Architecture and trace-fossil characteristics of a 10,000-20,000 year, fluvial-to-marine sequence, SE Ebro Basin, Spain

The Eocene Sant Llorenc del Munt fan delta complex lies on the southeastern edge of the Ebro Basin in NE Spain. Alluvial fans and fan deltas prograded northwestwards from the Catalan Coastal Range to form a series of elastic wedges that were interrupted by marine transgressions of period varying fro m 60,000 to a few hundred years, resulting in a highly cyclic succession. T he cyclicity of the succession was controlled by a combination of syndeposi tional basin-margin tectonics, high sediment supply, and eustatic sea-level changes. Analysis of one of the component (25-40 m thick) transgressive-to-regressiv e sequences (10,000-20,000 year duration) shows that within its transgressi ve part the alluvial and coastal-plain deposits are extensive and thickly d eveloped whereas shoreline lithosomes are narrow and thin. In the overlying regressive part the opposite relationship is documented. The transgressive and regressive tracts thus have an overall landward-expanding and seaward- expanding geometry, respectively. Six ichnofabrics have been documented in the succession, ranging from nonmarine to offshore-transition in origin, Th ese ichnofabrics are used to help define sequence boundaries, flooding surf aces, and maximum flooding surfaces, as well as to increase the resolution of the stratigraphic analysis. Within the transgressive tract of the main sequence, nonmarine deposits ate separated from marine deposits by a steep, landward-inclined complex: of w ave-ravinement surfaces. Successive ravinement surfaces have a vertical spa cing of about a meter, are offset landwards from each other by less than 10 0 m, and diverge slightly from each other as they climb through the section . These same erosion surfaces merge with each other basinwards, causing the transgressive tract to thin significantly, Five high-frequency sequences are recognized within the main sequence. The middle one is nearly symmetrical, the lower two are asymmetric and dominate d by their transgressive tract, and the upper two are asymmetric but domina ted by their regressive tract.