Relationships between tectonics and sedimentation on the northeastern margin of the Subpyrenean trough during the late Santonian
Résumé
In the eastern part of the Aquitaine Basin and to the South of the Toulouse high, the Subpyrenean trough is a narrow trench oriented N11 degrees E to N130 degrees E. The deposits on the northeastern side of this depression are preserved in the autochthonous Mesozoic cover of the Variscan Mouthoumet Massif, but also in the parautochthonous or allochthonous tectonic units that fringe to the north (Camps - Peyrepertuse slice, fig. 2) the North Pyrenean frontal thrust. From the Middle Cenomanian to the Lower Santonian included (96 to 85 Ma ago), the sedimentation in the Mouthoumet Massif indicates shallow marine carbonate or mixed (carbonate to terrigenous) conditions. The different facies depend mainly on two parameters : the variations of the accommodation space for sedimentation and the location of the numerous rudist buildups. The deposits are first organized in a homoclinal ramp until the Turonian. From the Coniacian up to the early Santonian, drowned platform patterns prevail. During the late Santonian and more precisely around 85 Ma with an other event around 84 Ma, the Mouthoumet Massif and its cover broke up under tectonic stresses. Positive and negative topographies reactivate the Variscan fault system. Platform - slope/basin morphologies substituted the preceeding ramp and drowned platform morphology. Looking to the south and in the direction N120 degrees E, the distal slope received gravitational and turbiditic sediments called the Gres de Labastide (fig. 7). The sediment supply shifted from north to south and from cast to west. To the north of this slope, the platform itself broke LIP into a mosaic of rhomboedric blocks, leading, to a graben and horst morphology. Those units are clearly different according to the character of their sedimentary facies. deltaic or reefal (Montagne des Cornes, Calcaires de Camps - Peyrepertuse). The detailed stratigraphic and sedimentologic studies of some of these systems reveal a tectono-sedimentary evolution involving two successive cycles Ss I (lower Upper Santonian) and Ss2 (Uppermost Santonian). In the western part of the Mouthoumet Massif this cyclic evolution is recorded from south to north, on the Parahou slope. the Rennes-les-Bains graben and the Bugarach horst. The lower cycle Ss1, located on the Rennes-les-Bains graben. is approximatively 85 Ma to 84 Ma in age. It starts with reworked deposits (lowstand systems tract) made up of sometimes several m elements derived from former sedimentary deposits (from TUronian up to Lower Santonian) even when the same deposits are in place on the adjacent horsts (e.g. the eastern horst of Bugarach). Those reworked deposits fill the bottom of the graben, principally in the transit zones (debris-flows of the Conglomerat de la Ferri re), or in the Parahou slope (slumps and debris-flows of the Cascade des Mathieux); then the deltaic complex of Rennes-les-Bains covers the older chaotic deposits; the blue marls and the overlying sandy facies (transgressive and highstand systems tracts) related to prodelta and deltafront deposits represent the infilling of the Rennes-les Bains graben. The upper cycle Ss2 developed probably between 84 Ma to 83,5 Ma; its geographical extension overlaps the limits of the lower cycle (e.g. the Bugarach horst), but its sedimentary organisation is still the same including: on the Parahou slope debris-flow and intrabasinal reworking (Conglomerat des Gaseous: lowstand systems tract); on the northern platform transgressive and highstand systems tracts, present in the Montague des Comes delta where the Marries bleues de Sougraigne represent the prodelta deposits, and the terrigenous and rudist buildups of the delta front deposits (fig. 7). The final infilling results from the spreading from NE to SW, of the (estuarine ? to) fluvial deposits of the Gres d'Alet Formation at around 83 Ma. In the eastern part of the Mouthoumet Massif, sedimentary development is punctuated by tectonic events. Nevertheless, it is possible to identify in some outcrops the main elements of the two tectono-sedimentary cycles. - The cycle Ss1 is partly preserved in the genetic sequence which links the Calcaires de Camps-Peyrepertuse (shelf margin wedge systems tract) and the Marnes du Pla de Sagnes (transgressive systems tract). The cycle Ss2 is only known through different facies of the Gres de Labastide Formation: reworked deposits on the slope; coarse-grained silicoclastic deposits on the transit zones. - In the cycle Ss1 differences appear between the western and the eastern parts of the Mouthoumet massif. When in the western area deltaic conditions prevailed, in the eastern area a shallow carbonate and buildup facies developed. Such differences disappear in the cycle Ss2 by the general establishment of fore slope deltaic deposits. The geodynamic reconstruction resulting from plate kinematics indicates a major change between the early Coniacian (89 Ma) and the Middle Campanian (79 Ma), when the sinistral/divergent motion of Iberia with respect to stable Europe turned to a dextral/convergent movement. The tectono-sedimentary events presented here took place during this period (85 Ma to 83 Ma). The tectono-sedimentary evolution of the subpyrenean trough and the shift of the European and Iberian plates are thought to be intimately linked. The new chronological and geodynamical data proposed herein show that the genesis and the evolution of the subpyrenean sedimentary processes related to the northern Aquitanian margin of the Subpyrenean trough allow to draw some basic conclusions: - the opening of the Subpyrenean trough occurred in two steps, the first around 85 Ma and the second around 84 Ma; - this caused a change in the sedimentary setting with platform environments replacing the earlier ramp geometry; - the Subpyrenean trough formed and evolved under transtensive tectonic conditions; - during the late Santonian two tectono-eustatic sequences marked the former stages of the eastward opening and infilling of this basin; - the diachronous infilling which began here around 83,5 Ma prograded to the western Plantaurel and Petites-Pyrenees area; - no significant northward shifting of the depositional-axis of the Senonian basins occurred; - only a gradual westward shift of the depositional centers, along the subpyrenean direction of the slope area (N110 degrees E to N130 degrees E) was noticed.