The principal results for this site bear little relationship to the proposed objectives. If these objectives had read, "Determine whether the Campbell Drift is actually a Neogene contourite drift," we could at least have declared a limited success with the answer, "At least the top 10 to 30 m is strongly current influenced, but it represents 10 to 20 m.y." Henceforth, in this instance, Campbell "drift" will require quotation marks.
Carter and McCave (1997) introduced the name "Campbell Drift" for an extensive sediment accumulation up to 170 km wide, 850 km long, and a maximum of 1.1-s thickness, which occurs along the margin of the Campbell Plateau at depths of 4000-4800 m. It sits under the path of the combined Deep Western Boundary Current (DWBC) and Antarctic Circumpolar Current (ACC). Site 1121 lies downstream of the first significant input of sediment to the DWBC, namely from turbidity currents flowing down Solander Channel into Emerald Basin to the west of Campbell Plateau (Carter and McCave, 1997) (Fig. F1). Scour in the Emerald Basin provides sediment that was presumed to be transported over the Campbell "drift" and partly deposited there. Part of the powerful ACC detaches from the Campbell Plateau boundary at ~55° S (Orsi et al., 1995), but the rest of this flow continues north to Bollons Seamount, where it too veers east to eventually link with the southern branch. Further north, still more tranquil conditions occur under the DWBC alone. Under the ACC, the seabed shows signs of scour and slow deposition in an extensive field of ferromanganese nodules and pavement (Carter, 1989).
The underlying crust at Site 1121 dates from the rifting of Campbell Plateau from Marie Byrd Land at 85 Ma (Lawver and Gahagan, 1994). Immediately overlying that crust is up to ~1000 m of sediment, presumed to be a wedge of nonmarine clastics followed by uppermost Cretaceous and Paleogene marine sediments and Neogene contourites. In particular, Carter and McCave (1997) surmised that the uppermost sequence of closely spaced reflectors (~160 m thick) was of Pliocene-Pleistocene age. DSDP Site 276 was located in the erosional moat between the western edge of the "drift" and the Campbell Plateau escarpment; sampled gravel, sand, and a few rock chips suggest the presence of underlying siliceous siltstones. DWBC/ACC erosion has apparently cut down to the Paleogene in the moat, apart from a thin skin of sediment smeared on the drill bit that bears Miocene to Pleistocene microfossils (Kennett, Houtz, et al., 1975). It was thought that this indicated a good chance of obtaining a continuous Neogene-Paleogene section by drilling through the crest of the "drift" at Site 1121. Such a section would record the history of the ACC-DWBC and associated water masses as they approached the Southwest Pacific gateway.
There are few modern seismic profiles across the area. Older Eltanin profiles reveal only one shallow reflector, thought to delimit Pliocene-Pleistocene strata. Modern multichannel data show two principal reflectors, which lie close to the levels drilled, the first at a seafloor depth of ~120 ms two-way traveltime (TWT), 90-100 m, and the second (R1) at 316 ms TWT or ~250 to 300 m, depending on velocity (Fig. F2). Before drilling, the deeper of these was inferred to represent mid- to late Oligocene erosion. The upper reflector is not truncated in the moat along the foot of the scarp and thus fails to indicate profound erosion.
Site 1121 was drilled to obtain a sediment record of flow and sediment supply changes in the DWBC system. Special interest was to be focused on a comparison of paleocurrent speeds between this site and sites farther north, as Site 1121 is located where the DWBC is presently reinforced by the ACC. It was intended that this comparison would allow the effects of the ACC and DWBC to be distinguished. This site, uniquely among the sites planned to be drilled on Leg 181, was hoped to yield data pertaining to the deepest layer of the DWBC which originates mainly as Weddell Sea Deep Water.
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Ms 181IR-105