1. INTRODUCTION, PRINCIPAL RESULTS—LEG 35 DEEP SEA DRILLING PROJECT1 Charles D. Hollister, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts and Campbell Craddock, University of Wisconsin, Madison, Wisconsin Leg 35 was the third DSDP cruise to undertake drill- Gansser (1973a) argues that block-faulting rather than ing in Antarctic waters and the first in the Southeast compression has predominated in the Andes during the Pacific Basin; the others were Legs 28 and 29 in the Cenozoic. Southwest Pacific which took place during the austral The intersection of the Chile Ridge separates the ac- summer of 1972/1973. tive central Andes from the relatively quiescent southern Glomar Challenger departed Callao, Peru, on 13 Andes. The central Andes are marked by many active February 1974 and sailed to Valparaiso, Chile, to take volcanoes and earthquakes, but the southern Andes on additional personnel, equipment, and supplies prior contain only two active volcanoes and the infrequent to departing for Antarctic waters. The cruise ended on earthquakes are widely spaced (Gonzalez-Ferran, 1972). 30 March 1974 at Ushuaia, Argentina, after steaming The rocks of coastal West Antarctica record a com- approximately 5239 n. mi. at an average speed of 7.7 plex history of Phanerozoic sedimentation, volcanism, knots. plutonism, metamorphism, and orogenic deformation. During this unusually short leg a variety of mechani- However, no rocks of certain Precambrian age are cal and weather problems severely limited our drilling presently known. Several orogenic cycles have been program and only 8 days were spent with pipe in the sea identified; the youngest is the Andean of Cretaceous to floor. Drilling operations were carried out at four sites early Tertiary age. Cenozoic volcanic rocks are in water depths between 3745 and 5026 meters. Two widespread in coastal West Antarctica; for example, sites (322 and 323) lie on the Bellingshausen Abyssal Peter I Island (see Craddock, this volume), within the Plain, and two (324 and 325) lie on the continental rise southern part of our area (Figure 2), is a basaltic of Antarctica (Figures 1 and 2). Figure 3 shows location volcano of probable Miocene age, and other seamounts of the sites with relation to the acoustic provinces recog- are known to occur here. Thurston Island, south of our nized in the region. A single hole was drilled at each site. Site 324, consists of Paleozoic and Mesozoic igneous A total of 193 meters of sediment was recovered which and metamorphic rocks. Upper Tertiary basaltic ex- represents only 37% of the stratigraphic sections pene- trusives rest upon a glaciated unconformity in the Jones trated. Basalt was reached at two sites and 13.43 meters Mountains, just inland from Thurston Island. were recovered. A summary of coring operations is The geologic history of the Antarctic Peninsula and given in Table 1 and results are summarized graphically its relation to the evolution of the Scotia Arc have been on Figure 4. treated by Dalziel and Elliot (1973). They suggest that the Scotia Arc probably formed by Cenozoic disruption BACKGROUND AND OBJECTIVES of an earlier, nearly linear, belt which connected South America and the Antarctic Peninsula during the Regional Geology Mesozoic. Paleozoic and Mesozoic orogenic cycles have formed widespread igneous plutons and deformed meta- Present knowledge of the sediments beneath the basin sedimentary and metavolcanic rocks in the Antarctic and the history of physical and biological events which Peninsula and westward along the Antarctic coast they record are summarized by Goodell et al. (1973). (Adie, 1970; Craddock, 1972). Although there has been The geology of Antarctica has been compiled in a extensive Cenozoic volcanic activity in this part of Ant- geologic map (Craddock, 1972) and that of the Antarc- arctica, there is very little evidence for Cenozoic com- tic Peninsula is shown on two larger scale maps by Adie pression and folding. Except for a few earthquakes in (1970). the western Drake Passage, the Antarctic plate appears The Andes Mountains of South America and the to be aseismic at present. coastal zone of West Antarctica mark segments of the Pacific continental margin of Gondwanaland (Crad- Objectives of Leg 35 Drilling dock, 1975), and recurrent orogenic cycles from the ear- The sites drilled on Leg 35 were selected to study a ly Paleozoic through the latest Mesozoic have been number of particular regional problems as well as to add recognized. The modern Andean chain, at 9000 km the to general knowledge of the area. Some specific objec- longest on earth, serves as an ideal model for a tives of the cruise and reasons for selecting the sites "cordilleran-type" range (Gansser, 1973b); it is com- follow: monly explained as the result of oceanic plate subduc- tion beneath an active continental margin. However, 1) To study the nature and age of oceanic basement in an attempt to learn more about the pre-breakup con- figurations of Gondwanaland 'With Operations Summary by Lamar Hayes, Western