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Site U1508

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      Abstract

      International Ocean Discovery Program (IODP) Site U1508 (34°26.89ʹS, 171°20.59ʹE; 1609 m water depth) is ~130 km west of Cape Reinga, which is the northern tip of the Northland region of New Zealand. This location on the northeast margin of Reinga Basin contains folded Eocene strata and is inferred to be near the southern end of where subduction initiation occurred. The site was chosen to sample a proximal record of deformation, uplift, subsidence, and early arc volcanism. Reinga Basin is a topographic depression with water depths of 1500–2500 m. It is bounded to the southwest by West Norfolk Ridge and Wanganella Ridge and to the northeast by Reinga Ridge and South Maria Ridge. Topographic highs at the edges of the basin are composed of deformed basin sediment and basement rock with typical water depths in the range of 500–1000 m but are locally shallower. Seafloor sediment of the basin is mostly carbonate sand derived from surrounding highs such as South Maria Ridge, where material comprises ~50% bryozoan fragments. Much of the sediment was probably transported by storms during lowstand sea level conditions. The sediment is also sorted by the Tasman (Eastern Australian) Current, which in places has created megaripples with >100 m wavelength and >5 km axial length that are visible on site survey swath bathymetry data. The subseafloor of Reinga Basin was first highlighted on reconnaissance seismic reflection surveys but had not been drilled. The geology of onshore Northland has been mapped, and an extensive grid of seismic reflection data exists adjacent to Northland. The Waka Nui-1 offshore petroleum exploration borehole was drilled ~275 km southeast of Site U1508 in Northland Basin. The geology of Northland and its adjacent basin provides context for interpreting Site U1508 because it is along strike structurally, presumably has a similar tectonic and stratigraphic history, and provides insight into the nature of terrigenous and volcanic sediment sources. Northland autochthonous basement rock is overlain by Eocene coal measures and Oligocene limestone, which is in turn tectonically overlain by deformed marine strata and Northland Allochthon volcanic rocks. Basement rock is known from magnetic surveys to have a strong northwest-striking fabric. The Junction magnetic anomaly is known from exposures farther south to mark the location of the Dun Mountain–Maitai terrane, which is a Permian ophiolite and volcaniclastic sequence, and its offshore location is close to Site U1508. Jurassic volcaniclastic sediments of the Murihiku terrane crop out onshore, were intersected at the base of the Waka Nui-1 borehole, and are correlated with the deepest reflectors imaged southwest of Site U1508. Northeast of the Junction magnetic anomaly, the Bay of Islands terrane basement rock is a deformed sequence of trench and trench-slope sandstone and mudstone of Permian to Jurassic age and may have represented a terrigenous sediment source for adjacent sedimentary basins. The Northland Allochthon was emplaced in the late Oligocene to early Miocene based on the minimum age of strata beneath the allochthon and the maximum age of strata above the allochthon; additionally, andesitic volcanism was active in the region during the early Miocene. Seismic reflection data near Site U1508 reveal five units. The deepest reflectors are truncated at the base of the basin fill and interpreted to be Jurassic sandstone, mudstone, and coal of the Murihiku terrane based on similarities to seismic reflection data near the Waka Nui-1 borehole. Localized areas of fanning reflectors at the base of the basin fill are interpreted to be Cretaceous extensional half grabens based on regional correlation with Taranaki Basin. The half-graben fill corresponds to seismic Unit U1 of Bache et al. A thick (>1 km) sequence of parallel folded reflectors is interpreted on the basis of regional correlations and dredged rock samples to be deformed Cretaceous to Eocene carbonaceous sandstone, mudstone, and bathyal carbonate that corresponds to seismic Units U2–U4 of Bache et al. Fanning, channeled, and onlapping reflectors at the basin margin record folding, uplift, erosion, and volcanic input during the Eocene–Miocene. A drape sequence overlies folded strata with angular unconformity and records conditions similar to present day, when terrigenous or reworked sediment sources are minimal and the primary source of sediment is biogenic carbonate from adjacent rises. Site U1508 was selected on the basis of seismic reflection data collected in 2009 and the Eocene ages obtained from samples of folded sedimentary rock dredged from the seabed elsewhere in the Reinga Basin. The objective at Site U1508 was to sample the Miocene–Eocene sequence, including the unconformity that marks the onset of deformation. The sequence contains a record of uplift, erosion, and subsidence and can constrain the age and type of nearby volcanic activity. The high-amplitude reflector near the base of the planned borehole is known to have regional significance for seismic stratigraphic correlation and was hypothesized to represent a major change in sedimentation during the Eocene.

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            Author and article information

            Journal
            10.14379/iodp.proc.371.2019
            Proceedings of the International Ocean Discovery Program
            International Ocean Discovery Program
            2377-3189
            2 February 2019
            10.14379/iodp.proc.371.105.2019

            This work is licensed under a Creative Commons Attribution 4.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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            Self URI (journal page): http://publications.iodp.org/

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