hinsdale Formation

Bighorn Peak Quadrangle (SW'/4 sec. 2, T. altered from volcanic ash, or fissile, light- from Broke Off Mountain to La Madera, 31 N., R. 7 E.), although boulders as long as brown silty claystone. and with the tuffs of Cañada del Agua and 1 m are present in conglomerates exposed In the northern Tusas Mountains, the Las Tablas in the Las Tablas Quadrangle on Tusas Ridge near the head of Esquibel Esquibel Member is difficult to distinguish (Manley and Wobus, 1981a). In the north- Canyon (Fig. 2). Clast compositions include from the Treasure Mountain Tuff sedimen- ern La Madera Quadrangle, the Cordito felsic and intermediate volcanic rocks, pum- tary facies (Lipman, 1975, p. 16). Some also interfingers with flow breccias. ice fragments, minor basalt, and a variety of areas have undoubtedly been mismapped The Cordito Member is a coarse, volca- Precambrian rocks including schist, granite, because of the poor exposures. The primary, niclastic sandstone to boulder conglomerate and quartzite. Intermediate volcanic rocks although not always reliable, distinguish- containing some mudflow deposits. The typically contain large (<1.8 cm) plagio- ing characteristic of the Treasure Mountain sandstone is light brown, moderately clase phenocrysts. Compositions of gener- Tuff sedimentary facies is the presence of well sorted, and bedded; poor cementation ally fibrous pumice clasts vary: common highly altered and weathered, subrounded, makes good exposures rare. phenocrysts are sanidine and biotite, or pla- blue-gray, red, gold, and brown volcanic The conglomerate is generally poorly gioclase and hornblende, with or without clasts, and the absence of abundant volcanic sorted, locally crudely laminated or cross- biotite. In some exposures, especially in the clasts of intermediate composition with bedded, and is well cemented with silica. northern Tusas Mountains, clasts include large felspar phenocrysts. Clasts, angular to well rounded, consist of highly silicified volcanic rocks that weather intermediate- and felsic-composition vol- white or gold. These may be reworked from Cordito Member canic rocks, especially ash-flow tuffs, pum- clastic units in the Treasure Mountain Tuff. ice fragments, minor mafic volcanic rocks, Conglomerates commonly fill channel cuts The Cordito Member was named by and Precambrian schist, quartzite, and into underlying sandstone or mudflow de- Butler (1946, p. 70) for a canyon 6.5 km metarhyolite. Boulders as long as 2 m are posits. south of Tres Piedras, Canyon de Cordito, present in beds above the Rio Tusas Box; Near or at the base of the Esquibel which is shown on the U.S. Geological Sur- 1.0 to 1.2-m-long boulders are present in Member is a generally well cemented con- vey topographic map of the area (Petaca exposures throughout the area: in Cleve- glomerate composed primarily of Precam- Peak Quadrangle) as Cañón de Tio Gor- land Gulch, Lamy Canyon, Esquibel Can- brian clasts. A few volcanic clasts can dito. The Cordito Member extends along yon, and Cañada del Borracho. On Tusas be found in large exposures, but their the east half of the Tusas Mountains from Ridge, both north and south of U.S. High- rarity makes many outcrops indistinguish- Broke Off Mountain southward to within 1 way 64, there are single 4.5-m-long boulders able from the El Rito Formation. Northeast km of El Rito. Thickness varies from 0 to of banded gray rhyolite. Ash-flow-tuff clasts of the Rio Tusas Box, this basal conglomer- 120 m. The absence of the Cordito in the are abundant in the conglomerates and ate is as thick as 45 m and is composed of north is due to nondeposition; in the west, it serve to distinguish the Cordito Member moderately well sorted, subangular pebbles appears to have been removed by