High-resolution Seismic Lines Research Articles

Using shallow hydroacoustic data to image seafloor mass transport deposits on the North West Shelf of Australia: links to neotectonics

Abstract Mass transport deposits have long been known on the Exmouth Plateau, offshore NW Australia, identified in 2D and 3D industry seismic lines. The expedition SO257 in 2017 collected 30 high-resolution, shallow seismic lines along targeted transects on the northern Australian margin. Many of these imaged mass transport deposits, with the top 700–800 m of the section captured in detail not available with industry seismic data. We present nine new high-resolution seismic lines from three separate areas of the North West Shelf. Slides in the Roebuck Basin show complex anastomosing ductile extensional mechanisms, with multiple slip surfaces and no headscarps or adjacent faults. Slides on the Exmouth Plateau have fault control, with surface fault offsets of up to 300 m, indicating seismicity as a likely triggering mechanism. Slumps along the western margin of Western Australia are more limited in extent, associated with surface notches, with indications of previous activity at depth. All areas show a repeated history of mass transport deposits. The area of the active landslide province offshore of NW Australia is far larger than the individual slides recognized on the Exmouth Plateau.

Late Miocene-Quaternary sedimentary environment evolution of the Kerch seep area, Black Sea

A set of 2D high resolution seismic lines was acquired near the Kerch Peninsula during R/V Meteor Cruise M72/3 in March/April 2007 to the Eastern Black Sea. The high resolution seismic data were used for analysis of seismic facies and seismic sequences to gain insight into the sedimentary evolution of the study area. Seven seismic facies types were identified and six seismic units could be mapped. Based on seismic line interpretation, isopach and seismic facies distribution maps, a chronostratigraphic framework could be established for the study area. The study reveals that the sediment deposits were influenced by syndepositional tectonic movements, sea level and climate. During the late Miocene and the early Pliocene, possible slope fan deposits developed. At the boundary between the Miocene and Pliocene, the Messinian erosional truncation surface was observed. After the Messinian sea level fall, accompanied by a major transgressional phase during the Pliocene, the Paleo-Don and Kuban Rivers delivered a large terrigenous input to the Black Sea from the uplifted Crimean Mountains. This formed thick submarine fan deposits near the mouth of Kerch Strait during the late Pliocene. Facies variations within the seismic units are related to the sea-level and climate curves, and are reflected by grain size variations in the sediments and the shift of the river mouth. During the Quaternary, hemipelagic sediments interbedded with coarse-grained fluvial sediments developed in the study area, along with slumps, river channels and fan deposits.

Seismic monitoring for subsurface uncertainties at the Endurance CO2 store

The development of the Endurance structure in the southern North Sea as a CO2 store offers a potentially large decarbonization opportunity for industry along the east coast of England. The Bunter Sandstone Formation in the Endurance structure has been assessed as an excellent candidate for CO2 storage due to its position, depth, reservoir properties, and extensive seal. Monitoring CO2 during active injection and after closure is vital to confirming containment and conformance of the CO2 within the store. There are two key subsurface uncertainties: compartmentalization and vertical permeability. Current seismic resolution does not provide imaging of thin, low-permeability layers that may create low relative vertical permeability, and the high net to gross of the sandstone limits imaging of any minor faulting. However, the strong CO2 response on 4D seismic likely will illuminate baffles and barriers to flow along with variations in reservoir permeability. In 2020, four two-dimensional high-resolution (2DHR) seismic data test lines were acquired to assess the best technology for imaging and monitoring Endurance. High-resolution seismic was concluded to provide quality imaging from the base of the reservoir (approximately 1500 m true vertical depth subsea) up through the overburden to image the entire subsurface and to demonstrate safe storage of CO2.

