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Geology of the Ultima Esperanza Fold- Thrust Belt, Southernmost Andes

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Geology of the Ultima Esperanza Fold- Thrust Belt, Southernmost Andes Geology of the Ultima Esperanza fold- and on the structural development of the Ultima Esperanza fold-thrust belt. thrust belt, southernmost Andes The oldest unit in the area is the Middle to Upper Jurassic Tobifera Formation, which forms part of an extremely wide- spread silicic volcanic unit in southern South American (Bruhn, T. J. WILSON and I. W. D. DALZIEL Stern, and de Wit 1978; Feruglio 1949). In Ultima Esperanza, the Tobifera Formation consists predominantly of silicic vol- Lamont-Doherty Geological Observatory of caniclastic rocks intercalated with euxinic marine argillites. Columbia University Hypabyssal rhyolite intrusions and peperites are abundant and Palisades, New York 10964 minor pillow basalt is also present. The sedimentology indi- cates a relatively deep marine environment of deposition During the austral summers of 1979-1982, stratigraphic map- (Wilson 1983). The marine volcanism and sedimentation record- ping was undertaken within the foothills of the southern Pa- ed in the Tobifera Formation of the Ultirna Esperanza district tagonian Andes at 51°S latitude, in the Ultima Esperanza district reflect the earliest stages in the formation of the Late Jurassic- of southern Chile (figure). The foothills belt is part of the fore- Early Cretaceous back-arc basin that developed along the Pacific land fold-thrust belt of the southernmost Andes and includes margin of southernmost South American (Allen 1982; Bruhn et stratigraphic units to Late Jurassic to Neogene age (figure). The al. 1978; Daiziel, de Wit, and Palmer 1974; and Katz 1973). study focused on the sedimentology, stratigraphy, and paleoen- The Lower Cretaceous Zapata Formation rests conformably vironmental setting of the oldest part of the stratigraphic se- on the Tobifera Formation and consists of a generally euxinic quence (including units to Late Jurassic to Late Cretaceous age) finegrained muds tone-siltstone sequence containing fine- 7 MA ARE_ AMER -; At 0 I ^11\)' - ¼-\ - rkl fri >- ( Cr EASALTU I- Cr RIO BANDURRIAS FM \ \\\," >615 m) \ A UOROTEA FM r \ (300—I200 m) ' \ ii LAGO TORO IRES PASOS FM (2000 — 2500m) LAGO GEIKIE N iIt cr 1 CERRO TORO FM 0 10km 73" W (2000-2500 m) LOWER TO UPPER M 10 C EN E CRETACEOUS PUNTA GRANITIC THRUST BARROSA FORMATION ROCK ANTICLINE ,7 FAULT PUNTA BARROSA FM j REVERSE (0— I000 m) LUWER CRETACEOUS UPPER SYNCLINE FAULT ZAPATA FORMATION CRETACEOUS - CERRO TORO 7 ZAPATA FM FORMATION CONTACT (>630 m( ?MIDDLE TO UPPER U, TOBIFERA FM JURASSIC TOBIFERA ( CONGLOMERATE — ,) I (500-1000 m FORMATION LAKES GLACIERS Cr (A) Simplified geologic map of part of the Ultima Esperanza fold-thrust belt. Location shown in inset maps. (B) Generalized stratigraphic section of the Ultima Esperanza foothills belt. Random dash denotes rhyolite; black denotes mudstone and shale; light stipple denotes sandstone; heavy stipple denotes conglomerate; "v" denotes basaltic volcanic rocks. 1983 REVIEW 75 grained turbidites, coarse-grained channelized turbidites and del Fuego (Nelson et al. 1980) may also represent a duplex as has chaotic slump deposits. The nature of the sediments, trace recently been suggested for the Shushwap complex in the Ca- fossils, and the presence of slumps suggest deposition within a nadian Cordillera (Mattauer, Collot, and Van den Driessche bathyal submarine slope environment, and the Zapata Forma- 1983). tion is interpreted to be part of a thin clastic sedimentary wedge This research was supported by National Science Foundation that developed on the subsiding cratonward margin of the back- grants DPP 78-20629 and INT 79-20213. We would like to thank arc basin (Wilson 1983). the Instituto de la Patagonia, the Parque Nacional Torres del The Albian-Cenomanjan Punta Barrosa Formation con- Paine, and the Empresa Nacional del Petroleo, Chile, for logis- formably overlies the Zapata Formation and consists of alternat- tical support during this project. ing coarse-grained sandstone and shale intervals that are inter- preted as lobe, lobe fringe, and interlobe facies deposited in the mid-fan region of a deep-sea fan system that prograded south- References ward within the northern part of the Magallanes basin (Wilson 1983). The Magallanes basin is a deep-water foreland basin Allen, R. B. 1982. Geologia de la Cordillera Sarmiento, Andes Pa- (Natland et al. 1974) that developed on the site of the Early tagonicos entre los 51°00 y 5215 lat. 5, Magallanes, Chile. Servicio Cretaceous back-arc basin slope concommitantly with mid-Cre- Nacional de Geologia y