8. Site 1124: Rekohu Drift--from the K/T Boundary to the Deep Western Boundary Current1

Shipboard Scientific Party2


Site 1124 is located ~600 km east of Gisborne, North Island, New Zealand (Fig. F1), on the 250-km-long, north-south-trending ridge of Rekohu Drift, at a water depth of 3978 m. Seismic line NZ 8802 through the site (Fig. F2) shows that the main drift is onlapped from the west, with angular unconformity, by a sequence of turbidites that have overspilled from the right bank of the Hikurangi Channel. These turbidites form part of the Hikurangi Fan Drift of Carter and McCave (1994), and are inferred to be of Pleistocene and perhaps late Pliocene age (cf. Lewis, 1994; Lewis et al., 1998). The Rekohu Drift ridge has clearly acted as a recent barrier to the eastward dispersal of terrigenous sediment from the Hikurangi Channel, which turns abruptly to the north against the drift. Until about the late Pliocene, the channel is thought to have flowed northeast directly along the axis of the Hikurangi Trough toward Kermadec Trench and to have become diverted eastward when a major slide blocked the trough-axis off Hawkes Bay (Lewis et al., 1998).

The Rekohu Drift sequence occurs above a regional reflector (R4), which before drilling was correlated with reflector X of presumed Oligocene age (Davey, 1977). Sediments of seismic Unit C, between R4 and the basement, are locally up to 200 ms thick, and overlie the basement (including fault-angle depression infills) as a simple drape. The upper part of Unit C is characterized by heavy, subparallel reflectors, indicating the probable presence of sands, cherts, or carbonates. As summarized in Table T1, the main Rekohu Drift comprises seismic Units A (65 m thick), B1 (78 m thick), B2 (150 m thick), and B3 (129 m thick). Unit B3 is characterized by the presence of "blotchy" reflecting zones within an otherwise almost transparent acoustic background. The seismic Unit A/B1 boundary may lie near the level of regional unconformity Y (Davey, 1977). A high-resolution 3.5-kHz profile (Fig. F3) indicates that the shallow stratigraphy comprises cyclic pelagic sediments, subparallel to the seafloor, with a particularly prominent reflector at a depth of 12 m (reflector of Carter and McCave, 1994; McCave and Carter, 1997).

Before drilling, it was hoped that Site 1124 would yield a mainly biopelagic carbonate record of the Miocene paleohydrography of the Deep Western Boundary Current (DWBC). It was anticipated that in the upper part of the hole tephra would provide important information about the Neogene volcanic history of the active arc system in nearby North Island (e.g., Alloway et al., 1993; Shane et al., 1996). Finally, and if drilling penetrated reflector R4, the initiation of the DWBC and related current systems might be dated within the context of the regional stratigraphy.

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