erosion. from all other units in the area. Several tuffs and cobbles; boulders are rare. Farther The Abiquiu Tuff of Smith (1938) is proba- are present; most contain quartz and/or north, along the Rio Tusas at the southern bly a remnant of the portion of the Cordito sanidine phenocrysts, and biotite with or margin of the Broke Off Mountain Quad- Member that extended southwestward to- without hornblende. rangle, this unit is thinner (generally 9 m) ward the present-day Nacimiento Moun- Mudflow deposits in the Cordito gener- and composed of moderately well rounded, tains. ally include a variety of unsorted gravel predominantly pebble-sized clasts. In the The Cordito overlies Precambrian rocks, clasts and sand, but some deposits are com- latter area, and in Mule Canyon Quadran- the El Rito Formation, the Conejos(?) posed almost exclusively of angular clasts of gle (N'/2 sec. 5 and SE'/4 sec. 9, T. 28 N„ Formation and the Esquibel Member. The gray rhyolite. In places, this gray rhyolite R. 8 E.), this Precambrian-clast-rich con- basal contact of the Cordito was considered breccia overlies the Conejos(?) Formation glomerate in the Esquibel Member can be to be the top of the Jarita Basalt Member by or lower Esquibel Member, elsewhere it readily observed overlying the Treasure Butler (1946, p. 71), lut I prefer to place the occurs within the Cordito Member. It is Mountain Tuff and Masonic Park Tuff. contact at the base of the tuff of Cañada del found in small outcrops in the Mule

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Canyon, Las Tablas, and Petaca Peak tains but particularly common in the north. The Abiquin Tuff of Smith (1938), which Quadrangles (Manley and Wobus, 1981a, As volcaniclastic material began accumulat- consists of three members, according to 1981b). It consists of angular, unsortecl, ing, erosion of the Precambrian-cored hills Vazzana (1980) and Vazzana and Ingersoll unbedded fragments of nearly aphanitie, continued, as evidenced by the clast compo- (1981), contains numerous clasts of ash- nonbanded gray rhyolite clasts 1 to 1.5 cm sition of the basal Esquibel Member. flow tuffs in its Upper Member. Many are long on the average (15 cm is maximum During the late Oligocene, several ash- the same as those in the Cordito Member of observed); traces of other lithologies include flow tuffs, the Treasure Mountain Tuff and the Los Pinos Formation, including the tuff basalt, intermediate-composition volcanic the Masonic Park Tuff, erupted from the of Cañada del Agua. The Upper Member of rocks, Precambrian schist, and Treasure Platoro and Mount Hope calderas, respec- the Abiquiu Tuff is considered by me to be Mountain Tuff. An exposure near Guydo tively (Lipman, 1975). These tuffs extended an equivalent in age and source area to the Canyon (Mule Canyon Quadrangle) con- 37 km south of the Colorado line to the Cordito Member and was deposited as a tains roughly 40% clasts in sand-sized matrix vicinity of the Rio Tusas (Manley and tongue to the southwest from the Tusas and is at least 10 m thick. In other expo- Wobus, 1981b; Wobus and Manley, 1981). Mountains area (Fig. 3, unit a). East of the sures, such as that north of Petaca Mesa A third tuff, the tuff of Cañada del Agua, town of Abiquiu, the Abiquiu Tuff grades (Petaca Peak Quadrangle), the rhyolite erupted from an unknown source area to upward into the Chama-El Rito Member clasts formed an open-work gravel that is the east of the Tusas Mountains, and possi- (Galusha and Blick, 1971) of the Tesuque now cemented with silica. It is possible that bly south of Tres Piedras. Formation (May, 1980). The upper contact the gray rhyolite breccia represents a vol- The Los Pinos Formation was deposited here is dated between 16 m.y. and 17 m.y. canic flow breccia