A mud-filled submarine canyon cutting through a sand-prone turbidite succession: A Lower Oligocene field analogue of stratigraphic-structural trap in the Western Alpine Foreland Basin, France

A review of the sequence stratigraphic development of the Tertiary basins of the North and West Alpine Foreland domains shows that their structural and depositional history was episodically affected by brief tectonic phases. These were associated with intermittent deformation events induced by the collisional convergence and compressional coupling of the Apulian and Iberian microplates with the European Plate. The plate kinematics-related episodicity was essentially isochronously recorded in the basin fills of the Alpine Foreland region. These are generally correlative with changes in eustatic sea level. The ensuing correlative successions of so-called Cenozoic Rift and Foredeep (CRF) sequences and phases can be traced throughout the European Cenozoic Rift System and Alpine Foreland Basin. Their temporal correlation indicates that, apparently, the changes in the plate collision-related stress regime of the Alpine Foreland were repeatedly accompanied by coeval changes in eustatic sea level. To test and substantiate the validity of this inferred causal relationship between intraplate deposition, plate kinematics and eustacy, the tectono-sedimentary evolution of the basins of the Mediterranean plate-boundary zone has been analysed in conjunction with a review of the plate-boundary events in the North Atlantic. Within the uncertainty range of available datings, synchroneity could thus be demonstrated for the punctuated tectonostratigraphic development of basins of the western Mediterranean (comprising the Liguro-Provençal Basin, Valencia Trough, Sardinia Rift and Tyrrhenian Basin), the Apenninic-Calabrian Arc, the Betic domain (including the Alboran Basin) and the North and West Alpine Foreland regions. Similar temporal correlations of plate tectonicsrelated events near the Mid-Atlantic Ridge in the North Atlantic and tectonostratigraphic sequences and phases of the Alpino-Pyrenean Foreland basins are further evidence of a common causal mechanism. The driving mechanisms appear to have been the northward drift of Africa and the resulting mechanical coupling of Apulia and Iberia with the southern passive margin of Europe, as well as the stepwise opening of the North Atlantic and accompanying episodic plate re-organisations of the Mid-Atlantic Ridge.

The studies carried out on tectonically confined turbidite systems in Mediterranean-type foreland basins have shown that these deposits can be dominated by supercritical flows and by their transformation into subcritical and/or transitional (mud–sand) flows. In these confined turbidite systems, flow deceleration is favored especially by morphologies transversal to paleocurrents, e.g., slope breaks or adverse slopes that can vary in scale from regional tectonic structures to depositional features such as thick mass-transport complexes and lobes. Based on data of more than fifty years of outcrop studies in foreland and wedge top basins, a new facies-tract scheme is presented that includes the occurrence of supercritical-flow deposits and hybrid event beds in turbidite successions. A review of the main turbidite facies schemes available in the literature is given, and detailed field examples of the Apennine and Alpine (Peïra Cava) foreland basins are given that demonstrate the effect of basin morphology on the type of facies tracts. The concept of flow efficiency is revisited as sediment transport depends not only on flow behavior but also on basin size and basin-floor morphology (e.g., large foredeeps and small piggyback basins are characterized by facies that reflect high-efficiency and low-efficiency transport, respectively).

Oligocene rocks are one of the most important sources of hydrocarbons within the Paratethyan realm. In the Alpine Foreland Basin (Central Paratethys) the main Oligocene source rock is the Schöneck Formation, but organic-rich rocks occur in the entire Lower Oligocene succession. Based on well-log calibration by core data, the spatial distribution and thickness variations of different Lower Oligocene source-rock facies are investigated. The deeper-water sediments are characterized by lateral continuity, but exhibit vertical variability. The latter reflects major palaeoceanographic changes in the Central Paratethys, such as the closure of seaways, basin-wide changes in salinity and in redox conditions. The upper shaly part of the Schöneck Formation has the highest source potential (>5% TOC, initial HI: 500–600 mgHC g −1 TOC) and reaches its maximum thickness ( c . 5 m) in a narrow belt parallel to the palaeo-shoreline. The present-day distribution of Lower Oligocene rocks is controlled by submarine erosion which affected the northern passive slope of the foreland basin. Erosion climaxed during the late Early Oligocene. The eroded material was re-deposited along the lower basin slope (Oberhofen facies). The source-rock potential of the re-deposited sediments is relatively low. The oil kitchen (4–7 km burial depth) is located beneath the Alpine nappes where the Lower Oligocene succession was removed locally by the advancing nappes. Both submarine erosion at the northern basin slope and tectonic erosion beneath the Alps have to be considered in the evaluation of the prospectivity of the basin. Because deposition of the Lower Oligocene succession in the Alpine Foreland Basin is controlled by basin-wide processes, it may serve as a model for source- rock deposition in foreland basins of the Paratethyan realm (e.g. Carpathians, Terek–Caspian Foredeep).

