François Guillocheau

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.

Découverte d’un dépôt paralique à bois fossiles, ambre insectifère et restes d’Iguanodontidae (Dinosauria, Ornithopoda) dans le Cénomanien inférieur de Fouras (Charente-Maritime, Sud-Ouest de la France)

Resume Un gisement paralique inedit, d’âge Cenomanien inferieur, avec du bois fossile, de l’ambre insectifere et des restes de vertebres, a ete decouvert sur l’estran de la presqu’ile de Fouras (Charente-Maritime, France), a la suite d’une tempete qui a temporairement ote du littoral les nappages de cordons sableux et de vasieres. L’assemblage de bois fossiles contient trois taxons de coniferes ( Agathoxylon , Podocarpoxylon , Brachyoxylon ) et un Ginkgoxylon . Les insectes de l’ambre correspondent a des Dipteres, des Hymenopteres et des Homopteres. Les restes de vertebres sont principalement representes par des carapaces de tortues terrestres (Solemydidae), des vertebres de serpents ( Simoliophis ) et des ossements de dinosaures, appartenant probablement au genre Iguanodon . Pour citer cet article : D. Neraudeau et al., C. R. Palevol 2 (2003).

Large‐scale relief development related to Quaternary tectonic uplift of a Proterozoic‐Paleozoic basement: The Armorican Massif, NW France

The topography of basements located in intraplate domains has often been interpreted in terms of planation theory by examining the remnants of paleosurfaces. In this paper, we address the significance of the present-day drainage networks that erode such topographies by looking at the relief development on the Armorican Massif basement (northwestern France). Using digital elevation model (DEM) analysis, the topography of this massif is characterized by large-scale (∼250 km) relief variations that define high elevation domains (HEDs). These domains control the pattern of drainage networks by setting the location of the main drainage divides and the polarity of the regional slopes. Large-scale relief is strongly associated with the existence of scarps along inherited fault zones, suggesting that relief development is mainly controlled by tectonics through fault reactivation. A surface evolution study shows that drainage networks developed during the Quaternary, which corresponds to the timing of the tectonic activity which also led to relief development. The study of Quaternary fluvial erosion through a new method for measuring incision shows that the drainage basins of the HEDs record greater amounts of base level fall during Quaternary times. This implies that differential uplift is responsible for the large-scale relief development of the Armorican Massif during the Quaternary. Our study suggests that as seen in active settings, the topography of basements corresponds to a dynamic signal between tectonics and erosional processes which can be used, for example, in the study of intraplate deformation.

Rapid South Atlantic spreading changes and coeval vertical motion in surrounding continents: Evidence for temporal changes of pressure-driven upper mantle flow

The South Atlantic region displays (1) a topographic gradient across the basin, with Africa elevated relative to South America, (2) a bimodal spreading history with fast spreading rates in Late Cretaceous and Eo-Oligocene, and (3) episodic regional uplift events in the adjacent continents concentrated in Late Cretaceous and Oligocene. Here we show that these observations can be linked by dynamic processes within Earths mantle, through temporal changes in asthenosphere flow beneath the region. The topographic gradient implies westward, pressure-driven mantle flow beneath the basin, while the rapid spreading rate changes, on order 10 million years, require significant decoupling of regional plate motion from the large-scale mantle buoyancy distribution through a mechanically weak asthenosphere. Andean topographic growth in late Miocene can explain the most recent South Atlantic spreading velocity reduction, arising from increased plate boundary forcing associated with the newly elevated topography. But this mechanism is unlikely to explain the Late Cretaceous/Tertiary spreading variations, as changes in Andean paleoelevation at the time are small. We propose an unsteady pressure-driven flow component in the asthenosphere beneath the South Atlantic region to explain the Late Cretaceous/Tertiary spreading rate variations. Temporal changes in mantle flow due to temporal changes in regional mantle pressure gradients imply a correlation of horizontal and vertical motions: we find that this prediction from our models agrees with geologic and geophysical observations of the South Atlantic region, including episodes of passive margin uplift, regional basin reactivation, and magmatic activity.

