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Stratigraphical Studies in Orville Coast and Eastern Ellsworth Land

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Stratigraphical Studies in Orville Coast and Eastern Ellsworth Land ,t. Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. the average thickness being about 1 meter. Internal lamina- 1972. Geology of the Lassiter Coast area, Antarctic Peninsula— tion is absent in most places, but where present it is generally Preliminary report. In: Antarctic Geology and Geophysics (R. J. Adie, parallel and laterally persistent. Cross-bedding occurs occa- ed.). Universitetsforlaget, Oslo. pp. 143-148. sionally, and there are rare examples of convolute lamination. Intraformational shale-pebble conglomerate, usually in beds a few centimeters thick, occurs frequently at the bases or tops of massively bedded sandstone units. In many places these shale-pebble conglomerates are fossiliferous and contain belemnites or buchiid bivalves. True conglomerates, apart Stratigraphical studies in Orville from fine granule types, are generally absent. Carbonaceous Coast and eastern Ellsworth Land shales and minor coals are largely restricted to the lagoonal or lacustrine facies in the northern part of the area. Despite the variation in sandstone types and bedding units, the sequence is repetitive and monotonous and con- M. R. A. THOMSON tains no obvious marker beds. Thus a detailed stratigraphy is British Antarctic Survey likely to be established only by a great deal of painstaking Madingley Road work. However, wherever possible, sections were measured to demonstrate variations in lithologic composition and bedding Cambridge CB3 OET, U K thicknesses from one area to another. Maximum stratigraphic thickness measured in a single section in the Or- T. S. LAUDON ville Coast was 830 meters. Department of Geology The base to the Latady Formation has never been University of Wisconsin-Oshkosh positively identified. The lower parts of the Latady Forma- Oshkosh, Wisconsin 54901 tion, exposed in the Sweeney Mountains, contain interbedded andesitic and felsic volcanic rocks, and the sedimentary rocks appear partly to overlie large masses of porphyritic M. BOYLES2 J. rhyodacite. However, this probably represents a sedimentary Department of Geological Sciences sequence banked against an island arc, rather than a true University of Texas at Austin basal contact. Likewise, no top to the formation was seen in Austin, Texas 78712 the area, although apparently overlying dacitic and andesitic volcanic rocks have been reported in the Lassiter Coast (Williams et al., 1972). The marine faunas are dominantly molluscan, and large The 1977-78 geological reconnaissance of Orville Coast collections of ammonites, belemnites, and bivalves were and eastern Ellsworth Land (Rowley, 1978) found that most of this area is underlain by sedimentary rocks of the Latady made, notably in the Hauberg Mountains. Fossil floras, as well as the invertebrate faunas, also were obtained from the Formation. Although the Latady Formation also is Behrendt Mountains and northern Sweeney Mountains. widespread farther north, in the Lassiter Coast of eastern Studies of the faunas will bridge the geographical gap be- Palmer Land, the exposures there are separated by many tween those being completed by R. W. Imlay and E. G. plutonic intrusions surrounded by wide metamorphic Kauffman (unpublished data) of the U.S. Geological Survey aureoles (Rowley and Williams, in press; Vennum, 1978), for the Lassiter Coast area and earlier investigations in and fossils become less abundant to the north. In the Orville et al., 1969). Field identifica- Coast area, by contrast, exposed portions of intrusive bodies eastern Ellsworth Land (Laudon tions indicate that the bulk of the faunas collected this season are smaller and more scattered, and fossils are locally abun- are Late Jurassic in age, but the hitherto unique Middle dant, making the Orville Coast an excellent place to study the Jurassic marine faunas of the southern Behrendt Mountains Latady Formation. A north-south cross section of nearly 200 (Quilty, 1970) were re-collected to see if they might be recog- kilometers is exposed intermittently in mountain ranges and nized elsewhere in the area. These faunal collections will add isolated nunataks of the Orville Coast. Depositional facies significantly to our knowledge of the Mesozoic rocks in Ant- along this section range from an island arc with lagoonal or arctica and the distribution of fossil faunas around the world. possibly lacustrine deposits in the northern Sweeney Moun- This work was supported by National Science Foundation tains to deltaic deposits in the Hauberg and Wilkins Moun- grant DPP 76-12557 to the U.S. Geological Survey. tains to a more distal shelf fades at Cape Zumberge. In most places the Latady Formation consists of fine- to medium-grained sandstone and subordinate siltstone and shale. Fossils occur sporadically throughout the sequence and References are locally abundant. The massive to fissile sandstones are generally arkose, but they also include subarkose and quartzite. They exhibit a wide range of weathering colors (white, gray, tan, brown, and red) and are in beds ranging in Laudon, T. S., L. L. Lackey, P. G. Quilty, and P. M. Otway. 