The long‐term (50 Myr) development of orogenic thrust wedges and their neighbouring foreland basins are inextricably linked. Foreland basins, such as occur nonh of the Alps, flanking the Pyrenees, east of the Apennines and south of the Himalayas, are characterized by an early underfilled (deep‐water) phase, followed by a filled (shallow‐marine) or overfilled (continental) phase. The extent to which a foreland basin is filled to sea‐level can be understood by comparing, over time, the rate at which sediment is delivered to the basin with the rate at which accommodation space is generated within the basin. This approach is applied to the Alpine thrust wedge/North Alpine Foreland Basin (NAFB) system. Using simple geometrical calculations assuming a critically tapered thrust wedge sliding over a foreland plate, preceded by a flexurally induced trough (foreland basin), it is possible to develop some general parameter relationships for the system. The thrust wedge is described firstly in terms of the history of exhumation which is directly linked to denudation and hence sediment generation, and secondly to thrust front advance rates. The accommodation space can be approximated by the product of the thrust front advance rate and the deflection of the basin at the thrust front. The extent to which a basin is, at one instant, being undersupplied or oversupplied (F) can then be described by the ratio of sediment supply to accommodation space generated:F=exhumation rate. width of thrust wedgeladvance rate. deflection at thrust frontWhen F> I, the basin is on a trend towards overfilling, and when F< 1 the basin is on a trend towards underfilling. The value for the width of the thrust wedge undergoing exhumation is held as a constant for most of the evolution of the NAFB.The development of the Alpine thrust wedge/NAFB system is characterized by an initial submarine phase (Cenomanian to mid‐Oligocene) of rapid thrust front advance and slow exhumation rates; this resulted in deep‐water and shelfal sedimentation in the foreland basin (F< 1), and is described as the occretionary wedge phase. By the mid‐Oligocene, exhumation rates were accelerating associated with major backthrusting, and frontal advance rates were slowing down; this resulted in the foreland basin being filled to sea‐level (F← 1), accumulating shallow‐marine and continental sediments. This period is described as the continental wedge phase. Although F‐values probably fell back to close to 1 following the period of major Oligocene exhumation, the basin remained in its filled state for the remainder of its geological history.Modern studies indicate that rapid exhumation rates lead to increased local relief and denudation. The increased relief causes increased maximum elevations, so enhancing orographic precipitation and glaciation leading to further increases in denudation. This positive feedback loop between denudation, exhumation and climate may have enhanced the rapid inversion of the core of the orogenic wedge during the latter part of its growth.

The Miocene marine basins of Central and Southeast Europe, once comprising the Paratethys Sea, were gradually filled with sediments during the Neogene and turned to be the catchment area of the proto‐Danube and finally that of the modern Danube. Seismic data from various parts of the large Danube catchment area show that these several hundred meter deep basins were filled by lateral accretion of river‐transported sediments, appearing as shelf edge scale clinoform sets in seismic profiles. The direction of shelf edge progradation is NW to SE (N to S, W to E) in each basin, except for the Dacian basin where NE to SW direction prevails. The age of the clinoform sets is generally younging downstream: 19–18 Ma in the North Alpine Foreland basin, 14–13 Ma in the Vienna basin, 10–9 Ma in the Danube (Kisalföld) basin, 8.6–4 Ma in the Central Pannonian basin (Alföld), ?9–5 Ma in the Dacian basin, and 6–0 Ma in the Euxinian (Black Sea) basin. In spite of this geographical and temporal pattern, only the Danube (Kisalföld) and the western and central part of the Central Pannonian basin were filled by the proto‐Danube shelf accretion. Formation of the Danube, as a longitudinal river of the Alpine foreland that gradually elongated to the east and followed the retreating shoreline of the Paratethys, most probably took place at the beginning of the Late Miocene, ca. 11 Ma ago, thus the Early and Middle Miocene shelf advance in the North Alpine Foreland and Vienna basins, respectively, cannot be attributed to a „paleo‐Danube”. The clinoform systems of the Dacian basin are coeval with those of the upstream Central Pannonian basin, indicating that by the time the Danube sedimentary system reached the Dacian basin, it was already a shallow basin. The vast clinoforms of the northwestern Euxinian shelf also significantly overlap in age with the Pannonian basin ones; only the <4 Ma part of the shelf accretion can be attributed to the Danube sensu stricto.

