Sur l’origine tectono-karstique et l’âge cénozoïque, tardi (?) et post tectonique, du marbre de Sarrancolin (Hautes-Pyrénées)

Abstract

In the Sarrancolin area (Hautes-Pyrénées) the folded Cenomanian limestones provide, since the roman period, a well known marble specially exploited in the open cuts of Beyrède-Jumet and Ilhet. These carbonate rocks are located in a vertical position between the Camous Permo-Triassic cover of the High Pyrenean range to the south and the Jurassic-Lower Cretaceous limestones of the Internal Metamorphic Zone to the north. The marble consists mainly of coloured karst infillings within the massive or brecciated Mid- Cretaceous carbonates. These materials have been recently considered as marine Turonian sediments located at the top of the Cenomanian formation. Our observations lead us to different conclusions concerning the same the structure as the origin and the age of the Sarrancolin marble. Breccias and coloured infillings which characterize the Sarrancolin marble are absent in the lowermost outcrops of the vertically-bedded Cenomanian limestones, near the Neste river. They appear in a higher position, on both sides of the valley, where the Beyrède-Jumet and Ilhet quarries can be found. The Ilhet working face shows massive or fractured and brecciated limestones crossed by centimetric to decimetric (rarely metric) cracks filled up by coloured microbreccias, carbonates and clays. These fractures have been observed, not only on the top of the limestones but also across the whole carbonate series. They are organized into a complex network which mesh is less and less developed downwards. This organization gives evidence for brecciation and fracture infillings subsequent to the tectonic processes which led the carbonate beds to their vertical position. Such structural conditions didn’t exist during the Mid-Cretaceous interval. The carbonate breccias are interpreted as monogenic raw materials related to hydraulic fracturing in subsurface karst conditions. In the Ilhet quarry, the fractures which cross the limestones and the breccias are closely linked to surface karst processes. Their filling deposits consist of: 1) beige to brown, gravelly or microbrecciated beds showing frequently a graded bedding organization; 2) yellow to ochre thin laminated mudstones; 3) red argilaceous spreadings. These deposits are involved into small, more or less complete, prograding sequences. In some places, the fractures are crossed by younger pipes which can rework their infilling deposits. The still working Beyrède pits show coloured infillings mainly made up of laminated mudstones and clays. These materials indicate continental exokarstic conditions which can involve rainy periods leading to gravelly or brecciated spreadings and fine weather times inducing the deposit of clays and laminated limestones. Iron which gives these sediments their typical red, blood-like colour has been probably provided by the leached Triassic sandstones which form the bordering outcrops. So, in our opinion, the supposed marine origin of these sediments must be rejected. Closely related to the recent topographic surface, the breccias and the brecciated, carbonate or clay fracture infillings which cross the vertical Cenomanian formation are obviously post-tectonic in age. The Mid-Cretaceous bedded limestones have been straightened up during the main Pyrenean folding phase, towards the Middle to Upper Eocene. So, the Sarrancolin marble formed later on, during the Oligocene and Neogene period. In our opinion, it must be involved in the fracturing and karst brecciated zone which developed along the Pyrenees during and after the continental collision between Europe and Iberia.