Mineria, Santiago, Chile, Boletin 38, 45. taceous deformation and uplift within the Andean Cordillera to Bruhn, R. L., C. R. Stern, and M. J. de Wit. 1978. Field and geochemical the west. The coarse clastics of the Punta Barrosa Formation are data bearing on the development of a Mesozoic volcanotectonic rift the first deposits within this foreland basin. The succeeding zone and back-arc basin in southernmost South America. Earth and stratigraphic sequence consists of deep to shallow marine clastic Planetary Science Letters, 41(1), 32-46. Cortés, R. 1964. Estratigrafia y un estudio de paleocorrientes del flysch sedimentary rocks that were deposited in the foreland basin in Cretaceo del Depto. de Ultima Esperanza. (Tesis de grado), Santiago: Cretaceous and Tertiary time (Cortés 1964; Dott, Winn, and Univ. Tecnico del Estado. Smith 1982; Katz 1963). Daiziel, I. W. D., M. J. de Wit, and K. F. Palmer. 1974. Fossil marginal The structural geometry of the Ultima Esperanza fold-thrust basin in the southern Andes. Nature, 250, 291-294. belt is characterized by megascopic and mesoscopic folds asso- Dott, R. H., Jr., R. D. Winn, Jr., and C. H. L. Smith. 1982. Relationship ciated with bedding-plane detachment faults (Wilson 1983). A of Late Mesozoic and Early Cenozoic sedimentation to the tectonic structure section across 50 kilometers of the fold belt indicates evolution of the southernmost Andes and Scotia Arc. In C. Craddock that the folding may be related to displacement along blind (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. thrusts in the subsurface. One regional cleavage is developed in Feruglio, E. 1949. Descripcion Geologica de la Patagonia. Direccion Gen- eral Yacimientos Petroliferas Fiscales, Buenos Aires, Argentina, Tomos Upper Jurassic to Neogene units, and is parallel to the Andean 1.-I". cleavage in the Main Cordillera to the west of the study area Katz, H. R. 1963. Revision of Cretaceous stratigraphy in Patagonian (Allen 1982). Structural and radiometric evidence, together Cordillera of Ultima Esperanza, Magallanes Province, Chile. Amer- with the sedimentary history of the foreland basin, suggest that ican Association of Petroleum Geologists Bulletin, 47, 506-524. the Andean deformation may have been a progressive event Katz, H. R. 1973. Contrasts in tectonic evolution of orogenic belts in the beginning with back-arc basin closure in the mid-Cretaceous southeast Pacific. Journal Royal Society of New Zealand, 3, 333-362. and continuing into the Pliocene. Mattauer, M., B. Collot, and J. Van den Driessche. 1983. Alpine model An east-dipping thrust zone along the boundary between the for the internal metamorphic zones of the North American Cor- Main Cordillera and the fold-thrust belt is interpreted as the dillera. Geology, 11, 11-15. east limb of an antiformal duplex stack (Wilson 1983). The de- Natland, M. L., E. Gonzalez, A. Canon, and M. Ernst. 1974. A system of Magallanes basin sediments. Geological Society of velopment of such a stack along this boundary may have stages for correlation of America Memoir 139. provided the tectonic thickening and crustal loading to produce Nelson, E. P., I. W. D. Dalziel, and A. G. Milnes. 1980. Structural subsidence within the foreland basin to the east of the fold- geology of the Cordiller Darwin; collision style orogensis in the thrust belt. southernmost Chilean Andes. Ecloage Geologicae Helvetiae, 73(3), There is rapidly accumulating evidence of large-scale trans- 727-751. port on low-angle thrust faults in many mountain belts. It there- Wilson, I. J. 1983. Stratigraphic and structural evolution of the Ultima fore seems likely that the highly deformed and metamorphosed Esperanza foreland fold-thrust belt, Patagonian Andes, southern pre-Tobifera basement window of Cordillera Darwin in Tierra Chile. Unpublished doctoral thesis, Columbia University, New York. A two-phase program of stratigraphic and oceanographic Expedition to Valdes: study around Peninsula Valdes (Patagonia, Argentina) was car- Miocene stratigraphy and Recent ried out between 4 and 24 September 1981. Use of the Rlv Hero siDed basins (cruise 81-4) allowed ready access to isolated coastal outcrops and to submarine deposits from Golfos San Matias, San José, and Nuevo (see figure, inset). THOMAS J. DEVRIES, JOHN M. ZAWISKIE, Miocene stratigraphy. Our stratigraphic study of the middle to and WILLIAM J. ZINSMEISTER late Miocene Entrerio and Rionegro Formations expands upon that of Zinsmeister and others (1981). They concluded that the Institute of Polar Studies The Ohio State University two formations were conformable and laterally equivalent, re- Columbus, Ohio 43270 cording the progradation of mudflat, beach, and supratidal 76 ANTARCTIC JOURNAL.
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