New Caledonia basin seismic amplifies earlier DSDP findings

A seismic survey program designed to evaluate the marine economic resources of New Caledonia (French Protectorate west of Australia) was conducted by Ifremer using the French research vessel L'atalante. The cruise was conducted within the framework of the ZoNeCo Program.

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The ZoNeCo 5 cruise allowed the mapping of the area located between the Lord Howe Rise to the west and the New Caledonia Basin to the east.
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A seismic survey program designed to evaluate the marine economic resources of New Caledonia (French Protectorate west of Australia) was conducted by Ifremer using the French research vessel L'atalante. The cruise was conducted within the framework of the ZoNeCo Program.

The cruise was led by Jean-Marie Auzende from Ifremer-Noumea in charge of the scientific coordination of ZoNeCo Program. The French scientific contingent aboard included Dr. Sabrina van de Beuque who is at present at AGSO (Australian Geological Survey Organization), and Dr Yves Lafoy of the New Caledonia mines department. Australian participants included Dr. Neville Exon (AGSO) and Dr. Jerry Dickens (James Cook University), whose prime responsibility was for taking sediment cores in the search for gas.

The ZoNeCo 5 cruise allowed the mapping of the area located between the Lord Howe Rise to the west and the New Caledonia Basin to the east. The ZoNeCo 5 cruise covered the Fairway Basin and its southern prolongation up to 26°40'S.

The seismic lines surveyed in the Fairway Basin show a sedimentary sequence characterized by five main units already described in the neighboring basins such as the New Caledonia, Loyalty, and Tasman Basins (unit 5 in accompanying graphic overlies seismic basement within the basins and is bounded at its top by a strong reflector).

Geologic history

With respect to tectonic history, the regional deposits are Early to Late Cretaceous, created in response to Early Cretaceous rifting throughout the area. Unit 4 either overlies unit 5, or in some places, mainly on the ridges, directly overlies the basement. Its thickness varies from 0.5 seconds in the Middleton Basin to a maximum of 0.6 seconds in the New Caledonia Basin. This sequence has been partly sampled by DSDP hole 208, where it consists of a Maastrichian to Late Paleocene section. The seismic character correlation indicates a Late Cretaceous to Late Paleocene section.

Unit 3, also sampled by DSDP (Deep Sea Drilling Program) 208, is extremely variable in thickness due to the uplift and erosion of the crests of high-standing features as a result of Middle to Late Eocene compressive events. In the basins, Unit 3 is more consistent in thickness and is believed to include Late Paleocene to Late Eocene section. Units 1 and 2 are more continuous throughout the area and constitute the post-tectonic cover. They can be correlated directly with DSDP 208. The DSDP results and seismic correlation indicate that Unit 2 was deposited from the Late Eocene to Late Oligocene, and Unit 1 from the Late Oligocene to the present day.

The total thickness of the sedimentary cover varies from 2 seconds (TWT) at the northern tip of the Fairway Basin up to 3.5 seconds (TWT) at the southern end of our survey. Two peculiar features appears within the sedimentary sequence:

(1) The first feature is a bottom simulating reflector (BSR) which has been mapped over a survey area of about 70,000 sq km. This BSR, located at about 500-600 ms-depth, was discovered in 1998 during the Faust 1 Cruise of the Australian RV Rig-Seismic. At that time, the BSR area surveyed was about 25,000 sq km, at the seabed boundary between New Caledonian and Australia. This BSR is interpreted as indicating the existence of gas hydrates.

Fairway Basin

The thickness of the gas hydrates could be as much as 200 meters. In the surveyed area, the BSR is known in the Fairway Basin from the northern tip limited by the Fairway and Lansdowne Ridges for about 400 km southward. The southern end is not yet known, but there are good reasons to expect its prolongation southward for the whole length of the basin. A deep seismic profile shot by the Rig Seismic around 27°S clearly shows the presence of the BSR.

(2) The second feature, within the sedimentary unit 5, is a continuous layer characterized by diffuse reflections, about 0.4 seconds thick and feeding diapirs. This layer occupies roughly the same area as the BSR, which is about 70,000 sq km. From our interpretation of the sedimentary sequence, this layer is Early to Middle Cretaceous in age. More than 100 diapirs have been counted in the surveyed area. Some of them are below the BSR, but others pierce the BSR and nearly reach the sea floor. The size of diapirs varies from 3 km to 15 km in diameter. In some places, the diapirs coalesce and constitute elongated ridges up to 50 km long. These diapirs can be interpreted either as salt diapirs or mud diapirs.

Deep diapirs of salt have been described in other continental margin settings, such as the north Atlantic rim and Mediterranean Sea. The diapirs of the Fairway Basin may originate from Lower to Middle Cretaceous salt deposited during the early stages of rifting along the eastern Australian margin. However, with available information, the diapirs could also be composed of mud. However, there are no direct indications of the age of the series except the correlation of seismic profiles with the DSDP 587 hole in the northern tip of Fairway Basin.

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Seismic line ZNC5-011 shows diapiric structures and a bottom simulating reflector (BSR) indicating gas hydrates.
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The ZoNeCo 5 cruise has confirmed the existence of a prominent BSR over about 70,000 sq km, a larger region than previously predicted. The depth of the BSR is consistent with pressure and temperature conditions for an interface between gas hydrate and free gas given a standard sediment velocity (1600 m/s) and the thermal gradient (0.04°C/meter) at DSDP site 587 in the northern tip of the Fairway Basin. The Fairway Basin appears to hold one of the most widespread marine gas hydrate deposits known.

Hydrate deposit

Most gas hydrate deposits found on continental margins contain predominantly biogenic methane. The presence of a very large gas hydrate deposit in the Fairway Basin is intriguing, because there is no apparent source for this gas. Surface waters in the region are relatively unproductive, and organic carbon contents are very low (<0.5%) in sediment at sites 587 and 588.

The area of diapirs in the Fairway Basin generally coincides with the region of BSRs suggesting a connection between the two phenomena. That connection may simply be that and/or organic-rich shale was laid down deep in the basin during early rifting, and that source rocks in this sequence has produced the gas that has generated the BSR. Thermogenic gas produced at depth may be migrating along diapir-related faults to the shallow subsurface. The shoaling of the BSR above some diapirs is consistent with upward advection of warm, saline fluids from depth.

Salt layers and diapirs have already been described in deep settings for a long time, when they were interpreted as indicators of the first stages of oceanic opening. During the first stages of stretching of the continental crust, a pre-oceanic shallow basin is created. The limited water supply and the high evaporation results in the periodic dessication of the basin for a long period of time and the deposition of successive layers of salt. Later, the salt, being less dense than the overlying sediments, will tend to move upward. The existence of faulting in the underlying basement considerably favors the upward motion.

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