from local vents. Further simultaneously from several source areas. (Manley and Mehnert, 1981). This age is in study is needed. The Esquibel Member formed as a detrital reasonably close agreement with that of 18 apron spreading southward from the San m.y. suggested by Baldrige and others HISTORY Juan Mountains of Colorado. The Cordito (1980) and May (1980). Member spread from the east into the area The Chama-El Rito Member has a clast Prior to the deposition of the Los Pinos of the southern Tusas Mountains. The basal composition different from the Cordito and Conejos(?) Formations, the area of the contact of the Cordito is about 2 m.y. Member: porphyritic, intermediate-compo- Tusas Mountains was one of highly irregu- younger than that of the Esquibel. Both sition volcanic clasts dominate, and rhy- lar topography consisting of hills of Pre- members continued to be deposited into olitic ash-flow-tuff clasts are sparse in the rock partially buried by remnants the late Miocene. Compositions of rhyol:itic former. This compositional difference gives of Mesozoic strata, especially along the west tuff clasts in the Cordito suggest several the Chama-El Rito Member a resem- side, and by the Eocene El Rito Formation. additional ash-flow tuffs in the source area. blance to the Esquibel Member of the Los In the Oligocene, local eruptions of flow One is similar to a tuff that is presumed Pinos; however, it is my opinion that the breccia began, and volcaniclastic detritus to be from the caldera near Questa, New source of the Chama-El Rito volcanic accumulated around volcanic centers that Mexico (P. W. Lipman, 1980, personal rocks is not the San Juan Mountains. Addi- were scattered throughout the Tusas Moun- commun.). tional study is needed. The Los Pinos Formation generally inter- fingers with the Santa Fe Group (Manley, N s 1979; May, 1980). The Santa Fe Group in the southern Tusas Mountains also overlies the Cordito Member. The Santa Fe Group was derived primarily, although not entire- ly, from nonvolcanic sources such as the Sangre de Cristo Mountains. Both the Los Pinos Formation and the Santa Fe Group were deposited in a broad Miocene basin that extended to at least the west edge of the 15.9 m.y.- old present Tusas Mountains and Nacimiento basalt dike uplift. These units were deposited prior to the formation of the Rio Grande rift as a fault-bounded structure (Manley and I I Los Pinos Formation • Treasure Mountain Tuff Mehnert, 1981). [M Tuff of Canada del Agua Conejos Formatiori-volcaniclastic facies In the late Miocene and Pliocene, tecto- Masonic Park Tuff F:I Conejos Formation-flow breccia facies nism changed the depositional pattern in 1/ of Smith, 1938 the area of the Tusas Mountains. The Tusas Mountains were relatively uplifted as the Figure 3. Diagrammatic, longitudinal cross section of the Tusas Mountains, New Mex- Rio Grande rift downdropped and the San ico. Precambrian-clast-rich bed in the Esquibel Member indicated by q; possible position Juan Basin was excavated. of Abiquiu Tuff of Smith (1938) indicated by a. Arrows indicate direction of transport from The Tusas Mountains were uplifted and different source areas. faulted following deposition of the Los

