Serge Berné

Pleistocene forced regressions and tidal sand ridges in the East China Sea

Abstract Tidal sand ridges are common features on modern shelves but only few examples of such preserved sand bodies are described in Pleistocene deposits. In the stratigraphic record, some sand bodies encased in shales, previously interpreted as sand ridges, have been reinterpreted as shoreface deposits. More than 5000 km of high-resolution seismic data from the East China Sea, correlated to geotechnical boreholes and shallow cores, demonstrate the potential of sand ridge preservation and allow reconstruction of the depositional history of Pleistocene fourth order (100-kyr) depositional sequences. A high subsidence rate of about 300 m/Myr allows the preservation of three elementary sedimentary facies, constitutive of a ‘motif’ which was repeated during glacio-eustatic cycles. They consist of (1) regressive marine prodeltaic prograding wedges, (2) estuarine and continental (deltaic) facies, and (3) transgressive sand ridges, similar in shape and orientation to modern sand ridges. Major discontinuities, traceable over the entire outer continental shelf along distances of hundreds of kilometers, are transgressive and regressive surfaces of marine erosion, whereas sequence boundaries formed by fluvial erosion are difficult to identify on this low-gradient shelf. Because of the asymmetry of the Pleistocene glacio-eustatic cycles, most of the preserved sedimentary record (with the exception of sand ridges) corresponds to forced regressive deposits (deposits that formed during a seaward shift of the shoreline due to relative sea-level lowering).

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.

Morphology, Internal Structure, and Reversal of Asymmetry of Large Subtidal Dunes in the Entrance to Gironde Estuary (France)

ABSTRACT The Gironde is a macrotidal estuary characterized by important changes in tidal-current velocity and river discharge. Two surveys were made in the lower estuary in June 1987 and October 1989 in an area with large dunes. The symmetry of these dunes (1.5-6.7 m high) changes from flood-dominated (in upstream areas) to ebb-dominated (in downstream areas). A transition zone consists of symmetrical dunes with a very low vertical form index. The internal structure of the dunes, revealed by seismic records, shows a hierarchy of bounding surfaces related to fluctuations in movement speed and asymmetry of the dunes. The lowest part of symmetrical dunes consists of ebb-oriented reflectors, while the upper part has flood-oriented reflectors. The same profile lines run in 1989 show that each large dune can be identified, despite an upstream shift of the transition zone of about 1000 m. Comparison of seismic records shows that some lower reflectors are preserved between the two surveys, and that the dunes moved downstream a mean distance of 30 m. In 1989 the internal structure of symmetrical dunes was reversed, with flood-oriented reflectors in the lower part overlain by ebb-oriented reflectors. Both seasonal changes in river discharge and fortnightly oscillations of the tidal range control the magnitude and orientation of the net bed-load transport in the study area. Because the response time of the large dunes is longer than that of the superimposed small dunes, the reversal of asymmetry of the large dunes requires a long-period process, while fortnightly oscillation or even semi iurnal reversal of the tidal current (during spring tides) could cause reversal of small dunes.

Multi-process generated sediment waves on the Landes Plateau (Bay of Biscay, North Atlantic)

Detailed analyses of recently collected bathymetric and sparker seismic data, support a new interpretation of the Landes Plateau field of sediment waves located on the Aquitaine upper continental slope (Bay of Biscay). The wave geometry, previously described as the result of a major sediment failure, is interpreted as a structure with a complex origin including the interaction of depositional and gravity deformation processes. Depositional processes are mainly recorded by the upslope migrating pattern of the waves resulting from oblique or sigmoid downlap reflections on the upslope flank of the waves and by toplapping and truncated reflections on the downslope flank. Hemipelagic and turbiditic sedimentation may be involved in the wave building as well as contouritic processes that could be related to the existing northward polar current and internal waves. Gravity deformations are syndepositional, discontinuous and of low amplitude, affecting thick layers which alternate with undeformed layers. They seem to correspond to gentle sediment creeping or stretching associated with minor listric or compaction-like faults, and possible limited back rotation of sediment blocks. These multi-process generated sediment waves could be rather common on the continental margins as they could have been mistaken with either depositional or deformational structures.

