Ancient fluvial deposits typically display repetitive changes in their depositional architecture such as alternating intervals of coarse-grained highly amalgamated (HA), laterally-stacked, channel bodies, and finer-grained less amalgamated (LA), vertically-stacked, channels encased in floodplain deposits. Such patterns are usually ascribed to slower, respectively higher, rates of base level rise (accommodation). However, “upstream” factors such as water discharge and sediment flux also play a potential role in determining stratigraphic architecture, yet this possibility has never been tested despite the recent advances in the field of palaeohydraulic reconstructions from fluvial accumulations. Here, we chronicle riverbed gradient evolution within three Middle Eocene (~ 40 Ma) fluvial HA-LA sequences in the Escanilla Formation in the south-Pyrenean foreland basin. This work documents, for the first time in a fossil fluvial system, how the ancient riverbed systematically evolved from lower slopes in coarser-grained HA intervals, and higher slopes in finer-grained LA intervals, suggesting that bed slope changes were determined primarily by climate-controlled water discharge variations rather than base level changes as often hypothesized. This highlights the important connection betweenclimate and landscape evolution and has fundamental implications for our ability to reconstruct ancient hydroclimates from the interpretation of fluvial sedimentary sequences.
<p>The geological evolution of the Mozambique Channel sensu largowas controlled by three major transfer zones (Davie, Mozambique and Agulhas/Falklands), (1) related to the migration of four continents (Africa, Madagascar, Antarctica, South America), (2) recording five major volcanic episodes from 186 Ma to today and (3) contemporaneous of the uplift of several plateaus (e.g. Southern African Plateau), affecting a quite heterogeneous lithosphere of Archean to Neoproterozoic ages. This is therefore a unique area for a better understanding of (1) the evolution of transform margins in a volcanic setting and (2) the relationships between deformation, relief growth and sediment routing evolution. We established in the frame of the PAMELA (Passive Margin Experiment LAboratory) project (TOTAL, IFREMER, CNRS) a chart and nine paleogeographic maps (with tectonic structures, magmatism, catchments and sediment routing system) to better constrain the timing of evolution of this domain. The main results are as follows:</p><p>(1) 255-240 Ma: a first E-W extension (Karoo &#8220;Rifts&#8221;) with no ocean opening;</p><p>(2) 185-160 Ma: a second NW-SE extension between Antarctica/Madagascar and Africa coeval of the Karoo Large Igneous Province and initiation of volcanic margins along the future Somali Ocean;</p><p>(3) 160-145 Ma: major change of the plate migration toward a N-S extensional initiation of very oblique margins along the Mozambique Fracture Zone (FZ), indicating an Antarctica motion toward SSE;</p><p>(4) 134 Ma: onset of the migration of the Falkland continental domain along the Agulhas FZ;</p><p>(5) 115 Ma: major deformation of the four plates with (i) end of the southward migration of Madagascar and (ii) major inversion along the Davie FZ (initiated around 135 Ma) and uplift of Madagascar;</p><p>(6) 92-70 Ma: uplift of the Southern Africa Plateau first eastward (92 Ma) and second westward (81-70 Ma);</p><p>(7) 40 Ma: onset of the uplift of the Zimbabwe/Zambia/Malawi Plateaus, East African Dome and Madagascar Plateau &#8211; last uplift of the Southern African Plateau;</p><p>(6) 11-5 Ma: acceleration of the uplift of the Zimbabwe/Zambia/Malawi Plateaus and East African Dome &#8211; growth of a dome crossing the Mozambique Channel from Madagascar to southern offshore of the Limpopo Plain.</p>
<p>The knowledge acquired on the exhumation of the Pyrenean mountain belt and the evolution of the adjacent foreland basins makes this Alpine-type domain a good laboratory to better constrain a full sediment routing system in a compressive context and to apprehend the driving processes controlling the sediment routing in space and time. This integrated approach aims at enhancing our basin mastering approach as well as improving our predictions of reservoir properties.</p><p>This Source-to-Sink study seeks to understand the evolution of sedimentary routing from the Source (orogenic relief, craton, basin recycling) through the transfer zone (peripheral or internal to the basin) to the final sink (flexural basin, deep turbiditic margin). Within this new cartography, we propose to compile the data over the entire peri-Pyrenean domain. We produced large scale quantitative and qualitative maps to better observe and interpret the tectonic, climatic and surface processes impacts of the SRS behavior.