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

The Eocene Sant Llorenç 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 clastic wedges that were interrupted by marine transgressions of period varying from 60,000 to a few hundred years, resulting in a highly cyclic succession. The cyclicity of the succession was controlled by a combination of syndepositional basin-margin tectonics, high sediment supply, and eustatic sea-level changes. Analysis of one of the component (25-40 m thick) transgressive-toregressive sequences (10,000-20,000 year duration) shows that within its transgressive part the alluvial and coastal-plain deposits are extensive and thickly developed 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. These ichnofabrics are used to help define sequence boundaries, flooding surfaces, and maximum flooding surfaces, as well as to increase the resolution of the stratigraphie analysis. Within the transgressive tract of the main sequence, nonmarine deposits are separated from marine deposits by a steep, landward-inclined complex of wave-ravinement surfaces. Successive ravinement surfaces have a vertical spacing of about a meter, are offset landwards from each other by less than 100 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 dominated by their transgressive tract, and the upper two are asymmetric but dominated by their regressive tract.