Michel Tesson

Late Quaternary deltaic lowstand wedges on the Rhône continental shelf, France

Abstract Shallow seismic profiles on the Rhone continental shelf have shown the existence of stacked progradational lowstand wedges on the outer shelf accumulated during late Quaternary glacioeustatic lowstands. The wedges are bounded by sharp to erosional sequence boundaries and onlap onto the middle shelf. The thickness of individual wedges attains 50 meters at the shelf edge, where they are sheared by slumping. The wedges exhibit well-developed progradational clinoforms, episodically interrupted by internal unconformities on which there is a downward shift of the succeeding clinoforms. These represent small-scale (in the order of a few meters) falls in sea-level. During the Quaternary eustatic cycles, the durations of the stable highstand conditions appear to have been too brief to permit the regressive highstand tract to prograde to the outer shelf. Therefore, the shelf edge builds up entirely with lowstand wedges.

Stratigraphic Organization of Late Pleistocene Deposits of the Western Part of the Golfe du Lion Shelf (Languedoc Shelf), Western Mediterranean Sea, Using High-Resolution Seismic and Core Data

Detailed analysis of shallow penetration single-channel seismic data, integrated with piston core data, reveals that the stratigraphic architecture on the Rhone shelf of the western Mediterranean Sea is characterized by a complex stratigraphy comprising both the regressive and transgressive parts of late Pleistocene depositional sequences. Several cycles of deposition are observed and are interpreted to be associated with fourth-or possibly fifth-order cycles of relative change of sea level. The regressive parts of the sequence are inferred to have been deposited either during late highstand or during periods of relative fall of sea level. These deposits are characterized, in some instances, by discrete downstepping wedges and internal downward shift surfaces; however, in other instances this evidence is not present. The transgressive parts of the depositional sequences are characterized by backstepping wedges and isolated sand bars. In general, these types of deposits are more common on the western part of the Rhone shelf and are largely absent on the extreme eastern part of the shelf. The transgressive deposits seem to be preferentially preserved at both inner and outer shelf locations and less so in the middle shelf. The exception to this is the area near the Rhone Delta depocenter, where transgressive deposits are observed across the entire shelf. Key surfaces separating stratigraphic units include ravinement surfaces, downlap surfaces, and subaerially formed erosional surfaces. These stratal discontinuity surfaces constitute the basis for analysis of the stratigraphic architecture. This area is dominated by seismic reflection geometries suggesting a high-energy depositional environment. Relatively steeply dipping seismic reflections bounded by horizontal to irregular erosional surfaces characterize most of the upper Pleistocene section in this area. The high-energy seismic facies correspond to three types of deposits: (1) thick regressive sands of lobate delta-front origin, (2) retrogradational beach barriers overlying the regressive wedge on the outer to middle shelf, and (3) late transgressive beach sands and ridges observed at the inner shelf. The stratigraphic complexity illustrated here is likely analogous to similar, although commonly undetected, complexity that characterizes petroleum fields in shallow-shelf settings. A variety of geomorphic elements have been interpreted here. These include transgressive sand bars, wave-dominated distributary mouth bars, recurved spits, isolated shelf edge shoreface/beaches, and distributary channels. These elements are consistent with an interpretation of a depositional environment characterized by wave-dominated delta deposition. The morphology of the delta in the eastern part of the shelf (i.e., near the depocenter) seems to be consistent from the outer to inner shelf location, suggesting that the position of the shoreline relative to the shelf edge and the presence of submerged shelf outboard of the shoreline had only minor impact on deltaic deposition; moreover, climatic change and changes of fluvial discharge that likely characterized this area during the End page 119 ---------------- late Pleistocene similarly had only minor effect on delta morphology.

Sequence stratigraphy, seismic profiles, and cores of Pleistocene deposits on the Rhône continental shelf

Abstract High resolution seismic profiles on the Rhone deltaic shelf has shown the existence of a complex of Pleistocene superimposed prograding wedges interrupted by a major incised valley system cutting across the shelf. The individual wedges are interpreted, from internal geometry and coring data, as prograding shoreface deposits accumulated during periods of relative sea level lowering punctuated by small-scale relative sea level falls or forced regressions. They are related to 5th-order glacio-eustatic cycles and are stacked to form a composite 4th-order sequence. Radiocarbon dating (Tandetron AMS) of the upper wedge (to 40 ka BP) indicates a stratigraphic hiatus across the shelf corresponding to the glacial maximum lowstand at the end of the Wurmian cycle (isotopic stage 2) during which sea level had dropped to 120 m below present sea level. Seaward tilting of the wedge boundaries indicate that the shelf has been subjected to differential subsidence that provides the necessary accommodation space for deposits to accumulate on the mid-outer shelf. The seaward tilting results from a cumulative effect of sediment and hydrostatic loading. Tectonism is also expressed by a westward shore-parallel tilting in the area east of the incised valleys. The amount of tilting, the stacking pattern of the wedges, and the location of incised valleys are structurally controlled, by the alpine orogen which dips westward under the Rhone margin. The tectonic component interacts with glacio-eustatic sea level fluctuations and accounts for the partition of the shelf into two areas with specific features separated by the Rhone incised valleys.

