Michel Voisset

Hydrate dissolution as a potential mechanism for pockmark formation in the Niger delta

Received 17 February 2010; accepted 9 March 2010; published 11 August 2010. [1] Based on acquired geophysical, geological and geotechnical data and modeling, we suggest hydrate dissolution to cause sediment collapse and pockmark formation in the Niger delta. Very high‐resolution bathymetry data acquired from the Niger delta reveal the morphology of pockmarks with different shapes and sizes going from a small ring depression surrounding an irregular floor to more typical pockmarks with uniform depression. Geophysical data, in situ piezocone measurements, piezometer measurements and sediment cores demonstrate the presence of a common internal architecture of the studied pockmarks: inner sediments rich in gas hydrates surrounded by overpressured sediments. The temperature, pressure and salinity conditions of the studied area have allowed us to exclude the process of gas‐hydrate dissociation (gas hydrate turns into free gas/water mixture) as a trigger of the observed pockmarks. Based on numerical modeling, we demonstrate that gas‐hydrate dissolution (gas hydrate becomes mixture of water and dissolved gas) under a local decrease of the gas concentration at the base of the gas‐hydrate occurrence zone (GHOZ) can explain the excess pore pressure and fluid flow surrounding the central hydrated area and the sediment collapse at the border of the GHOZ. The different deformation (or development) stages of the detected pockmarks confirm that a local process such as the amount of gas flow through faults rather than a regional one is at the origin of those depressions.

Detailed Study of Three Contiguous Segments of the Mid-Atlantic Ridge, South of the Azores (37° N to 38°30′ N), Using Acoustic Imaging Coupled with Submersible Observations

Using a new tool of seafloor characterisation (sonar images from FARA-SIGMA cruise; Needham et al., 1992), coupled with submersible observations (DIVA1 cruise) we compare, at different scales of observation, three contiguous segments of the Mid-Atlantic Ridge, South of the Azores Triple Junction, between 37° N and 38°30′ N.The two northernmost segments (‘38°20′ N’ and Menez-Gwen) show unusual morphological features for the MAR; the rift valley is absent and the present-day magmatism is focused on shallow axial volcanoes. On the third segment (Lucky Strike), the morphology is the one usually found on the MAR. On the Menez-Gwen and ‘38°20′ N’ segments, volcanic constructional activity can obliterate, during periods of high magmatic supply, the morphology inherited from tectonic activity. The dive results constrain the recent evolution of each segment and show that a temporal variability in volcanic dynamics exists. On the three segments, outcrops of eruptive lavas alternate with large areas of explosive volcanic ejecta. This cycle in volcanic activity is influenced by changes in water depth, both spatially (i.e. between segments) and temporally (i.e. for the same segment through time).Each segment has known a specific history in its accretionary processes with a succession of tectonic and volcanic predominance and changes in its volcanic phases between volcanic ejecta and effusive dynamics.The hydrothermal activity is focused at the central part of each segment and is controlled by the presence of fresh lava and major tectonic features.

Past deep-ocean circulation and the paleoclimate record-Gulf of Cadiz

Deep marine currents are strongly influenced by climatic changes. They also deposit, rework, and sort sediment, and can generate kilometer-scale sedimentary bodies (drifts). These drifts are made of thoroughly bioturbated, stacked sedimentary sequences called contourites [Gonthier et al., 1984]. As a consequence, change in the direction or intensity of currents can be recorded in the sediments.

Geological and mineralogical study of Co-rich ferromanganese crusts from a submerged atoll in the Tuamotu Archipelago (French Polynesia)

The Tuamotu Archipelago in French Polynesia is a Co-rich ferromanganese crust province. The NODCO I survey (1986) provided detailed data on Co-rich crusts in this environment through the exploration of a restricted zone in the vicinity of Niau Island on the southern flank of the archipelago. This flat zone is a fossil atoll which, under the action of subsidence and tectonic movements, has collapsed to a water depth of 1000 m. The plateau is partially filled with coralline sediments. Outcrops of ferromanganese crusts, associated with rare nodules and slabs, are located on the inner side of the coral reef which bounds the ancient lagoon. The successive episodes of plateau history have been recorded in the different growth periods of the ferromanganese crusts. The crusts, nodules and slabs belong to the same morphological, mineralogical and geochemical family. Cobalt contents vary from 0.7 to 1.3%. The highest values belong to the thinnest ferromanganese crusts which are located on the flanks of the plateau. Average Ni contents are about 0.5% and Cu contents about 0.1%; Pt contents vary from 0.2 to 1.3 ppm. Platinum and Co are enriched in the outermost oxide zone of the crusts. Poorly crystallized δ-MnO2 is the dominant mineralogical phase. Cobalt enrichment seems to be related to δ-MnO2 particle size. The greatest contents are located in the finest material where the particle size is less than 0.1 μm. Cobalt-rich crusts of the Niau Zone have the same characteristics as the Co-rich crusts from the Equatorial North Pacific. They differ in original setting: the reefal environment in the Niau Zone is superficial, overlying a volcanic substrate.

