Tania Marsset

Tidal deposition systems of China's continental shelf, with special reference to the eastern Bohai Sea

Abstract The continental shelf of China has undergone a large-scale marine transgression. Tidal currents have become the main active agent and have had a profound influence on the deposition and geomorphology of the shelf. The continental shelf of China has five modern tidal deposition systems: (1) the eastern part of the Yellow Sea, (2) the eastern part of the Bohai Sea, (3) off the mouth of Yangtze River; (4) to the west of Taiwan, and (5) the Strait of Qiongzhou. When the tidal current speed is above 150 cm/s, erosion dominates and reciprocating currents form deep scour furrows. When the tidal current speed is 50 to 150 cm/s, deposition is dominant, and tidal currents form tidal shoals (i.e., tidal sand ridges and tidal sand sheets) whose margins are near an absolute ellipticity value for the M2 tidal component of 0.4. When the absolute M2 value is less than 0.4, a reciprocating current is strong enough to form tidal sand ridges. When the absolute value is above 0.4, a rotating current increases so as to form a tidal sand sheet. Tidal deposition material came mainly from tidal erosion and locally redeposition of Late Pleistocene sediments, and from the neighbouring sea bottom. The tidal deposition system in the eastern Bohai Sea is a typical example. It consists of the Laotieshan Channel scour furrow, the Liaodong Shoal sand ridges and the Bozhong Shoal sand sheet. The maximum tidal current speed in the Laotieshan Channel where strong erosion occurs, is up to 250 cm/s. In the Liaodong Shoal, the maximum tidal current speed is 64–115 cm/s and the absolute ellipticity value of the M2 tidal component is less than 0.4. In the Bozhong Shoal, the maximum tidal speed is 58–79 cm/s and the absolute ellipticity value of the M2 tidal component is above 0.4. The small-scale bedforms surveyed in detail in the area show that the sand ridges and sand sheet are still under the action of tidal current processes at the present time. Based on the interpretation of tidal currents, bedforms and sediment thickness, it is inferred that the general direction of sediment transport within the study area is NNW from the Laotieshan Channel to the Liaodong and Bozhong Shoals. The small-scale bedforms in the detailed survey area indicate a local net sediment transport from south to north.

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.

Architecture of an active mud-rich turbidite system: The Zaire Fan (Congo-Angola margin southeast Atlantic): Results from ZaïAngo 1 and 2 cruises

Multichannel seismic data newly acquired during two ZaAngo surveys now provide an almost complete view of the Quaternary architecture of the Zaire Fan. Extending laterally from the southern Gabon margin to the Angola margin and longitudinally more than 800 km, the overall fan consists of three main individual fans that were deposited successively as overlapping depocenters. The individual fans are composed of channel/levee systems exhibiting similar seismic facies, external configurations, and organization to those described in other large mud-rich systems (e.g., the Amazon Fan). In particular, high-amplitude reflection units with a high oil-reservoir potential are recognized almost systematically as a basal sole for channel/levee systems. They possibly include true high-amplitude reflection packets related to avulsion processes below the avulsion points and coarse-grained basal levees related to the initial stages of levee aggradation subsequent to the avulsion. Correlations with Ocean Drilling Program Leg 175 Site 1077 indicate that the studied part of the Zaire Fan began to build in the late Pleistocene (780 ka). During the upper Quaternary, a great number of channel/levee systems (more than 80) were developed, possibly explained either by its permanent activity even during high sea level conditions or by the low Zaire River inputs. The frequent occurrence of channel entrenchment of either old or recent channels is another characteristic specific to the fan. Overdeepening of channels is probably partly caused by regressive erosion inside the parent channel in response to an avulsion and also in part because of other causes that are not fully understood.

Pockmark formation and evolution in deep water Nigeria: Rapid hydrate growth versus slow hydrate dissolution

In previous works, it has been suggested that dissolution of gas hydrate can be responsible for pockmark formation and evolution in deep water Nigeria. It was shown that those pockmarks which are at different stages of maturation are characterized by a common internal architecture associated to gas hydrate dynamics. New results obtained by drilling into gas hydrate-bearing sediments with the MeBo seafloor drill rig in concert with geotechnical in situ measurements and pore water analyses indicate that pockmark formation and evolution in the study area are mainly controlled by rapid hydrate growth opposed to slow hydrate dissolution. On one hand, positive temperature anomalies, free gas trapped in shallow microfractures near the seafloor and coexistence of free gas and gas hydrate indicate rapid hydrate growth. On the other hand, slow hydrate dissolution is evident by low methane concentrations and almost constant sulfate values 2 m above the Gas Hydrate Occurrence Zone.

Present-day rhythmic deposition in the grand Rhone prodelta (NW Mediterranean) according to high-resolution pollen analyses

Abstract A high-resolution pollen study (136 samples) has been performed on the KTR05 core (738 cm long) located in the modern Grand Rhône prodelta. The objectives were (1) to evaluate the palynological and sedimentological record quality of a prodelta in comparison with fluvial inputs (2) and to quantify lost sediment (resuspensions) in this area. The core covers more than the last thirty years. By comparison with the modern pollen rain both in the Rhône delta and in the mid-drainage basin (Lyon), a seasonal forcing in pollen deposition has been demonstrated. Monthly pollen successions can be evidenced in a well preserved sequence, providing an estimate of the true sedimentation rate (60 cm.yr−1). Importance of disturbed or incomplete sequences suggests that most of the sediment first deposited in the prodelta (around ⅔) has been resuspended. Sedimentological and palynological data record not only seasonal but fluvial impact. The progressive shift of the prodelta has been evidenced firstly with sedimentology and secondly with palynology. Despite high fluvial impact, pollen grains in the KTR05 core are representatives of the vegetation of the Rhône drainage basin. The apparent homogeneity of the pollen spectra evidences the high reliability of palynology applied on prodeltaic area, at least in a microtidal environment.