Oceanics, The tectonic relationships and interactions between Inc., Houston, Texas. the Antarctic plate and the Antarctic Peninsula, the C. D. HOLLISTER, C. CRADDOCK THURSTON RIDGE ISLAND FLANK (after Heezen Tharp, 1972) Figure 1. Generalized physiography of the Southeast Pacific Basin and location of Leg 35 Sites (after Heezen and Tharp, 1972). Scotia Arc, the southern Andes, and the Chile Ridge are deep waters. Changes in the production of Antarctic poorly understood. Prior to this cruise, no direct bottom waters together with changes in sea-floor evidence was available on the age of the sea floor in the bathymetry, due to sea-floor-spreading processes, have Southeast Pacific Basin. played a major role in determining patterns of deep-sea For the vicinity of Sites 322 and 325 three models sedimentation and in producing variations in sediment were considered, based on the limited magnetic data characteristics that are responsible for the acoustic available: (1) Pitman et al. (1968) show the known reflectors. Seismic profiles in the basin reveal many magnetic anomalies which parallel the East Pacific different acoustic provinces (Figure 3) and reflecting Ridge, and projection of their data suggests a Mesozoic horizons; we hoped to establish the basic acoustic basement, probably Early Cretaceous in age, for the stratigraphy of the area in an attempt to infer patterns of eastern margin of the basin; (2) Herron (1971) interprets paleocirculation and variations in processes of ac- the Chile Ridge as a spreading axis and if the crust at cumulation. Sites 322 and 325 is formed by southwestward spreading The Antarctic Circumpolar Current flows eastward at away from the axis, an early Tertiary age is probable; more than 200 million m3/sec (Gordon, 1967, 1972; and (3) Griffiths and Barker (1972) describe a northeast- Reid and Nowlin, 1971). Site 321 lies near the western trending spreading axis in the Drake Passage and if Site entrance to the Scotia Sea and close to the present axis 322 and 325 crust came from this source, a late Cenozoic of this current; Site 323 lies about 250 miles south of the age is possible. axis. Both sites are situated to provide data on the early 2) To determine deep and surface patterns of paleocir- history of this massive current system. The earliest time culation which cause changes of productivity patterns and that the circumpolar current, as we know it now, could sedimentation events that in turn determine regional have existed was about 55 m.y.B.P. determined from acoustic stratigraphy magnetic interpretation of ocean crust production be- Global patterns of abyssal circulation appear to be tween Antarctica and Australia (Weissel and Hayes, closely connected to spreading patterns of Antarctic 1972). BATHYMETRY OF THE BELLINGSHAUSEN BASIN 105° 100° 95° 90° 85° Figure 2. Detailed bathymetric map of the Bellingshausen basin by B.E. Tucholke. Contours in uncorrected fathoms (1 fm = 1/400 sec). C. D. HOLLISTER, C. CRADDOCK IRREGULAR HILLS AND HYPERBOLA | HIGHLY REFLECTIVE, INTERM. LAMINATED FLAT, REFLECTIVE, LAMINATED STEEP REFLECTIVE SLOPE SLOPING, MOD. LAMINATED FLAT REFLECTIVE SHELF BASEMENT RIDGE * BURIED CANYON INACTIVE CANYON FINELY LAMINATED - ACTIVE CANYON TOO0 EIGHTS COAST AND JONES MOUNTAINS Figure 3. Acoustic provinces of the Bellingshausen basin from Tucholke and Houtz. this volume. Kennett et al. (1972) report a regional Oligocene un- LeMasurier (1972) described the volcanic history of conformity in the southwestern Pacific Ocean Basin. Marie Byrd Land and postulated the existence of a West They attribute it to flow of a proto-circumpolar current Antarctic ice sheet since the Eocene. Margolis and initiated during the separation of Australia from East Kennett (1970) studied the quartz grains and foramini- Antarctica. Sites 324 and 325 lie near the westward- fers in 18 piston cores from the South Pacific and in- flowing near-bottom, contour current on the continental ferred that Antarctic glaciation occurred during much of rise (Hollister and Heezen, 1967). This current with a the time since the Eocene, but that there was a warm epi- source in the Weddell Sea probably started soon after sode in the Miocene. During the DSDP Leg 28 glacio- the Drake Passage opened or about 25 m.y.B.P. marine sediments were found in holes in the Ross Sea, 3) To understand the history of continental glaciation including beds as old as Oligocene at Site 270. of Antarctica The Jones Mountains have yielded important evi- Evidence of Tertiary glaciation in the Jones Moun- dence for Miocene glaciation of ice sheet dimensions in tains (74°S, 94° W) was first reported by Craddock et al. this part of West Antarctica. K-Ar radiometric ages on (1964) and later summarized by Rutford et al. (1972). basaltic rocks overlying a fresh glacial pavement give INTRODUCTION TABLE 1 Coring Summary, DSDP Leg 35 Water Penetra- No. Age of Dates Hours Latitude Depth tion of Cored Recovered Recovered Oldest Deepest Site Occupied on Site Longitude (m) (m) Cores (m) (m) (%) Sediment Unit 322 Feb.