KENNETH SEGERSTROM U. S. Geological Survey, Federal Center, Denver, Colo.

Quaternary Geology of Chile: Brief Outline

Abstract: The Quaternary deposits of Chile are of the Pacific they are mostly water-laid tuffs. Flows generally classified according to their mode of and pyroclastics are distributed throughout large deposition and the physiographic province in areas of the Altiplano and the Cordillera de los which they occur. Lacustrine, fluvial, mudflow, Andes, and in a few isolated areas in Magallanes. and eolian deposits occur in the Cordillera de la The composition of the Quaternary tuffs of Chile Costa, Central Valley, Altiplano, Cordillera de los ranges from rhyolitic to basaltic, but the lavas are Andes, and in Magallanes. In addition, deltaic and basaltic or andesitic. terrace deposits occur in coastal areas, and glacial The deposits have been very little warped or deposits in the Central Valley, Cordillera de los tilted, but they have been affected by displace- Andes, and Magallanes. ments of fault blocks and by epeirogenic move- Quaternary volcanic deposits are present in all ments. The net results of epeirogeny have been physiographic provinces, although in the Central general uplift of the coast north of Puerto Montt Valley, Cordillera de la Costa, and along the shore and general subsidence to the south.

CONTENTS Introduction 157 Crustal movements 167 Relief 157 References cited . . 168 Climate and present glaciers 162 Acknowledgments 162 Sedimentary deposits 162 Figure General statement 162 1. Map of northern Chile showing principal areas of Marine and coastal (excluding Magallanes) . 162 Quaternary deposits 158 Cordillera de la Costa 163 2. Map of central Chile showing principal areas of Central Valley 163 Quaternary deposits 159- Altiplano and Cordillera de los Andes . . . 165 3. Map of southern Chile (excluding Magallanes) Magallanes 166 showing principal areas of Quaternary de- Volcanic rocks 166 posits 160 Distribution 166 4. Map of Magallanes and adjacent areas showing Nature and composition 167 principal areas of Quaternary deposits . . 161

INTRODUCTION northward through western South America practically to the Caribbean Sea (10° N. lat.). Relief A matureland on the western flank of the Throughout much of the length of Chile the Cordillera de los Andes in northern Chile is topography is characterized by two parallel, 5-50 miles wide, 1000 to about 2000 m above north-trending chains of mountains and an sea level, and dates from middle Tertiary time intervening Central or Longitudinal Valley (Segerstrom, 1963b). About 75 km off the (Figs. 1-4). The Cordillera de la Costa, the coast of Taltal, at 25°30' S. lat., the Atacama western chain of mountains, is of moderate or Richards Deep extends to 7635 m below height, 1000-2000 m above sea level, and sea level, whereas Cerro Ojos del Salado, at disappears completely in northern Chile near about 27° S. lat. and 335 km to the southeast, Arica (18°30' S. lat. approximately). An attains an altitude of 6970 m. The Altiplano erosion surface, of Tertiary age, is well denned of northern Chile, about 4000 m high, is a in Central Chile by concordant summits, some southwesterly extension of the broad Bolivian of them flat and extensive. The Cordillera de plateau. los Andes, to the east, is higher, 1500-4000 m in The Longitudinal Valley, present from 41°30' southern Chile, 4000 to nearly 7000 m in to 33° S. lat., is absent in a zone of transverse central and northern Chile, and continues valleys drained by rivers that flow directly

Geological Society of America Bulletin, v. 75, p. 157-170, 4 figs., March 1964 157

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EXPLANATION

22° Sedimenfory deposits

Volconic rocks Basalt and anaesite, in cones and lava flows

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68° Figure 1. Map of northern Chile showing principal areas of Quaternary deposits. This and Figures 2, 3, and 4 were traced from an unpublished geologic map of Chile compiled by the Instituto de Jnvesrigaciones Geofogicas in 1963, with modifications by the author.

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EXPLANATION

Sedimentary deposits Cross-hatched oreos ara those where deposits are chiefly glaciofluvial, glacioloCLJStrine, or morainal

Volcanic rocKs Basalt and andesite, in cones and lava flows

72° 70° Figure 2. Map of central Chile showing principal areas of Quaternary deposits

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EXPLANATION

Seditnentary deposits Cross-hatched areas are those where deposits are chiefly glaciofluvial, glaciolacustrine, or morainal

Volcanic rocks Basalt and andesite, in cones and lava flows

Figure 3. Map of southern Chile (excluding Magallanes) showing principal areas of Quaternary deposits

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EXPLANATION -48=

Sediir.entory deposits Cross-hatched areas are those where deposits are chiefly qlaeiofluviol, glaciolacustrine, or morainol.

