A branch of a time-calibrated late Quaternary compound incised valley complex is investigated using high- and very-high-resolution seismic data. The incised valley system is confined on the inner shelf, and entrenched parallel to the shore in unconsolidated Pliocene deposits. The infilling of the incised valley system comprises three sigmoidal bodies dipping progressively downstream representing depositional sequences. The lowermost sequences are less well preserved at their downstream extremity and the whole system is both aggrading and prograding. Older Pleistocene/Late Quaternary sequences could be preserved under the coastal plain. Individual sequences are closely similar to the classic model of a microtidal incised valley fill. Nevertheless, the central estuary/bay basin muds are seen to interfinger locally with high-energy deposits that represent potential reservoirs. The properties (prograding and aggrading architecture and occurrence of high energy deposits) and preservation of these compound incised valley fill deposits are attributed to general (glacio-eustatic cycles) and local (atmospheric and oceanic regime and proximity of the hinge line) conditions. Data acquisition strategy is a determining factor to interpret such systems.

Terra Nova, 24, 181–188, 2012AbstractSinkholes are frequently observed where soluble rocks are present, in meteoritic recharge areas or when subsurface dissolution causes ground collapse. Seismic profiles on the inner shelf of the Gulf of Lions (W. Mediterranean) display a giant atypical sub‐circular structure, up to 800‐m thick and 2‐km wide. It is characterized by down‐warped internal seismic reflections forming a concave V‐shaped depression rooted in the substratum, most likely carbonated. This structure is interpreted as a poly‐phased solution‐subsidence/collapse structure of Plio‐Quaternary age. The last phase of deformation probably occurred during the last sea‐level low‐stand phase. The mechanism proposed for the creation of this structure is collapse/dissolution of the soluble substratum, as a result of a combination of several possible factors including pre‐existing deep Messinian palaeo‐karst with active groundwater flows during Plio‐Quaternary lowstands, and deep‐rooted acidic fluids ascending through the substratum.

A buried system was observed using seismic data along the northern side of the Roussillon coastal plain and inner shelf (Western Gulf of Lion, Mediterranean Sea) far from the nearest Agly and Tet rivers. It was interpreted as an example of a "compound" type, which is subject to controversy.A core 60 meters long was obtained on the sandy beach barrier between sea and lagoon approximately on the axis of the basal surface of erosion mapped from seismic data and long core logs obtained previously on the coastal plain.In the lower part of the core (-64 m to 26 m b.p.s.l.) the main estuarine muddy-silt facies and thin levels of fluvial gravels intercalated in the mud suggest that successive cycles of base-level and sea-level falls and rises were recorded. This interpretation is confirmed by pollen diagrams showing several successive warming periods during climatic changes (differentiated as "interglacial" phases) recorded by the estuarine muds. Owing to these correlated data we can attest that this incised-valley system is of a "compound" type.Every vertical succession of fluvial gravels and estuarine muddy silts represents a fundamental depositional sequence typical of the "simple" model of wave-dominated incised-valley filling, truncated at the top by a subsequent base-level fall. Several depositional sequence remnants are stacked upon each other.The chronostratigraphic benchmark is based on microforaminiferal biomarkers which indicate that the time period covered by the successive phases of base-level and sea-level cycles extends from MIS 16 (about 600,000 years B.P.) to MIS1 (present day). The exact correlation of base level and sea level cycles with identified climate cycles remains partly approximate, but the cycles which contain well-known plant associations are quite reliable.The internal geometry of the incised-valley system, based upon high-resolution seismic data, shows that the lateral migration of the successive phases of incision and filling explains both the preservation of several cycles and the incomplete preservation of the typical facies due to the reworking of the upper part of all individual depositional sequences.Eustasy is the dominant controlling factor, and differential subsidence, an important factor on the mid and outer shelf, has a reduced impact at this location. The preservation of the "compound" system depends upon the variable maximum depth of erosion reached at each maximum sea-level lowstand. We propose that different interglacial conditions took place in the drainage basins, rather than differences in the lowstand-sea-level values. The transverse shape of the incisions, with lateral terraces and deeper channels, combined with a continuous lateral shift of the successive incisions, also contributed to preservation. The lateral shifting is partly due to a normal fault and substratum tilting, and partly to the oceanic regime.

