Sylvie Bourquin

Meso-Cenozoic geodynamic evolution of the Paris Basin: 3D stratigraphic constraints

Abstract 3D stratigraphic geometries of the intracratonic Meso-Cenozoic Paris Basin were obtained by sequence stratigraphic correlations of around 1 100 wells (well-logs). The basin records the major tectonic events of the western part of the Eurasian Plate, i.e. opening and closure of the Tethys and opening of the Atlantic. From earlier Triassic to Late Jurassic, the Paris Basin was a broad subsiding area in an extensional framework, with a larger size than the present-day basin. During the Aalenian time, the subsidence pattern changes drastically (early stage of the central Atlantic opening). Further steps of the opening of the Ligurian Tethys (base Hettangian, late Pliensbachian;...) and its evolution into an oceanic domain (passive margin, Callovian) are equally recorded in the tectono-sedimentary history. The Lower Cretaceous was characterized by NE–SW compressive medium wavelength unconformities (late Cimmerian–Jurassic/Cretaceous boundary and intra-Berriasian and late Aptian unconformities) coeval with opening of the Bay of Biscay. These unconformities are contemporaneous with a major decrease of the subsidence rate. After an extensional period of subsidence (Albian to Turonian), NE–SW compression started in late Turonian time with major folding during the Late Cretaceous. The Tertiary was a period of very low subsidence in a compressional framework. The second folding stage occurred from the Lutetian to the Lower Oligocene (N–S compression) partly coeval with the E–W extension of the Oligocene rifts. Further compression occurred in the early Burdigalian and the Late Miocene in response to NE–SW shortening. Overall uplift occurred, with erosion, around the Lower/Middle Pleistocene boundary.

Influence of syntectonic sedimentation on thrust geometry. Field examples from the Iberian Chain (Spain) and analogue modelling

Abstract Steep thrusts are usually interpreted as oblique-slip thrusts or inverted normal faults. However, recent analogical and numerical models have emphasised the influence of surface mass-transfer phenomena on the structural evolution of compressive systems. This research points to sedimentation and erosion during deformation as an additional explanation for the origin of steeply dipping thrusts. The present study uses both field observations and analogue modelling to attempt to isolate critical parameters of syntectonic sedimentation that might control the geometry of the upper part of thrust systems. A field study of thrust systems bounding two compressive intermountain Tertiary basins of the Iberian Chain is first briefly presented. We describe the surface geometry of thrusts surrounding the Montalban Basin and the Alto Tajo Syncline at the vicinity of deposits of Oligocene–Early Miocene alluvial fans at the footwall of faults. Field observations suggest that synthrusting sedimentation should influence the structure of thrusts. Indeed, the faults are steeper and splitted at the edge of the syntectonic deposits. Effects of sedimentation rate on footwall of thrusts, and of its change along fault strike are further investigated on two-layer brittle-ductile analogue models submitted to compression and syntectonic sediment supply. Two series of experiments were made corresponding to two end-members of depositional geometries. In the first series, the sedimentation was homogeneously distributed on both sides of the relief developed above the thrust front. In the second series, deposits were localised on a particular area of the footwall of thrust front. In all experiments, the sedimentation rate controls the number and the dip of faults. For low sedimentation rates, a single low-angle thrust develops; whereas for high sedimentation rates, a series of steeper dipping thrust is observed. In experiments with changing sedimentation rate along fault strike, the thrust geometry varies behind the areas with the thickest sediment pile.

Paleoenvironment reconstructions and climate simulations of the Early Triassic: Impact of the water and sediment supply on the preservation of fluvial systems

Paleoenvironmental reconstructions and climatic modelling allow us to investigate the influence of water and sediment supply on the preservation of fluvial systems within a given geodynamic context. To simulate climate, we need global-scale paleoenvironmental and paleotopographic reconstructions. However, the present study only covers the West-Tethys domain, where sedimentological and stratigraphic data allow us to check climate simulation results against geological data. We focus our modelling on the Olenekian, with the aim of characterizing the impact of climate on fluvial sedimentation in the West-Tethys domain. The climatic simulations show that paleoclimates differ between Western Europe and North Africa. A more humid climate is simulated over North Africa, whereas a rather arid climate prevails over Western Europe. In Western Europe, the sediments are preserved for the most part in endoreic basins and the presence of rivers in an arid environment suggests that these rivers are mainly fed by precipitation falling on the North Africa Variscan Mountains. In North Africa, sedimentation is exclusively preserved in exoreic basins (coastal plain sediments). Consequently, the lack of preserved fluvial systems in endoreic basins in North Africa either could be due to a shortage of accommodation space in this area, or is linked to the climatic conditions that controlled the water and sediment supply.

