Downloaded from gsabulletin.gsapubs.org on May 4, 2011 Geological Society of America Bulletin Structural Evolution of the Cordillera Huayhuash, Andes of Peru PETER J CONEY Geological Society of America Bulletin 1971;82;1863-1884 doi: 10.1130/0016-7606(1971)82[1863:SEOTCH]2.0.CO;2 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. Downloaded from gsabulletin.gsapubs.org on May 4, 2011 PETER J. CONEY Department oj'Geology, Middlebury College, Middlebury. Vermont 05753 Structural Evolution of the Cordillera Huayhuash, Andes of Peru ABSTRACT 5,000 m high, extending in a northerly direc- An expedition to the Cordillera Huayhuash tion close to long. 75°55' W., between lat 10° in the north-central Andes of Peru has shown 4' and 10°25' S. Along its central portion the the range consists of complexly folded and range is dominated by six peaks more than 6,- thrust-faulted Cretaceous sedimentary rocks, 000 m high, culminating in Nevado Yerupaja and less deformed early to middle Tertiary(?) (6,634 m), the second highest mountain in andesitic volcanic rocks, all intruded by granitic Peru. The axial ridge of the Cordillera Huay- plutons thought to be as young as 9 m. y. B.P. huash forms the continental divide drained on A younger sequence of felsic ash flows has es- the west by tributaries of the Rio Pativilca, and caped folding and intrusion. Unlike the Cordill- by tributaries of the Rio Maranon on the east. era Blanca to the north (a batholith), the major Total relief in the region is about 4,000 m. 6,000 m summits of the Cordillera Huayhuash The Cordillera Huayhuash is most easily are carved from a complex synclinorium of Up- reached by 158 km of dirt road from Para- per Cretaceous Jumasha Formation carbonates. monga on the Pacific coast to Chiquian (3,400 The region has suffered two deformations. m), which is 35 km northwest of the range (Fig. The first deformation was the most severe, pro- 2). From the road-head at Chiquian, travel is on ducing tight flexural-slip and flexural-flow folds foot or horseback. Trail time to the foot of and east-directed thrust faults between Late glaciers is a two- to five-day effort. Cretaceous and middle(?) Tertiary. During No published systematic geologic study of deformation, detachment of the Cretaceous the Cordillera Huayhuash has appeared prior prism from some unexposed "basement" oc- to this report. In 1936, three Austrian geogra- curred at the horizon of basal Cretaceous Oyon phers explored the range, compiled a topo- Formation shale. Less severe middle(?) Terti- graphic map, and wrote a report (Kinzl and ary deformation warped andesitic volcanic others, 1942) including descriptions of orogra- rocks into broad gentle folds, and may have phy, glaciology, and cultural geography. Heim either reactivated or initiated thrust faults. (1948) commented on brief geologic observa- Field data and hypsometric (area altitude) tions in the range. Kinzl returned in 1954 and analysis suggest the middle to late Tertiary later published a book of photographs (Kinzl, Puna erosion surface is represented in the Cor- no date). Bodenlos and Ericksen (1955, p. 140- dillera Huayhuash by accordant flanking ridge- 153) published descriptions of several mines in tops and a landmass concentration below 5,000 and about Cerro Culebras on the northwest m. The 6,000 m axial ridge stood above this flank of the range. surface as a residual mass around which ash CRUSTAL CONSTITUTION flows were deposited. Latest Tertiary to Quat- ernary erogenic uplift, perhaps during the last General Statement 6 m.y., incised drainage and represents the Rocks of the Cretaceous, Tertiary, Quater- most recent deformation of the region. nary, and Holocene are exposed in the Cordill- era Huayhuash with a total thickness of about INTRODUCTION 4,500 m (Fig. 3). More than 80 percent of the The Cordillera Huayhuash (Fig. 1) stands in outcropping rocks are Cretaceous (Fig. 4) and splendid isolation on the crest of the north- are composed of 3,000 m of marine sediments. central Andes of Peru, 110 km northeast by Tertiary rocks include minor continental red east of the Pacific coast, and 200 km north of beds, more than 1,500 m of older intermediate Lima (Fig. 2). The range forms a serrate snow- volcanic rocks, and over 750 m of