Submarine canyons are commonly controlled by tectonic structures and, therefore, are key elements of the evolution of convergent margins such as the Southern French Alpine Foreland Basin. Here we use the outcrops of Grès d’Annot and Schistes à Blocs formations of the Sanguinière-Restefond and Trois Eveches sub-basins, to study the morphology of ancient canyons respectively in relation to extensive and compressive tectonics. The Grès d’Annot Upper Erosion Surface (GAUES) and faults have been mapped in the field and using airborne and drone pictures. Moreover, the deposition age of the Schistes à Blocs Formation has been constrained by the analysis of calcareous nannofossils and benthic foraminifera coming from 9 samples. We also compared ages of detrital zircons by U-Pb thermochronology from 4 samples. One of them was sampled within Annot Sandstones while the other come from the turbidites of the Schistes à Blocs Formation that seals the GAUES.The Colombart Structure in the Sanguinière-Restefond area is composed of two normal faults with a N80°E orientation and a southern vergence, bordering a northward dipping rollover anticline. The Colombart Structure axially controls the 700 m deep La Bonette Canyon cutting through the underlying Annot Sandstones. The submarine canyon is made of a succession of sharp erosive features, such as erosive walls, ramps and terraces. The cross-section profile of the canyon exhibits a tectonic control at several scales: it is asymmetric as well as the thalweg is. Faults also commonly control smaller scale morphologies, but also the capture of tributaries at right angles with the canyon axis, which testifies for a rectangular drainage pattern. The preliminary study of the GAUES in the Trois Eveches Sub-basin also exhibits a strong relationship between tectonics and submarine erosion. The last shows a 300 m-high scarp frontally eroding a NW-SE oriented thrust which affects the underlying sandstones. Moreover, biostratigraphic dating of the Schistes à Blocs Formation indicates NP22-lower NP23 biozones, i.e. the Early Rupelian. Detrital zircons analysis by U-Pb method show that Annot Sandstones and Schistes à Blocs Formation have the same signal. Finally, within both sub-basins, the thin bedded turbidites of the Schistes à Blocs Formation exhibit paleocurrent directions which are almost opposed to those measured within the Annot Sandstones. Paleocurrents within the Trois Eveches Sub-basin also locally change depending on which thrusts is located below the Schistes à Blocs Formation.Consequently, the GAUES mainly results from retrogressive erosion affecting partially lithified turbidites following two main triggering factors which are: i) the foreland deformation with a deformation direction that potentially locally changes, and ii) the 3rd order eustatic fall linked to the Oi1a δ18O event. The creation of submarine canyons affecting previously deposited turbidite lobes testifies of a strong paleogeographical modification of the foreland before the Autapie nappe emplacement. This change is also evidenced by the paleocurrent reorganization after the submarine erosion. Nevertheless, the complete understanding of the whole Early Rupelian source-to-sink system would need to enlarge the study of the Schistes à Blocs Formation to the whole foreland, including the use of other methods.