BLACK SEA SUBMARINE VALLEYS – PATTERNS, SYSTEMS, NETWORKS

The article presents a detailed analysis of the underwater morphology of the entire Black Sea basin beyond the shelf break. The focus is on submarine valley systems on continental slope and rise zones, and partially in the abyssal plain area. The present research is among the very few studies that have undertaken a morphological analysis on a regional scale, for an entire marine basin. This achievement was possible by using the publicly available EMODnet bathymetric map of the Black Sea. The Black Sea submarine valleys networks are presented in a map-sketch. It includes 25 valley systems, 5 groups of simple first order channels, and other number of simple, not associated channels. The 25 valley systems are adding up more than 110 main channels and tributaries. Morphological description and analysis of each of the mapped systems is given – shape and plan view morphology, dimensions (length and surface) and slope gradient. Some considerations about the amount of sediments supplied by the valleys from the shelf to the basin floor, forming the deep-sea fans, are presented. For a more detailed and precise image of the Black Sea network of submarine valleys additional work would be necessary to cover the entire basin with a minimum standard network of modern bathymetric mapping and of high resolution seismic survey lines. To take into account the composition and geological structure of different sectors, as factors that influence significantly the morphology of the seabed, would be also particularly useful.

GAS ESCAPE FEATURES ON THE ROMANIAN BLACK SEA SHELF – EXAMPLES FROM 2D SEISMIC INTERPRETATION

Gas escape features are a common presence on any young sedimentary basins where large amounts of organic-rich sediments are deposited within a short time. The fast burial process conducts to maturation and carbonization of the sedimentary organic matter by bacterial and, later, by temperature transformations. The Black Sea is one such basin, where numerous shallow gas accumulation and gas escape features were observed and analyzed, ranging from gas hydrates, gas chimneys, flares, and mud volcanos. The study of such geological phenomena is important for identifying present seabed geohazards, and nonetheless for hydrocarbon exploration. Our study integrates newly acquired 2D high-resolution seismic lines, further used to correlate the gas escape features and their relationship with deep-seated faults, such as Peceneaga-Camena, and petroleum system of the Romanian Black Sea Shelf area. Our results show that in the studied area the gas escape features are confined into the Pliocene-Quaternary sequence, hence not related to Peceneaga-Camena Fault or to a deeper thermogenic petroleum system, and most probably, the gas is sourced from the shallower biogenic system.

The imprint of sedimentary processes in the acoustic structure of deposits on a current-dominated continental shelf

This study aims to investigate the spatial variability of surficial sedimentation processes on a large sector of the Brazilian shelf by using echo-character mapping. To reach this goal, almost 3200 km of high-resolution seismic lines collected with a 3.5 kHz Chirp source are compared with surficial sediment-related distribution maps. Based on seismic characteristics, such as sharpness, amplitude, and continuity of sub-bottom reflections, six echo types were identified in the study area. These echo types were grouped into two main classes according to the nature of seafloor morphology. Class 1 is characterized by flat-bottom echoes with a variable amplitude of reflections and acoustic penetration. This class is predominantly found on the inner and middle shelves. Class 2 consists of irregular seafloor morphologies and is classified based on the occurrence of internal reflections and relief variations. This class is mainly found on the outer shelf. The distribution of echo types and associated surficial sediment maps exhibit three main shelf domains: inner, middle, and outer. The echo types are mostly controlled by the influence of along-shelf currents, as well as by mid-shelf fronts associated with the entrance of cold-water tongues under changing wind conditions and upwelling phenomena. Whereas the inner shelf is mainly affected by variable wind-driven currents, the outer shelf is more persistently influenced by the southwestward movement of the Brazil Current. Between these areas, the middle shelf is affected by seasonal intrusions of the South Atlantic Central Water and northward extensions of the Brazilian Coastal Current. The studied area can be considered a case model for a shelf driven by hydrodynamic processes.

Plio-Quaternary transpressive tectonics: a key factor in the structural evolution of the outer Apennine–Adriatic system, Italy

We used data from seismic reflection profiles and well logs from both onshore and offshore locations along the Adriatic coast to investigate the structural style and recent active deformation in the external area of the northern Apennines adjoining the Po Plain. We prepared four NE–SW sub-parallel transects based on a detailed interpretation of the available subsurface data, supported by surface geological data. Through analysis of the well logs, we identified two significant seismo-stratigraphic markers, together with some unconformities within the Middle Pliocene/Quaternary succession. We propose a detailed structural scheme for the area that identifies and interprets both the outcropping and buried Apennine structures. NW/NNW–SE/SSE high-angle transpressive or flower structures, referred to the recent and actual deformation history, may be responsible for surface features and features at shallow depths, with possible implications for active tectonics. We show the continuity of some of these structures towards the Conero area to the south, suggesting a wider and more regional meaning for the proposed tectonic style. These Apenninic structures close against the active NNE–SSW Cattolica fault system in the northern offshore part of the area to form an asymmetrical arch. This study contributes to the interpretation of the deformation style of the outer northern Apennines. Supplementary material: High resolution seismic lines not interpreted are available at https://doi.org/10.6084/m9.figshare.c.4497542 Thematic collection: This article is part of the ‘Apennines and Tyrrhenian system’ collection available at: https://www.lyellcollection.org/cc/the-apennines-tyrrhenian-system