Volcanic rocks Basalt and andesite, in cones and lava flows

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72» 70o Figure 4. Map of Magallanes and adjacent areas showing principal areas of Quaternary deposits

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from the Cordillera de los Andes westward to tion range from small and widely separated the sea (between 29° and 33° S. lat.). North- in the north to large and continuous between ward, the valley reappears near 29 S. lat and 48°10'and51°40'S. lat. disappears again near Copiapo, at about 27°15' S. lat. With an offset of approximately 60 km ACKNOWLEDGMENTS to the east, the Longitudinal Valley reappears The author has profited greatly from dis- at Carrera Pinto, near 27° S. lat., and con- cussions with Carlos Ruiz Fuller, the Director tinues northward through the Pampa del of the Institute de Investigaciones Geologicas, Tamarugal. South of Puerto Montt (41°30' who made suggestions regarding the content, S. lat.) the Central Valley is drowned by the and with G. E. Ericksen, W. D. Carter, and sea and the lowered Coast Range becomes the R. W. Lemke, of the U. S. Geological Survey, Island of Chiloe, extending to about 43°30' R. U. Cooke, of the University of London, and S. lat. Farther south the Canal Moraleda and R. F. Flint, of Yale University, who critically its southern tributary extend to Laguna San reviewed the manuscript. H. J. Harrington, of Rafael (46°45' S. lat.), which is at the extreme the Tennessee Overseas Company, aided in southern limit of the Central Valley alignment. compiling references and improving the text. In general the coast of Chile exhibits few indentations north of the Latitude of Puerto SEDIMENTARY DEPOSITS Montt (41°30') and hundreds of channels, fiords, straits, gulfs, bays, peninsulas, and General Statement islands south of Puerto Montt, where the The following paragraphs on sedimentary floor of the Central Valley also descends below deposits are essentially descriptive rather than sea level. A similar coincidence between the interpretative. Because of lack of knowledge western shore and the Central Valley is noted little attempt is made to correlate between in northern Chile, where high terraces along regions, to develop chronology, or to show the uplifted coast and deeply entrenched relations of tectonic and eustatic movements to quebradas in the "pampa" or Longitudinal deposition and erosion. The descriptions are Valley to the east occur. In Central Chile, only of certain known deposits which are be- however, the relationship between coastal and lieved to be typical; the arrangement is first inland uplift is not the same; at about 33°30' geographical and second by types of deposits. S. lat., for example, marine terraces are as much as 200 m or more above sea level, but Marine and Coastal (Excluding Magallanes) the Rio Mapocho is not appreciably entrenched DELTAIC DEPOSITS: The Rio Bio Bio, Chile's in the Central Valley at Santiago. At the port largest river, has deposited a broad delta of San Antonio (33°30' S. lat.) a submarine (36°45' S. lat.) that consists largely of andesitic canyon descends steeply seaward to a depth basalt sand and silt. Grains of plagioclase and of 100 m or more (Briiggen, 1950, p. 198-199). olivine in these sediments are little altered, indicating that their erosion and transportation Climate and Present Glaciers were very rapid and that their deposition was Few countries, if any, exhibit the great recent (Veyl, 1961, p. 27-28). contrasts of climate and vegetation represented TERRACE DEPOSITS: Most of the exposed by the Atacama Desert north of the 27th marine deposits of the Quaternary Period crop Parallel, where the combination of the cold out in terraces 40-120 m above the present sea Humboldt Current and a prevailing wind level. Some of the notable localities where from the southwest results in very little rain- Quaternary marine terraces are recognized are fall, and the Aysen rain forests, south of the north of Antofagasta (23°-23°30' S. lat.), south 43rd Parallel, where the mean annual precipita- of Caldera (27°-27°15' S. lat.), south of Co- tion exceeds 4 m. Much of central Chile, on quimbo (30°10'-30°40' S. lat.), near La Liqua the other hand, enjoys a mild Mediterranean (32°20' S. lat.), and near San Antonio (33°45' type of climate, with warm, dry summers and S. lat.). The deposits consist of sand, for the cool, moist winters. most part, with interbedded silts, clays, The Cordillera de los Andes has glaciers, gravels, and coquina locally. The sediments from Cerro del Potro, at 28°30' S. lat., to the are generally poorly indurated, but the coquina southern tip of the continent, at altitudes rang- beds are loosely to firmly cemented, and in ing from 5000 m or more in the north to sea places limonite has indurated the sand beds. level in the south. Their extent and distribu- The age of the deposits is difficult to determine,

