Abstract

In the South Aquitain sector, salt bodies of varying sizes present two geographical gradients. The first gradient, south to north, associated with the proximity of the Pyrenees shows a decreasing deformation of the structures towards the north. The second gradient explains their increasing depth of burial from the west (extrusive structures) to the east (intrusive structures). This organization has been studied from the western sector (Tercis-Bénesse and Bastennes-Louer) to the eastern sector (Viella-Labarthète and Nogaro-Lussagnet). Other more modest structures, revealed by hydrographical anomalies, illustrate halokinetic events during the Quaternary. Then the detailed study of the Geaune dome shows a neotectonic phenomenon dating from the Mio-Pliocene. The radial annular drainage pattern of this structure is guided by recurrent faulting during the Quaternary. Thus the halokinetic dynamics consist in three phases: deformation of the diapirs (overthrusts) during the Oligocene, resuming of halokinesis during the middle of the Miocene and recurrent faulting during the Riss. A model of geomorphological evolution of the domes is then proposed. The first phase (stages of positive relief) is characterized by the centrifugal drainage pattern of the dome. The second phase (stages of inverted relief) is distinguished by the erosion of the center of the structure, by an inversion of the drainage pattern and by the sedimentary accretion in the central depression.

Regional frame of the southern-Aquitaine diapirism. – Figure 1 shows the main saliferous bodies. They present a double geographical gradient. The first, S-N, is linked to the Pyrenean tectonics. On the south of the Frontal Overthrust the Trias participated in the overthrusts (St-Jean-de-Pied-de-Port Basin, St-Palais and Salies-de-Béarn structures, etc…). Then in the Outer Sub-Pyrenean zone, the salt forms the axial part of the E-W Pyrenean structures ; finally in the north, in the Aquitaine basin, the diapirs are only slightly deformed. The second gradient, E-W, is linked to the Gulf of Gascogne dynamics where the height of the salt structures decreases from west to east. In Aquitaine, diapirism is a nearly continuous event that has followed the different stages of the history of the basin. The evaporites are deposed in Keuper (Initiation = rifting). The real opening begins in the Albian with the initiation of the turbiditic basins. The main compression begins in the Palaeocene. During the Miocene, the tectonic movements take over with a left lateral transpression on direction N110oE conjugated with recurrent faults N140. Diapirism is reactivated. Certain structures come to the surface. From this rapid historic overview we must retain not only the permanence of diapirism but also the interactive movements between the ascending domes and the subsiding of compensation synclines

Evolution of salt structures from West to East. – The cases studied correspond to megastructures (anticlines N140, N090 and multi-kilometric domes) and to the extrusive or intrusive mesostructures (kilometrical domes) which are organized on the ancient Triassic Armorican fault trough. The fact that a E-W global gradient exists for these salt structures is worthnoting: in the eastern sector, the domes are covered with 800 m to 2 000 m of molasse (Lussagnet, Nogaro, Viella-Labarthe, and Geaune) ; coexisting in the western sector are structures of varied forms with an exposed (Tercis, Bénesse, Bastennes) or sub-exposed (Dax) Triassic core. The presence of numerous drainage anomalies (annular and radio-annular drainage patterns) in the east as well as in the west reveals the existence of more modest deep structures. Their morphological expression on the surface shows the continuance of salt deformations during the Quaternary.

An exemplary structure: the Geaune salt dome. – It is situated on a trend oriented N060oE. This “ ripple ” has existed since the Upper Jurassic. In fact it concerns an ancient Triassic rift where evaporites accumulated. Afterwards, this Upper Trias deposit underwent a diapiric process which spanned from the Jurassic (middle ? Upper ?) until the present.

Geomorphological organization. – The sub-circular structure corresponds both to the valley or dale floor and to flat or convex interfluves. The organization of the drainage shows anomalies characterized by linear and curved structures. Several lineaments in a N140 direction are visible on the eastern half of the dome. They appeared as parallel drains aligned on both sides of the Marcusse brook. Sub-circular structures clearly appear on the sectors A and B of the eroded dome. These curved lineaments are arranged in arcs inside and following the form of the dome. This corresponds to the drainage patterns of the sedimentary domes or exposed diapirs. In the case of the Geaune dome, it concerns a type of complex radio-annular drainage pattern for which the radial drains are largely controlled by the recent fracturing.

Dynamics. – Since the Miocene, the history of the Geaune sector has integrated the evolution of the entire gascon piedmont with the deposing of continental sediments (Upper Cenozoic) generating a higher surface followed by, from the Quaternary, the development of the drainage pattern, the formation of present relief and the deposition of alluvial terraces.

The diapiric dynamics since Oligocene. – In the Pyrenean foreground, the Oligocene is the paroxysmal phase. The diapiric structures already formed are picked up in the overthrusting ramps (Tercis, Audignon, Riscle).

Mio-Pliocene. – In Gascogne, the N140 faults move (in extension ?): they favour a renewal of diapirism which deforms the middle Miocene deposits (Bastennes and Geaune). An inversion with detumescence of the domes then occurs ; the whole is enclosed by the truncation of the higher surface.

Quaternary. – The diapir takes over weakly and becomes more pronounced during the Riss. It is associated with toppled boulders orientated N140. The motor is the mobility of the Gulf of Gascogne which participates in the rotation of the Pyrenean rivers towards the west. The inversion occurs at the end of the Riss with a centripetal drainage on the Geaune dome.

A type of original model: the inversion ring. – In the northern Pyrenean piedmont, intrusive and extrusive saliferous domes coexist. The analysis of the surface morphostructures which are generated by the rising of these Triassic elements makes the proposal of a detailed model of evolution possible (fig.13). There are five stages regrouped in two phases. In the course of phase I (stage I of positive relief) the drainage is principally centrifugal and radial. During phase II (inversion stages 3, 4 and 5) the center of the dome grows and centripetal radial drains form. The final stage is marked by the filling of the depression. As well as the interest given to the evolution of these structures, the analysis of drainage pattern anomalies and of the forms constitutes an efficient tool of reconnaissance permitting the illustration of, or confirming the presence of deep intrusive saliferous domes.

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