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New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/22 Tertiary volcanic stratigraphy of the eastern Tusas Mountains, southwest of the San Luis valley, Colorado-New Mexico Butler, Arthur P., Jr., 1971, pp. 289-300 in: San Luis Basin (Colorado), James, H. L.; [ed.], New Mexico Geological Society 22nd Annual Fall Field Conference Guidebook, 340 p. This is one of many related papers that were included in the 1971 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. Free Downloads NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks. Copyright Information Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. 289 TERTIARY VOLCANIC STRATIGRAPHY OF THE EASTERN TUSAS MOUNTAINS, SOUTHWEST OF THE SAN LUIS VALLEY, COLORADO-NEW MEXICO by ARTHUR P. BUTLER, JR. U.S. Geological Survey Denver, Colorado INTRODUCTION sociates (Lipman and Steven, 1970; Lipman and others, 1970), and with some results of work in New Mexico by Tertiary rocks in the upland of the southeast part of the Barker (1958), Bingler (1968), and Muehlberger (1968). San Juan Mountains, Colorado, and the Tusas Mountains, New Mexico (fig. 1), southwest of the San Luis Valley, consist of effusive and pyroclastic volcanic rocks that alter- OLDER ROCKS AND SURFACE BENEATH nate and interfinger with fluvially transported debris THE TERTIARY VOLCANIC SEQUENCE eroded from them. Locally, some fluvial beds consist en- The Tertiary rocks west of the southern part of the San tirely of material derived from Precambrian crystalline Luis Valley rest with marked unconformity on older rocks. rocks. Volcanic rocks predominate near the large centers In Colorado, the older rocks are exposed only at the of eruption in southern Colorado (Lipman and Steven, present southwest limit of the Tertiary volcanic sequence, 1970, fig. 1) but are generally subordinate to fluviatile but in New Mexico some parts of the basement were never sedimentary rocks in eastern Rio Arriba County, New completely covered by the Tertiary rocks, and other parts Mexico. Along the eastern margin of the upland south of have been exhumed by subsequent erosion. In most of the the San Luis Valley in New Mexico the top of the se- area described in this report (fig. 2) the exposed basement quence is represented by widespread flows of basaltic lavas. consists of metamorphic and igneous rocks of Precambrian Many major units of the sequence were first delineated age. Near their present western limit in southern Colorado during extensive investigations of the San Juan volcanic and in New Mexico the Tertiary rocks overlie unconform- field by Larsen and Cross (1956). Subsequent study by ably the beveled edges of Mesozoic sedimentary rocks, T. A. Steven and P. W. Lipman and their associates have which dip southwestward into the San Juan Basin (Larsen resulted in better definition of the relations among inter- and Cross, 1956; Muehlberger, 1968). fingering volcanic units erupted from different centers, in The Precambrian terrane beneath the volcanic sequence establishing a chronology of events based on radiometric in New Mexico represents the southeastern continuation determinations of age that fits well with observed strati- of the Uncompaghre-Needlc Mountain highlands of Colo- graphic relations (Lipman and others, 1970) and in an rado and, like them, was uplifted during Laramide orogeny. improved understanding of the petrologic evolution of Prior to deposition of the Tertiary volcanic rocks erosion the rocks. had removed any Paleozoic and Mesozoic rocks that may The units of the volcanic sequence established by Lar- have once covered the older rocks and had sculptured a sen and Cross in the southeastern San Juan Mountains in fairly rugged topography on them. The maximum relief of Colorado were traced southward by the author (Butler, this topography approached 2,000 feet in the vicinity of 1946) and mapped in the eastern part of the Tusas Moun- T. 28 N., R. 8 E., and was 1,500 feet near the Rio de Los tains, New Mexico, and along the western margin of the Pinos in the northwest part of the mapped area. Taos Plateau. This mapping resulted in an interpretation of the rela- tion between the volcanic sequence and the Santa Fe TERTIARY ROCKS