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as most of the fauna resemble Recent forms. In the Cordillera de la Costa between 34° Changing associations of fauna, plus superpo- and 35° S. lat. extensive areas are covered sition of beds, have given relative ages, but not with pumiceous sand deposited from standing to the degree where it is easy to distinguish water in poorly drained or temporarily un- between late Pliocene and early Quaternary drained parts of the Rio Rapel watershed, beds, for example. Fauna with more or less particularly in the valleys of the Rio Cachapoal modern forms are found as high as 268 m above and Rio Tinguiririca. Four extinct lakes where sea level near Caldera (27° S. lat.). the deposition took place are mentioned by In the coastal region west of Santiago (north- Karzulovic (1962). east of San Antonio) two unfossiliferous for- FLUVIAL AND MUDFLOW DEPOSITS.' Valleys mations of high terrace deposits of probable and depressions in the Coast Range are partly Quaternary age have been defined and mapped filled with stream deposits. In Central Chile by H. Fuenzalida and J. Varela. The San (Casablanca) and southern Chile (near Valdivia) Sebastian Formation, which attains a maximum some of the river gravels are auriferous. Ter- thickness of more than 60 m and overlies raced sedimentary deposits as much as 200 m fossiliferous beds of Pliocene age, consists of above the valley floor occur along the Maipo highly quartzose sand, crossbedded near the and Rapel rivers and their tributaries, near base, with intercalated silt and pebble gravel. (34° S. lat.). Most of the deposits are fluvial The overlying Paraguas Formation, about 10 sands and gravels, but near a new hydro- m thick, consists largely of coarse gravel. electric power site on the Rapel and along the These formations, whose top altitudes are lower course of Estero Puangue north of the 80-189 m above sea level, are truncated by a Maipo, poorly sorted conglomerates were de- marine-abraded terrace (Fuenzalida and Va- posited either by torrential floods heavily rela, 1963). charged with sediment or by mudflows. EOLIAN DEPOSITS: Sand dunes are a common EOLIAN DEPOSITS: A fine-grained red eolian feature of the Chilean coast, but considering deposit called "coba" covers parts of the I he desert nature of much of the terrain, the Coast Range in the vicinity of El Toco and in dunes are not as extensive as might be be- the middle of the Cordillera de la Costa south- lieved. A noteworthy dune immediately south east of Iquique, between 20°30' and 22°10' S. of Iquique (20°15' S. lat.) is about 200 m high. lat. (Briiggen, 1950, p. 142). Near 27° S. lat. Dunes of several generations have been studied chains of dunes extend inland from the coast recently on the central Chilean coast near La as far as 60 km, and to a maximum altitude of Ligua by Roland Pascoff of the Institute de 1600 m above sea level at Cerro Medanoso Geografia of the University of Chile (1962), (Segerstrom, 1962). and near Valparaiso by Leonardo Alvarez, of the Institute de Investigaciones Geologicas Central Valley (Oral communication, 1963). LACUSTRINE DEPOSITS: In the lowest tectonic depressions in the northern part of the Central Cordillera de la Costa Valley and in areas scoured by ice and dammed LACUSTRINE DEPOSITS: The westernmost by glacial and volcanic deposits in the southern salares or playa lakes between 20° and 24° S. part, large volumes of sediments were deposited lat. lie in tectonic basins of the Cordillera de from standing water. la Costa. The unconsolidated sand, silt, and In the north these form typical desert playa clay deposited in the salares are generally (salar) deposits of silt-size material with clay crusted with a "salt pan" less than 1 m thick. and sand components and beds of evaporites: The salt consists mainly of sodium chloride and chlorides, sulfates, nitrates, and other salts. sodium sulfate, but also contains small varying The deposits are commonly covered with as amounts of potassium-bearing salts and borates much as 40 cm of "chuca," sandy soil with a which in times past have been exploited (G. desert "pavement" of rock fragments. E. Ericksen, written communication, 1962). Over an extensive area now transversed by "Nitrate deposits, in contrast to other saline the Rio Loa west of Calama (22°30' S. lat.) deposits, are along the lower eastern slopes of lacustrine beds of fresh-water limestone, the coastal range and the lower hillslopes kieselguhr, red clay, and sand are in the Loa bordering salt pans within the coastal range Formation. Gastropods in the limestone are in the region extending from near Iquique believed by Wetzel (1927) to indicate a southward to Taltal" (Ericksen, 1961). Quaternary age. Diatoms in the kieselguhr