The Permian-Triassic boundary and Early Triassic sedimentation in Western European basins: an overview

At the scale of the peri-Tethyan basins of western Europe, the “Buntsandstein” continental lithostratigraphic units are frequently attributed to the “Permian-Triassic” because, in most cases, the lack of any “Scythian” (i.e. Early Triassic) biochronological evidence makes it very difficult to attribute the basal beds of the cycle to the Permian or to the Triassic. A careful recognition of unconformities and sedimentary indications of clearly arid climate provide powerful tools for correlation within non-marine successions that are devoid of any biostratigraphic markers, at least on the scale of the West European Plate. From a review of the “Buntsandstein” series of several basins we can characterize the Permian-Triassic boundary and the beginning of Triassic sedimentation at the scale of Western Europe. We clearly show that, except for the Central Germanic Basin, an unconformity can be observed between the Permian and the Triassic. Apart from the Germanic Basin, there is a total lack of typically “Scythian” fossils in the rest of West European basins, and the oldest biochronological markers yielded by these units are palynomorphs allowing to assign an Anisian age generally to the upper part of the “Buntsandstein”, but also its lowermost in a few cases. In the peri-Tethyan basins of western Europe, the Permian-Triassic boundary corresponds to an unconformity overlain by conglomerates containing ventifacts (followed by fluvial sandstones, sometimes rich in paleosols and sometimes totally devoid), which are attributed mainly to the lower Olenekian, i.e. Smithian. Alternatively, the succession passes up directly into fluvial sandstones containing the first paleosols, and then plant debris and palynomorphs attributed to the Anisian. In this way, the lack of typically Early Triassic fossils in most of the peri-Tethyan basins, at the scale of the west European Plate, can be explained by a true stratigraphic hiatus in the earliest Triassic (i.e. Induan) and by arid conditions unfavourable for the development of flora and fauna and their preservation during the Olenekian.

Keuper stratigraphic cycles in the Paris basin and comparison with cycles in other peritethyan basins (German basin and Bresse-Jura basin)

Abstract High-resolution sequence stratigraphy of the Keuper, Paris Basin, is used to establish correlations between the basin-centre evaporite series and the basin-margin clastics series. The high-resolution correlations show stratigraphic cycle geometries. The Keuper consists of five minor base-level cycles whth occur in the upper portion of the Scythian-Carnian major base-level cycle and the lower part of the Carnian-Liassic major base-level cycle. The maximum relative rate of subsidence for the base-level fall phase of the Scythian-Carnian major cycle occurs in the eastern part of the Paris Basin. During the base-level rise phase of the Carnian-Liassic major cycle, the area of highest rate of subsidence shifted westwards and northwards. This shift records the first occurrence of an independent Paris Basin which was no longer merely the western margin of the German Basin. Two phases of tectonic movement influenced evaporite sedimentation and sequence geometries by creating areas of subsidence where halite could accumulate. The second, within the ‘Marnes irisees superieures’, induced a general westward and northward tilt of the basin. Concurrent migration of depocentres to the west and north produced an intra-‘Marnes irisees superieures’ truncation. Comparison of the stratigraphic records of the Paris Basin and of other Triassic Peritethyan basins (German Basin, Bresse-Jura Basin and South-East Basin) reveals numerous similarities. The coastal onlap curve of the German Keuper (Aigner and Bachmann, 1992) exhibits many similarities with the sequence evolution of the Paris Basin. But the Triassic succession is more complete in the German Basin and more cycles are observed. The major difference between these two basins during the Keuper is that the ‘Marnes irisees inferieures’ minor base-level cycle does not occur in the German Basin. In the Bresse-Jura Basin, the major difference concerns the Lettenkohle. One minor base-level cycle is recorded in the Paris Basin while no cycle is observed in the Bresse-Jura Basin.

Mio–Pliocene to Pleistocene paleotopographic evolution of Brittany (France) from a sequence stratigraphic analysis: relative influence of tectonics and climate

