Site U1511

Abstract

International Ocean Discovery Program (IODP) Site U1511 (37.5611°S, 160.3156°E; 4858 m water depth) is located on the Tasman Abyssal Plain, ~945 km east of Australia and ~990 km northwest of New Zealand. Site U1511 lies west of Lord Howe Rise on oceanic crust that formed during the Late Cretaceous (Chron 33n; 74–80 Ma). Regional seismic reflection data reveal a thick (>800 m) sequence of sediments deformed by reverse faults and folds. Site U1511 was chosen to determine the age of this deformation and to provide the first comprehensive record of sedimentation on the areal extensive Tasman Abyssal Plain. The only previous scientific borehole into Tasman Abyssal Plain sediment was drilled with spot coring at Deep Sea Drilling Project (DSDP) Site 283, which is located on conjugate oceanic crust of Late Cretaceous age near southeast Australia. Hole 283 collected 61.6 m of core from a total penetration of 592 m. The drilled sequence included 16 m of Pleistocene to late Miocene zeolite clay above a major unconformity. Strata beneath the unconformity included upper Eocene diatom ooze with calcareous nannofossils (~164 m thick), middle Eocene silty clay (~225 m thick), and sparsely fossiliferous Paleocene silty and/or pyritic claystone (283 m thick). Highly altered basalt was sampled at the base of Hole 283, where seismic reflection data image 0.72 s two-way traveltime (TWT) of sedimentary strata. Three seismic units were identified at Site U1511 from a grid of seismic lines that image folded and reverse-faulted strata. The upper seismic unit (1) is acoustically transparent, except for a thin layer observed just (0.05 s TWT) beneath the seabed. The unit varies in thickness along seismic profiles such that it fills in lows of underlying folded units. The middle seismic unit (2) is defined by a reflective package in its upper part and a high-amplitude, normal polarity reflector at its base. The upper reflective part of seismic Unit 2 contains fanning relationships and variations in thickness associated with folds and faults that are interpreted to be syntectonic. The lower seismic unit (3) contains low-amplitude reflections with moderate continuity and is bounded at its base by high-amplitude reflectors that are interpreted to be oceanic basement. Localized packages of high-amplitude reflections in the lower unit are interpreted to be basaltic intrusions. Regional mapping of the seismic grid reveals a negative correlation between the thicknesses of Units 1 and 3. Likely, Unit 3 depocenters were controlled by normal faults that were active during crust formation (seafloor spreading). These faults were reactivated as reverse faults during deposition of the upper part of seismic Unit 2. The primary drilling objectives at Site U1511 were to sample (1) the top of the middle seismic unit to constrain the age of folding and (2) thick intervals deeper in the sedimentary sequence to develop an understanding of this abyssal location in the past.