Messinian Erosion Research Articles

Seaway restriction, sea level drop and erosion in the Alboran Basin from a paleotopographic reconstruction for the Messinian Salinity Crisis

The Messinian Salinity Crisis (MSC) resulted from changes in the Atlantic-Mediterranean connectivity in the Alboran Basin, a region with a complex and debated geodynamic configuration. Since the MSC, this basin's topography and its record of the Messinian Erosional Surface have been subject to vertical motions due to sediment accumulation, tectonic deformation, isostasy, and latent effects of thermal cooling after extension and magmatic arc formation. The objective of this work is to restore these contributions to post-Messinian subsidence in order to quantify the original depth of formation of the MSC features. We do this by performing a pseudo-3D planform flexural isostatic reconstruction of the Messinian Erosion Surface mapped from an extensive set of seismic reflection data. We focus on identifying the most likely position of the gateway between Atlantic and Mediterranean, the effect of a drawdown on gateway topography and connectivity, and the depth of proposed erosional features related to the Messinian lowstand. The results indicate that the depth of the Alboran Basin by the end of the Messinian was about 500 m shallower than nowadays, but over 500 m deep on average, reaching depths of >1000 m in most subbasins, even when accounting for the possible ∼300 m isostatic rebound caused by water unloading in a largely desiccated Alboran Sea during the MSC. Although these results are compatible with volcanic cones locally emerging above sea level at the East Alboran Volcanic Arc during the Messinian, several lows remaining in the reconstruction suggest that that region is unlikely to have been the sill between Atlantic and Mediterranean at that time, unless the basin saw unconstrained dynamic topography contributions of over −500 m. Full desiccation of the Alboran Basin implies an uplift of up to 100 m at the Strait of Gibraltar, and uplift rates too high to be counteracted by erosion, suggesting that full disconnection and the main corresponding evaporative drawdown took place only once. The terraces and canyons in the West Alboran are restored to depths between 250 and 550 m (shallowest terrace) and 750–1500 m (deepest terrace), and cannot be clearly linked to a single, stable water level during the MSC, pointing to climate-controlled variations in the water level during the isolation phase.

Messinian canyons morphology of the Rhône and Ardèche rivers (south-east France): new insights from seismic profiles

Twelve available two-way time high-resolution seismic reflection profiles located in the central part of the middle Rhône valley are interpreted. In addition, one of the profiles was reprocessed to determine the P-wave velocities of the main geological units and to convert this profile into a depth cross section. The Lower and Upper Cretaceous units are clearly identifiable on all the profiles, along with the Messinian Erosion Surface (MES) carved out during the Messinian Salinity Crisis (MSC) by the paleo-Rhône and its western tributaries, the Ardèche and Cèze paleo-canyons. The Plio-Quaternary fill of these paleo-canyons shows at least 4 main units with an overall transgression. The combination of geological data from geological maps, geological field surveys and borehole data made it possible to model the MES in 3D at the scale of the region, and to produce depth/elevation model. From a geological point of view, the interpretation of the seismic profiles enabled us to reconstruct the stages in the sub-aquatic filling of the Messinian-Pliocene aggradation of the paleo-river. Several Mass Transport Deposits (MTDs) were identified both during the drop and during the rise in the Mediterranean Sea level. From a geomorphological point of view, this study provides new insights in the route and longitudinal profile of paleo-rivers and, in particular, it deepens the profile of the Paleo-Rhône at the latitude of the Tricastin region (up to −700 m b.s.l.) and significantly modifies the course and depth of the Ardèche proposed in previous studies. The N-Ardèche river, known to develop a karstic system during the MSC, is connected to a deep canyon, most likely through a karstic pocket valley, as suggested by the very steep longitudinal profile of the MES. Finally, from a structural point of view, our interpretation of the seismic profiles shows a broad ENE-trending anticline structure associated with a normal fault which apparently did not affect the Mio-Pliocene fill. In the southern part of the area, near the Uchaux anticline, the imaged structures suggest the presence of a recent (syn- to post-Pliocene) fault propagation fold. In addition to all the new information on the geology, morphology and methods of excavation and filling of the Messinian paleo-canyon, the proposed topographic model of the paleo-canyon is crucial for modelling seismic movement in the context of a basin with a complex geometry and, in particular, for the numerical assessment of site effects in a context of low seismicity.