younger fel- and ice-clad ridge 40 km long and more than sic volcanic rocks. Cretaceous rocks, continen- Geological Society of America Bulletin, v. 82, p. 1863-1884, 13 figs. July 1971 1863 1864 P. CONEY—EVOLUTION OF CORDILLERA HUAYHUASH, PERU Downloaded from gsabulletin.gsapubs.org on May 4, 2011 Figure 1. Cordillera Huayhuash from above on snow-clad axial ridge are carved from folded Upper Chiquian. View southeasterly from 3,600 m. Rio Pati- Cretaceous limestones. Summits to right on Tsacra Spur vilca runs left to right in deep canyon (P). Quebrada are carved from Tertiary volcanics. Ridges and valley Llamac (L) is tributary to Rio Pativilca at 2,700 m. High- walls in foreground, and across Rio Pativilca, are carved est summit is Nevado Yerupaja (6,634 m). Snow line is from folded Lower Cretaceous sandstone and shale, at about 5,000 m. Nevado Yerupaja and summits to left tal red beds, and the older volcanic rocks are p. 9), made up of basal Oyon Formation shale, intruded by scattered late Tertiary granitic plu- Chimu Formation orthoquartzite, Santa Forma- tons. Quaternary and Holocene rocks are gla- tion limestone, Carhuaz Formation sandstone cial deposits, valley alluvium, and talus. and shale, and the Farrat Formation orthoquart- Regional considerations suggest the Cretaceous zite. This group is followed above by Albian to sedimentary prism is floored by a Paleozoic Turonian carbonates with interspersed shale "basement" composed of metamorphic rocks made up of the Pariahuanca Formation lime- equivalent to the Excelsior Group (Jenks, stone, Chulec and Pariatambo Formations lime- 1956, p. 222; McLaughlin, 1924, p. 598; Har- stone and shale, and Jumasha Formation rison and Wilson, 1960, p. 34-35) of central limestone. The pre-Albian Goyllarisquisga Peru with the possibility of some Jurassic rocks Group thins just east of the range, loses lime- just below the Cretaceous (Steinmann, 1930, p. stone and shale, and changes facies into the 84-86; Bodenlos and Ericksen, 1955, p. 18- Goyllarisquisga Formation (Wilson, 1963, p. 21). 9-14). West of the range the early Tertiary coastal batholith obscures correlations, but Cretaceous Rocks Cretaceous detrital and volcanic rocks of The 3,000 m of Cretaceous sedimentary eugeosynclinal aspect crop out along the Pacific rocks in the Cordillera Huayhuash are the pre- coast (Wilson, 1%3, p. 6-8). The Cretaceous Albian Goyllarisquisga Group (Wilson, 1963, stratigraphy described by Benevides (1956) Downloaded from gsabulletin.gsapubs.org on May 4, 2011 Figure 2. Generalized location and geologic map of and others, 1956). the north-central Andes of Peru (geology after Belltdo Downloaded from gsabulletin.gsapubs.org on May 4, 2011 —4000 -3000 -2000 — 1000 METERS Figure 3. Generalized composite geologic column for the Cordillera Huayhuash. Downloaded from gsabulletin.gsapubs.org on May 4, 2011 CRUSTAL CONSTITUTION 1867 and Wilson (1963) for the central Andes of dillera Huayhuash. Peru was found to extend into the Cordillera The Chimu Formation (Benevides, 1956, p. DownloadedHuayhuas from gsabulletin.gsapubs.orgh with only minor variations on May. 4, 2011 365) is prominent in the Cordillera Huayhuash Goyllarisquisga Group. Wilson (1963, p. with a thickness of at least 650 m. It is brought 12) described the Oyon Formation as more than to surface by all anticlines and upthrust belts 100 m of thin-bedded, dark, fine-grained sub- throughout the range (Fig. 5). The formation is graywacke shale interbedded with lenticular medium- to thick-bedded, occasionally mas- seams of coal exposed in the core of a thrusted sively bedded, very light gray to white, medi- anticline near Oyon, 30 km south of the Cor- um- to coarse-grained, occasionally conglo- dillera Huayhuash. In the Cordillera Huay- meratic, orthoquartzite interbedded with huash the Oyon Formation is restricted to the subordinate laminated beds of dark gray, silty, core of a complex anticline 1 m east of Car- carbonaceous shale and scattered seams of coal. huacocha, and possibly several other small The ratio of sandstone to shale beds is about highly sheared exposures in cores of anticlines 10:1. Sandstone units exhibit well-developed in the Chimu Formation in other parts of the diagonal and concave cross-bedding indicating range.