SEDIMENTARY UNCONFORMITIES AND THEIR USE FOR TIME CORRELATIONS - EXAMPLE IN THE MIDDLE LIAS OF QUERCY (NE AQUITAINE)

The middle and upper part of the northern Quercy Domerian series (Gram at and Martel areas), shows three types of sedimentary unconformities each with a specific lithologic context: intra-marl unconformity, lime stone-marl unconformity and intra-limestone unconformity. Their study, combined to the analysis of the sedimentological, palaeoecological an d micropalaeontological characteristics of adjacent units, allows to i nterpret their significance in terms of sequence stratigraphy. All the intra-marl unconformities indicate maximum flooding surfaces. Limesto ne-marl unconformities characterize transgressive surfaces. Intra-lime stone unconformities, less obvious, express an abrupt fall in relative sea level or a first transgression. Three successive depositional seq uences have been recognized in the studied Domerian sections (Margarit atus zone and Spinatum zone). The ammonite faunas are exclusively pres ent within transgressive systems tracts. Thus, the different types of unconformities cannot be dared with the same precision : transgressive surfaces can be precisely dated by the overlying deposits, and maximu m flooding surfaces by the underlying ones. The less precise datations correspond to the sequence boundaries.