Nummulitic Limestones deposits are preserved along the tectonic contact between the Variscan basement and Alpine units of Corsica. These marine carbonates, dated from the Late Palaeocene to the Middle Eocene, were deposited within a foreland flexural basin that is considered to be the southern continuation of the Alpine foreland basin of southeast (SE) France. However, in contrast with the Nummulitic Limestones of SE France, those of Corsica are far less documented. This field‐based study constrains the sedimentology, stratigraphy and structure of the Nummulitic Limestones of Corsica in three localities (Balagne, Corte and Sari‐Solenzara) to identify factors that controlled foreland basin development and to clarify its significance within the early alpine orogen. The microfacies, microfaunal assemblages and siliciclastic fractions are characterised throughout the succession at each locality. The results indicate the existence of an important Variscan basement relief to the west of the basin (West Corsican Massif) that supplied early alluvial fans found at the base of the foreland succession in the northernmost Balagne area. Continuous high clastic input strongly reduced the development and diversity of the overlying Nummulitic Limestones facies and fauna. Further south, limestones in the Corte and Sari‐Solenzara areas are thicker and contain richer fauna. Three depositional models corresponding to the carbonate ramp system are proposed for the Nummulitic Limestones and used to construct paleogeographic maps illustrating the transgressive evolution of the Corsican foreland basin from the Early to the Late Eocene. Based on our results and available regional tectonic data and LT thermochronological data, we propose that the Nummulitic marine transgression took place within a continuous foreland basin encompassing southern Corsica and SE France during the early development of the western alpine arc.

The Western Alpine Foreland Basin ("French Molasse Basin") is located along the Western Alps and is composed of Oligo-Miocene formations resulting, at least to some extent, from the erosion of the alpine range. The distribution of sedimentation area, drainage network and sedimentary sources have strongly varied during its development. Late Eocene and Miocene marine formations are well-constrained as longitudinal basins with some transverse sedimentary transfer: the Eocene turbiditic basin was fed from the South, whereas the Miocene molasse basin was flowing southward. The Oligocene time period corresponds to the beginning of continental collision and to the exhumation of internal crystalline massifs. The erosion of first Alpine landforms causes the transport of sedimentary materiel in the basin with the transition from flysch (underfilled) to molasse (overfilled) deposits. The paleoenvironment is mainly continental and sediments are preserved in both internal and external position, which attests of a complex drainage network. Oligocene is therefore an important period of reorganisation in the foreland basin but has been poorly studied at the scale of the whole Western Alps and remains under-documented, mainly because of scarce outcrops probably due to early deformation in the basin. Here, we provide a new tectono-sedimentary study of these deposits based on new field work, seismic and well data interpretations, palynological analyses and bibliographic synthesis. This work led us to propose an exhaustive synthesis of the Oligocene foreland basin (or sub-basins) with synthetic logs and detailed palaeoenvironmental maps. Our results show that the Oligocene Western Alpine Foreland Basin can be divided in two main sedimentation areas: (1) an internal area which is mainly influenced by the alpine range evolution and (2) the Rhône Valley which has been structured by both the European Rift and the Pyrenean orogeny ("Pyrenean-Provence phase") and receives autochthonous sediments but also erosional products from the Massif Central, the Pyrenean Chain and the Alps. Palaeoenvironments and nature of sedimentation have strongly changed during the entire Oligocene. The internal basins (i.e., in the footwall of the Penninic Frontal Thrust) are connected with the South Rhône Valley since the early Rupelian thanks to E-W transverses valleys possibly inherited from the Pyrenean orogeny. Sedimentary supply remains mixt (Massif Central/Alps) until the end of Oligocene. A final longitudinal system set up at the beginning of the Aquitanian in which all the Alpine material was flowing to the south and the Mediterranean Sea. Two episodes of marine incursion have been identified (Early Rupelian and Early Chattian) thanks to biostratigraphy in the Rhone Valley which was probably already connected to the Mediteranean Sea before the Miocene. To sum up, the West Alpine Foreland Basin experienced during Oligocene (and Early Miocene) times transient basin dynamics with sub-basins controlled by westward propagation of the wedge front due to frontal accretion, a complex transverse routing system along with global flow inversion from north to south.

