A profile across the Cordillera de los Andes between Villavicencio, Mendoza, Argentina, and Los Andes, Chile, shows the following stratigraphic units: (1) A basement complex composed of strongly folded beds of graywackes and slaty shales, Palaeozoic in age, slightly metamorphosed by stocks of granodiorite not younger than Middle Triassic; (2) A complex of strongly consolidated pyroclastics, intruded by large laccolithic bodies of quartz-porphyry and dikes and sills of andesites, diabase, etc. Age: Late (?) Triassic; (3) A marine series, ranging from Upper Liassic to Lower Cretaceous, formed by limestones, sandstones, anhydrite (gypsum), tuffites, shales, and conglomerates; sills and dikes of dacite, and andesite porphyries and diabase are common; (4) A thick complex of pyroclastic rocks of andesitic composition, largely breccias, traversed by dikes and sills of andesitic or dioritic rocks; partly intercalated in the upper part of the marine series. Age: Neocomian to Late Cretaceous. (5) Conglomerates and sandstones of Tertiary age. The units 2, 3, and 4 are roughly conformable with each other, and rest unconformably on 1. Unit 5 is unconformable with respect to all. The structure is characterized by a large flexure, the monoclinal part of it being parallel to and broadly coincident with the International boundary. A thrust fault transects the monocline near the base, whereby the Mesozoic beds rest partly on Tertiary conglomerates; another thrust fault marks the limit of the Cordillera to the east. The sequence of events is as follows: (1) deposition of the sedimentary-pyroclastic Mesozoic complex over an irregular peneplain of Palaeozoic rocks; (2) flexuring in the Early (?) Tertiary; (3) erosion and deposition of conglomerates and sandstones; (4) thrusting (Late Tertiary); (5) uplifting and erosion. The loftiest peak of the Cordillera, the Aconcagua (ca. 23,300 ft.), located about 20 kilometers north of our profile along the boundary, is made up of breccias belonging to unit 4 (Porphyritic formation) and structurally forms part of the huge cuesta that carries the frontier line.

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