High-resolution seismic imaging of Paleozoic rocks in the Mora area, Siljan Ring structure, central Sweden

The Late Devonian Siljan Ring structure in Sweden is the largest known impact structure in Europe. The present-day structure comprises a central dome that is about 20–30 km in diameter, which is surrounded by a ring-shaped depression. In this study, we focus on the southwestern part of the Siljan Ring with the aim to map the structure of the Paleozoic sedimentary rocks. Four 2D high-resolution seismic lines with a total length of about 3 km were acquired in the Mora area. A three component eighty-unit land streamer, combined with wireless recorders, was used for data acquisition along with a weight drop source. Processing of the data shows that clear reflections are present, but results are less distinct where external noise was present during acquisition or the maximum source-receiver offset was too short. Petrophysical measurements on core samples, core log data and a density model along one line were used to guide the interpretation of the seismic sections. These data demonstrate that fault blocks are present in the study area and that the individual blocks have been affected differently by impact-related tectonics.

High-resolution seismic imaging in complex environments: A comparison among common-reflection-surface stack, common-midpoint stack, and prestack depth migration at the Ilva-Bagnoli brownfield site, Campi Flegrei, Italy

I used the common-reflection-surface (CRS) stack technique together with conventional common-midpoint processing and prestack depth migration (PSDM) to process three parallel high-resolution seismic reflection lines acquired in a closed industrial plant. This brownfield site, now undergoing remediation, is located within the Bagnoli district of the city of Naples, Italy, near the southeastern border of Campi Flegrei caldera. Compared with the other results, the CRS stack, followed by poststack depth migration, produced a seismic image more consistent with the expected subsurface structures and with outcropping units present in the area. The survey confirmed the presence under the brownfield site of some buried steps of Coroglio Fault, a main structure formed approximately [Formula: see text] ago, during the Neapolitan Yellow Tuff supereruption. Results have important implications for hazard assessment and geothermal exploration because the imaged faults certainly are preferential passageways for geothermal fluids and washed pollutants. These tests revealed that the common-reflection-surface stack can be a fast and cost-effective alternative to PSDM in settings in which structural complexity and high levels of ambient noise make it challenging to obtain a reliable background velocity model; therefore, allowing high-resolution reflection seismology to be successfully used in those environments.

Constraints on the dynamics of pockmarks in the SW Barents Sea: Evidence from gravity coring and high-resolution, shallow seismic profiles

Abstract Newly acquired sediment cores and high-resolution seismic lines were studied in order to constrain the timing and mechanism of pockmark formation in the Loppa High area in the southwestern Barents Sea. The evolution of the pockmarks after their formation was also investigated, together with the possibility that they are currently active features. The cores retrieved within the pockmarks lack a layer of ice-rafted debris and an overlying sequence of laminated glaciomarine sediments that were deposited on the undisturbed seafloor at the inception of the Bolling interstadial (~ 15 cal kyr BP). The pockmarks are suggested to have formed in that period, probably due to climate change-induced destabilisation of methane hydrates and the subsequent release of free gas. The depressions deepened as a result of reduced sedimentation above active seeps. The pockmark cores contain a series of gravel-rich layers, which are attributed to the winnowing of sediment during episodic fluid venting activity. The seismic images reveal little disturbance of the most ancient sediments underneath the pockmarks, indicating that the pockmark chimneys are localised features and were not imaged. The Holocene marine sediment cover in the pockmarks is thinner than on the surrounding seafloor, indicating relatively low sedimentation rates inside the depressions. The gas seepage may thus have been active until the recent past, although no indications of present-day methane flux were found.