New onshore/offshore evidence of the Messinian Erosion Surface from key areas: The Ibiza-Balearic Promontory and the Orosei-Eastern Sardinian margin

As the Messinian sea-level draw down associated with the Messinian Salinity Crisis is still questioned, we propose to show that the widely spread erosion surface affecting the Mediterranean margins is indeed linked to an exondation demonstrated from offshore and onshore data. Our study presents a comprehensive onshore to offshore correlation of the Messinian erosional surface. It is focused on small drainage systems or interfluve areas, outside of evaporite basins or incised canyons, where the Messinian erosion had not yet been studied previously: around Ibiza on the Balearic Promontory and around Orosei on the Eastern Sardinian margin, Tyrrhenian Basin, both areas where new offshore data were recently acquired. We show that the late Messinian erosion formed in subaerial settings, as testified by evidence of continentalization events, and attests for a regression phase that was correlated from the offshore continental slopes to the onshore paleo-platforms in both areas. Characteristics of this erosion in both study areas strengthen the scenario with at least one important low-stand sea-level for the Messinian Salinity Crisis with evaporites subbasins lying at different depths and possibly disconnected.

Neogene-Quaternary onshore record in the lower Ebro river incised palaeovalley (Ebro margin, Catalan Coastal Range, NE Iberia)

The lower Ebro is a bedrock-alluvial mixed incised valley with a persistent degradational stacking architecture developed from latest Serravallian(?) to Holocene. This degradational pattern was probably controlled by isostatic rebound in NE Iberia and punctuated by major relative sea level changes that temporally accentuated or attenuated the palaeovalley entrenchment and sediment retention. Six allostratigraphic units in the palaeovalley constitute the onshore record of its evolution and the opening and connection of the Ebro Basin with the Mediterranean. This paper deals with the analysis and reinterpretation of these units in order to precise the sequence of events that took place on the onshore part of the Catalan continental margin during the Ebro River drainage entrenchment. Plausible chronology and palaeogeographic evolution of the Neogene-Quaternary drainage incision in the lower Ebro are also proposed. The early evolutionary stages of the incised palaeovalley (Latest Serravallian?-Tortonian-Early Messinian, from 11.63–9? to near 5.6Ma) were dominated by entrenchment and intense sediment transfer from the onshore to the offshore zones (erosion surface S2). These processes were only punctuated by the sedimentation of the alluvial palaeovalley unit M2 (late Messinian). The polygenetic onshore erosion surfaces S2 and S3 are linked here with the onshore erosive processes that fed the sedimentation of the terrigenous shelf-slope system of the offshore Castellon group and considered coeval with the offshore Messinian erosion surfaces (reflectors M and m). During a further evolutionary stage (Pliocene to Early Pleistocene from 5.3 to approximately 2Ma) the Early Pliocene major Mediterranean reflooding caused some sediment retention in the incised palaeovalley (sedimentation of unit P) but sediment transfer into the offshore remained very effective. In the last evolutionary stage (Early Pleistocene-Holocene, from 2Ma to present) the palaeovalley became again mainly degradational (generation of erosion surfaces S4 to S6 and sedimentation of stepped alluvial terraces Q1-2 to Q4). The onshore stratigraphic record, including the allostratigraphic units P and Q1-2 to Q4 and the related bounding surfaces S3 to S6, is correlated with the sedimentation of the terrigenous shelf-slope system of the offshore Ebro group.