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were described by Hustedt (1927) and Fren- large fragments, yet high proportions of clay guelli (1936). The age of these beds is discussed occur in the matrix; whereas in the lower parts, at some length by Fuenzalida (1957). the sediments tend to show better sorting. In the vicinity of Santiago, silts, clays and Each alluvial fan seems to have characteristic sands of lacustrine origin have been encoun- proportions of lithologic types, chiefly andesite tered in wells located near Pudahuel and and granodiorite, and these proportions differ Quinta Normal (western part of Santiago), for the various fans (Castillo and others, 1963). 150-236 m below the surface. Small, fresh- In the Chilian area of southern Chile two water gastropods were found in microlaminated types of alluvium occur: (1) an ancient "tosca," clays at a depth of 157 m (Octavio Castillo, well cemented in part and consisting of gravel, oral communication, Nov. 1962). sand, silt and reworked basaltic ash; (2) a The Lircay Formation, the type locality of modern alluvium, which disconformably over- which is a little north of Talca (35°30' S. lat.), lies the "tosca" and consists of unconsolidated consists of lacustrine clays and tufFaceous sand gravel, sand, and silt. Most of the clastic fill with some intercalations of fluvial gravel deposits in the Longitudinal Valley south of (Mufioz C., 1960, p. 45-46). Three kilometers Chilian and in the Puerto Montt region are southeast of Puerto Montt (41°30' S. lat.) glaciofluvial. glaciolacustrine deposits exposed in a. highway MUDFLOW AND LANDSLIDE DEPOSITS: Poor cut contain wood the age of which is 15,400 sorting and rare stratification suggest that a + 400 years, as determined from its C 14 con- high proportion of the piedmont deposits of tent; these beds are overlain locally by till the Longitudinal Valley \vas deposited by (R. W. Lemke, written communication, Oct. mudflows, although it may be that the trans- 15, 1961). Because of the greater volume and porting medium was transitional between a transporting power of the rivers that feed the heavily loaded stream of water and a true mud- southern lakes, coarse gravels are more com- flow. These deposits are most extensive in monly seen on the modern lake beaches of the northern Chile, where many of them are south than on the ancient playas of the north. entirely preserved. The best vertical sections FLUVIAL DEPOSITS: The nature of fluvial can be observed where streams have dissected deposits in the Longitudinal Valley is closely the thick mudflows. related to the climate, and since the differences Mudflows in central Chile near Santiago are in climate between northern and southern both tonguelike and fan-shaped and cover the Chile have been great during all of the Qua- base of the Andean front at the eastern edge ternary Period the fill deposits of the south of the Central Valley to a maximum depth of are strikingly different from those of the north. 100 m or more. Light-weight products of mud- In the north the fluvial deposits are in general flow deposition with high admixtures of poorly washed, are perhaps inferior to mud- pumiceous ash form deposits as much as 35 m flows in volume, and contain high percentages thick in the low part of the Valley, west of of salts; in the south the fluvial deposits are Santiago. in general well washed by glacial meltwater Mudflows, including lahars, between Chilian (glaciofluvial deposits in part). and Temuco are composed chiefly of fragments Typical of much of the Longitudinal Valley of Quaternary basalt and andesite. The Banco pampa of the north is the region of Pica del Laja Formation is a breccia containing (20°30' S. lat.) where the quebradas that subangular fragments and blocks 1-50 cm debouch from the high Cordillera have de- across in an ashy matrix of fine-grained volcanic posited great alluvial fans. The fan of the material. A few of the blocks are granitic Quebrada Chacarilla (south of Pica), for ex- (Mufioz C., 1960, p. 45). ample, covers about 350 km2. Its apex is made Landslides with volumes as great as 100,- up of poorly sorted material and its toe is 000,000 m3 occur in the vicinity of Lago covered with clayey, saline sediments, al- Rinihue. Three slides, produced by earth- though at depth even the lower part of the quakes on May 22, 1960, blocked the outlet of fan contains large clasts (Galli and Dingman, the lake and caused the water level to rise 1962, p. 64). 26.5 m; the largest of the 1960 slides contains In Central Chile near Santiago, the alluvial about 30,000,000 m3 of unconsolidated sedi- fans of the Rio Maipo, Rio Mapocho, and two ments (Davis and Karzulovic, 1963). smaller rivers form a large percentage of the In the northern and middle sectors of the total bulk of valley-fill materials. In upper Central Valley most major mudflow and land- parts of the fans these sediments contain very slide deposits are probably Pleistocene. There