1969. Geology of eastern Ellsworth Land (sheet 3, eastern Ellsworth thickness from a few centimeters to more than 10 meters, with Land). In: Geologic Maps of Antarctica (Antarctic Map Folio Series, folio 12, V. C. Bushnell and C. Craddock, eds.). American British exchange scientist, Orville Coast field party. Geographical Society, New York. 2Work done while employed by the U.S. Geological Survey, Quilty, P. G. 1970. Jurassic ammonites from eastern Ellsworth Land, Denver. Antarctica. Journal of Paleontology, 44(1): 110-116. October 1978 Rowley, P. D. 1978. Geologic studies in Orville Coast and eastern Dalziel, I. W. D. In press. Pre-Jurassic history Ellswcirtn Land. of the Scotia Arc Antarctic Journat of the Us., 13(4): 7-9. region. In: Proceedings of the Antarctic Geology and Geophysics Sym- Rowley, P. D., and P. L. Williams. In press. Geology of the northern posium, August 1977. (C. Craddock, ed.). University of Wisconsin Lassiter Coast and southern Black Coast, Antarctic Peninsula. In: Press, Madison, Wisconsin. Third Symposium on Antarctic Geology and Geophysics (C. Craddock, Dalziel, I. W. D., M. J. de Wit, and K.F. Palmer. 1974. ed.). University of Wisconsin Press, Madison. A fossil marginal basin in the Southern Andes. Nature, 250: 291-294, Vennum, W.R. 1978. Igneous and metamorphic petrology of the Forsythe, R. 1978. Geologic reconnaissance of the Pre-Late Jurassic southwestern Dana Mountains, Lassiter Coast, Antarctic Penin- basement: Patagonian Andes, R/v Hero cruise 76-5. Anartic Journal sula.Journal of Research of the US. Geological Survey, 6(1): 95-106. of the US., 13(4): 10-12. Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. Nelson, E., R. Forsythe, F. Herve, M. Surez, E. Valenzuela, and T. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula— Wilson. 1977. Observaciones Estructurales en la Cordillera Dar- Preliminary report. In: Antarctic Geology and Geophysics (R. J. Adie, win, Provincias Antarctica y de Tierra del Fuego: Crucero 77-4, ed.). Universitetsforlaget, Oslo. 143-148. del pp. Riv Hero. Notas Cien4ficas, Communicaciones, 21 (Santiago, Chile). pp. 32-35. Scotia Arc Tectonics Project, Geologic reconnaissance of the 1977-78 Pre-Late Jurassic Basement: Patagonian Andes IAN W. D. DALZIEL Lamont -Doherty Geological Observatory RANDALL D. FORSYTHE Columbia University Palisades, New York 10964 Lamont-Doherty Geological Observatory of Columbia University Palisades, New York 10964 During the austral winter of 1977 and the austral summer of 1977-78, Scotia Arc Tectonics Project (Dalziel, 1975) fieldwork was conducted by scientists from Lamont-Doherty Over 3 months of field work have been done to define the Geological Observatory at the South Shetland Islands, Ant- regional geologic framework of the Pre-Late Jurassic base- arctica; at Cordillera Darwin, Tierra del Fuego, Chile; in the ment exposed along the outer perimeter of southernmost Andean foothills, Magallanes Province, Chile; and the Chile. This work has been carried out through the RJv Hero Patagonian coastal cordillera, Chile. The fieldwork involved cruise 76-5, and more recently through the logistical support the history of the Scotia Arc region from the late Paleozoic to of the limestone quarry operated by the Compania de Acero present day. del Pacifico within the region of concern. These investiga- In his work in the Patagonian coastal cordillera, Randall tions were part of the continuing study of the structural and Forsythe (1978) mapped detailed pre-Middle Jurassic base- tectonic history of the Scotia Arc supported by the National ment rocks of southern South America to tie in with work in Science Foundation. (See, for example, Daiziel et al., 1975; the South Orkney and South Shetland Islands (Daiziel, in Dalziel, 1975.) press). Margaret Winslow and Terry Wilson undertook field The Pre-Late Jurassic basement forms an almost con- studies of the tectonic evolution of the well-developed Andean tinuous belt of exposure from 47° S. to 54S. along the outer foreland fold and thrust belt. The first detailed study of the belt of islands that comprise the southern Chilean structural geology and tectonic history of Cordillera Darwin archipelago (see figure). The islands containing basement ex- in Tierra del Fuego, a highly deformed region on the conti- posures that have thus far been investigated are Desolacion, nental side of the Early Cretaceous marginal basin in the Donas, Madre de Dios, and Duque de York. southern Andes
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