Burial diagenesis, organic maturation and tectonic loading in the French subalpine chains, S.E. France

Abstract

The French Subalpine Chains of southeastern France lie on the western edge of the Western Alps. The area is the foreland fold/thrust belt to the main orogenic hinterland to the east. The Mesozoic and Cenozoic geological history of the area is typical of many foreland regions, with a former gently subsiding passive margin altered to a destructive margin through compressional tectonics and ocean closure. This thesis details the results of a case study into particular aspects of the sedimentology and basin history of the Subalpine Chains fold/thrust belt. The two main aspects to this thesis are burial diagenesis and the causes of palaeo-temperature variation within the fold/thrust belt. A principal aim of this research was to document the burial diagenetic history of the mid-Cretaceous rudist-bearing 'Urgonian' limestone. Similar Cretaceous rudist platform limestones elsewhere in the world are good reservoir rocks. Often their good porosity is due to the selective dissolution associated with the bi-mineralic rudist shells. Although later cement precipitation often has modified this. Regional dolomitization of platform carbonates has also been known for a long time to increase the reservoir potential of such sediments. In the Urgonian, fabric selective secondary porosity, equant burial cements and dolomitization are important components of the platform's diagenetic history. The petrography and geochemistry of rudist shells, the equant calcite cements responsible for occluding the majority of porosity and replacive dolomitization within the basal section of the Urgonian platform are described. On the basis of the field, petrographic and geochemical data, a diagenetic model for the Urgonian is proposed. The second principal aim was to study the regional variations in maximum burial temperatures and depths. To this end several palaeo-thermal indices were studied (clay minerals, vitrinite reflectance and spore fluorescence) and the thermal history of the region is modelled using a PC-based modelling package (Basinmod). Regional trends in thermal indices are presented and quite clearly indicate an along strike (of the foreland fold/thrust belt) variation in burial temperatures, with a less important across strike trend. A simple thermal history model is used to estimate the maximum depths of burial of specific locations and the results compared to measured values of maturity. Both the modelling and field relationships suggest that major overthrust sheets emplaced only in the northern part of the study area are the principal cause of the greater depth of burial of that part of the Subalpine Chains. The causes of thermal perturbations in fold/thrust belts are discussed. A final third part of this thesis details the results of a pilot study of the geochemistry of syn-tectonic veins. The petrography and outcrop nature of these veins indicate that cyclic hydraulic fracturing (the so-called "crack-seal mechanism") was very important in producing fractures within the rocks during Alpine deformation. These were healed by calcite cements, the stable isotope composition of which indicates the significance of water/wall rock interaction and the temperature of the precipitating fluids on the isotopic composition of the precipitate. Taken as a whole the geochemistry strongly suggests that syn-tectonic fluid flow was highly channelised.