Seismic stratigraphy and geodynamic evolution of Christiana Basin, South Aegean Arc

The analysis and interpretation of vintage and recent seismic profiles across Christiana Basin in the South Aegean arc provided information on the seismic stratigraphy and chrono-stratigraphy of the basin, the occurrence of volcanic deposits within the sedimentary basin infill, and its post-Miocene geodynamic evolution. Plio-Quaternary sediments overlie a Messinian erosion surface on the basin's margins, while in the basin, they have been deposited above the Messinian evaporites. The basin evolved in two stages. During the early stage, subsidence of the three main depocenters in the NW, W and SE part of the basin has been controlled by E-W to ESE-WNW trending faults. The end of the early stage is marked by the cessation of activity on the southern marginal fault and the migration of the northern faults towards north where the basin expanded significantly towards south and north. The seismic stratigraphy of the younger stage deposits is characterized by the presence of three thick, pyroclastic flows. They occur only in the eastern part of the basin and have therefore derived from Santorini volcanic center since Middle Pleistocene. The youngest one is associated with the Minoan eruption while the next one is slightly younger than 0.42 Ma and may be linked with activity in Peristeria volcano. The lower pyroclastic flow has deposited shortly after 1.2 Ma and is the thickest (100 msec) and widest distributed pyroclastic flow deposit in the basin. It is most probably the biggest pyroclastic flow known in the South Aegean Sea. It may be associated with an eruption of the early volcanic centers of Akrotiri. No evidence of volcanic or volcano-clastic deposits derived from west, i.e. from Milos volcanic center has been found in Christiana Basin. By taking into consideration the similar, two stage structural evolution of the Santorini-to-Amorgos area proposed in the literature, it is suggested here that since Early to Middle Quaternary, the E-W trending Christiana Basin may be considered as an area of extension developed on the right side of the southwestern termination of the NE-SW trending, dextral Santorini-Amorgos fault zone.

The Messinian salinity crisis: open problems and possible implications for Mediterranean petroleum systems

A general agreement on what actually happened during the Messinian salinity crisis (MSC) has been reached in the minds of most geologists but, in the deepest settings of the Mediterranean Basin, the picture is still far from being finalized and several different scenarios for the crisis have been proposed, with different significant implications for hydrocarbon exploration. The currently accepted MSC paradigm of the ‘shallow-water deep-basin’ model, which implies high-amplitude sea-level oscillations (> 1500 m) of the Mediterranean up to its desiccation, is usually considered as fact. As a consequence, it is on this model that the implications of the MSC events on the Mediterranean petroleum systems are commonly based. In fact, an alternative, deep-water, non-desiccated scenario of the MSC is possible: it (i) implies the permanence of a large water body in the Mediterranean throughout the entire Messinian salinity crisis, but with strongly reduced Atlantic connections; and (ii) envisages a genetic link between Messinian erosion of the Mediterranean margins and deep brine development. In this work, we focus on the strong implications of an assessment of the petroleum systems of the Mediterranean and adjoining areas (e.g. the Black Sea Basin) that can be based on such a non-desiccated MSC scenario. In particular, the near-full basin model delivers a more realistic definition of Messinian source-rock generation and distribution, as well as of the magnitude of water-unloading processes and their effects on hydrocarbon accumulation.

The Miocene–Pliocene boundary and the Messinian Salinity Crisis in the easternmost Mediterranean: Insights from the Hatay Graben (Southern Turkey)

The Hatay Graben is one of three easternmost basins in the Mediterranean that preserve sediments that span the Miocene–Pliocene boundary, including gypsums from the Messinian Salinity Crisis (MSC). Here we integrate existing data and present new sedimentological and micropalaeontological data to investigate the palaeoenvironments of late Miocene to early Pliocene deposits and place this important area into a regional stratigraphic framework. Six sections are described along a ~W–E transect illustrating the key features of this time period. Late Miocene (Pre-MSC) sediments are characterised by open marine marls with a benthic foraminiferal fauna suggestive of water depths of 100–200m or less. Primary lower gypsum deposits are determined to be absent from the graben as sedimentological and strontium isotopes are characteristic of the Resedimented Lower Gypsums. The intervening Messinian erosion surface is preserved near the basin margins as an unconformity but appears to be a correlative conformity in the basin depocentre. No Upper Gypsums or ‘Lago Mare’ facies have been identified but available data do tentatively suggest a return to marine conditions in the basin prior to the Zanclean boundary. Sediments stratigraphically overlying the Messinian gypsums and marls are coarse-grained sandstones from coastal and Gilbert-type delta depositional environments. The Hatay Graben is not only strikingly similar to Messinian basins on nearby Cyprus but also to the overall model for the MSC, demonstrating the remarkable consistency of palaeoenvironments found in marginal basins across the region at this time. This research also raises questions as to the timing of the Mediterranean reflooding and the significance of the widespread mega-breccias of the resedimented gypsum deposits.