A detailed study of a Pliocene to Quaternary incised-valley system located under the Roussillon coastal area (Gulf of Lion) is carried out by means of a high-resolution seismic data set (coastal lagoons and adjacent shelf area), drill reports and analysis of a cored drill (Leucate SCI) performed in 2007 on the barrier beach in the North of Leucate lagoon.The lowermost surface (S100/S200), correlated with a pebbles level on the Leucate core SCI (RO), erosionnally overlies pliocene deposits. It is incised by a thalweg 15 m depth and deepens progressively from the coastal plain to the outer shelf and dips under the Quaternary forced regressive wedges.Above this basal surface, the infilling of the incised valley corresponds to the seismic unit U1. The lower part of U1 shows continuous sub-horizontal reflectors and is correlated (Leucate SCI) to marine muds with levels of mud-supported gravels (body B). The upper part of unit U1 comprises seismic erosional reflectors that are almost amalgamated under the barrier beach. It is correlated on the Leucate well to body C comprising coarse levels (gravels and pebbles) alternating with marine muds (lower part) and marine coarse sands (upper part). The upper part of the unit is dated 12900 cal yr B.P. This unit is interpreted as resulting of successive phases of incision and infilling due to base-level changes during Pleistocene glacial and interglacial periods. Coarse levels of sand and gravel corresponding to river stages and sands and muds shelly levels representing marine stages.The overlying units represent post-glacial late transgressive (S650) and highstand (U660, U661, U662) deposits.This system is a rare example of well preserved compound incised valleys correlated offshore with Quaternary lowstand wedges.By comparison, the incised valleys along the Atlantic coast of France are "simple" incised valleys where only the last episode of incision/infilling is observable. The Quaternary "compound" incised valleys cited in the literature represent examples of the fluvial part of incised systems, whereas the Languedoc-Roussillon incised valleys probably correspond to estuary or embayment, successively reoccupied during the various Quaternary eustatic cycles.Tectonics is the main factor controlling the depositional stratigraphic architecture. The studied area is located at the hinge point between continental uplift and marine subsidence and favoured the preservation of successive phases of erosion/infilling. The geometry of the Pliocene deposits has also an impact on the shape and orientation of the buried paleovalleys.

La quantification des séquences sédimentaires holocènes du delta du Rhône est obtenue par calcul d'accumulation verticale, par mesure de la progradation horizontale de la plaine deltaïque et estimation du volume total accumulé. Nous utilisons pour cela un ensemble de données : profils sismiques haute résolution réalisés sur la plate-forme interne, carottes sur la plaine deltaïque et cartographie géomorphologique à haute résolution chronologique des cordons littoraux. Les variations du stockage sédimentaire sont fortement contraintes par les forçages géomorphologiques (position des embouchures, variation du niveau marin relatif). Le calcul des volumes effectué sous-estime fortement le stockage sédimentaire si l 'on se réfère aux apports actuels. Cela est probablement lié au trop faible nombre de données carottées à terre, à l'incertaine corrélation des faciès sismiques et à la difficulté d'estimer la porosité des sédiments dans la colonne sédimentaire. La période 2800/2525 cal BP semble cependant clairement identifiée comme période de rapide stockage et progradation deltaïque et pourrait correspondre à la détérioration climatique du premier Age du fer identifiée ailleurs dans le bassin versant et en Europe. Cette tendance, moins marquée, se prolongera durant le second âge du Fer. L 'apparent fonctionnement antithétique du bassin versant durancien amont avec les apports sur le delta du Rhône est à noter, mais le caractère non exhaustif de la quantification de l 'accumulation sédimentaire sur le plateau continental interne laisse à l 'état d 'hypothèse cette observation.

At a regional scale, high-resolution seismic dataset analysis provides an accurate image of the stratigraphic organization of the Post-Glacial transgressive deposits of the Gulf of Lions inner-shelf. Architectural and stratigraphic characteristics are different in four main sectors, clearly demonstrating that depositional models have to be adapted from place to place following the interplay of various genetic factors.The southern part of the Gulf of Lions presents minor development of the transgressive deposits mainly controlled by paleo-topography, in addition to glacio-eustatism. Northward, complexity and sediment thickness increase. Additional factors, such as oceanographic regime and sedimentary supply, interact with glacio-eustatism and paleo-topography. In the occidental and oriental Rhône-shelf sectors, sedimentary supply and glacio-eustatism control the deposition and the preservation potentials. The paleo-topography controls the type of stacking pattern.At the regional scale, this study further emphasizes the role of two usually underestimated factors which are the drainage characteristics and the sedimentary response to climatic variations. The stacking pattern of the transgressive systems tract with several parasequences emphasizes a pulsed Post-Glacial transgression. The main parasequences seems to be related to two periods of decreasing rate of sea-level rise and increasing rate of sediment discharge.

