Towards IODP deep riser stratigraphic drilling in northern Zealandia: outcomes of pre-drilling site surveys on the Lord Howe Rise continental ribbon

The Lord Howe Rise, the backbone to northern Zealandia, is a ribbon of submerged and extended continental crust that separated from Australia during the Late Cretaceous. Sedimentary basins on the Lord Howe Rise preserve a 100 million year record of tectonics, climate and ancient microbial life in one of Earth’s last remaining geoscience frontiers. Proposal 871-CPP, approved by the International Ocean Discovery Program (IODP) in 2017, aims to drill and core a Lord Howe Rise rift basin using the riser-capable drilling vessel CHIKYU. In preparation for this proposed riser drilling, Geoscience Australia and JAMSTEC have undertaken two site surveys to acquire pre-drilling data that constrain the stratigraphic, structural, geotechnical and environmental characteristics of the sites under consideration.
Multichannel seismic reflection data acquired across proposed drill sites in 2016 provide a more detailed view of basin stratigraphy and pre-rift basement. At the preferred drill sites, syn-rift sediments with a thickness of 700–1000 m are overlain by up to 1000 m of post-rift sediments. At the northern drill sites, the new data show prominent sub-horizontal reflectivity within pre-rift basement beneath the syn-rift strata. Seismic stacking velocities range from 2–3 km/s in the sedimentary sequence and up to 4 km/s in basement. Based on these velocities and experience from previous CHIKYU drilling operations, the anticipated rate of coring will be 40–60 m/day through syn-rift sediments and 30–40 m/day in pre-rift basement.
At the southern drill sites, the inferred base of the rift-fill sediments coincides with a strong reflector that is interpreted to represent a sill. As seismic penetration below high impedance contrasts is normally dramatically reduced, depth-to-basement estimates at these sites remains uncertain. However, gravity modelling suggests that low-density sediments are likely to be present beneath the sill, thereby requiring drilling to a greater, un-realistic depth in order to reach the basement. 
Given the ambiguity in the depth to basement estimate for the southern depocentre and the new evidence for the existence of pre-rift strata in the northern depocentre, drilling in the northern depocentre is the preferred option. Drilling at these northern sites has the highest chance of intersecting rocks that record the full 100+ million year history of the region.
A second site survey in late-2017 acquired high-resolution multichannel seismic data around priority drill sites. These data not only indicated the absence of any significant drilling hazards in the shallow sediments, but also provide a higher-resolution image of the sedimentary fill. Seismic refraction data recorded on closely-spaced (800 m) ocean-bottom seismometers across the preferred drill site will provide a rare, public-domain dataset that can be used to generate a high-resolution velocity model that will help to better constrain depth to key reflectors at the preferred drill hole.  
Piston cores acquired during the second survey penetrated to a hard layer at a depth of ~7 m below the seafloor. This layer corresponds to a reflector that is evident in sub-bottom profiler data surrounding the drill sites. Strength tests to be conducted on whole-round samples from the piston cores and also from rotary cores collected from greater depths during IODP Expedition 371 in 2017 will provide important constraints for planning riser top-hole construction.  
Multibeam bathymetry data from a ship-mounted system and, at some sites with a deep-tow configuration, reveal a relatively smooth seafloor around the drill sites. These data will help ensure that drill sites are not located on structures related to sediment dewatering. Imagery from the deep tow bathymetry also confirms that there are no significant biological communities on the seafloor around the proposed drill sites.
The site surveys conducted on the Lord Howe Rise demonstrate the benefits of acquiring a broad range of data that allows a detailed geological and geotechnical understanding of the area. This  supports the prospect of successful IODP riser drilling at the sites.
Abstract submitted and presented to The 2018 Japan Geoscience Union (JpGU) Conference (https://www.jpgu.org/meeting_e2018/)

Tags: EARTH SCIENCES, International Ocean Discovery Program - IODP, Lord Howe Rise - LHR, Zealandia, ocean drilling, site survey
Organization: Geoscience Australia Data