New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/19 Multiple intrusions in the San Miguel Mountains, Colorado Calvin S. Bromfield and Alfred L. Bush, 1968, pp. 94-99 in: San Juan, San Miguel, La Plata Region (New Mexico and Colorado), Shomaker, J. W.; [ed.], New Mexico Geological Society 19th Annual Fall Field Conference Guidebook, 212 p. This is one of many related papers that were included in the 1968 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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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. 94 MULTIPLE INTRUSION IN THE SAN MIGUEL MOUNTAINS, COLORADO' By CALVIN S. BROMFIELD AND ALFRED L. BUSH U.S. Geological Survey, Denver, Colo. INTRODUCTION Precambrian to Mesozoic. On the west this mountainous region descends abruptly to the lower, generally flat-lying, H. Holmes, the famous artist-geologist, visited the W. sedimentary rocks of the great Colorado Plateau. western San Miguel Mountains briefly in the summer of The San Miguel Mountains are a west-trending outlier 1876 and recognized the laccolithic features of some of the of the San Juan Mountains that rises abruptly from the intrusions. Cross and Purington (1899) established the mesa surface of the Colorado Plateaus Province to eleva- basic geologic framework of the eastern half of the San tions of over 14,000 feet ( fig. 1 ). Their principal geologic Miguel Mountains, as a part of Cross' and co-workers' ex- feature is an extensive east-west zone of intrusion into tensive geologic studies in the San Juan Mountains. In nearly flat-lying sedimentary and volcanic rocks of Meso- recent years the U.S. Geological Survey has undertaken zoic and Tertiary age. At the west end a group known as studies in the western San Juan Mountains which includes the Dolores Peaks is formed by the Dolores Peak stock and the San Miguel Mountains. This articles describes briefly associated laccoliths; about 3 miles to the east the Wilson the regional geologic setting and some aspects of multiple Mountains, a larger group, have the Wilson Peak stock and intrusion in the San Miguel Mountains and is chiefly a the dual eastward-trending prongs of the Black Face and summary of published data (Bromfield, 1961, 1967; Bush Ames plutons as their dominant geologic feature. The and Bromfield, 1966). size and relation of these intrusive bodies can best be GENERAL GEOLOGIC SETTING understood by reference to figures 2 and 3. The San Juan Mountain region of southwest Colorado, STRATIGRAPHY an area of several thousand square miles, is composed in In the San Miguel Mountains the outcropping sedi- large part of a vast volcanic pile of Tertiary age which has mentary rocks range in age from Late Jurassic to Eocene been deeply dissected into a rugged mountainous region. and aggregate approximately 4,000 feet in thickness. Over- Erosion has exposed underlying rocks ranging in age from lying them are about 1,000 feet of Tertiary tuffs, breccias, 'Publication authorized by the Director, U. S. Geological Survey and flows. Various surficial deposits of Quaternary age— erosion remnants of a volcanic sequence once continuous talus, rock streams, landslide deposits, glacial deposits— with the thick San Juan volcanic pile just east of the map locally cover the older rocks. area. The oldest sedimentary unit exposed is the Upper The relation and relative thickness of the stratigraphic Jurassic Salt Wash Sandstone Member of the Morrison units are shown in table 1. Formation which is found along the valley of the Lake Fork of the San Miguel River in the extreme northeast INTRUSIVE ROCKS corner of the area. This is overlain by the Brushy Basin BLACK FACE AND AMES PLUTONS Shale Member of the Morrison Formation. The Morrison The Black Face pluton forms an elongate east-trending Formation in turn is overlain disconformably by the body southeast of the Wilson Peak stock (fig. 2). The Lower (?) and Upper Cretaceous Dakota Sandstone and the thick marine shales of the Upper Cretaceous Mancos Ames pluton, east of the stock and in contact with it, Shale, the most widespread of the outcropping formations. forms a narrow outcrop which crosses the northeast part The Eocene Telluride Conglomerate overlies the Mancos of the area (fig. 2); the outcrop makes a spectacular cliff Shale with a very slight angular unconformity. The high- along the west side of the Lake Fork of the San Miguel est rock units, both stratigraphically and topographically, River about a mile west of Ophir where it takes the form are the San Juan Formation of Oligocene or older age, and of a sill as much as 800 feet thick. The Ames and Black a part of the Silverton Volcanic Group of middle to late Face plutons probably connect in the subsurface. Both Tertiary age. These units overlie the Telluride Conglom- plutons are composed of granodiorite porphyry of identical erate and along some of the high ridges form scattered appearance. 96 NEW MEXICO GEOLOGICAL SOCIETY-NINETEENTH FIELD CONFERENCE The typical rock is medium to dark gray on fresh frac- and at the east end of the intrusive mass. At both places ture, and its texture is everywhere porphyritic. Through the cap of Mancos Shale lies at relatively low angles in con- increase in coarseness of the groundmass, much of the rock cordant relation to the underlying granodiorite porphyry. is seriate porphyritic, whereas in some places the contrast To the north the strata gradually arch over into steeper between phenocrysts and groundmass is more distinct and dips, concordantly following the intrusive's roof. the rock is more nearly a true porphyry. Phenocrysts of In contrast, along the south side of the Black Face plagioclase, clinopyroxene, hornblende, and rarely ortho- pluton the Mancos Shale is at steep angles, commonly pyroxene, which range in size from 0.5 to 3.0 mm, are set vertical to overturned, and probably in part is faulted in a groundmass of plagioclase, potassic feldspar, and quartz parallel to the contact. Locally adjacent to the pluton, which ranges from 0.01 to 0.2 mm in grain size. The mafic overturned Mancos Shale beds dip at 30°-70° into the con- phenocrysts tend to cluster together and give the rock a tact. The Mancos Shale is crumpled into small-scale over- characteristic mottled or dark flecked appearance. turned folds which diminish in intensity outward and pass In a few places the granodiorite porphyry contains into normal flat-lying strata within a thousand feet of the swarms of inclusions, principally of Precambrian rocks intrusive contact. The pluton itself apparently has a some- carried up from the basement, which here is a minimum what irregular but steep south-dipping contact along much of 6,000 feet below the outcrop. One of these inclusion of its south side, but in places it is vertical. swarms is strikingly exposed, near the field trip route just As with the Black Face pluton, the Ames pluton has south of Ames, along the abandoned narrow-gauge rail- both concordant and discordant relations to the enclosing road bed at the tight switchback known as Ophir Loop flat-lying sedimentary rocks and thus is not readily class- (about half a mile west of Ophir). ified, but in general it has the form of a thick irregular The form of the Black Face pluton and its relations to sill. Near the Wilson Peak stock, the Ames pluton is a the country rock suggest that it is an asymmetric laccolith, discordant body adjacent to which the strata are domed though no floor has been exposed by erosion. The roof and crumpled. The body grades northeastward into a sill rocks are still preserved near the summit of Black Face 600 to 800 feet thick. NEW MEXICO GEOLOGICAL SOCIETY-NINETEENTH FIELD CONFERENCE 97 WILSON PEAK STOCK boundary of the stock west into Navajo Basin, then east The highest and most impressive group of peaks in the and north along the stock contact into Silver Pick Basin. San Miguel Mountains, several of which reach above 14,- On the northeast, however, along the ridge leading east 000 feet, have the Wilson Peak stock at their core ( fig.