The purpose of this study is to build efficiently and automatically a three-dimensional geometric model of the stratigraphic units of the Gulf of Lion margin on the basis of geophysical investigations by a network of seismic profiles, using geostatistics. We want to show that geostatistics can produce unbiased maps of the morphology of submarine stratigraphic units, and furthermore that some specific features of these units can be found, that classical manual mapping may ignore. Depth charts of each surface identified by seismic profiling describe the geometry of these units. The geostatistical approach starts with a statistical analysis to determine the type and parameters of the variograms of the variable "depth" of each identified surface. The variograms of these surfaces show that they are mostly non-stationary. We therefore tried the following two non-stationary methods to map the desired surfaces : (i) the method of universal kriging in case the underlying variogram was directly accessible; (ii) the method of increments if the underlying variogram was not directly accessible. After having modelled the variograms of the increments and of the variable itself, we calculated the surfaces by kriging the variable "depth" on a small-mesh estimation grid. The depth charts of each surface calculated with the geostatistical model are then interpreted in terms of their geological significance, which makes it possible to suggest hypotheses on the influence of major processes, such as tectonics and rivers (Rhone, Herault, etc.) on the sedimentary structure of the gulf of Lion margin. The added value of geostatistics for this interpretation is emphasized. These unusual geostatistical methods are capable of being widely used in earth sciences for automatic mapping of "non-stationary" geometric surfaces, i.e. surfaces that possess a gradient or a trend developing systematically in space, such as piezometric or concentrations surfaces.

High resolution (HR – sparker) and very high resolution (VHR – boomer) seismic reflection data acquired in shallow water environments of the Roussillon coastal area are integrated to provide an accurate image of the stratigraphic architecture of the Quaternary deposits. The complementary use of the two systems is shown to be of benefit for studies of shallow water environments. The HR sparker data improved the landward part of a general model of Quaternary stratigraphy previously established offshore. They document an incised valley complex interpreted as the record of successive late Quaternary relative sea-level cycles. The complex is capped by a polygenetic erosional surface developed during the last glacial period (>18 ky) and variably reworked by wave ravinement during the subsequent post-glacial transgression. The overlying transgressive systems tract is partly preserved and presents a varying configuration along the Roussillon coastal plain. The VHR boomer data provide information on the architecture of the uppermost deposits, both in the near-shore area and in the lagoon. These deposits overlie a maximum flooding surface at the top of the transgressive systems tract and constitute a highstand systems tract composed of two different architectural elements. In the near-shore area, a sandy coastal wedge is subdivided into a lower unit and an upper unit in equilibrium with present day dynamics. In the Salses-Leucate lagoon area, the sedimentary architecture is highly complex due to the closure of a former embayment and the formation of the present beach barrier.