3D architecture of Quaternary sediment along the NW Atlantic Moroccan Rharb continental shelf: A stratal pattern under the dual control of tectonics and climatic variations

This study focuses on the evolution of the Atlantic NW Moroccan Rharb continental shelf during the Neogene and Quaternary. This region is part of a foreland basin bounded by the Rif mountain belt and thus provides an interesting geological setting to study the interactions between eustasy and tectonics and the driving mechanisms controlling stratigraphic patterns. The results are supported by an interpretation of new data including high-resolution seismic lines coupled with an interpretation of industrial seismic lines and detailed logs of industrial wells completed by micropaleontologic analysis of cuttings. The stratigraphy reveals a succession of three mega sequences related to the transition from an underfilled to an overfilled stage reflecting the long-term evolution of the foreland system. Moreover, evidence of cyclical sea-level changes are visible in the upper megasequence composed of three depositional sequences assumed to be fourth-order sequences generated in response to the most recent 100-ka glacio–eustatic cycles. This study also shows the peripheral deformation of the Rharb shelf responsible for changes in the geometry of the deposits and thicknesses of the sedimentary fill during the Pliocene and Pleistocene. The most important change was triggered by the uplift of the Lallah Zahra Ridge corresponding to a major Quaternary kinematic boundary and the broad uplift of the southern shelf interpreted as a flexural uplift of the forebulge domain. The deformation-controlled sediment dispersal pattern consists of a progressive growth of the shelf accompanied with a progressive shift of depocenters from the North East to the South West and a general progradation to the North West along the southern border. This progressive filling has led to the confinement of the Rharb paleo-valley across the continental shelf. The complete filling of the palaeo-valley was followed by the development of a more than 70-m thick prodeltaic lobe at the front of the Oued Sebou river mouth during the Holocene.

Investigation on the geochemical dynamics of a hydrate-bearing pockmark in the Niger Delta

A joint industrial project with IFREMER and Total provides insights into the tectonic setting and the geochemistry of a large active pockmark offshore W-Africa in the Niger Delta. The study combines both geophysical (seismic) and geochemical data to infer the dynamics of a 300 m wide pockmark located at 667 m water depth. Two Calypso cores, one at the centre and one outside the pockmark, along with three box-cores, two inside the pockmark and one outside, were collected to study the geochemistry of gas hydrates, carbonates and pore waters. Two seismic lines, a 3D high resolution random line and a 2D high resolution seismic line, provide a good description of the plumbing of the system. The integrated results depict a very active pockmark characterized by several gas charged-bodies, a well identified BSR marking a hydrate accumulation zone, charged from deeper sources via a network of faults. Hydrates and carbonate concretions have been detected inside the pockmark while only the latter has been found outside. The hydrate analyses show that the hydrate-forming gases are of thermogenic origin, albeit overwhelmingly composed of methane. Pore-water analyses reveal the occurrence of anaerobic oxidation of methane coupled with sulphate reduction at shallow depth (∼200 cm). The chloride concentration profile exhibits both values lower than that of seawater which is inherent to hydrate dissociation after core recovery and positive anomalies (values 7% higher than that of seawater background) at the lowermost part of the core. The latter evidence suggests current or recent hydrate formation at this area. The carbonate dating provides additional temporal information and indicates that this pockmark has been active since ∼21 kyr.

Revisiting the Appalachian structural front and offshore Anticosti Basin (northern Gulf of St. Lawrence, Canada) by integrating old and new geophysical datasets

The Anticosti Basin is a large, predominantly marine sedimentary basin centered within the northern Gulf of St. Lawrence. Poor-quality vintage seismic reflection data collected more than 25 years ago in the offshore part of the basin failed to depict its geometry. However, recently acquired multibeam bathymetry and magnetic data can be used in combination with high-resolution seismic lines to map the Paleozoic succession within the Honguedo Strait. The upper part of the offshore sedimentary succession consists of approximately 1140m of sediments younger than those exposed on Anticosti Island. The offshore part of the basin includes a 40km wide fold and fault belt sub-parallel to the coastline of the Gaspé Peninsula. The structural style is characterized by broad open synclines, narrow anticlines and NW-striking faults with dextral strike separations, which bears similarities to that of the Gaspé belt that formed mainly during the Middle Devonian Acadian Orogeny. This study indicates that the Appalachian deformation front is located to the north of its classically inferred location. Moreover, it suggests that the curvature of the early Paleozoic assemblages in the Gulf of St. Lawrence is partly a late (Devonian) feature. An improved understanding of the offshore architecture of the basin may help to develop new paleogeographic reconstructions and hydrocarbon exploration models in this under-explored frontier basin.

