(Carter and McCave, 1994) has a particularly hard acoustic response, and before drilling, was suspected of being either an unconformity or a sand unit.
Site 1123 is located 410 km northeast of the Chatham Islands, on the deep northeastern slopes of Chatham Rise (Fig. F1). The site was drilled in a water depth of 3290 m. Site 1123 is located on National Institute of Water and Atmosphere seismic line NIWA 3034-7 (Fig. F2).
Four main seismic units occur above basement, which is not well imaged on this or nearby lines (seismic Units D-A; Table T1). In ascending order, these units consist of sediment with short, irregular reflectors that passes up into sediment with strong, irregular, and laterally discontinuous reflectors (Unit D; 5.70-ms two-way traveltime [TWT] thick). Next, Unit C comprises 205 ms TWT of irregularly reflecting sediment, with stronger, more continuous reflections concentrated toward the top. Unit B, similarly to Unit D, comprises a lower unit of acoustically homogeneous sediment with markedly discontinuous reflectors in short segments (120-ms TWT thick, Subunit B2), and an upper unit of stronger, more continuous reflectors (180-ms TWT thick, Subunit B1), many displaying diffraction hyperbolae. At the top, Unit A comprises 180 ms of more regular but sometimes discontinuous reflectors, which are subparallel to the seafloor. The reflectors chosen to separate these units and subunits (R1 through R4) are all composite, in the sense that individual segments of each "reflector" can only be traced laterally for short distances, their extended correlation depending upon the presence of another similar reflector at about the same level. The zones of harder reflectors in the upper parts of Units B and C are made up of large numbers of short, concave-downward segments, some of which represent refractions from hard zones lying within the sediments, possibly chert or carbonate concretions. These refractions obscure the precise relationships between Units A and B and B and C in particular, and, before drilling, were viewed as candidate unconformities.
The 3.5-kHz profiles through and near the site (Fig.
F3) show the seafloor to be underlain by ~40 m of light, 2- to 6-m-spaced, semi-continuous reflectors at depths of ~3, 10, 23, 33, and 35 m. Reflector (Carter and McCave, 1994) has a particularly hard acoustic response, and before drilling, was suspected of being either an unconformity or a sand unit.
Little previous information exists to precisely correlate or date these seismic units. Although the geology of the Chatham Islands is now well described (Campbell et al., 1993), the thin, shallow water and the atypically volcanic-influenced nature of the Cenozoic stratigraphy there makes it difficult to extend into deep water. Regional interpretations of the offshore seismics have been attempted (Wood et al., 1989; Wood and Herzer, 1993) but are tenuous. Carter and McCave (1994) inferred an age of Pliocene-Pleistocene for regularly bedded Unit A, and correlated reflector R1 with regional Unconformity Y, of late Miocene age, with reference to the seismic stratigraphy of the outer Bounty Trough (Carter et al., 1994), which is itself an equivocal extension of that unconformity from Deep Sea Drilling Project (DSDP) Site 594 and petroleum exploration holes under the Canterbury shelf. Carter and McCave (1994) also suggested that Unit B represented Miocene, and perhaps older, sediment drifts deposited from the Pacific Deep Western Boundary Current (DWBC). The uppermost part of Unit A has also been sampled in a piston-coring transect some distance west of Site 1123 (Fenner et al., 1992) and in nearby core CHAT 1K at station S924 (Weaver et al., 1998; Lean and McCave, 1998). All cores have contained rhythmic glacial/interglacial hemipelagic/biopelagic couplets with an average sedimentation rate of ~2 cm/k.y.
Site 1123 was drilled to document the stratigraphy of the northern slopes of the Chatham Rise and to establish the effects of the DWBC on Neogene sediment deposition. For parts of the column influenced by drift sedimentation, grain-size, geochemical and isotopic signals will be used to determine water-mass movement and DWBC velocity. Because the site is just north of the Subtropical Convergence (STC), information from Site 1123 will help indicate the position of the STC between glacial and interglacial times and test for associated variations in oceanic productivity (Fenner et al., 1992; Nelson et al., 1993; Weaver et al., 1997). It will also test the coherence of the paleoclimatic record with Milankovitch cycles.
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