Lower cretaceous basalt and sediments from the Kerguelen Plateau

Geological samples from the southern Kerguelen Plateau include Lower Cretaceous basalt and lava breccia, probable Lower Cretaceous conglomerate and shelf limestone, Upper Cretaceous chert with dolomite, Upper Cretaceous-Eocene ooze, and Tertiary conglomerate. Neogene sediments are only a few hundred m thick, and include foraminiferal and diatomaceous ooze, and ice-rafted debris. In conjunction with seismic reflection profiles, the samples indicate Early Cretaceous near-shore volcanism, followed by erosion, sedimentation, and subsidence through Cretaceous; arching of the plateau at the end of Cretaceous; subsidence through Paleogene; widespread emergence in mid-Tertiary; and slow subsidence through Neogene.

Morphological evolution and time-varying bedrock control of main channel at a mixed energy tidal inlet: Maumusson Inlet, France

Abstract Maumusson Inlet, located on the French Atlantic coast, connects the Atlantic Ocean with the Marennes–Oleron tidal bay. The tidal range (2–6 m) and wave climate (mean height 1.5 m) place this tidal inlet in the mixed energy, tide dominant, category of Hayes [(1979) Barrier island morphology. In: Leatherman, S.P. (Ed.), Barrier Island, Academic Press, New York, pp. 1–28]. An innovative method, combining high quality bathymetric data (nine accurate Digital Elevation Models since 1824) with a very high seismic resolution, demonstrates a major tidal inlet evolution from 1824 onwards and its dramatic acceleration since 1970. The chronology of those morphological changes suggests strong coupling between the location of the tidal channel and the behaviour of the adjacent shorelines. The recent shoaling and migration of the inlet channel can be attributed to a decrease in tidal prism due to the filling in sediment of Marennes–Oleron Bay. Seismic data give evidence that the inlet was located on a major incision of the bedrock. It can be inferred that the bedrock exerts control of channel location, this control varying in time as a function of channel depth. A conceptual model is proposed, including the inlet, its adjacent shorelines, the tidal bay and the time-varying bedrock control of main channel location. Such a model could be considered valid in similar cases along other coastlines, i.e. coastlines with a fine unconsolidated sediment sheet.

Analysis of the postglacial deposits of the Rhône shelf (golfe du Lion). Implementation to the study of the late Quaternary depositional sequences

Postglacial deposits of the Rhone 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 Rhone 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 Rhone 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-Rhone river mouth. Laterally, on either side of the Rhone 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. 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 Rhone 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.

La marge méridionale de la mer d'Alboran: caractères structuro-sédimentaires et évolution récente

Abstract High-resolution seismic reflection profiles (2000 km) were collected across the southern continental margin and major structural trends of the Alboran Sea. Seismic units are dated on the basis of well-known marine Neogene onshore sections and offshore dredged samples. A better assessment of margin structuration and Neogene chronology is made. A conformable succession of Pliocene-Quaternary and older series provides evidence for the non-evaporitic nature of the Upper Miocene, similar to the facies from the onshore Neogene Boudinar and Melilla basins. It also suggests that Alboran Ridge is in part sedimentary rather than only volcanic. This Ridge appears as an “en echelon” anticlinal complex. Conformable Upper Miocene and Lower Pliocene-Quaternary strata are folded and faulted. Primarily important manifestation of the movements is dated as Pliocene, but the main vertical relative movements are of Quaternary to recent age as emphasized by well-developed divergent strata lying above. Submarine volcanism, probably Miocene in age, often controls the morphology and structure of the eastern Moroccan continental margin and adjacent south Alboran Basin. The latter one is characterised by a thick sedimentary accumulation; it is tectonically bounded, except on the southwestern part seaward of the Bokkoyas cliffs. Vertical movements occur during much of Plio-Quaternary time, prevailing during the Quaternary. The structure and related morphology of the study area are mainly the result of neotectonic events.

Les dépôts postglaciaires de la plate-forme rhodanienne : organisation stratigraphique et conditions de mise en place

Postglacial deposits of the Rhone shelf are organised into a set of retrograding units representing parasequences of the Transgressive Systems Tract. They are correlated with periods of reduced rate of eustatic sea level rise but their properties (location, nature) are also controlled by local factors (paleotopography, dynamic conditions, sedimentary input). They are overlaid on the deltaic plain by the prograding parasequences of the Highstand Systems Tract. The poor development of Pleistocene equivalent is due to the short duration and low-frequency occurrence of quaternary highstand periods as well as the subsequent subaerial erosion, particularly on the inner shelf.

L'enregistrement des cycles climatiques et eustatiques quaternaires de marges récentes du bassin Nord-Méditerranéen

Abstract 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 Rhone-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.

Carte morpho-bathymétrique de la plate-forme du golfe du Lion : un outil pour la reconnaissance et l'analyse des modifications environnementales récentes

A detailed regional bathymetric map has been analysed and correlated with seismic and sedimentological data. The morphostructural features identified between 90 and 120 m are considered as well preserved coastal enviromnent deposits accumulated during the last glacial maximum and the eustatic sea level rise of the Holocene. They are analogues of Pleistocene seismic units and reservoirs previously described in the area.