Architecture and long term evolution of a tidal sandbank: The Middelkerke Bank (southern North Sea)

Abstract The internal structure of the Middelkerke Bank (one of the Flemish Banks located in the southern North Sea off the coast of Oostende, Belgium) has been studied in the framework of the Marine Science and Technology (MAST) program co-funded by the European Community. A dense grid of high and very high resolution seismic profiles has been used, as well as several vibrocorings. Seven major seismic units can be identified in the Quaternary sediments, bounded by major discontinuities correlated across the whole study area. The lower units clearly appear as being deposited during periods of relative low sea level (channel infillings, shoreface, estuarine and/or ebb-tidal delta deposits). The present shape of the bank results partly from recent erosional processes, reworking the underlying deposits. Thus, the lower part of the bank as a morphological feature does not consist of “offshore tidal sands”. The master bedding of the upper part of the bank consists of inclined reflectors, dipping at an angle of about 5° in the same direction as the banks “steep” face. These reflectors, very similar to those described by Houbolt (1968), are interpreted as being the result of alternating periods of deposition and erosion related to the episodic combination of tidal currents and storms.

The 100‐ka and rapid sea level changes recorded by prograding shelf sand bodies in the Gulf of Lions (western Mediterranean Sea)

Thick forced regressive units on the wide continental shelf of the Gulf of Lions (western Mediterranean) recorded the composite effect of sea level changes during the Quaternary. They are mostly composed of coastal siliciclastic and bioclastic wedges showing clinoform geometry. These deposits have been intensively explored through high-resolution seismic investigations, but only recently it was possible to ground truth seismic interpretations, based on a long (100 m) borehole that crossed the succession and recovered a large part of the mainly sandy deposits (similar to 84% recovery). A multiproxy analysis of the sedimentary succession shows that (1) the stratal architecture of the shelf margin is defined by major bounding surfaces that are polygenic erosion surfaces associated with coarse-grained material incorporating abundant and diverse shells, including cold-water fauna (presently absent from the Mediterranean Sea). Between each surface, coarsening upward units with steep (up to 5 degrees) foresets are made of massive (more than 20 m thick) sands with possible swaley and hummocky cross-stratification, passing seaward to sands with muddy intervals and, further offshore, alternating highly boiturbated sands and silts. Each prograding wedge corresponds to a forced-regressive shoreface (or delta front/prodelta), deposited during the overall sea level falls occurring at (relatively slow) interglacial/glacial transition and therefore represents the record of 100 ka cyclicity. Higher-frequency Milankovitch cyclicities are also probably represented by distinct shoreface/delta front wedges; (2) detailed examination of the architecture and chronostratigraphy of the most recent sequence shows that minor bounding surfaces, corresponding to abrupt shallowing of sedimentary facies, separate downward stepping parasequences within the last 100 ka sequence...

Subseafloor stratigraphic profiling and soil classification from piezocone tests: A case study in the Gulf of Lion (NW Mediterranean Sea)

We show the results provided by piezocone tests in determining the stratigraphic profile and the soil classification of two drilling sites in the outer shelf and the upper slope of the Gulf of Lion, PRGL2 and PRGL1, respectively. Correlations with grain-size data indicate that sleeve friction can be used for profiling fine-grained sediments (site PRGL1), whereas cone tip resistance is the most adequate for sequences made of alternations of coarse- and fine-grained intervals (site PRGL2). Normalized cone resistance and friction ratio proved to be also appropriate for soil stratigraphy as it depicts trends in the coarse fraction of the tested soil. Silts and clays present in similar proportions at site PRGL1 responded to piezocone testing as pure clays usually do. Consequently, classical soil classification methods resulted in erroneous interpretation of these sediments as clays, whereas classification of the heterogeneous deposits at PRGL2 was consistent with the grain size. When tied to a high-resolution seismic reflection profile, the stratigraphy interpreted from the piezocone profile matches with the main seismic sequences and discontinuities defined from seismic stratigraphy analysis. Graded bedding also matches with cone tip resistance and sleeve friction data.

