R. Sutherland , G.R. Dickens , P. Blum , C. Agnini , L. Alegret , J. Bhattacharya , A. Bordenave , L. Chang , J. Collot , M.J. Cramwinckel , E. Dallanave , M.K. Drake , S.J.G. Etienne , M. Giorgioni , M. Gurnis , D.T. Harper , H.-H.M. Huang , A.L. Keller , A.R. Lam , H. Li , H. Matsui , H.E.G. Morgans , C. Newsam , Y.-H. Park , K.M. Pascher , S.F. Pekar , D.E. Penman , S. Saito , W.R. Stratford , T. Westerhold , X. Zhou
2 February 2019
International Ocean Discovery Program (IODP) Site U1509 (34°39.13ʹS, 165°49.66ʹE; 2911 m water depth) lies ~640 km west of the northern tip of New Zealand, ~300 km south of Deep Sea Drilling Project (DSDP) Site 206, and ~200 km north of DSDP Site 592. It is located on the western margin of New Caledonia Trough (NCT) at the base of the Lord Howe Rise slope, near the mouth of a canyon that drains northeast around the end of a small spur. Site U1509 was chosen to determine the timing of Cenozoic folding in the southern NCT and to constrain the timing of vertical tectonic movements and volcanism. In particular, the site was chosen to test alternate hypotheses for the relative magnitudes of Cretaceous versus Eocene subsidence in the region. The sedimentary sequence at the southern end of NCT is contiguous with that in Taranaki Basin, where flat-lying (undeformed) strata overlie a Cretaceous rift sequence in most places. Samples dredged from West Norfolk Ridge and Wanganella Ridge include Cretaceous (~95–86 Ma) terrestrial arkosic sandstone containing coal and silicic volcanic fragments and Cretaceous (~86–66 Ma) upper bathyal (400–600 m water depth) sandstone and mudstone containing carbonaceous debris. Mapping of seismic reflectors tied to petroleum wells in Taranaki Basin shows that similar facies and a relict Cretaceous to Eocene shelf-edge morphology is buried across a large area (50,000 km2) of southern NCT. Of particular relevance to Expedition 371 are observations such as a buried shelf edge that suggest anomalously large tectonic subsidence (>2 km) since the Eocene in southern NCT and, moreover, that such subsidence was rapid and approximately coincided with local folding and reverse faulting (Tectonic Event of the Cenozoic in the Tasman Area [TECTA]). The spur on which Site U1509 is located is inferred to have been created by TECTA deformation and is underlain by the northeast-dipping limb of a west-verging fold. Paleocene bathyal claystone and late Oligocene (~25 Ma) basaltic volcanic material were dredged from the eastern edge of a canyon that appears to have been diverted by growth of the fold. The Paleocene claystone is early Teurian (Danian) in age (dinoflagellate Zones NZDP1–NZDP2) based on the presence of Trithyrodinium evittii and Palaeoperidinium pyrophorum and the absence of Alterbidinium pentaradiatum. Four seismic units were identified in the vicinity of Site U1509. The upper unit found beneath the axis of NCT (seismic Unit 1a1) comprises continuous parallel reflectors with moderate or high amplitude that onlap the basin margin and are interpreted to be turbidites. Seismic Unit 1a1 was drilled at Site 206 and is composed of clay-bearing nannofossil ooze of Oligocene to recent age. Seismic Unit 1a2 is the upper unit beneath the spur and is composed of moderate- or low-amplitude reflectors that have relatively complex geometries, including wavy geometries with a stacking pattern suggesting upslope-migrating bedforms. Seismic Unit 1a2 is interpreted to be a pelagic or hemipelagic unit that is time equivalent to the basin fill sequence, particularly the basal part of seismic Unit 1a1, and has been winnowed by bottom currents. Seismic Unit 1b1 comprises high-amplitude, discontinuous reflectors with complex geometries that occur in packets, in some places with downlap relationships. The unit thins dramatically away from the mouth of the canyon, appears to be locally deformed and tilted, and truncates reflectors of seismic Unit 2, below. The upper surface of seismic Unit 1b is marked as a downward transition to higher amplitudes and is not a discrete reflector in most places. Seismic Unit 2 comprises moderate-amplitude, parallel reflectors with moderate or high continuity. This unit correlates with strata that crop out in the canyon, from which samples of early Paleocene mudstone were dredged. The drilling objectives at Site U1509 were to sample the sequence of seismic Units 1a2–2 in a location with a relatively expanded seismic Unit 1b and minimal burial by seismic Unit 1a2. The predrilling working hypothesis was that high-amplitude, chaotic reflectors of seismic Unit 1b represented a syntectonic (TECTA) unit that contained material derived from the local canyon and was subsequently uplifted by folding.