– Postglacial deposits of the Rhône shelf have been studied from high-resolution seismic data and Kullenberg piston cores. They are organised into a set of transgressive units or parasequences backstepping from the outer shelf to the subaerial deltaic plain. On the deltaic plain, they are overlain by the prograding deltaic parasequences deposited at the end of the Holocene sea level rise. At regional scale, given the short time span covered by the late Quaternary deposits, tectonic subsidence has played a minor role and sediment deposition on the Rhône shelf was chiefly controlled by glacio-eustatic sea level changes (120 m between the maximum lowstand and present highstand). Progradational phases correspond to periods of reduced rate of eustatic sea level rise while the flooding surfaces bounding the regressive units form during periods of increasing rate of sea level rise and landward shoreline migration. At local scale, location, geometry, nature of deposits, and lateral variations of the stratigraphic pattern are controlled by the interaction between eustasy and local factors as sediment supply, antecedent morphology and ocean dynamics. Seaward of the Rhône river, terrigenous input was important during deglaciation and transgressive deposits extend continuously from the outer shelf to the inner shelf along the retreating path of the paleo-Rhône river mouth. Laterally, on either side of the Rhône incised valley, because of the reduced sediment supply, parasequences only develop on the outer shelf and inner shelf : the combination of the very low inherited gradient of the mid/outer shelf and a very high rate of sea level rise favoured a very rapid migration of the shoreline from outer to inner shelf. Bull. Soc. géol. Fr., 2003, t. 174, n o 4, pp. 401-419 Bull. Soc. géol. Fr., 2003, n o 4 Université de Perpignan, BDSI, EA 1947, 52 Av. de Villeneuve, 66860, Perpignan, France. gensous@univ-perp.fr Manuscrit déposé le 26 septembre 2001 ; accepté après révision le 3 mars 2003. Ocean dynamics has been controlled, as in present time, by the E to SE prevailing waves that are the only ones which can develop on an extended fetch. The westward alongshore drift accounted for the development of parasequences west of the incised valley. The sandy material needed for the construction of the outershelf parasequence was supplied by wave-reworking and westward long-shore drift of deposits from the Rhône delta front and the uppermost forced regressive unit. The decreasing sand content of parasequences from outer shelf to inner shelf results from flattening of the equilibrium river profile that led to a decrease in competence and a change in the character of the sediment caliber (relative increase of suspension load). The underlying Pleistocene depositional sequences comprise both lowstand prograding units, that characterize most of the Mediterranean shelves, and intercalated units which are analogs of the postglacial transgressive deposits here presented.

Sedimentary architecture of the Roussillon shelf (SW Gulf of Lions, France): sequence stratigraphy considerations FRANCISCO LOBO , MICHEL TESSON , BERNARD GENSOUS , & JAVIER HERNÁNDEZ-MOLINA 3 CIACOMAR-Univ. Algarve, Avenida 16 de Junho, s/n, 8700-311 Olhão, Portugal ( mailto:pacolobo@ualg.pt ) CEFREM-ERS1745-Univ. de Perpignan, 66860 Perpignan, France Dpto. de Geociencias Marinas y Ordenación del Territorio, Univ. Vigo, 36200 Vigo, Spain

Preliminary results of high-resoluti on Sparker seimic investigations (SIFADO) along the Charente shoreline are described. An erosion troncature is observed which is interpreted as the result of a strong incision due to the last sea level fall. Active sand waves are observed which indicate a strong remobilization of Holocene deposits induced by present-day hydrodynamic processes.

The object of this study is to build a three-dimensional (3D) geometric model of the stratigraphicunits of the margin of the Rhone River on the basis of geophysical investigations by a networkof seismic profiles at sea. The geometry of these units is described by depth charts of eachsurface identified by seismic profiling, which is done by geostatistics. The modeling starts bya statistical analysis by which we determine the parameters that enable us to calculate thevariograms of the identified surfaces. After having determined the statistical parameters, wecalculate the variograms of the variable Depth. By analyzing the behavior of the variogramwe then can deduce whether the situation is stationary and if the variable has an anisotropicbehavior. We tried the following two nonstationary methods to obtain our estimates: (a) Themethod of universal kriging if the underlying variogram was directly accessible. (b) Themethod of increments if the underlying variogram was not directly accessible. After havingmodeled the variograms of the increments and of the variable itself, we calculated the surfacesby kriging the variable Depth on a small-mesh estimation grid. The two methods then arecompared and their respective advantages and disadvantages are discussed, as well as theirfields of application. These methods are capable of being used widely in earthsciences forautomatic mapping of geometric surfaces or for variables such as a piezometricsurface or aconcentration, which are not “stationary,” that is, essentially, possess a gradient or a tendencyto develop systematically in space.

Studies of Quaternary deposits on shelves lead us to propose simplified stratigraphic models, far from the global models, whose cyclic pattern is reduced to a regressive unit built up during a relative sea level fall (cooling period). Analysis of neighbouring areas of the Gulf of Lions show a more complex pattern. In the Languedoc area, the stratigraphic model shows the record of falling stage, maximum lowstand, early and late sea level rises. These attributes evolve laterally towards the Rhône-Est and Ampurdan areas, into a simple model. Thus, Quaternary models are relevant to the global models and represent, depending upon area, end members associated with the interaction of local factors and global factors.