A volcaniclastic deep-sea fan off La Réunion Island (Indian Ocean): Gradualism versus catastrophism

A new geophysical data set off La Reunion Island (western Indian Ocean) reveals a large volcaniclastic submarine fan developing in an open-ocean setting. The fan is connected to a torrential river that floods during tropical cyclones. Sediment storage at the coast is limited, suggesting that the sediments are carried directly to the basin. The fan morphology and turbidites in cores lead us to classify it as a sand-rich system mainly fed by hyperpycnal flows. In the ancient geological record, there are many examples of thick volcaniclastic successions, but studies of modern analogues have emphasized mechanisms such as debris avalanches or direct pyroclastic flow into the sea. Because the Cilaos deep-sea fan is isolated from any continental source, it provides information on architecture and noncatastrophic processes in a volcaniclastic deep-sea fan.

Identification of Shear Zones and Their Causal Mechanisms Using a Combination of Cone Penetration Tests and Seismic Data in the Eastern Niger Delta

In a site investigation of the eastern part of the offshore Niger delta, cone penetration tests (CPTU) showed significant drops in tip resistance, associated with decreases in sleeve friction and induced excess pore pressures at the interface between superficial sediments and the underlying deposits of a mass-transport complex (MTC) called NG1. Such signature characteristics of weakened zones are clearly expressed at three sites where the drop in tip resistance reaches more than 40% over 2–3 m-thick intervals. Correlations between CPTU profiles and both 3D and ultrahigh-resolution 2D seismic data suggest that the weakened zones surround syndepositional the within the frontal part of NG1. Hence, weakening appears associated with the remobilization of thrust faults, inducing localized plastic shear. Relatively recent, deep-seated structural movements affecting NG1 are suspected to have remobilized these thrusts faults. When considering the sole influence of gravity, the fact that shear strength is mobilized within scattered, limited zones along steeply dipping syndepositional faults is not favorable for the further development of a continuous slope-parallel failure surface above NG1.

Morphology and Depositional Processes of the Celtic Fan, Bay of Biscay

The first important studies of the Celtic Fan, located in the Bay of Biscay (Fig. 1), were initiated in 1996 by the SHOM and IFREMER institutes. The complete survey of the area by IFREMER was completed in 1997 (Auffret et al. 2000).The fan is mapped with multibeam echosounder (SIMRAD EMl2), seismic 3.5 kHz profiles and 12 Kullenberg cores (Zaragosi et al. 2000). Open image in new window Fig. 1. Bathymetry and morphological setting of the Bay of Biscay.Bathymetry after Sibuet et al. (1994). The Celtic Fan located in the northwestern part of the Bay of Biscay is bound to the east by the Trevelyan Escarpment and the Armorican Turbidite System, to the south by the Biscay Sea Mount, and to the west by the BiscayAbyssal Plain. It lies at the foot of the Celtic continental margin between 4,200 and 4,900 m water depth and is approximately 200 km long and 250 km wide. The fan is connected with the Celtic margin slope by two major deep-water channels: the Whittard Channel (4) which is supplied by the Great Sole drainage area (7) linked to the southern end of the Irish Sea system and the Shamrock Channel (5) which is supplied by the Little Sole drainage area (8) linked to the western end of the English Channel system

The Celtic and Armorican Margins — a New View

The Celtic and the Armorican margins constitute an important part of Europe’s western frontier at middle latitudes. Since the pioneering work of Berthois and Brenot which began in the early 1950s (Berthois and Brenot 1962; Berthois 1974) and the first maps including data obtained by Seabeam surveys (Pastouret et al. 1982; Lallemand et al. 1985; Sibuet et al. 1994), the Western Approaches and the northern Bay of Biscay margin sea floor have been intensively investigated as part of French (IFREMER, EPSHOM), Belgian and European programmes (RESECUSED, STARFISH, ENAM I and II, CORSAIRES, OMEX). Each of these phases have benefited from technical advances, such as: (a) the transition from single beam echo-sounding based on astronomic and radar dead recognitions to radio assisted navigation (DECCA); (b) seabeam (2/3 of water depth) and satellite-assisted navigation; (c) multibeam (up to 15 km range) and (d) high resolution Global Positioning System (GPS).