The Pliocene Mediterranean infilling of the Messinian Erosional Surface: New biostratigraphic data based on calcareous nannofossils (Bajo Segura Basin, SE Spain)

The Bajo Segura Basin (eastern Betic Cordillera) is a Mediterranean marginal basin where the Messinian Erosional Surface (MES), formed during the Messinian Salinity Crisis sea-level fall, is well developed. Overlying this major discontinuity the lower Pliocene transgressive sediments record the reflooding of the Mediterranean and the return to an open marine environment, the continental shelf being rebuilt after the Messinian erosion. The stratigraphic and biostratigraphic study of seven sections allow two transgressive-regressive sequences filling the MES to be distinguished, correlated with the previously distinguished Mediterranean offshore seismic units. Ten calcareous nannofossil bioevents have been identified. The lower sequence can be dated according to nannofossil biozones NN12 to NN14 and the upper sequence by NN15 to NN16. The boundary between both lower Pliocene sedimentary sequences is dated by the First Common Occurrence (FCO) of Discoaster asymmetricus in the uppermost sediment of the lower sequence and the First Occurrence (FO) of Discoaster tamalis in the lowermost part of the upper sequence. This discontinuity can be calibrated at between 4.1 and 4.0 Ma ago.

The Hecataeus Rise, easternmost Mediterranean: A structural record of Miocene-Quaternary convergence and incipient continent-continent-collision at the African-Anatolian plate boundary

The Hecataeus Rise represents a plateau-like structure, adjacent to the southern Cyprus margin and directly next to the Cyprus – Eratosthenes Seamount convergence zone, where incipient continent-continent-collision is believed to occur. Based on newly acquired seismic reflection and bathymetric data, we report on the Miocene to Quaternary structural evolution of this yet underexplored sector along the African-Anatolian plate boundary. A period of pre-Messinian compression has significantly deformed the western and southern part of the plateau area. Offshore continuation of onshore Cyprus lineaments attests the existence of a pre-Messinian structural link between Cyprus and the Hecataeus Rise. Owing to its relatively elevated position during the Messinian Salinity Crisis, the Hecataeus Rise experienced wide-spread Messinian erosion and only local deposition of evaporites which were precipitated within small, isolated sub-basins. Post-Messinian convergence was accommodated along the southeastern flank of the Hecataeus Rise, where NE–SW trending anticlinal structures experienced reactivation and significant growth. A prominent intra-Pliocene-Quaternary unconformity in the northwestern part of the plateau area may correlate with the Pliocene–Pleistocene transition and indicates the near synchronous occurrence of several tectonostratigraphic events. We suggest that these events represent a chain of structural and depositional changes initiated by incipient collision of Eratosthenes Seamount with Cyprus and the northwestern corner of the Hecataeus Rise. Collision-derived uplift of Cyprus resulted in vertical separation between Cyprus and the Hecataeus Rise and the development of a prominent unconformity in the northwestern part of the plateau area. Increasing the slopes adjacent to Cyprus facilitated more energetic downslope sediment transport, expressed by the development of sediment waves and the evolution of a prominent canyon structure at the northeastern limit of the plateau area.