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the present climate is more arid than during Laja sands ("trumaos") of the Rio Bio Bio parts of Pleistocene time, and individual basin and an adjacent area to the north are storms are generally not of sufficient intensity derived from ash, as a result of wind and stream and/or duration to produce landslides of the transport. magnitude of those of 1960 in the presently humid southern part of the valley (R. W. Altiflano and Cordillera de los Andes Lemke, oral communication, Jan. 1964). SALAR DEPOSITS: Along the Bolivian border GLACIAL DEPOSITS: South of Santiago the east of Arica and Iquique several salares occur lower limit of till drops rapidly until in the that contain the borate ulexite (G. E. Ericksen, Curico-Talca area (35°-35°30' S. lat.) it written communication, 1961). Halite derived reaches the Longitudinal Valley. The name from the solution of the older salt strata of Morena de la Montana Formation has been the Salar de Atacama, in Antofagasta Province, given to till of the region of Talca and farther is being deposited today (Dingman, 1963, in south. At Talca the formation was deposited press). In the Salar de Maricunga (26C45' S. at the mouths of Andean valleys along the lat.) borax is found beneath a halite crust eastern edge of the Central Valley; farther about 60 cm thick. south the till extends well out toward the FLUVIAL, MUDFLOW, AND LANDSLIDE DEPOS- middle of the Valley, and finally in the Osorno ITS: The salares of the Altiplano of northern region (40°30' S. lat.) it reaches the Cordillera Chile are rimmed by coalescing fan deposits de la Costa (Mufioz C., 1960, p. 46). The formed chiefly during heavy storms by ephem- Cerritos del Teno, near Curico, have been eral streams and mudflows. Stratification of the described as small drumlins by Briiggen (1950, fan deposits is poor, with individual beds p. 221) because of their form and composition, wedging out in a few meters (Dingman, 1963). but they are probably hummocky mudflow Peat deposits are formed in the vegas, or deposits. naturally irrigated parts of the high Cordilleran In the Chilian area there is good evidence valleys of northern and central Chile; at La of two glaciations: a surficial till is separated Piadosa, in the eastern part of Atacama prov- from a subsurface one by fluvial deposits ince, salts of copper and uranium are being (Robert Devaul, oral communication, Mar. precipitated in the peat by circulating ground 1961). Both tills contain decomposed boulders water. and cobbles which can be easily sliced with a The Altos de Pica Formation and associated spade, as well as (locally), fresh rock fragments. rock units extend along the western base of Near Lago Laja (37°30' S. lat.) "two stages the Andes from the Peruvian border southward of glaciation are shown by a 'U'-shaped glacial to a latitude of 24° or 25°. At its type locality valley filled with lava flows exposed in a vertical (20°30' S. lat.) the formation is made up of 5 section which is the wall of another 'U'-shaped members, chiefly torrential and mudflow de- glacial valley; in the same area is inconclusive posits about 700 in thick, but containing 2 evidence of a third glaciation" (G. E. Ericksen, ashflow members 17 and 23 m thick. The age written communication, June 1963). is stated to be late Tertiary to Quaternary On the west side of Lago Llanquihue (41° S. (Galli and Dingham, 1962), although earlier lat.) three morainal ridges 2-3 km wide and workers believed that the beds were as old as 30-50 m high probably represent different Miocene ("Formacion Liparitica," Briiggen, stages of glacial retreat (Briiggen, 1950, p. 1950). 231). The high Cordilleran valleys have hum- The till that overlies lacustrine deposits mocky surfaces due to the influx of fans from 3 km southeast of Puerto Montt contains tributary valleys on either side. In low places relatively unweathered rock fragments and is between the fans ponded silts and clays con- less than about 15,000 years old (late Wiscon- trast with the coarse fan materials. The fans sin). are composed in part of mudflow sediments, EOLIAN DEPOSITS: Windblown sand and silt some bodies of which resemble glaciers, with are widespread in the Central Valley, but their well defined flow lines and lateral and terminal bulk is minor compared to that of sediments moraine-like ridges. An area of mudflows on ol other origin. A zone of dunes 10 km long the south slope of Cerro Cadillal (27°50' S. lat.) by 2 km wide occurs north of Pica (20°30' covers 40 km2 or more. S. lat.); the dunes strike east-northeast, paral- In the upper Rio Maipo canyon, southeast lel to the direction of dominant wind (Briig- of Santiago, mudflows, debris avalanches, and gen, 1950, p. 162). In southern Chile the black rockfalls resulted from earthquakes in Sep-

