New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/22 Geology of the San Luis Hills, south-central Colorado Richard L. Burroughs, 1971, pp. 277-287 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. 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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. 277 GEOLOGY OF THE SAN LUIS HILLS, SOUTH-CENTRAL COLORADO by RICHARD L. BURROUGHS Geology Department Adams State College Alamosa, Colorado INTRODUCTION at 5 percent normative quartz after recalculation of end members to 100 percent. The San Luis Hills occupy an area of 428 square miles The author is greatly indebted to the Adams State Col- in the center of the San Luis Basin. They consist of lege Research Council and to a Penrose Bequest Research volcanic rocks of the Tertiary Conejos Formation intruded Grant provided by the Geological Society of America. He by Late Oligocene stocks dated at 27.4 ± 0.6 and 27.9 ± gratefully acknowledges their generous financial support. 0.6 m.y. Three local members of the Conejos Formation I would like to thank J. Paul Fitzsimmons for reading the are recognized; the Wildhorse, La Sauses, and Manassa manuscript. Members. The Conejos volcanics are rhyodacites of inter- mediate composition having a Si0 2 content ranging from PREVIOUS WORK 53 to 65 percent. The rock suite has a Peacock alkali-lime Very little previous work has ever been done in the San index of 55.8, falling on the boundary between the alkali- Luis Hills although the hills have been noted by several calcic and calc-alkalic classes. authors: Hayden (1869), Stevenson (1875, 1881), Con- Beginning with Wildhorse time the eruptions became kling (1876), Endlich (1877), Siebenthal (1910ab), Bryan more silicic through La Sauses time. With the beginning (1927ab, 1928, 1938), Atwood and Mather (1932), Upson, of eruptions in Manassa time the relationship was reversed J. E. (1938ab, 1939, 1941), Pearl (1941ab, 1942, 1957), as volcanism progressed, suggesting differentiation of the Larsen and Cross (1956), Powell (1958). Of the above magma chamber during an interval of quiescence in which the most extensive notations were by Atwood and Mather the La Sauses Member was subjected to considerable (1932), Upson (1938) and by Larsen and Cross (1956) erosion. The center of this volcanic activity appears to be although only two or three paragraphs from each reference in the general vicinity of the King Turquoise Mine. apply to the San Luis Hills. Larsen and Cross (1956, p. After volcanism a major north-south fault zone de- 87) published six chemical analyses from this area and veloped passing through the center of previous eruptions. Upson (1938, p. 48) mentioned seeing a field map of the This faulting resulted in uplift of the eastern San Luis area by E. S. Larson, Jr. This map, however, has never Hills relative to those west of the fault zone. Large dikes been published and its whereabouts is not known to the intruded along planes of weakness produced by the fault- author. ing. The course of the Rio Grande presently follows the fault zone although it is not controlled by it. PHYSIOGRAPHIC SETTING As subsidence and tilting of the San Luis Basin took The San Luis Basin, an intermontane structural depres- place, beginning in Miocene time, the hills were further sion, is made up of the San Luis Valley in south-central uplifted resulting in those east of the Rio Grande being Colorado and the Taos Plateau in north-central New broken into a series of southeastward tilting fault blocks. Mexico. On the east it is bounded by the Sangre de Cristo The hills were subsequently eroded to a mature topography Mountains and on the west by the San Juan Volcanic and surrounded by sediments of the Santa Fe Formation province and the Tusas uplift. The northern portion of as the basin continued to subside. In Late Pliocene time the San Luis Basin from Villa Grove to Poncha Pass was tholeiitic olivine-tholeiitic basalts of the Servilleta For- referred to as Homan's Peak by Endlich (1877, p. 140). mation flooded around the south and east margins of the Siebenthal (1910a, p. 9 and 10) described the basin as hills and became islands in a sea of lava. After deposi- a great lowland about 150 miles long and 50 miles wide, tion of additional sediments around the hills the area arbitrarily placing its southern limit about 15 miles south of was rejuvenated resulting in the Rio Grande being super- the Colorado border. Bryan (1938, p. 199) marks the posed across the hills and forming the La Sauses Gorge. southern end of the basin at the San Luis Hills where a The hills are presently being exhumed. shallow canyon has been developed by the Rio Grande 15 Normative classification of rock types is based on a miles north of the Ncw Mexico line. Upson (1939, p. 722) modified Johannsen (1939, p. 144) triangular diagram subdivided the basin into five physiographic provinces (see (fig. 2) in which normative quartz, potassium feldspar and Upson, this guidebook); the Alamosa Basin, the San Luis plagioclase feldspar are plotted to determine rock name. Hills, the Taos Plateau, the Costilla Plains and the The rhyodacite-trachyandesite boundary has been placed Culebra reentrant, a subdivision generally accepted today. 278 NEW MEXICO GEOLOGICAL SOCIETY—TWENTY-SECOND FIELD CONFERENCE Qa1 Mesita Mbr (Servilleta) Servilleto Manassa Mbr (Conejos) Lc Souses Mbr (Conejos) Sierra del Wildborse Mbr (Conejos) Oj ito li Mor ita + + -f- + + Stocks Dikes o /u Faults Syncline Colorado New Mexico Scale 1/250,000 FIGURE 1. Generalized geologic map of the San Luis Hills, Colorado. After perhaps a long, hard day in the field Stevenson one sees nothing but a dull repulsive plain of lava, from (1875, p. 422) described the basin along the Colorado- which rise the high basaltic domes known as Ute Peak and New Mexico state line as he looked eastward from the foot- Cerro San Antonio." Although I can at times understand hills of the San Juan Mountains: "From these mountains Stevenson's feelings, I would tend to agree more with the eastward, across San Luis Valley to the Spanish Range, descriptions of Pearl (1942, 1957) as he pointed out the NEW MEXICO GEOLOGICAL SOCIETY-TWENTY-SECOND FIELD CONFERENCE 279 Quartz The San Luis Basin includes all the physiographic pro- vinces of Upson (1939, p. 722) with its northern boundary at Poncha Pass and its southern boundary being defined by Kelley (1956, p. 109) as that of the Embudo constric- tion. On the east the basin is bounded by the Sangre de Cristo uplift along the high-angle Sangre de Cristo fault zone. On the west the boundary is that of the San Juan volcanic province in Colorado and the Tusas uplift in New Mexico. The basin, a complex hinged graben having an eastward tilt, began subsiding in Miocene time and has continued to subside to the present. Siebenthal (1910a, p. 51), and Bryan (1938, p. 204) noted that wide-scale volcanic activity and uplift of the adjacent mountains along with corresponding sedimentation began in Miocene time. This was also suggested by Kelley (1956, p. 113) who attributed the initial development of the Rio Grande depression to Potassium Feldspar Plagioclase F•Idspor the Late Miocene. From studies of the upper Arkansas FIGURE 2. River Valley, Chapin and others (1970, p. 159) found a series of paleovalleys trending at right angles to the present Normative composition of rocks from the Conejos Forma- day major structural and topographic features. Studies of tion plotted on a Johannsen diagram. Numbers refer to these valleys along with studies of the Thirtynine Mile analysis given in Table 1. volcanic field to the east show that Ash-Flow 1 of that area, dated at 36 m.y. (Early Oligocene), erupted west of remarkable combination of scenic beauty and natural the Salida-Buena Vista region prior to the development of wealth which the valley possesses—a land of many con- the upper Arkansas River Valley.