</p><p>These maps include kinematic reconstructions of the Iberian-European-Mediterranean system, restored sequential cross-sections, history/magnitude of exhumation by thermochronology, source tracking, characterization of weathering and erosion surfaces, synthesis of the major structures activity, paleogeographic reconstructions, analysis of sedimentary geometries and transport directions as well as the quantification of volumes preserved in the basins. Their interpretation is combined with a time representation along the routing system, linking classical basin wheeler diagram representation to source erosion and lithologies to obtain a continuous view on the sediment journey.</p><p>The time steps chosen for these 5 maps account for the different stages of tectono-sedimentary evolution of the peri-Pyrenean system at the early-, syn- and post-orogenic stages. The compilations carried out compare exhumed domains and sedimentation zones in terms of fluxes and volumes and make it possible to map the routing systems and point out the main drivers for the surface evolution during the construction/destruction cycle of an orogen. This research work was financed and carried out as part of the BRGM-TOTAL Source-to-Sink program</p>
<p>Transform continental margins known across the Earth represent 31% of passive margins. Resulting from first-order plate tectonic processes, transform margins record a diachronous evolution mainly defined by three successive stages, including intra-continental transform faulting, active and passive transform margin. Due to their high complexity and a lack of large hydrocarbon discoveries (i.e. not a target for oil industry), they have only been sparsely studied, especially when compared with other margin types (i.e. divergent or convergent).</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; We present the structure and evolution of the NS-trending Limpopo Transform Fault Zone (LTFZ), corresponding to the main fracture zone from western part of the Africa-Antarctica Corridor (AAC). Here, we combine published and unpublished dataset (seismic reflection profiles, wells, multibeam bathymetry, gravity, magnetic data) in order to propose an interpretation of the LTFZ structure and adjoining segments and their evolution through time, from rifting to spreading.</p><p>The LTFZ is composed of two main segments: the East Limpopo segment and the Astrid conjugate one and the North and South Natal segment including the Dana-Galathea Plateau (Mozambique side) and the Maud rise/east of Grunehogna craton (Antarctica margin). The LTFZ offsets the segments of divergent conjugate margins (Southern Natal-off Grunehogna craton in the west and Beira High Angoche-Riiser Larsen Sea in the east) since&#160;155 Ma (chron M25). We focus on the evolution of the transform fault zone from its initiation at chron M25 up to chron M0 (~126 Ma, Barremian). Oceanic spreading onset at chron M25 in the south of Beira High segment and Dana-Galathea Plateau triggered the uplift and erosion of the proximal parts of the margin and the formation of several seaward dipping reflectors wedges. Plate kinematic implies an NNW-SSE opening of the LTFZ. The oblique component of opening promotes the setting up of several volcanic wedges. These wedges rejuvenate southward trough time, which is consistent with the sliding of Antarctica with respect to Africa and thus confirm the diachronous evolution of the transform fault zone.</p>
Abstract 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.
<p>The Jurassic-Cretaceous boundary corresponds to a major step in the Gondwana dispersal. The deformation regime indeed changed from localized, along the incipient ocean (Atlantic Tethys, Somali-Mozambique Ocean), to a highly distributed deformation along several rifts spanning from India to southern South America through Africa including Arabia. The last step of extension is marked by a major unconformity of Late Aptian in age known, since the pioneering work of Edward Suess at the end of the nineteenth century, as the Austrian Unconformity that corresponds to a world-scale plate kinematic reorganization.</p><p>We compiled a new map of the Early Cretaceous (Berriasian-Aptian) rifts in Africa and austral South America with a particular emphasis on southern Africa and the Falkland-Malvinas plateau:</p><p>At middle wavelength (few tens of kilometers) deformation scale, this Late Aptian event may have stopped the rift regime, corresponding to the transition to a sag setting (Chad and Sudanese rifts), and/or reactivated basement structures (e.g. neoproterozoic faults in the Illizi and Ghadames basins in southern Algeria and Libya). In the central segment of the future South Atlantic Ocean, Late Aptian corresponds to the end of the hyperextension period and the onset of the passive margin coeval with salt deposition.</p><p>At a longer wavelength of deformation (several hundreds to thousand of kilometers), the highlighted deformation regime may have changed regional subsidence pattern with for example the overall subsidence of northern Africa and the onset of large marine floodings (e.g. deposition of Nubian sandstones).The Late Aptian unconformity therefore records a major change in the stress within the African plate likely related, considering the scale of deformation, to a reorganization in mantle convection processes.</p>