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tember 1958 and heavy rainfall in April 1959. Farther south, till is found at progressively The largest deposits resulting from these events lower altitudes. Moraines existing at more are in the Quebradas Sonambulo and Caballos southerly latitudes are described in the sections and the Estero Yesillo, west and south El on the deposits of the Central Valley and Volcan. The debris avalanche of Estero Yesillo Magallanes. is 10 km long, 100 m thick, and about 1 km wide (Pierre St. Amand, written communica- Magallanes tion, Feb. 1963). MARINE DEPOSITS: The Puerto Ramirez The Rio Pangal, a little farther south, is Formation crops out along the bay immediately entrenched in a prehistoric mudflow about 1 west of Mt. Burney (52°20' S. lat.) up to 2 m km downstream from the junction of the Rio above sea level. The rock unit is a poorly Blanco and Rio Paredones. The flow is more cemented conglomerate made up largely of than 100 m thick and contains angular blocks andesitic pumice. Pelecypods of Quaternary as much as 6-7 m across (Robert Swain, oral age, including Mytilus edulis, M. chorus, and communication, 1963). Still farther south Chione sp., are abundant (Cecioni, 1957). (35° S. lat.) "an immense section of mountain GLACIOLACUSTRINE AND PEAT DEPOSITS: Most fell in the valley . . ." (of the Rio Tinguiririca, of the bays and sounds of the region of the during the middle of the 19th Century) "dam- Estrecho de Magallanes were formerly glacial ming the river with a pile of debris a quarter lakes. The clay, silt, and sand deposited in of a mile thick" (Perez Rosales, 1859). A these lakes can be observed in cliffs and islands terrace on the north side of the Rio Laja, of the Strait. West of Punta Arenas (53° S. west of Lago Laja, is made up largely of mud- lat.) peat has been deposited in lowlands flows. These mudflows or lahars consist of separated by morainal ridges (Briiggen, 1950, "unsorted volcanic material in which irregular p. 247). Extensive peat deposits are encoun- blocks and rock fragments of as much as a tered on the Isla Grande de Tierra del Fuego meter in diameter are enclosed by a matrix (Munoz C., 1950, p. 158). of ash-size material" (G. E. Ericksen, written GLACIOFLUVIAL DEPOSITS: All of the flat area communication, March 1962). of the Punta Arenas region is covered with GLACIAL DEPOSITS: In northern Chile glacial poorly sorted clay, sand, and gravel of glacio- deposits are of very small extent and exist only fluvial deposition. During the period of Pleisto- at very high altitudes. In the Lake Titicaca cene glaciation some valleys were cut and region of Peru and Bolivia (16° S. lat.) the partly filled with these deposits (W. W. Doyel, moraines did not descend below 4100 m above oral communication, August 1962). sea level (Newell, 1949, p. 84) and farther GLACIAL DEPOSITS: The till of Magallanes south a more arid climate during the Pleisto- and the western part of Patagonia has been cene than that of Lake Titicaca resulted in related to four systems of end moraines. The even higher minimum altitudes of glacial first is situated on interfluves and is apparently deposits. The northernmost terminal moraines unrelated to modern valleys. The second, of Chile are said to be in the upper valley of third, and fourth systems are found in large the Rio Turbio (30° S. lat.), the principal transversal valleys, indicating that there was a source of the Rio Elqui, at 3100 m above sea long interglacial epoch of valley formation level (Briiggen, 1950, p. 214). between deposition of the till of the first Along the Trans-Andean railroad between system and that of the second system (Caldeni- Los Andes, Chile (32°50' S. lat.), and Mendoza, us, 1932). Argentina, a step-like group of three or more moraines descends from 2980 to 2250 m above VOLCANIC ROCKS sea level (Briiggen, 1950, p. 216). Despite statements to the contrary by Distribution earlier workers (Briiggen, 1950; Munoz Cristi, The Pleistocene and Recent volcanoes of 1950), the minimum altitude of till at the Chile are distributed through a nearly con- latitude of Santiago is about 1600 m. A moraine tinuous belt along and near the Bolivian and in the valley of the Rio Yeso, in the Cordillera Argentine borders from about 17°20' to 27°30' southeast of Santiago, is 12 km long, at least S. lat., a distance of 1050 km, and at intervals 200 m thick, and consists of blocks of grano- of 35-80 km from 33° to approximately 45° diorite and andesite in a matrix of rock flour; S. lat. The isolated Mt. Burney, at 52°20' S. the two types of rocks tend to be segregated lat. has shown recent volcanic activity. It is in narrow bands parallel to the valley. estimated that there are between 800 and 1000