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Pinos Formation. Prominent faults in the New Mexico Geological Society Guidebook Survey map (in press). area are dip-slip, trend between N35°W and 22, p. 289-300. 1981b, Reconnaissance geologic map of the N50°W, and have relative movement down Galusha, Ted, and Blick, J. C„ 1971, Stratigraphy Mule Canyon Quadrangle, New Mexico: of the Santa Fe Group, New Mexico: Amer- U.S. Geological Survey map (in press). to the southwest. The faults bound a series ican Museum of Natural History Bulletin May, S. J., 1980 Geology of the Ojo Caliente-Rio of blocks tilted eastward toward the Rio 144, 127 p. Chama area, Rio Arriba and Taos Counties, Grande rift. Within blocks, there is little Larson, E. S., Jr., and Cross, Whitman, 1956, New Mexico [Ph.D. thesis]: Albuquerque, deformation; folding is rare and confined to Geology and petrology of the San Juan re- New Mexico, University of New Mexico, 198 p. fault zones. At the south end of the Tusas gion, southwestern Colorado: U.S. Geo- logical Survey Professional Paper 258, 303 p. Smith, H.T.U., 1938, Tertiary geology of the Abi- Mountains, the fault trend changes abruptly Lipman, P. W., 1975, Evolution of the Platoro quiu Quadrangle, New Mexico: Journal of to a dominant N40°E to N70°E direction. caldera complex and related volcanic rocks, Geology, v. 46, p. 933-965. southeastern San Juan Mountains, Colo- Steven, T. A., Mehnert, H. H., and Obradovich, rado: U.S. Geological Survey Professional J. D., 1967, Age of volcanic activity in the REFERENCES CITED Paper 852, 128 p. San Juan Mountains, Colorado: U.S. Geo- Lipman, P. W., and Mehnert, H. H., 1975, Late logical Survey Professional Paper 575-D, Atwood, W. W„ and Mather, K. F., 1932, Phy- Cenozoic basaltic volcanism and develop- p. D47-D55. siography and Quaternary geology of the ment of the Rio Grande depression in the Vazzana, M. E., 1980, Stratigraphy, sedimentary San Juan Mountains, Colorado: U.S. Geo- : Geological So- petrology and basin evolution of the Abiquiu logical Survey Professional Paper 166, 176 p. ciety of America Memoir 144, 119-154. Formation, north-central New Mexico [M.S. Baldridge, W. S., Damon, P. E., Shafiqullah, M., Lipman, P. W., Steven, T. A., and Mehnert, thesis]: Albuquerque, New Mexico, Univer- and Bridwell, R. J., 1980, Evolution of the H. H., 1970, Volcanic history of the San sity of New Mexico, 115 p. Rio Grande Rift, New Mexico: new po- Juan Mountains, Colorado, as indicated by Vazzana, M. E., and Ingersoll, R. V., 1981, Strati- tassium-argon ages: Earth and Planetary K-Ar dating: Geological Society of America graphy, sedimentology, petrology and basin Science Letters, v. 51, p. 309-321. Bulletin 81, p. 2329-2352. evolution of the (Oligo- Barker, Fred, 1958, Precambrian and Tertiary Manley, Kim, 1979, Stratigraphy and structure Miocene), north-central New Mexico: Geo- geology of Las Tablas Quadrangle, New of the Española basin, Rio Grande rift, logical Society of America Bulletin, v. 92 Mexico: New Mexico Bureau of Mines and New Mexico, in Riecker, R. E., ed., Rio (this issue). Mineral Resources Bulletin 45, 104 p. Grande rift: Tectonics and magmatism: Wobus, R. A., and Manley, Kim, 1981, Reconnais- Bingler, E. C., 1968, Geology and mineral re- Washington, D.C., American Geophysical sance geologic map of the Burned Mountain sources of Rio Arriba County, New Mexico: Union, p. 71-86. quadrangle, New Mexico: U.S. Geological New Mexico Bureau of Mines and Mineral Manley, Kim, and Mehnert, H. H„ 1981, New Survey map (in press). Resources Bulletin 91, 158 p. K-Ar ages for Miocene and Pliocene vol- Butler, A. P., Jr., 1946, Tertiary and Quaternary canic rocks in the northwestern Española geology of the Tusas-Tres Piedras area, New basin, and their relationships to the history Mexico [Ph.D. thesis]: Cambridge, Mas- of the Rio Grande rift: Isochron/West, no. sachusetts, Harvard University, 188 p. 30, p. 5-8. 1971, Tertiary volcanic stratigraphy of the Manley, Kim, and Wobus, R. A., 1981a, Recon- MANUSCRIPT RECEIVED BY THE SOCIETY eastern Tusas Mountains, southwest of the naissance geologic map of the Las Tablas SEPTEMBER 21, 1981 San Luis valley, Colorado-New Mexico: Quadrangle, New Mexico: U.S. Geological MANUSCRIPT ACCEPTED SEPTEMBER 21, 1981

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