New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/22 The Summer Coon volcano, eastern San Juan Mountains, Colorado Mertzman, Stanley A., Jr, 1971, pp. 265-272 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. 265 THE SUMMER COON VOLCANO, EASTERN SAN JUAN MOUNTAINS, COLORADO by STANLEY A. MERTZMAN, JR. Case-Western Reserve University Cleveland, Ohio FIGURE 1. Central core area of Summer Coon volcano. INTRODUCTION underlie approximately seventy square miles, which include parts of four 1:24000 quadrangles (Twin Mountains, Twin The San Juan volcanic field extends over approximately Mountains S.E., Indian Head, and Del Norte). Pre- 5,000 square miles in the southwestern part of Colorado. vious work in the area involves only two field-based The stratigraphic section, which is applicable to the cen- studies; a fast reconnaissance visit to the area by Larsen tral and eastern segments of the volcanic field, is listed in and Cross (1956) and a detailed reconnaissance project Table 1. The pioneering work in this area was performed by Lipman (1968 ), which involved outlining the basic by E. S. Larsen, Jr. and Whitman Cross, spanning nearly field relationships of the complex, examination of thin forty years and culminated in U.S. Geological Survey sections and several chemical analyses. Subsequently Doe Professional Paper 258, which summarized the geology et al. (1969) published lead and strontium isotope data and petrology of the entire region. Since 1960 an extensive for three samples from the Summer Coon area and Lip- program of field mapping has been carried on by the Den- man et al. (1970) reported three potassium-argon age ver Branch of the U.S. Geological Survey. The data pub- dates. lished by the various coworkers of this program, including R. G. Dickinson, W. R. Hansen, P. W. Lipman, R. G. Luedke, J. C. Olsen, J. C. Ratte, and T. A. Steven, are SUMMER COONS RELATIONSHIP listed in the bibliography. TO REGIONAL GEOLOGY The specific area of interest is called the Summer Coon In a general manner the geologic map of the Summer volcano. The intrusive rocks that appear to mark the cen- Coon area (see plate 1, back pocket) shows two relation- tral conduit of the volcano are located 61/2 miles north of ships which are important to the interpretation of the over- Del Norte, Colorado (fig. 1). Rocks related to the volcano all geologic development of the eastern San Juan volcanic Presently Division of Volcanology, Smithsonian Institute, Wash- field. The first is the nearly perfect pattern of radial dikes ington, D.C. about the central intrusive area. No transverse or ring dike 266 NEW MEXICO GEOLOGICAL SOCIETY—TWENTY-SECOND FIELD CONFERENCE TABLE 1. FIELD GEOLOGY GENERALIZED VOLCANIC STRATIGRAPHY OF THE GENERAL AND EASTERN PARTS OF THE The initial development of the Summer Coon volcano SAN JUAN MOUNTAINS COLORADO was upon a terrain of older volcanic rocks of rhyodacitic (with mean ages of dated units). composition. A window of these rocks outcrops just north (Modified from Lipman et al., 1970) of La Garita Creek along the boundary between NE1/4 Late Basalts and Rhyolites Sec 6, T41N, R6E, and NW 1/4 Sec 5, T41N, R6E (see Servilleta Formation of Montgomery, 1953 (3.6 to 4.5 m.y.) Hins- Plate 1, hack pocket). A rock from this outcrop has been dale Formation dated at 34.0 ± 1.5 m.y. by Lipman et al. (1970), which Basalt (4.7 to 23.4 m.y.) provides a maximum age for the complex. Rhyolite (4.8 to 22.4 m.y.) The Summer Coon volcanics are part of the Conejos Lavas and Related Rocks Erupted Concurrently with Formation as outlined in Table 1. This correlation is the Ash-flow Tuffs based on the potassium-argon dates reported in Lipman Local andesitic-quartz latitic flows and breccias that intertongue et al. (1970). The stratigraphic nomenclature that will be with the ash-flow sequence in and near the central San Juan com- used in this paper is outlined in Table 2. This sequence plex. The Fisher Quartz Latite (26.4 m.y.) overlie the entire ash- flow sequence. TABLE 2. Ash-flow Tuffs VOLCANIC STRATIGRAPHY OF THE Snowshoe Mountain Quartz Latite (greater than 26.4 m.y.) SUMMER COON SHIELD SEQUENCE, COLORADO Rat Creek and Nelson Mountain Quartz Latite Wason Park Rhyolite `(.3 Late Upper Andesite Member Mammoth Mountain Rhyolite (26.7 m.y.) Intermediate Carpenter Ride Tuff-Bachelor Mountain Rhyolitc –o" Unit Lower Pyroclastic Member Fish Canyon Tuff-La Garita Quartz Latite (27.8 m.y.) 0 0 Tuff of Masonic Park (28.2 m.y.) LT-, Middle Upper Rhyolite Member Treasure Mountain Rhyolite (29.8 m.y.) C5 O Silicic cl.) Early Intermediate Lavas and Breccias 0o Unit Lower Rhyodacite Member Conejos Formation (31.1 to 34.7 m.y.) 0 Early Mafic Unit pattern has been discovered, nor any indication that a graben type structure was ever developed in the core area. is based solely upon field geologic relationships since no These characteristics, by analogy with a scale model study radiometric dating was performed. of salt dome intrusion by Parker and McDowell (1951) Rocks derived from the Summer Coon volcano can be indicate that only the mildest doming took place during divided into two distinct groups: a shield sequence of the Summer Coon intrusive and extrusive activities. In flows, breccias, and dikes, which can be further subdivided addition, the regional stress regime must have been iso- into three units and a central intrusive complex which tropic or only mildly anistropic, otherwise one would ex- includes some intra-caldera lava flows. Each of the three pect the dike pattern to become more complex, as it did units of the shield sequence consists of varying proportions at Spanish Peaks where the regional stress system was at of breccias, flows and dikes. The lower part of this shield least moderatly anistropic. sequence is overlain by younger lava flows and breccias of The second important relationship is the asymmetrical the Conejos Formation to the northwest, by younger ash- distribution of dips about a north-northeast line through flow tuffs to the north and east, by volcaniclastic sediments the center of the complex. To the cast and southeast of the to the south (some of which is probably detritus eroded core area dips range from 25° to 35°; on the north and from Summer Coon itself ), and is intruded and hydro- west sides the dip vary between 5° and 15°. This demon- thermally altered by rhyolite plugs and vents to the east. strates that the Summer Coon area has been tilted east- southeastward toward the San Luis Valley. This regional EARLY MAFIC UNIT warping must have occurred after the cessation of volcanic The Early Mafic Unit is 2800-3100 feet thick, of which activity because the original dip of the flows and breccias 85-90% consists of weakly stratified breccia occurring in around the caldera was probably essentially symmetrical. beds 5 to 75 feet thick and having essentially uniform Moreover, if the volcano did form during the tilting appearance throughout the entire sequence. In general the process, the dike pattern probably would have reflected an breccia is composed of angular blocks 10 to 15 inches anisotropic stress condition. across which contain few if any vesicles. A matrix of coarse These facts demonstrate that the initial development of tuff (Williams and McBirney, 1969) is usually present, the Rio Grande depression could not have taken place although occasionally the intervening space between blocks prior to the extinction of the Summer Coon volcano (Lip- is empty. The tuff contains the same minerals as those man, 1968). Lipman and Mehnert (1969) have shown constituting the breccia blocks, as determined by oil im- from other evidence that subsidence of the Rio Grande de- mersion methods.