Stratigraphy and sedimentary environments during the Late Quaternary, in the Eastern Bohai Sea (North China Platform)

Abstract The Bohai Sea is a shallow sea at the east coast of China. It communicates with the Yellow Sea through a narrow strait. During and since the Late Pleistocene, the Bohai Sea has been a filled subsiding basin submitted to successive regressive-transgressive cycles. In the East Bohai Sea, a number of finger-shaped ridges are present near the strait, where tidal currents are dominant. Very-high resolution seismic reflection data have been interpreted relative to sedimentological data provided by a formerly drilled borehole. Seven seismic units are identified and correlated with the following events: units U7-U6 with the Bohai relative sea level rise (65,000–53,000 yr B.P.) and a relative sea level fall (53,000–39,000 yr B.P.); unit U5 with the Xianxian relative sea level rise (39,000–22,000 yr B.P.); unit U4 with the period of the late Wurm glaciation (22,000–15,000 yr B.P.); units U3-U2 with the transitional period from the end of the Pleistocene to the beginning of the Holocene (15,000–9000 yr B.P.); and unit U1 with the Holocene (Huanghua) marine intrusion in the Bohai Sea (since 9000 yr B.P.). In the Bohai Sea, formation of the tidal ridges is probably due to a substantial increase in tidal-current velocities during the late Holocene rise of sea level. Constriction due to the strait amplifies the tidal process, so that the sediments of the late Holocene transgressive sequence have been remodeled as tidal ridge.

The Celtic Sea banks: an example of sand body analysis from very high- resolution seismic data

Very high-resolution seismic data from the Kaiser-I-Hind sand bank (southern Celtic Sea) recently highlighted the internal structure of the enigmatic Celtic Banks, which are among the deepest and largest shelf sand ridges. The main body of the bank is made up of 4 seismic/depositional units which reflect a transgressive evolution. New data on the detailed architecture of two of these units allow discussion of bank growth in terms of either (1) a channel–levee system preserved both by lateral migration and aggradation of the channels, or (2) a package of large offshore tidal sediment bodies (bar chains and/or giant dunes). Careful geometrical observations of seismic discontinuities make the second hypothesis more likely. The unit architecture is analysed in terms of long- to short-term processes of build-up. Long-term processes are evinced by the landward stacking of erosive sub-units in response of the last post-glacial sea-level rise, whereas short-term processes control the seaward progradation of sand bodies and fills due to the ebb predominance of the Western Channel Approaches.

Morphobathymetry of Small-Scale Mud Reliefs on the Adriatic Shelf

Morpho-bathymetric and stratigraphic data reveal small-scale mud reliefs in the toe region of the late-Holocene mud wedge on the Adriatic shelf. The reliefs are elongate features with acoustically-transparent cores. They are present in two geologic settings: seaward of shore-parallel undulations within a thick mud wedge and seaward of basement highs where the mud wedge is thin. In both settings, the reliefs define clusters sub-perpendicular to the regional contours, possibly indicating an origin related to escape of fluids from an impregnated unit at the base of the late-Holocene wedge. Shore-parallel bottom-hugging currents appear to modify the reliefs following their episodic growth.

Detailed Anatomy of Late-Holocene Deposits on the Adriatic Shelf From 3D Very High Resolution Seismic Data (Triad Survey)

Very High Resolution 3D seismic data collected on the Adriatic shelf define the smallscale geometric complexity of late Holocene deposits. Three groups of units have been distinguished from deeper to shallower: the Transgressive System Tract (TST), a condensed interval at the base of the Highstand System Tract (HST) and the progradational units of the HST. The HST deposits show large-scale shore-parallel undulations and shore-normal reliefs. The genesis of these features seems to be related to a combination of two mechanisms: 1) sediment deformation in relation with the condensed section and 2) depositional process related to shore parallel currents that increasingly interact with topography.

Focused hydrocarbon‐migration in shallow sediments of a pockmark cluster in the Niger Delta (Off Nigeria)

The Niger Delta is one of the largest hydrocarbon basin offshore Africa and it is well known for the presence of active pockmarks on the seabed. During the Guineco-MeBo cruise in 2011, long cores were taken from a pockmark cluster in order to investigate the state of its current activity. Gas hydrates, oil and pore-water were sampled for geochemical studies. The resulting dataset combined with seismic data reveal that shallow hydrocarbon migration in the upper sedimentary section was focused exclusively within the pockmarks. There is a clear tendency for gas migration within the hydrate-bearing pockmarks, and oil migration within the carbonate-rich one. This trend is interpreted as a consequence of hydrate dissolution followed by carbonate precipitation in the course of the evolution of these pockmarks. We also demonstrate that Anaerobic Oxidation of Methane (AOM) is the main process responsible for the depletion of pore-water sulfate, with depths of the Sulfate-Methane Transition Zone (SMTZ) ranging between 1.8 and 33.4 m. In addition, a numerical transport-reaction model was used to estimate the age of hydrate-layer formation from the present-day sulfate profiles. The results show that the sampled hydrate-layers were formed between 21 and 3750 years before present. Overall, this work shows the importance of fluid flow on the dynamics of pockmarks, and the investigated cluster offers new opportunities for future cross-site comparison studies. Our results imply that sudden discharges of gas can create hydrate layers within the upper sedimentary column which can affect the seafloor morphology over few decades. This article is protected by copyright. All rights reserved.