Analysis of slope failures in submarine canyon heads: An example from the Gulf of Lions

To improve understanding of evolution of submarine canyons, a three-dimensional slope-stability model is applied to Bourcart Canyon in the western Gulf of Lions in the Mediterranean Sea. The model builds on previous work by Chen and others, and it uses the upper bound theorem of plasticity to calculate the factor of safety of a kinematically admissible failing mass. Examples of three-dimensional failure surfaces documented in the literature were used to test the model formulation. Model application to Bourcart Canyon employed the results of a detailed stratigraphic analyses based on data acquired by swath bathymetry, sub-bottom profiling, high-resolution seismic reflection surveys, and piston coring. The sediment layers were also characterized using in-situ geotechnical measurements and laboratory tests. The effects of three loading scenarios were analyzed: (1) earthquake shaking, (2) hemipelagic sedimentation, and (3) axial incision. These three mechanisms influenced the predicted volumes and shapes of slope failures along the flanks of Bourcart Canyon, and comparison of these predictions with failure geometries inferred from seafloor morphology showed that mass failures could account for the observed morphology along the canyon walls as well as a mechanism of canyon widening

Tide and wave dynamics on a sand bank from the deep shelf of the Western Channel approaches.

Abstract The sedimentary response of deep shelf sediments to the interplay of tides and waves, based on the example of the Celtic Sea sand banks is described. Grab samples and 1500 km of multibeam and side scan data have been collected from the 300 km2 densely surveyed Kaiser bank area. The bank is 140–170 m deep, 30 m high and 60 km long, oriented perpendicular to the shelf edge and nearly parallel to the major axis of the tidal ellipse. Surficial sediments of the bank consist of medium to gravely biolithoclastic sands. They are swept by tidal currents that reach 0.9 m/s 1 m above the seabed, and under the occasional influence of waves. The mobile sands are commonly bedformed and rest on a highly backscattering lag. The flanks below −140 m mainly exhibit transverse tidal dunes and sand ribbons, whereas the top of the bank mostly displays discontinuous sand patches and wave ripples. The sand patches are interpreted as the remnants of tidal bedforms reworked by waves. Calculations of the threshold of motion of four modal grain sizes under various conditions at the seabed show that tidal bedforms are active during spring tides sometimes associated with waves, whereas only the largest annual waves may explain the observed wave ripples. As in active tidal banks, there is a reversal of tidal bedload transport from one bank side to the other. The sedimentation rate is very low on the Kaiser bank. In addition, there is a loss of sediment at both bank extremities and the tidal bedload cross-bank transfer is very small, probably controlled by shelf residual currents and long-term drift of sediment resuspended by waves. However, the present-day action of waves at the bank top is less intense than it was at lower sea levels during the Holocene, as evidenced by seismic studies.

Mud volcanoes at the shelf margin of the East China Sea

Abstract Roughly circular sediment mounds, varying from tens to hundreds of meters in diameter and from a few meters up to 40 m in height, have been mapped for the first time at the shelf edge of the East China Sea (ECS). High-resolution seismic profiles, together with swath bathymetric and acoustic data, indicate that these features are common on the ECS outer shelf and on the upper western slope of the Okinawa Trough. They are associated with unusually large pockmarks, up to 40 m in depth and 600 m in diameter. On seismic records, bright spots, phase inversions and other acoustic anomalies indicate that gas and/or fluid escape plays an important role in the formation of these mounds. They are often associated with normal faults that occur along the western slope of the Okinawa Trough, these faults acting as conduits for migrating fluid. Based on regional stratigraphic correlation, we propose that the source of mud and fluid is deeper/older than 235 ka. The seepage process is suspected to be very recent, and probably still active, based on the facts that (1) despite high sedimentation rate, the mounds are not buried by recent sediments, and (2) Last Glacial Maximum deposits are reworked at the emplacement of the mounds. Carbonate-cemented sediments and deep-water reefs possibly associated with fluid seepage could be the origin of local patches of very high backscattering acoustic facies, mapped with the swath bathymetric system.