A sequence stratigraphic framework for a narrow, current-swept continental shelf: The Durban Bight, central KwaZulu-Natal, South Africa

High resolution seismic lines from the inner and mid-shelf of the Durban Bight reveal an unprecedented view of the seismic stratigraphy of the central KwaZulu-Natal uppermost continental margin. Seven units are recognised from the shelf on the basis of their stratal architecture and bounding unconformities. These comprise four incompletely preserved sequences consisting of deposits of the highstand systems tract (Unit B), falling stage systems tracts (Unit C), the transgressive systems tract (Units A, D and G) and lowstand systems tracts (early fill of the incised valleys and strike diachronous prograding reflectors of Unit A). Seismic facies recognised as incised valley fills correspond to the lowstand and transgressive systems tracts. When integrated with published accounts of onshore and offshore lithostratigraphy and local sea level curves, we recognise an Early Santonian transgression (Unit A to Unit B), superimposed by uplift-induced pulses of forced regression. A Late Campanian relative sea level fall (Unit C) followed. Sediments of the Tertiary period are not evident on the Durban Bight shelf except for isolated incised valley fills of Unit D lying within incised valleys of Late Pliocene age. Overlying these are two stages of Pleistocene shoreline deposits of indeterminate age. Erosion concurrent with relative sea level fall towards the last glacial maximum shoreline carved a third set of incised valleys within which sediments of the Late Pleistocene/Holocene have infilled.

High-resolution seismic stratigraphy of the late Neogene of the central sector of the Colombian Pacific continental shelf: A seismic expression of an active continental margin

Abstract The sedimentary prism of the central Pacific continental shelf of Colombia was affected by regional folding and faulting, and probably later mud diapirism, from the Late Miocene to the Holocene. Interpretation of high-resolution seismic lines (2 s/dt) revealed that the prism consists of 13 high-resolution seismic units, that can be separated into 5 seismic groups. Deposition of the prism and the associated stacking pattern, are probably the response to variable uplift and subsidence in a fore-arc basin that underwent important tectonic events by the end of the Miocene. Throughout the Pliocene, the continental shelf sedimentation was affected by the growing of a dome structure probable due to mud diapirism. This fact caused peripheral faults both normal and reverse that controlled the distribution of some of the seismic units. During the Late Pleistocene (Wisconsin stage?) a eustatic sea level fall caused the shoreline to advance about 50 km westward of its present position. Because of this eustatic sea level change, a strong fluvial dissection took place and is interpreted as the probable extension of the San Juan River to the south of the present day river mouth. Within this framework it is believed that the Malaga and Buenaventura Bays were the passageways of branches of the old drainage system of the San Juan River. The inner branch circulated through the present Buenaventura Bay and runs southward leaving the mark of an apparent valley identified in the seismic information in the eastern sector of the study area. This old fluvial valley and its filling material located in the present day inner continental shelf front of Buenaventura are postulated as important targets to find placer minerals such as gold and platinum.

The Application of Seismic Oceanography in Study of Ocean Water Mass Structure in the Northeastern South China Sea

AbstractSeismic reflection method is usually used to explore the underground geological structure. In the paper, by processing the data from a high resolution seismic survey line in the northeastern South China Sea (SCS), we have obtained the subtle velocity structure and clear reflection images of seawater mass in the survey line. From the seismic profile, we can distinguish that the thermohaline structure of upper layer seawater in the northeast SCS is relatively steady. There isn't obvious intrusion of other water mass. But in the middle layer seawater, internal waves can be found from the seismic profile. The result is in agreement with the research of physical oceanography in the northern SCS. Additionally, a reasonable data model has been designed to prove the feasibility of the seismic oceanography method; some opinions and suggestions are given, which will contribute to future research in seismic oceanography.