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.

Rapid erosion of the Southern African Plateau as it climbs over a mantle superswell

We present new sedimentary flux data confirming that a large pulse of erosion affected the Southern African Plateau in the Late Cretaceous and is likely to be related to a major uplift episode of the plateau. This short phase of erosion (i.e., less than 30 Myr in duration) has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift: given its size, the rise of the African superplume is likely to have lasted much longer. Here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent-wide drainage reorganization, leading to substantial denudation in a relatively short amount of time. The amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the Karoo sedimentary and volcanic rocks and the underlying basement. We also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse, suggesting that climate may have also played a role in triggering the denudation. The diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are, however, better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling.

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.

Relative uplift measured using river incisions: the case of the armorican basement (France)

River incision is a potential tool to quantify uplift of continental areas. However, river incision measurements using valley depths are difficult because of their variability at the catchment scale. We present a new method to quantify incision, by measuring valley depth according to their width, taking into account all the valleys in each catchment. Systematic differences in valley depths between catchments are quantified, related to relative uplift between them. An application on Brittany makes it possible to quantify a relative uplift of 30 m between two catchments during Quaternary times.

Les vallees fossiles de la baie de la Vilaine; nature et evolution du prisme sedimentaire cotier du Pleistocene armoricain

The study of a dense network of high resolution seismic profiles in the bay of Vilaine, INSU-CNRS cruise Geovill, have led to the characterization of the architecture of the sediment wedge preserved between the coast and the 50 m isobath. This wedge lies on a substratum composed of three seismic units, U1, U2 and U3 respectively attributed to metamorphic and magmatic rocks, Lutetian and Ypresian sandy carbonates and post-Eocene sediments. The coastal sediment wedge comprises three major units. A basal unit (U4), dated around 600 to 300 ky BP, interpreted as braided river sandy conglomerates. A median unit (U5) corresponding to estuarine and fluvial sandstones and clays that give way to the west to mouth bar sandstones. A sommital unit (U6) attributed to marine argillites and barrier island sandstones dated from 8110+ or -200 years at the base. These three units are bounded by two major surfaces: an unconformity between U4 and U5 and a marine (wave and tidal) ravinement surface between U5 and U6. The unconformity is interpreted as a sequence boundary between two depositional sequences: a lower one with U4 seismic unit and a topmost one with U5 and U6 seismic units. Based on the available datations, the lower sequence is attributed to the Saalian and/or Elsterian glacial cycles and, the upper sequence to the Weichselian (lowstand systems tract) and to the Holocene marine transgression (transgressive systems tract). The passage from one sequence to the other corresponds however to a drastic shift in the paleoflow directions (60 degrees ) in the Bay of Vilaine closely related to the main faults orientations. The tectonic activity in Brittany during the Pleistocene, linked to intraplate stress, seems to exert a control on sediment architecture in the coastal wedge. Indeed, the tilt of the Armorican Massif during that period has caused a complete rejuvenation of the fluvial profiles in land and the separation of the paleo-Vilaine from the Paleo-Loire river courses.

A better-ventilated ocean triggered by Late Cretaceous changes in continental configuration

Oceanic anoxic events (OAEs) are large-scale events of oxygen depletion in the deep ocean that happened during pre-Cenozoic periods of extreme warmth. Here, to assess the role of major continental configuration changes occurring during the Late Cretaceous on oceanic circulation modes, which in turn influence the oxygenation level of the deep ocean, we use a coupled ocean atmosphere climate model. We simulate ocean dynamics during two different time slices and compare these with existing neodymium isotope data (ɛNd). Although deep-water production in the North Pacific is continuous, the simulations at 94 and 71 Ma show a shift in southern deep-water production sites from South Pacific to South Atlantic and Indian Ocean locations. Our modelling results support the hypothesis that an intensification of southern Atlantic deep-water production and a reversal of deep-water fluxes through the Caribbean Seaway were the main causes of the decrease in ɛNd values recorded in the Atlantic and Indian deep waters during the Late Cretaceous.