New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/22 Stratigraphic relations between Bonanza Center and adjacent parts of the San Juan volcanic field, south-central Colorado Dennis L. Bruns, Rudy C. Epis, Robert J. Weimer, and Thomas A. Steven, 1971, pp. 183-190 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. 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STEVEN2 INTRODUCTION northwest of Saguache, Colorado ( figs. 1 and 2), where Saguache Creek and its tributaries East Pass, Sheep, Jacks, Stratigraphic relations of units derived from the Bonanza Cross, Middle and Ford Creeks have cut numerous val- volcanic center in the northeastern part of the San Juan leys into the Cenozoic volcanic rocks. These exposures volcanic field and units from sources in the central and show that many of the rocks from the Bonanza center are western parts of the field are well exposed in the Trickle generally equivalent in age to the early intermediate rocks Mountain and Lake Mountain NE quadrangles, 8-16 miles in the rest of the San Juan volcanic field, and are older Colorado Area of Fig.1 1 0 10 20 Miles 285 West Elk Scale Volcanic Field ° Gunnison Salida ° V I I , // -- r Cation City ° Rocks of the Bonanza Volcani / I Center CO Rosita Hills - ° Bonanza 0- co Trickle Mtn. and Silver Cliff Lake Mtn. NE Volcanic Field - Quadrangles u, 0 –_ C- . C.. .g.-- -- ., 0 Saguache ,.c. c. ...■ / .- -.... .e... / -1. 0 // C. -- San Juan Volcanic Field / „ -- ... --- CO--- ...c. - -- _ FIGURE 1. Location of the Trickle Mountain and Lake Mountain NE quadrangles in the San Juan volcanic field, Colorado. 1 Colorado School of Mines, Golden, Colorado 2 U.S. Geological Survey, Denver, Colorado •opu.moD `ping DILIU310A tonf ttus `saftuipunb utu4unow alul pui ItImunolAI °ppm, Jo dm ot2oloao • z axnom auliouAs 6u!6unld nnod -4 1- iso.ps -3 ss!au6 UD!.1qWD3aJd )199.10 pod jo aosaPuli U4 4 allop1 40 ai!sapuv smoo8 -4°1-,, owooloA jo d!P aNtilS pamoaJawpunsuoitow.iod uospioN-olomoo ;pi osavi oiau!dos iinl a6p9d Ja.tuadloo • 31 \ 0 z aysapuv Aa1mo8 4aaJD aoortos jo apsapuv (llosoci) uoq.owiod alopsuil-1 1\1 9100S 4 9111N ozuouoe ;pi uoAuop tis!d tunvo115 0 / / I too l pf 2Z.901. .siosoi 398 398 308 308 30a pEosot Of L .8f of .1.008f CZ) z 0 C/) z E-1 r Z.5 O 00 /-4 015b1 8501 0 Fa N 901 N901 C ;.T4 O C2, X • Ay .41 ...mu. so F. 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Much of the earlier work They used the term Conejos Formation for the early in- was summarized by Larsen and Cross (1956), who in- termediate rocks in the central and eastern parts of the cluded the volcanic rocks in and adjacent to the southern San Juan volcanic field, and on the basis of radiometric and western parts of the area described here in the Cone- ages, suggested that the Bonanza volcanic pile was cor- jos Ouartz Latite, Treasure Mountain Rhyolite, Sheep relative (fig. 3). Mountain Quartz Latite, Alboroto Rhyolite, and Piedra The volcanic rocks of the Bonanza center were first Rhyolite of the Potosi Volcanic Series (fig. 3). described by Patton (1916), and were treated in more de- Later work in the western San Juan Mountains by tail in a comprehensive report by Burbank (1932). A sum- Lucdke and Burbank (1963) led to redefinition of the mary of local stratigraphic terminology as proposed by Potosi Volcanic Series to Potosi Volcanic Group, and to Burbank is shown in figure 3. include a specific assemblage of ash-flow units there. Recent graduate thesis research by students from the Steven and Ratte (1965, p. 13) abandoned use of the Colorado School of Mines (Mayhew, 1969, Bruns, 1971, Potosi in the Creede area in the central part of the San Perry, 1971, and Marrs, in progress), has expanded the Juan volcanic field. Olson, Hedlund, and Hansen (1968) knowledge of the Bonanza pile outward from the central re-studied ash-flow tuffs in the Powderhorn-Gunnison mineralized area studied by Patton and Burbank. Burbank River region where they defined the Blue Mesa, Dillon (1932) considered the Hayden Peak Latite to be younger Mesa, Sapinero Mesa, Fish Canyon, and Carpenter Ridge than the Rawley Andesite, but additional work by May- Tuffs. These units were formerly mapped by Larsen and hew (1969) indicates that in large part the two units in- Cross as the Alboroto and Piedra Rhyolites. Olson, Hed- tertongue. Patton (1916) originally named the Bonanza lund, and Hansen (1968) retained the name Conejos Latite, and this usage was followed by Burbank (1932). Quartz Latite where previously used and included it, to- Mayhew (1969) identified the Bonanza Latite as an ash- gether with the Lake Fork Formation and the West Elk flow tuff and emphasized its stratigraphic and structural Breccia, as part of an assemblage of "older volcanic rocks importance. The ash-flow sheet has since been traced of Tertiary age" (fig. 3). from the northeastern and central parts of the Bonanza Recently, Lipman, Steven, and Mehnert (1970) inter- volcanic pile southwestward into the Trickle Mountain preted the volcanic history of the San Juan field on the and Lake Mountain NE quadrangles by work of Marrs basis of potassium-argon dating. They presented a major and Bruns, and students of the Geology Summer Field MODIFIED AFTER MODIFIED AFTER LARSEN AND CROSS OLSON, HEDLUND, AND LIPMAN, STEVEN, AND (1956) HANSEN (1968 ) MEHNERT (1970) THIS REPORT BURBANK (1932 ) Hinsdale Formation .. Hinsdale Formation Hinsdale Formation Hinsdale Formation Basalt and Basalt (4.7-2 3.4 my.) Rhyolite and Rhyolite (4.8-22.4 m.y.) ASH-FLOW TUFFS AND RELATED ROCKS Piedra Rhyolite Carpenter Ridge Tuff Carpenter Ridge Tuff-Bachelor Carpenter Ridge Tuff Mtn. Rhyolite Alboroto Rhyolite Fish Canyon Tuff Fish Canyon Tuff-La Garita Fish Canyon Tuff Sheep Mtn. Quartz Qtz. Latite (27.8 m.y.) Andesite of Saguache Porphyry Peak Latite Creek Rhyolite Water-laid and air-fall Brewer Creek tuffs of Saguache Creek Latite Treasure Mtn. Rhyolite Sapinero Mesa Tuff Sapinero Mesa Tuff Squirrel Gulch Dillon Mesa Tuff Tuff of Masonic Park (28.2 my.) Andesite of Ford Creek Latite Blue Mesa Tuff Treasure Mtn. Rhyolite (29.8 m.y.) Bonanza Tuff Bonanza Latite EARLY INTERMEDIATE LAVAS AND BRECCIAS "OLDER VOLCANICS" Conejos Formation ( 31.1- Conejos Quartz Latite Conejos Quartz Latite, 34.7 my.), Lake Fork Formation, Hayden Peak Bonanza volcanic pile Lake Fork Formation, West Elk Breccia and San Juan Fm.