Comparison of wave-and tide-dominated incised valleys: specific processes controlling systems tract architecture and reservoir geometry

Abstract This paper presents a comparison between the system tract architecture and the reservoir geometries of the Gironde and Leyre (Arcachon) incised-valley fills, both located within the Bay of Biscay 100 km apart. This study, based on high resolution seismic lines acquired by Ifremer on the continental shelf and onshore core and well data, illustrates that some features of the Gironde and Leyre valleys fills are similar while some others are not. The architecture of both valley fills is characterized by fifth order depositional sequences (corresponding to an interval from 120000 yr B.P. to present day). Both valleys are filled predominantly with transgressive systems tract, with associated poorly developed lowstand and highstand systems tracts. Key stratigraphic surfaces punctuate the valley-fill architecture and comprise deeply eroding tidal ravinement surfaces merged with and enhancing, earlier formed, fluvial-related erosive sequence boundaries. These tidal ravinement surfaces can be undulatory in form and in most places mark the basal boundary of the incised valleys. In contrast, nearly horizontal wave ravinement surfaces cap the incised-valley fills, extending over the adjacent interfluves. The Gironde and Leyre (Arcachon) valley fills exhibit two main stratigraphic differences: 1) transgressive systems tract sand bodies are ribbon shaped within the Gironde and tabular shaped within the Leyre; 2) lowstand systems tract deposits, represented by fluvial sediments, are preserved within the Gironde but absent within the Leyre. In a wave- and tide-dominated environment, the geometry of the sandbodies within the transgressive systems tract is a function of the tidal ravinement processes, which characterizes the estuary inlet. Two categories of tidal ravinement processes can be distinguished here: “anchored tidal ravinement” and “sweeping tidal ravinement”. The Gironde estuary is characterized by an “anchored tidal ravinement”. The tidal inlet has remained largely in a fixed location; littoral drift has not shifted the tidal inlet to the south because it is constrained by resistive Eocene carbonates that define the margins of the Gironde incised valley. In contrast, the Leyre estuary is characterized by a “sweeping tidal ravinement”. The inlet has been shifted approximately 30 km to the south by the formation of a littoral drift associated spit. This extensive lateral shifting was made possible by the fact that the incised valley was cut into unconsolidated, easily eroded Pleistocene sands. Within a wave- and tide-dominated environment, the preservation potential of the lowstand systems tract is a function of the size of the fluvial drainage basin. During lowstand time, the erosive power of the fluvial discharge was much greater within the much larger Gironde valley, consequently the fluvial sequence boundary was cut much deeper in the Gironde valley than within the Leyre valley and, correspondingly, the thickness of the associated fluvial deposits was commensurately greater. In response, the lowstand systems tract was not preserved within the Leyre valley fill because the depth of tidal ravinement erosion formed during the sea-level rise and associated transgression was greater than that associated with fluvial incision generated during the sea-level fall.

Correlação de dados sísmicos multiescala e integração com arcabouço tectônico regional: exemplo da área do Domo de Piratininga, SP

The study area covers the Piratininga Dome, a structural high composed by a center horst bordered by faults. The main objective of this work was to establish a systematic multiscale approach, in which high resolution seismic data was compared to conventional seismic, digital terrain models and geophysical potential data. The subsurface data include an 80km conventional seismic section and the well 1-PA-1-SP. The Kingdon (Seismic Micro-Technology) software was used to interpret the seismic data in order to map the main horizons and faults. To test the multiscale hipothesys was acquired a high resolution seismic line just over the regional seismic trace. This detailed line measures 1 km length and 360m depth. The seismic processing was based on a conventional flowchart for CDP technique with Vista (Gedco) software. Shuttle Radar Topography Mission (SRTM) and aeromagnetic data of Botucatu and Bauru projects were used to the lineaments interpretation. Comparison between observed horizons in the high resolution and conventional seismic lines made possible to test different alternatives to map structural and stratigraphic features. The obtained multiscale geological elements hierarchies enlarge the knowledge in reservoir resolution. The results of the interpretation indicate the close relationship between regional structural framework and features observed in seismic data, and can be applied to enhance and guide the studies of analogous to depth reservoirs.