The Mio–Pliocene in Western Europe is a period of major climatic and tectonic change with important topographic consequences. The aim of this paper is to reconstruct these topographic changes (based on sedimentological analysis and sequence stratigraphy) for the Armorican Massif (western France) and to discuss their significance. The Mio–Pliocene sands of the Armorican Massif (Red Sands) are mainly preserved in paleovalleys and are characterized by extensive fluvial sheetflood deposits with low-preservation and by-pass facies. This sedimentological study shows that the Red Sands correspond to three main sedimentary environments: fluvial (alluvial fan, low-sinuosity rivers and braided rivers), estuarine and some rare open marine deposits (marine bioclastic sands: ‘‘faluns’’ of French authors). Two orders of sequences have been correlated across Brittany with one or two minor A/S cycles comprised within the retrogradational trend of a major cycle. The unconformity at the base of the lower cycle is more marked than the unconformity observed at the top, which corresponds to a re-incision of the paleovalley network. A comparison of the results of the sequence stratigraphy analysis with eustatic variations and tectonic events during the Mio–Pliocene allows (1) to discuss their influence on the evolution of the Armorican Massif and (2) to compare the stratigraphic record with other west-European basins. The unconformity observed at the base of the first minor cycle may be attributed to Serravallian–Tortonian tectonic activity and/or eustatic fall, and the unconformity of the second minor cycle may be attributed to Late Tortonian–Early Messinian tectonic activity. The earlier unconformity is coeval with the development of a ‘‘smooth’’ paleovalley network compared to the jagged present-day relief. A single episode of Mio–Pliocene deformation recorded in Brittany may be dated as Zanclean, thus explaining the lack of the maximum flooding surface except in isolated areas. From this study, five paleogeographic maps were drawn up also indicating paleocurrent directions: three maps for the lower cycle (Tortonian retrogradational trend, Late Tortonian to Early Messinian maximum flooding surface and Messinian progradational trend) and two for the upper cycle (Pliocene retrogradational trend and Piacenzian maximum flooding surface). These maps show (1) the variations of paleocurrent directions during the Mio–Pliocene, (2) the extent of estuarine environments during the maximum flooding intervals and (3) a paleodrainage watershed oriented NNW–SSE following the regional Quessoy/Nort-sur-Erdre Fault during the retrogradational trend of the upper cycle and possibly during the progradational trend of the lower cycle. The present-day morphology of the Armorican Massif is characterized by (1) incised

Echelles de temps et d'espace du controle tectonique d'un bassin flexural intracratonique; le bassin de Paris

The Meso-Cenozoic intracratonic flexural Paris Basin is a good tool to determinate the time/space-scales of the tectonic control on the sedimentary record. This study is based on basin-scale 2D sedimentary geometries of the different orders of stratigraphic cycles and on 1D/3D accommodation space measurements (space available for sediments created by subsidence and/or eustatism). Tectonic controls occur at least at three different time-scales: 230 m.y. (basin evolution), 10-40 m.y. (major stratigraphic cycles) and 1-5 m.y. (minor stratigraphic cycles). The evolution of the flexural intracratonic Paris basin can be described as the result of a long term thermal subsidence component with superimpositions of short-term tectonic components, due to intraplate deformations of 10-40 Ma and 1-5 Ma frequencies.

The Upper Jurassic–Lower Cretaceous alluvial-fan deposits of the Kalaza Formation (Central Asia): tectonic pulse or increased aridity?

Abstract The topographical evolution of tectonic systems, as well as the sedimentation pattern and depositional environments in the associated basins, are controlled by both tectonics and climate. In the region of the Tien Shan (Central Asia), the Jurassic–Lower Cretaceous period was marked by complex, low-intensity tectonic deformation and major climate changes from humid to arid conditions (Jurassic) to semi-arid conditions (Cretaceous). Using the sediment record in the Junggar, Tarim and Fergana basins to describe the tectonic evolution of the Tien Shan area during the Mesozoic thus requires differentiation between the tectonic and climatic influences on sedimentation. The conglomerates of the Upper Jurassic–Lower Cretaceous Kalaza Formation were commonly associated with renewed tectonic activity resulting from the docking of the Lhasa block along the southern margin of Asia. From sedimentology and sequence stratigraphy analyses of several sections in the Junggar, Tarim and Fergana basins, we reassess the main factors controlling the deposition of this formation. We show that, while some tectonic activity persisted throughout the Jurassic–Cretaceous transition, the switch from the sandy deposits of the Upper Jurassic Qigu Formation to the coarse deposits of the Kalaza Formation is largely linked to the development of an arid climate.

Mise en évidence d'un réseau de paléovallées ennoyées (paléorias) dans le Massif armoricain: une nouvelle interprétation des sables pliocènes armoricains

Abstract The sedimentological analysis of sandy patches of Upper Miocene-Pliocene age in the Armorican Massif (‘redonian’ red sands) once again calls into question their marine origin. Two types of fluvial systems have been characterized: braided channels associated with alluvial fans and low sinuosity channels. Marine environments are poorly developed; they correspond to inner estuarine deposits. Those facies are vertically organised into two base-level sequences; the most marine facies are located on top of these sequences. This stratigraphic organisation is typical of a fluvial palaeovalley network flooded by the sea.

Palynostratigraphic Data for the Buntsandstein and Muschelkalk Facies from the Iberian Ranges (Spain)

This work presents results of a compilation of all published palynological data as well as other unpublished data, on the basis of which a unified palynological biozonation is proposed for the Buntsandstein and Muschelkalk facies from the Iberian Ranges (Spain).