Abstract
The nature and the geometry of Mesozoic deposits in the Ardeche region reflect the different stages of structural evolution of the western extensional margin of the Subalpine Basin. Through stratigraphic analysis of a broad transect passing from the Uzer block, through the GPF (“Géologie Profonde de la France”) borehole Balazuc 1 and two petroleum exploration wells on the different blocks of the margin, it has been possible to integrate data from borehole BAI in a sequential stratigraphic and regional dynamic framework, and to attempt to specify the modes of tectonic control on the organization of the deposits along a profile of this extensional palaeomargin. The Triassic and Jurassic deposits are composed of six major transgressive-regressive cycles for which the geometric and stratigraphic characteristics indicate that they were controlled by the tectonic evolution of the basin, and more specifically by variations in the type and rate of subsidence. The first two cycles, Anisian-early Sinemurian in age, correspond to the initial structural creation of the margin with a west-east sedimentary polarity. Extensional tectonism, which affected the Palaeozoic basement, in part controlled the duration and intensity of the transgressive phases, with the regressive phases reflecting a decrease in regional subsidence. The third and fourth cycles, early Sinemurian-middle Bathonian in age, were contemporaneous with the Tethyan rifting. In particular, they record evidence of decreasing regional subsidence, and even uplift, associated with block faulting. This caused strong tectonic decoupling and highly variable subsidence along the margin profile; episodes that are represented by that part of each sedimentary cycles deposited around the maximum regression, i.e. end of the regressive phase and beginning of the transgressive phase (Sinemurian and Aalenian). A regional phase of subsidence followed which allowed maximum expansion of the transgression (Toarcian and Bajocian transgressive peaks). The fifth and sixth cycles, middle Bath on ian-Kimmeridgian in age, were deposited during a period of widespread subsidence of the margin that was synchronous with the onset of Tethyain oceanic spreading. They preferentially record episodes of accelerated subsidence giving rise to transgressive phases generally associated with a synsedimentary tectonic activity that incurred relatively heterogeneous subsidence across the entire palaeomargin (late Bathonian and early Oxfordian). The regressive phases are subsequently marked by a decreased, but uniform subsidence with no evidence of uplift. The sediments of these last two cycles were deposited in a post-rift context and are liable to record effects of geodynamic events outside the basin. Characterization of the different stages in the evolution of the Ardeche palaeomargin by their stratigraphic expression provides geological constraints for modelling fluid migration throughout this extensional margin, the objective of the 'GPF Ardeche' project.
Published Version
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