The ebb-tidal delta of the Venice Lagoon, Italy

Very high-resolution seismic lines were collected offshore the Venice Lagoon in the framework of the Co.Ri.La `New very high resolution seismic methods to study the Venice Lagoon subsoil' project. The 140 km boomer profiles led to the identification of a convex-upward, lens-shaped body just outside the Lido inlet, one of the three lagoon inlets, which has been interpreted as an ebb-tidal delta. The comparison between the seismostratigraphic setting of this deposit, as revealed by the seismic lines, and the analyses of historical bathymetric maps highlighted the key role of human interventions in the formation and evolution of the Lido inlet ebb-tidal delta. To preserve the lagoon environment but also to ensure a navigable way, human interventions at the Lido inlet, performed since the fourteenth century, caused profound variations in the inlet dynamics, leading to a progressive increase in the sediment dispersion from the lagoon interior towards the sea. The ebb-tidal delta of the Lido inlet is thus a very recent feature compared with the formation of the Venice lagoon and formed mainly as a consequence of the construction of the two jetties that have bound the inlet from AD 1886 to the present day.

Small rivers contribution to the Quaternary evolution of a Mediterranean littoral system: The western gulf of Lion, France

Along the Mediterranean coasts and the Atlantic French coast, former and actual research programs focused on major river systems, estuaries and deltas, characterized by a last Glacial relative sea level lowstand and incision and a well developed sedimentary incised valley infilling deposited during the following sea level rise and highstand. This paper presents the preliminary results of a program focused on a particular area of the Gulf of Lion coast with a thin sedimentary cover over the substratum and only minor rivers with non-apparent and important sedimentary contribution during the Late Quaternary. The results show that in this area the best Pliocene to Actual sedimentary record is preserved. The paper rests on the analysis of an extensive database of recent high resolution and very high resolution seismic reflection lines and previously published core data. Seismic data show that a major complex of paleovalleys connected with the Orb, Aude and Agly rivers is preserved on the inner shelf and adjacent coastal plain. On the inner shelf, the separated incised valleys merged in a unique broad and shoreparallel incision dipping southward. At the southward extremity, the incision turns eastward and seaward. The basal surface of this incision extends seawards under six to seven Late Quaternary depositional sequences preserved on the mid and outer shelf. The infilling of the complex of paleovalleys is characterized by aggrading deposits attributed to periods of relative sea level rise (transgressive systems tracts), organized into several subunits bounded by internal discontinuities locally deeply incising. The discontinuities are amalgamated surfaces, including successive sequence boundaries (indicative of phases of relative sea level falls) merged with transgressive surfaces (indistinct tidal and wave ravinement surfaces). The subunits are the part of Late Quaternary depositional sequences preserved in estuarine environments. They are the lateral equivalents of landward fluvial terraces and seaward coastal and prodeltaïc deposits on the shelf. Using borehole dataset, the underlying and eroded deposits below the basal unconformity are correlated with Pliocene deposits outcropping landward in the hinterland. The top of the incised valleys complex is capped by the last Glacial lowstand surface of erosion (18 ky B.P.) reworked by the postglacial transgressive surface (TS), dissociated near the shoreline into a tidal and a wave ravinement surfaces. Above the TS, the very high resolution (VHR) seismic data in the lagoons, the tidal channels and cores, reveal in details the stratigraphic architecture of the deposits. At the base, a small wedge constitutes the postglacial transgressive systems tract (TST) locally thinning in the areas distant of the sediment point sources. The TST is capped by a flat surface of wave reworking (maximum flooding surface or mfs) prolongating under the Leucate lagoon and merging offshore at the seafloor. Boreholes and VHR seismic lines trough the coastal barrier and in the lagoon show that the shoreline probably migrated far landward at the end of the transgression. When the rate of sea level rise decreased strongly, the shoreline migrated seaward and prograding and aggrading sandy material, with landward muddy lateral equivalent facies, deposited early highstand systems tract (HST) above the MFS. Offshore, fine material deposited as a sigmoidal blanket of mud originating in part from the north-east and Rhône river under oceanic circulation (equivalent to a subaqueous prodelta). Subsequently, the modern beach barrier built up by wave reworking of the early HST. This new study of the western part of the Gulf of Lion inner shelf and littoral illustrates an incised valley complex and thus presents the best preserved example of the sedimentary record of the effects of the relative sea level changes during the Pliocene to Actual period. For the first time, the land to sea transition is preserved and the Late Quaternary depositional sequences are in a great part observed. The last post Glacial deposits present a simplified but very different organisation compared to a record front of the adjacent Rhône river. Consequently a synthesis is now possible.