Messinian and Zanclean continental formations of the Montpellier region and their relevance to the geodynamic model of Georges Clauzon (1996)

Abstract The Pliocene period in the Montpellier region, considered as being emblematic since the middle of the 19th century because of its diverse mammal fauna, has not received much innovative treatment over the last 30 years. In particular, there has been no reinterpretation of geological data in the light of the Messinian-Zanclean double eustasy and related mechanisms. And yet, the increase in the number of urban projects affecting the Pliocene terrains and substratum has allowed for a renewed analysis of these areas in conformity with the results obtained in the nearby valley of the Rhône river. Highlighting the discordance, the regressive Messinian erosion has allowed for the Pliocene fluviatile network of the Lez to emerge. This in turn has made possible the reconstruction of a digitate network that recorded all of the phases of the filling in of these Pliocene rias. This is particularly clear thanks to the study of the Messinian discordance in the hard rocks of the Montpellier fold on the one hand, and on the other, within the Neogene ditches. Since the Messinian, the course of the Mosson and Lez rivers, just as that of the Vidourle river, tracked heavily to the east. This is how the Pliocene Mosson river crossed the Montpellier fold. In fact these modifications of the river courses, made possible by the reuse, until the end of the Pliocene, of the erosion surfaces of the coastal links (the Montpellier fold, the Gardiole), correspond to the accentuation of the deformations of these surfaces. The amplitude and variability of these movements is a result of the altitude of the marine-continental Pliocene transition. This altitude is doubtless often rather approximate, but it differs considerably with respect to the coastal Jurassic ranges (the Montpellier fold to the north, the Gardiole to the south), which frame the Pliocene “Gulf” of Montpellier. The two coastal ranges, calcareous and sub-parallel to the sea surface, have succeeded in channeling the transgression at the point of the saddleback axes, conferring Dalmatian coastal characteristics to the Pliocene ria.

New constraints on the Messinian sealevel drawdown from 3D seismic data of the Ebro Margin, western Mediterranean

We present new 3D seismic and well data from the Ebro Margin, NW Mediterranean Sea, to shed new light on the processes that formed the Messinian Erosion Surfaces (MES) of the Valencia Trough (Mediterranean Sea). We combine these data with backstripping techniques to provide a minimum estimate of the Messinian sea level fall in the EBRO Margin, as well as coupled isostasy and river incision and transport modeling to offer new constraints on the evolution of the adjacent subaerial Ebro Basin. Four major seismic units are identified on the Cenozoic Ebro Margin, based on the seismic data, including two major prograding megasequences that are separated by a major unconfirmity: the MES. The 3Dseismic data provide an unprecedented view of the MES and display characteristic features of subaerial incision, including a drainage network with tributaries of at least five different orders, terraces and meandering rivers. The Messinian landscape presents a characteristic stepped-like profile that allows the margin to be subdivided in three different regions roughly parallel to the coastline. No major tectonic control exists on the boundaries between these regions. The boundary between the two most distal regions marks the location of a relatively stable base level, and this is used in backstripping analysis to estimate the magnitude of sea level drop associated with the Messinian Salinity Crisis on the Ebro Margin. The MES on the Ebro Margin is dominated by a major fluvial system, that we identify here as the Messinian Ebro River. The 3Dseismic data, onshore geology and modeling results indicate that the Ebro River drained the Ebro Basin well in advance of the Messinian.

Onshore to offshore reconstruction of the Messinian erosion surface in Western Sardinia, Italy: Implications for the Messinian salinity crisis

New outcrops in the Sinis peninsula (Sardinia, Italy) allow a detailed description of the Messinian erosion surface. The correlation between outcrops, industrial seismic data and data collected offshore Oristano from a recent cruise permits reconstruction of the geometry of this surface from presently emerged interfluves to the deep canyons offshore, with: - onshore: an eroded karstic plateau presently cropping out over a 20 km × 5 km area. Well-preserved karst structures occur locally and can form several metres high reliefs. The southernmost edge of the plateau is limited by a cliff. The karstic plateau and the cliff are sealed by transgressive deposits of Pliocene age; - offshore, an interfluve zone with a mostly flat and low-angle dipping plateau, westward dissected by paleovalleys which were identified down to a depth of 2000 m. In the Oristano Gulf, Messinian paleovalleys were also identified. The Capo San Marco paleocliff was probably located on the edge of a canyon. Onshore, only one major erosion surface was identified, located at the top the Messinian sequence. Offshore, this surface extends down to the abyssal plain. The surface was emplaced above deposits related to the Terminal Carbonate Complex. This indicates that the marginal evaporites are older than the major drawdown of the Mediterranean sea-level. The major drawdown is related to the deposition of the deep evaporites.

