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Deep Sea Drilling Project Inital Reports Volume 29

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Deep Sea Drilling Project Inital Reports Volume 29 43. SYNTHESIS — SEDIMENTS OF THE SOUTHWEST PACIFIC OCEAN, SOUTHEAST INDIAN OCEAN, AND SOUTH TASMAN SEA Peter B. Andrews,1 Victor A. Gostin,2 Monty A. Hampton,3 Stanley V. Margolis,4 and A. Thomas Ovenshine5 ABSTRACT The Leg 29 sediment sequences are dominated by burrow-mottled, open-ocean biogenic sediments and by burrow-mottled, very fine- grained terrigenous sediments. Coarse detritus, primary sedimentary structures and turbidites are rare. Silicification and chertification is characteristic of the upper part of the fine-grained terrigenous facies. Clear-cut evidence for contemporary volcanism was recorded at only one site. The succession of facies generally reflects the gradual evolution of the three ocean basins by sea-floor spreading and the related development of the oceanic circulation patterns as they now exist. INTRODUCTION TERRIGENOUS SILT AND CLAY FACIES This sediment was the first to accumulate on both The sediments drilled in the area covered by Leg 29 oceanic basement and continental basement during Late (Figures 1 and 2), generally reflect the gradual evolution Cretaceous and Paleogene time. It rests on basalt at of the three major ocean basins and the development of deep positions in the Tasman Sea and southeast Indian the oceanic circulation systems now characteristic of the Ocean. It also appears to have been the first sediment to basins. accumulate on the continental rocks of the Campbell Terrigenous silt and clay accumulated during the first Plateau. Here, the facies appears to underlie an phase of basin evolution. With continued sea-floor acoustically reverberant surface that is widespread spreading, the ocean basins enlarged, the supply of throughout the Campbell Plateau. At Site 275 (eastern terrigenous sediment diminished greatly, and persistent Campbell Plateau) late Campanian semilithified silt and current systems evolved. In the southwest Pacific (Site clay underlie this surface. On the western Campbell 275) and south Tasman Sea (Sites 280 and 283), Plateau (Site 277), drilling ceased about or just above terrigenous silt and clay was succeeded by siliceous the same acoustic surface. The lowest sediments sampl- ooze, indicating that cool Antarctic waters had ed were clay nannofossil chalk of middle Paleocene age. penetrated into these areas. However, contemporaneous It is reasoned that within a short stratigraphic interval in nannofossil ooze and mixed foraminiferal and nan- Hole 277, this clay nannofossil chalk would have passed nofossil ooze that accumulated at Site 277 suggest that down into terrigenous silt and clay, the top of which siliceous-rich Antarctic waters never penetrated this would coincide with the reverberant surface. Seismic area. With two notable exceptions (Sites 278, and profiles show that acoustic basement lies 1500 meters 283) succeeding Neogene time is characterized by below the reverberant surface at Site 277 and 500 meters calcareous oozes (mostly nannofossil oozes). below it at Site 275. It is likely that these intervals (to With the exception of rare occurrences of detrital acoustic basement) consist largely of terrigenous silt and sand or glauconitic sand, the sediments can be grouped clay; the lowest part of the sequence may consist of into three facies: terrigenous silt and clay, siliceous ooze, coarser terrigenous sediment. Such a sequence is expos- and calcareous ooze/chalk. At several sites they succeed ed on Campbell Island, midway between Sites 275 and each other in that order, thereby reflecting the gradual 277. development of the three ocean basins. On Campbell Island, the Garden Cove Formation consists of quartz pebble conglomerate and pebbly sand, which rest unconformably on the Complex Point Schist. The Garden Cove Formation grades up into interbedd- 'Sedimentation Laboratory, New Zealand Geological Survey, Christchurch, New Zealand. ed pebbly sandstone and mudstone, then into dark gray, department of Geology, University of Adelaide, Adelaide, South unbedded though locally laminated carbonaceous Australia. mudstone (Oliver et al., 1950). It is regarded as Late 3Geology Department, University of Rhode Island, Kingston, Cretaceous at the base, and Paleocene at the top (A. R. Rhode Island. Edwards, personal communication). The upper part of 'Department of Oceanography and Hawaii Institute of Geophysics, University of Hawaii, Honolulu. the sequence appears comparable with the terrigenous 5United States Geological Survey, Menlo Park, California. silt and clay facies of Leg 29. 