Formation, as used by Bryan (1938), that differed from INTRODUCTION earlier interpretations by Atwood and Mather (1932) and The Tertiary rocks within the area shown on the accom- Smith (1938) and forms the basis for the description of panying geologic map (fig. 2) comprise seven formations the Tertiary stratigraphy presented herein. The descrip- (table 1) and several local bodies of volcanic rock that tions presented here are modified, however, to conform accumulated around individual vents. Some of the forma- with the stratigraphy, nomenclature, and chronology as tions are composed largely of effusive and pyroclastic revised by P. W. Lipman and T. A. Steven and their as- volcanic rocks and subordinately of interbedded elastic Publication authorized by the Director, U.S. Geological Survey. sedimentary rocks; some largely of elastic sedimentary 290 NEW MEXICO GEOLOGICAL SOCIETY—TWENTY-SECOND FIELD CONFERENCE 107° 105° SOUTHERN ROCKY MOUNTAINS \ i I n 0°\)C\ / . co Rio 1 30°- z I \ QC\ Gronde,": a) , \ 1 \S .` 1 - I I O 0: ( v,. c > \ l:. 0 -1 i:- (r) \ \ -. k id \ Durango 0 I 0 \ 0 \ Z \ Z 0 k N CI U. 370 _COLORADO __ NEW MEXICO \r0 \ Ls+ 1 (1) I \ 0 , I — 1 \ tp LS 0 \ 4 Tres- i co 5 0/1 1Piedras a rea of .-, :::::... , 0 ,., / 10 (pf;;:r:-.... 1∎ cr Lii (i Figure 2 i ., /1 0._ a / <, ..77: , 1., 0 / - .- ?Taos I ) W Ll I (S) Ct \ -- 0 0 , a z cr a \ r 1 Id co I 0) . i it/ r C.) (1) l I tb \ L <I / / :,:Albuquerque 350 OCOrr0 Fi it 1 0 50 100 MILES FIGURE 1. Index map of south-central Colorado and north-central New Mexico showing locations of accompanying geologic maps, Figure 2 and of calderas mentioned in text; H, Mt. Hope; P, Platoro. NEW MEXICO GEOLOGICAL SOCIETY—TWENTY-SECOND FIELD CONFERENCE 291 TABLE 1. TERTIARY ROCKS OF THE EASTERN PART OF THE TUSAS MOUNTAINS AND ADJOINING PART OF THE TAOS PLATEAU, NEW MEXICO FORMATION THICKNESS AGE AND MEMBER DESCRIPTION ( FEET )1 Pliocene Servilleta Formation Flows of coarse-grained, porous, tholciitic basalt and interbedded gravel 10-100 Unconformity Pliocene Hinsdale Formation and Upper Basalt Member, Flows of xenocrystic, quartz-bearing alkalic basalt and basaltic andesite in 10-150 Miocene (Dorado Basalt of disconnected bodies Barker, 1958) Lower basalt member, Flows of fine-grained, slightly porphyritic, alkalic basalt in disconnected 10-50 (Cisneros Basalt of bodies Barker, 1958) I lypersthene Quartz Flows form San Antonio Peak and some other domes; relation to basalts 0-2000 Latite uncertain Rhyolitc of No Agua Dissected local mass of perlitic and spherulitic rhyolite undetermined Mountain Unconformity Pliocene (?) Santa Fe Formation as Fluvial and colian sandstone in southeast part of area 0-900 and used by Smith (1938) Miocene (?) Pliocene Los Pinos Formation to Oligocene Rhyolite Member Sandstone, conglomerate, tuff, and flows or ash flows in which rhyolitic rock 0-700 predominates Jarita Basalt Member Flows of basalt of locally different characteristics in many disconnected 10-100 of Barker (1958) bodies Undivided in northern Tuffaccous sandstone, conglomerate, and tuff, largely equivalent to two 0-700 part of area lower members in southern part of area Coarsely porphyritic Tuffaccous sandstone, conglomerate containing abundant clasts of coarsely 0-600 Ouartz Latite Member porphyritic quartz latite, and tuff Ouartz Latite-Andesite Breccia and conglomerate in which clasts of dark quartz latite or andesite 0-700 member predominate, tuff, and intrusive breccia Uncertain Ritito Conglomerate of Conglomerate of angular fragments of Precambrian rock. Present only in 0-400 Barker (1958) southwest part of area. Underlies rhyolite member of Los Pinos Forma- tion, correlation otherwise uncertain 0-275 Oligocene Tuff of Masonic Park Ash-flow tuff of quartz latite, mostly welded Oligocene Treasure Mountain Tuff Tuff, sandstone, conglomerate, and discontinuous welded ash-flow tuff near base. Absent south of center of T. 28 N., R. 8 E. Unconformity Oligocene Conejos Formation Breccia, flows, agglomerate, of varicolored andesite and quartz latite, tuff, 0-1000 and older (?) tuffaceous sandstone, and conglomerate, crudely and irregularly bedded 1The maximum thickness of different units are not superposed.

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