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volcanoes in the northern belt (Briiggen, 1950, It is well known that Chile forms part of a seismically active belt with frequent earth- . The distribution of volcanoes in Chile shows quakes. Many historic movements with dis- a close relationship to the Longitudinal Valley. placements of a meter or more have been Where the Valley is present there are Qua- described. Fault displacement of alluvium can ternary volcanoes to the east; where it is be seen in aerial photographs and on the absent, as in the zone of transverse valleys ground throughout much of the area north of (27°30'-33° S. lat.) there are none. 26° S. lat., and west of 69° S. long. In northern and Central Chile, late displacements along Nature and Composition north-striking faults are right-lateral and those Most of the volcanoes are extinct and deeply along northeast-striking faults tend to be left- eroded. Many of them are composite vol- lateral (Ruiz and St. Amand, 1961, p. 133). canoes, composed of both lava and pyroclastics. The best documented major earthquakes are Pure cinder cones are commonly small features those of November 1922 (Valparaiso-Iquique), built on edifices of mixed volcanic rocks. A January 1939 (Chillan-Concepcion), and May recent compilation listed 33 Chilean volcanoes 1960 (Concepcion-Chiloe). Precise leveling by that have been active in historic times (Caser- the Military Geographic Institute of Chile tano, 1963). showed that the 1939 earthquake produced The composition of the lavas ranges from vertical uplift as great as 2.5 m near Chilian biotite and augite andesite to pyroxene or and subsidence of over 0.5 m near Concepcion olivine basalt and that of the ash, from basaltic (Briiggen, 1950, Fig. 93). A complex pattern or andesitic to felsic pumice. It has been said of local uplift and subsidence resulted Irom that with few exceptions the oldest lavas are of the 1960 quakes, but in general a continental hypersthene basalt and the youngest are of block about 600 km long and 30 km wide was olivine basalt (Mufioz C., 1950, p. 124), but depressed 1.0-1.5 m along the coast (Weischet, it is probable that the exceptions are many, for 1963, p. 1240). andesites of both early and late Quaternary The Longitudinal Valley is of tectonic origin. age are common. The eruptive products com- The bedrock surface under the floor of the prise extensive lava flows (chiefly of the aa, or Valley is as much as 600 m below the surface blocky type) and ash deposits. The Laco of fill deposits in the Pampa del Tamarugal magnetite deposit, at 23°30' S. lat., is believed (R. J. Dingman, oral communication, July to be a flow (Park, 1961 ; Ruiz F. and Ericksen, 1962). In the vicinity of Santiago, the Valley 1962). Sulfur deposits of fumarolic origin are is probably several graben with a net downward widely distributed in the Cordillera de los displacement of about 2000 m after various up Andes and Altiplano of northern Chile. and down movements (W. D. Carter and L. Lahars, or volcanic mudflows, seldom recog- Aguirre, written communication, 1963), but nized in Chile in the past, have been important the maximum depth of sedimentary fill deposits results of volcanic eruptions. The eruption of there is probably not over 400 m, as indicated Calbuco in February 1961 produced three by seismic data (Hans Menardius, oral com- lahars which descended the north, northeast, munication, 1963). A gravimetric survey near and south flanks of the volcano. The largest Chilian has indicated that a fault on the west of the lahars contained about 30 per cent of side of the Valley at that latitude (36C30') has hot lava (Klohn, 1963). a net vertical displacement of 2000 m, with a correspondingly thick wedge of fill deposits CRUSTAL MOVEMENTS on the east side of the fault (Lomnitz, 1959). The sedimentary deposits and volcanic rocks The depth to bedrock may be 2000-3000 m of Quaternary age have been displaced by near Puerto Montt as indicated by geophysical faulting, but there has been little or no actual data (Hans Katz, oral communication, Apr., folding in Chile since the end of Tertiary time. 1963). It is likely that much of the great thick- In the northern part of the country gentle ness of clastic fill materials at those places has warping of the Pliocene(P) Altos de Pica For- been deposited during the sinking of fault mation, south of Quebrada de Tana, and of blocks in the Longitudinal Valley during the the Pleistocene(P) Loa Formation, just north Pleistocene Epoch. Evidence for this is the oi: the Rio Loa, is visible. Dips of terrace beds striking absence of fluviatile terraces in this as great as 37° indicate strong tilting of fault valley as compared to their ubiquity in the blocks near Caldera at about 27° S. lat. (Seger- other physiographic provinces of Chile. strom, 1963a). Abnormally high positions of Quaternary

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marine beds along the coast indicate vertical and setting of marine bays and sounds in uplift of hundreds of meters in places (Seger- Magallanes (Bahia de Lomas, Bahia Imitil, Seno strom, 1963a); as much as 100m of this altitude Otway, Seno de Skyring) indicate that they can be ascribed to eustatic sea-level change, were glacial lakes like those of Patagonia (Lago however. The emerged shoreline north of Viedma, Lago Argentine), and that because Puerto Montt and the submerged shoreline of the subsidence they have been drowned by south of Puerto Montt are the result of epeiro- the sea (Carlos Klohn, oral communication, genie uplift and subsidence. The size, shape, Mar., 1963).