Although plate flexure exerts a first-order control on facies patterns in foreland basins, the influence of other factors can overprint flexural effects on basin stratigraphy. We examine the influence of preexisting foreland structural elements on paleotopography and sedimentation in the distal Alpine foreland basin in southeastern France. The Eocene Nummulitic Limestone Formation records progressive backstepping of a shallow-marine carbonate ramp at the distal margin during flexurally induced transgression of the European foreland plate. Analysis of paleogeomorphic features preserved along the basal foreland basin unconformity permits reconstruction of paleotopography during early foreland-basin development. The Nummulitic Limestone unconformably onlaps basement in the hanging walls of a set of reverse faults in the foreland plate. The pattern of onlap indicates that the late Eocene land surface comprised a set of structurally controlled basement highs with an estimated relief of up to 500 m at the time of the Nummulitic transgression. Locally, bedrock-confined paleovalleys filled with fluvial conglomerates are preserved along the unconformity surface and indicate that the pre-Tertiary subcrop was exposed subaerially. Subsequent marine transgression took place across a foreland substrate that showed considerable variation in seafloor topography. Erosional and depositional processes adjacent to structural highs created a complex facies mosaic, which overprinted the simple backstepping carbonate-ramp facies succession that is typically associated with distal forelands. Although flexure determines large-scale facies trends, preflexural relief related to foreland structures can exert significant control on smaller scale depositional patterns. Our study has implications for the interpretation of the basal unconformity of the Alpine foreland basin in southeastern France. Regional subcrop patterns at the basal unconformity show that there were abrupt variations in the depth of erosion into the foreland plate. Field- and basin-scale stratigraphic relations demonstrate that the geometry of the basal unconformity is controlled by paleotopography related to relict basement structures. It is not exclusively compatible with an origin either by broad wavelength uplift of the foreland plate (as depicted by a migrating flexural forebulge) or by eustatic sea-level fall as has been proposed for other basal foreland-basin unconformities.

<p>Foreland basin sediments mirror the history of an orogeny. Deformation and geodynamic processes in low spatial extend (e.g. dozens of km) can be quantified using kinematic restoration. Processes happening deep underneath an orogen show a large spatial manifestation that is difficult to quantify in time and space. Marine units at surface outcrops show 900 m of net uplift since deposition in undeformed parts of the alpine foreland basin. Existing low-temperature thermochronology data from the Swiss part of the Molasse Basin show a thermal overprint that indicates exhumation of more than 1.5 km. We quantify the wavelength of deep seated processes of the Alpine orogen by generating and analyzing a holistic dataset of the entire alpine foreland basin. In addition to compiling existing data from the western part of the basin we have generated a new apatite (U-Th)/He and vitrinite reflectance data set from the central and eastern part of the basin. The new apatite (U-Th)/He ages in the German part of the basin show exhumation below or close to the detection limit (~1.5 km). Within the folded and thrusted Molasse, exhumation is localized along thrusts and the thermochronological data indicates thrusting between 10 to 20 Ma. Vitrinite reflectance data reveals a trend of exhumation increasing from East to West. Parts of the central German Molasse basin have been exhumed as well. Thus, on the large scale we can see longwave exhumation patterns in the western part of the basin that affect both the deformed and undeformed parts of the basin which cannot only be related to Jura thrusting.</p>

Late Paleogene syn-tectonic magmatic products are known from sandstones contained in the North Alpine (NAFB) and South Alpine (SAFB) foreland basins and in the South Alpine pelagic Trento Basin. The generally turbiditic and mass-flow deposits grade up from marly hemipelagic deeper water series. The source and amount of the reworked volcanic materials have been in debate for a long time. As a proxy for the magma-derived input we use the U–Pb geochronology and geochemistry [176Hf/177Hf(t) and Eu/Eu* ratios] of detrital zircons, and evaluate their temporal and genetic relationships with potential volcanic sources in the Periadriatic magmatic systems (Adamello, Bergell, Biella). The oldest volcanic sources (Lutetian–Priabonian) we identify are in the Trento Basin and Glarus NAFB. During most Rupelian, Bergell and Biella volcaniclastics were delivered to the NAFB in the Glarus, Alpe de Taveyanne and Haute-Savoie. Bergell and minor Adamello magmatic material were supplied to the Villa Olmo Conglomerate in the SAFB. During late Rupelian–Chattian in the entire NAFB, the influx of Paleogene volcanic material faded out. At the same time in the SAFB, the Como Conglomerate shows mixed input from Biella and Bergell. High numbers of old zircons (> 90%) in the NAFB document the asymmetry of the early Alpine orogenic wedge exposing large basement areas to the north of the Periadriatic intrusions. The syn-sedimentary right-lateral movement along the Periadriatic fault system is identified as the main driver of magmatic activity, uplift, and exposure to erosion and transport to the basins. On this base, the dynamics of the early Alpine drainage systems are reconstructed with new accuracy.