1147 P. B. ANDREWS ET AL. S35- 40- 45- 50- 55- 60- >z 65- ANTARCTICA i E130 140 150 160 170 180 160W Figure 1. Sketch map showing location of the Leg 29 drill sites. The terrigenous silt and clay facies is characteristically remains of an infauna; however, its burrowed nature a dark olive-gray to dark brown, organic carbon-rich, suggests the former presence of a relatively rich, soft- burrow-mottled, poorly sorted silty clay, with subor- bodied infauna. dinate clay and clayey silt. The facies has been described The facies is exceptionally developed at the foot of the in detail (Andrews and Ovenshine, Chapter 31, this continental slope west of Tasmania (Site 282) in that it volume) so only a brief review of its characteristics are contains moderate amounts of sponge spicules and nan- presented here. The intensity of burrow-mottling varies noplankton, scattered low diversity planktonic stratigraphically. Some distinctive burrow forms occur foraminifera and scattered shallow-water fossils. and primary stratification of any form is extremely rare. At all sites except 282, siliceous microfossils (especial- Diagenetic and authigenic minerals are very ly diatoms) appear in the uppermost part of the facies. characteristic of the facies and include: glauconite, They are few in number, poorly preserved and common- microspheres of manganese and/or iron, and pyrite. ly broken at first, but at some locations become in- The facies also is characterized by a very restricted creasingly diverse, abundant, and well-preserved at the fauna and flora. The restricted epibenthic fauna, and top of the facies. The facies grades up into terrigenous planktonic fauna and flora, include small numbers of silt and clay-rich siliceous oozes, in which well-preserved siliceous and agglutinating foraminifera. Planktonic diatoms are dominant. foraminifera do not occur, but other pelagic microfossils The zone below the diatom-rich upper part of the are present in modest numbers and varying diversity at terrigenous facies is extensively silicified. The zone is several horizons. The facies does not contain skeletal rich in the silica minerals cristobalite and tridymite, and 1148 SYNTHESIS 275 277 283 284 meters 2800m 1214m 4729m [066m meters PW>locβne 10 100 100 200 200 300 300 400 400 glauconitic or 500 500 dβtrital sand basalt basement schist basement 600 600 Figure 2. Simplified stratigraphy for each of the sites drilled, Leg 29. at Site 280 is succeeded by interbedded chert and deep oceanic positions. It consists of detrital minerals, silicified clay silt, both of which also contain and the sediment is land derived. However, the presence cristobalite, tridymite, and microcrystalline quartz. This of extensive burrow-mottling and manganese and iron sequence supports the thesis that chertification of micronodules indicates that bottom waters were ox- oceanic silt and clay is a three-step process. The details ygenated. The lack of primary sedimentary structures of this process are further presented in Chapter 31 (this indicate that bottom currents were weak. volume), Heath and Moberley (1971), and Greenwood (1973). SILICEOUS OOZE FACIES A variant of the terrigenous silt and clay facies occurs Siliceous ooze is variable in character. Three types at Site 282, which lies at the foot of the continental slope were encountered during Leg 29 drilling. One type is west of Tasmania. Here, units of a bedded subfacies characteristic of the late Eocene and early Oligocene at alternate with the burrow-mottled subfacies. Thin (20- three of the four South Tasman Rise and south Tasman 90 cm) color-graded and commonly size-graded beds Sea sites. At Sites 280 and 283 siliceous ooze succeeds characterize the subfacies. The beds consist of silty clay the terrigenous silt and clay facies, and at Site 281, it and clayey silt low in organic carbon; they are siltier forms the first thick unit to cover the thin basal units of than the burrow-mottled interbeds and rich in nan- angular pebble gravel and nannofossil chalk that overlie nofossils. Only small numbers of poorly preserved nan- the schist basement of the South Tasman Rise. At these nofossils occur in the interbeds. The presence of limited sites the facies contains abundant detrital sediment, numbers of neritic nannofossils and shallow-water which appears to be responsible for the characteristic benthonic foraminifera, and the size-graded character of greenish-gray and grayish-olive color. Silt and clay-size many beds, suggest that the beds are distal turbidites. detritus comprises 40%-45% of the facies at Site 280, and The turbidites must have originated on the outer con- 10%-25% at Site 283. The facies is soft to stiff, massive to tinental shelf or upper continental slope and ac- faintly fine mottled silty diatom, or siliceous ooze. At cumulated at the foot of the slope. In the absence of tur- Site 283 it is a silt-bearing nannofossil siliceous ooze at bidites the burrow-mottled facies is deposited. the base, a silt-rich diatom ooze over much of the rest of The terrigenous silt and clay facies is coarsest near the unit, and a silty diatom ooze at the top. At Site 280 land and in shallower positions, and finest in distant and the facies alternates between silty diatom ooze and 1149 P. B. ANDREWS ET AL. diatom detrital silt. The facies at Site 281 is nannofossil, prises most of the sequence drilled at Site 278, occurring foraminifera, and detrital sand-rich siliceous ooze at the at most stratigraphic levels (middle Oligocene to base, gradually changing to a glauconite-rich, siliceous Pleistocene). detrital sand-silt-clay at the top. Here, it is succeeded Site 278 is the most southerly drilled (latitude unconformably by glauconitic sand.
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