Offshore evidence of polyphase erosion in the Valencia Basin (Northwestern Mediterranean): Scenario for the Messinian Salinity Crisis

The Messinian Salinity Crisis at the end of the Miocene strongly affected the physiography of the Mediterranean margins, and in particular that of the Valencia Basin and its margins. The Valencia Basin, which is an aborted Tertiary rift related to the opening of the oceanic Provençal Basin, shows evidence of two major Messinian erosion surfaces — a Basal Erosion Surface (BES) and a Top Erosion Surface (TES) — bracketing the Messinian evaporite succession; the basin's margins show only a single erosion surface, here termed the Margins Erosion Surface (MES). In examining the geometrical relationships of these erosion surfaces with the deep basin facies deposited in the abyssal Provençal Basin during the Messinian Salinity Crisis, we have been able to establish a chronology for the Messinian in the Valencia Basin. Briefly, the Basal Erosion Surface extends the Margins Erosion Surface into the Central Valencia Basin and records an early and important sea-level fall at the beginning of the Messinian Salinity Crisis. The Miocene basin series were incised by canyons, while the exposed margins underwent substantial subaerial erosion. The basin's detrital products linked to this basal erosion grade laterally into the massive Messinian Salt layer of the Provençal Basin. The overlying Upper Evaporites unit appears to be widely transgressive beyond the edge of the Messinian Salt and onlaps the Basal Erosion Surface in the Valencia Basin. This disposition suggests a progressive sea-level rise during the deposition of the Upper Evaporites, possibly due to the rapid accumulation of thousands of metres of shallow-water evaporite facies in the Provençal Basin. The Top Erosion Surface at the top of the Upper Evaporites shows a network of incisions, and we suggest that this final erosion was linked to the Lago-Mare event recorded elsewhere in the Mediterranean at the end of the Messinian Salinity Crisis.

Upper Cenozoic stratigraphy and paleogeographic evolution of Myrtoon and adjacent basins, Aegean Sea, Greece

The southern Aegean Sea has undergone major subsidence as a result of back-arc extension during the Neogene. The details of this subsidence and the resulting sedimentation patterns have been examined in the Myrtoon basin and adjacent areas, including the south Evoikos, Saronikos and Argolikos gulfs, by comparing seismic stratigraphy and facies interpretation from multichannel seismic profiles with the stratigraphic and sedimentologic history of adjacent land areas. A detailed seismostratigraphy has been developed for the whole area in the late Neogene. The Messinian erosion surface is an important regional marker. Pliocene and early Quaternary markers are dated by comparison of marine transgressions with dated sections along the coast and dated volcanic rocks on islands. The region appears to have progressively subsided since the Miocene. Myrtoon basin and Saronikos Gulf were lake basins in the Messinian that were flooded to form deep-water marine basins during the basal Pliocene transgression. South Evoikos and Argolikos basins were progressively flooded during the Pliocene and early Quaternary. In most areas, deep-marine basins filled principally with hemipelagic sediment, but a submarine fan developed in the early Pliocene in the northern Myrtoon basin and fan deltas built into the basin from the east and southwest. Opening of marine ‘gateways’ as a result of subsidence led to enhanced oceanographic circulation and the formation of regional erosion surfaces at basin margins.

The Late Messinian salinity crisis and Late Miocene tectonism: Interaction and consequences on the physiography and post-rift evolution of the Gulf of Lions margin

The interpretation of Rhône maritime seismic profiles and the accurate mapping of all the structural features affecting the Miocene cover made it possible for us to develop herein a new structural model for the top of the eroded Miocene of the Gulf of Lions platform. Late Miocene extensional faults exerted a considerable structural control on the Messinian erosion. In particular, mapping of transfer zones has shown that these structures controlled retrogressive erosion of the platform. This late Miocene extension could have been caused by gravity-driven destabilization of the platform, associated with base-level lowering at the onset of the Messinian salinity crisis.

Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis

Current interpretation of the Messinian Salinity Crisis (MSC) involves partial “desiccation” of the Mediterranean Sea coupled with the deposition of thick evaporites in the deep basins. New sets of seismic reflection profiles in the western part of the Gulf of Lions confirm the basinward extension of the Messinian erosion and enable the mapping of distinctive seismic markers indicating the Messinian Erosional Surface (or Messinian unconformity), the basin-margin detrital deposits, and the deep evaporite sequence. The geometrical relationship between these three elements and their relationship to the paleogeography of the margin during the MSC provide new information about the evolution of the study area during the Messinian. The Messinian Erosional Surface (MES), commonly correlated with the “desiccation” phase and the deposition of deep evaporites during the apogee of the event, is generally interpreted as a subaerial feature. In the Gulf of Lions, it is a complex diachronic polygenic erosional surface observed at the base of the prograding Plio-Quaternary sequence beneath the shelf and slope; it extends downslope beneath the deep basin Upper Evaporites and the Salt, and possibly correlates conformably with the base of the so-called deep Lower Evaporites. The whole morphology of the MES reflects a buried drainage pattern, supporting the interpretation of fluvial erosion driven by a substantial drop in sea level. Our results also suggest that large submarine gravity flows occurred prior to any significant accumulation of Salt in the basin and prior to the Upper Evaporites. Consequently, interbedded clastic deposits may partly account for the parallel reflectors of the Lower Evaporites. Since river erosion persisted throughout the MSC, the Salt and Upper Evaporite units may also contain a large amount of detrital sediments. The good quality of the new seismic data clearly reveals fan-shaped Messinian deposits in the downstream part of the main Messinian valleys (i.e., the Nile, Var, and Spanish rivers). The depositional scenarios generally involve a substantial sea-level fall coupled with deltaic/prodeltaic accumulations. A chaotic seismic unit (Unit D) filling Messinian lows and extending beneath the Salt within the study area is interpreted as a Messinian clastic unit. We propose a polyphase scenario of detrital fan deposition involving pre-, syn-, and post-Salt deposition in subaqueous/subaerial environments. In the Gulf of Lions, a late Miocene tectonic phase that affected the western shelf also played an important role in controlling (a) the pattern of the Messinian fluvial network, (b) the location of maximum erosion on the shelf, and (c) the location of the detrital fan depocentre downslope.

Structure of the Sardinia Channel: crustal thinning and tardi-orogenic extension in the Apenninic-Maghrebian orogen; results of the Cyana submersible survey (SARCYA and SARTUCYA) in the western Mediterranean

Abstract The Sardinia Channel is located in a 400 km-long submerged section of the Apennine-Maghrebian branch of the Alpine chain. The Sardinia Channel connects the Algerian-Ligurian-Provençal to the Tyrrhenian oceanic basins. The structure of this region results from the superposition of two tectonic regimes: an earlier crustal thickening and a later crustal thinning. The crustal thickening is the consequence of the shortening which occurred in the late Oligocene–early Miocene during the build up of the Apennine-Maghrebian Orogen. This thickening is coeval with the rotation of the Corsican-Sardinian block and the opening of the Provençal-Algerian back-arc basin. All of these structures, as well as the magmatic arcs in Sardinia and Tunisia, i.e. the Galite Archipelago, are connected to the subduction of the Tethyan Ocean. The crustal thinning is associated with the rifting of the Tyrrhenian Basin, which occurred just before the Messinian salinity crisis and was accompanied by significant erosion throughout the region. This erosion was followed by a period of thermal subsidence linked to the opening of the Tyrrhenian oceanic basin in the Plio-Quaternary, interspersed with minor episodes of compression. On the Sardinian margin, the dives led to the discovery of a submarine volcano, dated at 12.6 Ma, and composed of shoshonitic andesites with lamprophyre inclusions, and to the characterization of the nature and structure of the underlying basement, consisting of tilted blocks of Hercynian metamorphic and granitic rocks and their sedimentary cover. The sea floor morphology reflects this structure. The other areas of the Sardinia Channel explored, i.e. its southern margin and central ridge, belong to the Calabrian-Peloritanian-Kabylian group (CPK). They are composed of a metamorphic and granitic Hercynian basement deformed during the Alpine orogeny, which is stratigraphically overlain by an Oligo-Miocene detrital cover of Peloritanian or Kabylian type, and tectonically overlain by the so-called “flysch nappe”. Throughout the CPK domain these formations were subjected during the Oligo-Miocene, at ca. 23 Ma ago, to a first denudation event, and during the Tortonian, ca. 10-8 Ma ago, to a second denudation, which has been connected to the opening of the Tyrrhenian basin. Structures, microstructures and thermochronological data indicate relatively low P-T conditions for the extensional deformations: this suggests that these units remained at shallow depths in the Apennine-Maghrebian Orogen, and were relatively preserved from the Messinian erosion. The age (12.5 Ma) and nature of the volcanic sequence in the Sorelles is closely comparable with the calc-alkaline suite of the Galite Archipelago, Tunisia. Thus, the data gathered during the dives in the Sardinia Channel give new constraints to the reconstruction of the kinematic evolution not only of the region, but also to the entire western Mediterranean.