REFERENCES CITED Briiggen, Juan, 1950, Fundamentos de la geologia de Chile: Santiago, Institute Geografico Militar, 374 p. Caldenius, C. zon, 1932, Las glaciaciones cuaternarias en la Patagonia y Tierra del Fuego: An. Soc. Cient. Argentina, v. 113, p. 49-70 Casertano, Lorenzo, 1963, General characteristics of active Andean volcanoes and a summary of their activities during recent centuries: Bull. Seismol. Soc. America, v. 53, p. 1415-1433 Castillo, Octavio, Falcon, Eduardo, Doyel, W. W., and Valenzuela, Manuel, 1963, El agua subterranea de Santiago; Segundo informe: Institute de Investig. Geol. Bol. 15, 65 p. Cecioni, Giovanni, 1957, I terrazi marina della Patagonia Cilena: Atti Soc. Tosc. Sci. Nat., Ser. A, v. 64, p. 33-39 Davis, S. N., and Karzulovic, Juan, 1963, Landslides at Lago Rinihue, Chile: Seismol. Soc. America Bull., v. 53, p. 1403-1914 Dingman, R. J., in press, Cuadrangulo Tulor: Carta Geologica de Chile, Institute de Investig. Geol. Ericksen, G. E., 1961, Rhyolite tuff, a source of the salts of northern Chile: U. S. Geol. Survey Prof. Paper 424C, p. C224-C225 Frenguelli, J., 1936, Diatomeas de la caliza de la cuenca de Calama: Rev. Mus. La Plata, N. S. v. 1, Pal. No. 1, p. 3-34 Fuenzalida, Humberto, 1957, Sobre la edad de las Calizas del Loa: An. Fac. Sci. Fis. Mat. Univ. Chile Fuenzalida, Humberto, and Varela, Juan, in press, Geologia de los yacimientos de arenas y limos en la region de San Antonio y Cartagena: Universidad de Chile, Escuela de Geologia publication Galli, Carlos, and Dingman, R. J., 1962, Cuadrangulos Pica, Alca, Matilla y Chacarilla; Carta Geologica de Chile: Inst, de Investig. Geol., v. 3, nos. 2, 3, 4, and 5, 125 p. Hustedt, F., 1927, Fossile Bacillariaceen aus dem Loa-Becken in Der Atacama-Wuste, Chile: Arch f. Hydrobiol., Bd. 18, p. 224-251 Karzulovic, Juan, 1962, Problemas geotecnicos de la Central Hidroelectrica Rapel: Sociedad Geologica de Chile, Resumenes, no. 3, p. 5-8 Klohn, Erik, 1963, The February 1961 eruption of Calbuco Volcano: Bull. Seismol. Soc. America, v. 53, p. 1435-1436 Lomnitz, Cinna, 1959, Investigaciones gravimetricas en la region de Chilian: Institute de Investig. Geol. Munoz Cristi, Jorge, 1950, Geologia, Chapter III in Geografia Economica de Chile: Santiago, CORFO, p. 55-187 1960, Contribucion al conocimiento geologico de la Cordillera de la Costa de la Zona Central: Miner- ales, no. 69, p. 28-47 Newell, N. D., 1949, Geology of the Lake Titicaca Region, Peru and Bolivia: Geol. Soc. America Mem. 36, 111 p. Park, C. F., 1961, A magnetite "flow" in northern Chile: Econ. Geol., v. 56, p. 431-436 Pascoff, Roland, 1962, Note preliminaire sur certaines analogies du Quaternaire Chilien avec le Quatern- aire marocain: Soc. des Sciences Naturelles et Physiques du Maroc, Comptes rendus des seances mensuelles, 1962, no. 7, p. 129-131 Perez Resales, Vicente, 1859, Ensayo sobre Chile: Santiago, 450 p. Ruiz, F., Carlos, and Ericksen, G. E., 1962, Metallogenetic provinces of Chile, S. A.: Econ. Geol., v. 57, p. 91-106 Ruiz, F., Carlos, and St. Amand, Pierre, 1961, Observations concerning the Chilean earthquakes of May 1960: 21st Internal. Geol. Cong. Rpt., pt. 26, suppl. vol. Sec. 1-21, p. 116-133 Segerstrom, Kenneth, 1962, Deflated marine terrace as a source of dune chains, Atacama province, Chile: U. S. Geol. Survey Prof. Paper 450C, p. C91-C93

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Segerstrom, Kenneth, 1963a, High marine terraces in the Caldera region of northern Chile (Abstract): Geol. Soc. America Spec. Paper 73, p. 237-238 1963b, Matureland of northern Chile and its relationship to ore deposits: Geol. Soc. America Bull., v. 74, p. 513-518 Veyl, Carlos, 1961, Contribucion al conocimiento de la geologia regional de la provincia de Concepcion: Minerales, no. 72, p. 1-51 Weischet, Wolfgang, 1963, Further observations of geologic and geomorphic changes resulting from the catastrophic earthquake of May 1960, in Chile: Bull. Seismol. Soc. America, v. 53, p. 1237-1257 Wetzel, W., 1927, Zur Erdgeschichte der mittleren Atacama: Neues. Jahrb. f. Miner., Geol. U. Palaeont., Beil. Bd. 58

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