The Miocene of the Western Alpine foreland basin were deposited in a north–south seaway along the active alpine orogenic front. In the subalpine massifs and the southern Jura mountains, the revised Miocene stratigraphy documents a detailed chronology of thrust propagation at the western alpine front, where tectonic activity had a primary influence on seaway palaeogeographical evolution. Here we propose nine palaeogeographical maps during the Miocene, the first of which depicts the initial Miocene transgression at c. 21.0 Ma. Between c. 18.05 and c. 12.0 Ma, a westward retreat of the Miocene Sea occurred in response to activation of the basal thrust of the Belledonne massif, which in turn triggered successive fault zones from east to west. At c. 10.0 Ma, a major uplift phase intervened and induced a rapid southward retreat of the Miocene Sea. The reconstructed palaeogeographical maps outline the main controls on the foreland basin seaway evolution: (1) the timing of the main thrusts; (2) the inherited palaeotopography; and (3) eustatic sea-level changes during the Miocene. These reconstructions are integrated at the basin scale, highlighting the southward- to westward-directed seaway migration in response to the Belledonne thrust activity that deeply shaped the palaeogeographical evolution during the early to middle Miocene.

Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc

It is hypothesized that lithospheric reorganisation, including slab breakoff and tearing, leads to shifts in crustal buoyancy, which then influences rock uplift, erosion and weathering on the surface. This process can be ideally studied in compressional orogenic settings with complex fluvial drainage systems, such as the European Alps. Changes in uplift and erosion can be studied using sedimentary provenance techniques, such as major element geochemistry and petrographic point counting. First we use modern-day fluvial sands to understand how major element geochemistry and petrography reflect the modern erosional pattern of the Alps. In a second step, the signatures of modern river sands are compared to those of sandstones deposited in the Alpine foreland basin, which was a major sedimentary sink throughout the Oligocene and Miocene.Here, we present two datasets consisting of major element geochemistry (ca. n=180) and petrography (ca. n=200) data of modern Alpine rivers. We use smaller rivers draining specific source rock types within the orogen to define geochemical and petrographic end-member lithological fingerprints. These fingerprints are subsequently used to deconvolve via unmixing modeling 9 larger fluvial drainage basins in the Alps: the Adige, Dora Baltea, Drau, Enns, Inn, Mur, Rhine, Rhone andSalzach rivers. We compare themodeled relative contributions of specific source rocks/areas with the modern-day erosion patterns in those drainage basins based on geological maps and published erosion rates. The comparison with detrital spectra in foreland basin deposits provides insights into the change of watershed locations and river networks from the Miocene to today. 

Abstract. Reconstructing Oligocene–Miocene paleoelevation contributes to our understanding of the evolutionary history of the European Alps and sheds light on geodynamic and Earth surface processes involved in the development of Alpine topography. Despite being one of the most intensively explored mountain ranges worldwide, constraints on the elevation history of the European Alps remain scarce. Here we present stable and clumped isotope measurements to provide a new paleoelevation estimate for the mid-Miocene (∼14.5 Ma) European Central Alps. We apply stable isotope δ–δ paleoaltimetry to near-sea-level pedogenic carbonate oxygen isotope (δ18O) records from the Northern Alpine Foreland Basin (Swiss Molasse Basin) and high-Alpine phyllosilicate hydrogen isotope (δD) records from the Simplon Fault Zone (Swiss Alps). We further explore Miocene paleoclimate and paleoenvironmental conditions in the Swiss Molasse Basin through carbonate stable (δ18O, δ13C) and clumped (Δ47) isotope data from three foreland basin sections in different alluvial megafan settings (proximal, mid-fan, and distal). Combined pedogenic carbonate δ18O values and Δ47 temperatures (30±5 ∘C) yield a near-sea-level precipitation δ18Ow value of -5.8±1.2 ‰ and, in conjunction with the high-Alpine phyllosilicate δD value of -14.6±0.3 ‰, suggest that the region surrounding the Simplon Fault Zone attained surface elevations of >4000 m no later than the mid-Miocene. Our near-sea-level δ18Ow estimate is supported by paleoclimate (iGCM ECHAM5-wiso) modeled δ18O values, which vary between −4.2 ‰ and −7.6 ‰ for the Northern Alpine Foreland Basin.