STRATIGRAPHIC RECORD OF THE STRUCTURAL EVOLUTION OF THE WESTERN EXTENSIONAL MARGIN OF THE SUB-ALPINE BASIN DURING THE TRIASSIC AND JURASSIC, ARDECHE, FRANCE

The nature and the geometry of Mesozoic deposits in the Ardeche region reflect the different stages of structural evolution of the western e xtensional margin of the Subalpine Basin. Th rough stratigraphic analy sis of a broad transect passing from the Uzer block, through the GPF ( ''Geologie Profonde de la France'') borehole Balazuc 1 and two petrole um exploration wells on the different blocks of the margin, it has bee n possible to integrate data from borehole BAI in a sequential stratig raphic and regional dynamic framework, and to attempt to specify the m odes of tectonic control on the organization of the deposits along a p rofile of this extensional palaeomargin. The Triassic and Jurassic dep osits are composed of six major transgressive-regressive cycles for wh ich the geometric and stratigraphic characteristics indicate that they were controlled by the tectonic evolution of the basin, and more spec ifically by variations in the type and rate of subsidence. The first t wo cycles, Anisian-early Sinemurian in age, correspond to the initial structural creation of the margin with a west-east sedimentary polarit y. Extensional tectonism, which affected the Palaeozoic basement, in p art controlled the duration and intensity of the transgressive phases, with the regressive phases reflecting a decrease in regional subsiden ce. The third and fourth cycles, early Sinemurian-middle Bathonian in age, were contemporaneous with the Tethyan rifting. In particular, the y 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 tha t are represented by that part of each sedimentary cycles deposited ar ound the maximum regression, i.e. end of the regressive phase and begi nning of the transgressive phase (Sinemurian and Aalenian). A regional phase of subsidence followed wh ich allowed maximum expansion of the transgression (Toarcian and Bajocian transgressive peaks). The fifth a nd sixth cycles, middle Bathonian-Kimmeridgian in age, were deposited during a period of widespread subsidence of the margin that was synchr onous with the onset of Tethyan oceanic spreading. They preferentially record episodes of accelerated subsidence giving rise to transgressiv e phases generally associated with a synsedimentary tectonic activity that incurred relatively heterogeneous subsidence across the entire pa laeomargin (late Bathonian and early Oxfordian). The regressive phases are subsequently marked by a decreased, but uniform subsidence with n o evidence of uplift. The sediments of these last two cycles were depo sited in a post-rift context and are liable to record effects of geody namic events outside the basin. Characterization of the different stag es in the evolution of the Ardeche palaeomargin by their stratigraphic expression provides geological constraints for modelling fluid migrat ion throughout this extensional margin, the objective of the 'GPF Arde che' project. Copyright (C) 1996 Elsevier Science Ltd