The Messinian event on the margin of the Mediterranean Sea in the Nice area, southern France

Seismic reflection profiles have been used to map the Messinian erosion surface and associated basin-margin deposits on the continental margin off Nice. A palaeovalley extends seaward from the incised Var palaeovalley mapped on land. The lower part of this valley is filled by an acoustically incoherent unit with a flat top and steep slope into the basin: this unit is interpreted as a coarse-grained deltaic deposit that accumulated synchronously with the upper Messinian evaporites in a ponded lake basin. Two submersible dives on strategic outcrops supported the interpretation of the seismic profiles. We examined and sampled outcrops of probably early Tertiary sandstones underlying the Messinian erosion surface and of a fining-up, predominantly conglomeratic sequence that fills Messinian channels on the continental slope and probably overlies the deltaic deposits. These conglomerates pass up into Early Pliocene marls and have turbidite sedimentary structures. These are prodeltaic deposits formed during the transgression at the end of the Messinian. Based on seismic facies mapping, sampling and submersible observations, a palaeogeographic reconstruction of the area is proposed. A basal erosion surface, including the Var palaeovalley, represents extreme dessication of the western Mediterranean and was followed by accumulation of the main Messinian salt. Following this, a lake was ponded in the Ligurian Basin, with accumulation of evaporites and shales. A Gilbert-type delta prograded into the basin from the Var palaeovalley. Relative sea (lake)-level lowering is estimated to be about 1200 m at this time. There has been little faulting on the margin since the Early Pliocene transgression, but there is estimated to have been about 800 m of flexural subsidence due to both thermal subsidence and water loading.

Geotechnical and Sedimentological Properties of Nice Slope and Submarine Var Canyon Deposits: ABSTRACT

The submarine topography of the Baie des Anges consists of a narrow shelf and a steep slope where about 1,000 m-thick Pliocene-Quaternary deposits overlie the Messinian erosion surface. After a stage of rapid development, this large sedimentary body was submitted during the Quaternary to degradation through active mass-wasting processes. Thirteen piston cores were taken in several parts along the flanks and the floor of the Var canyon. Logging of these cores included gamma-density measurements, sedimentary structure descriptions, and microfacies observations (nannofossils). The cores were also sampled for a first geotechnical analysis. Geotechnical parameters such as water content, bulk density, vane shear strength, and Atterberg limits allowed us to study the response of surficial sediments to the vertical effective pressure and to discriminate several geotechnical groups related to different genetic types of sediment. Particularly the consolidation state was evaluated with the calculated ratio between undrained cohesion and vertical effective pressure (C/sub u//P). The sediment stability was assessed using the infinite slope stability analysis method under undrained conditions. Results of this preliminary analysis show that sedimentary instabilities could occur on important slopes.