Anatomy of high-sediment supply, transgressive tracts in the Vilomara composite sequence, Sant Llorenç del Munt, Ebro Basin, NE Spain

Abstract

Abstract The Sant Llorenc del Munt fan-delta succession forms a clastic wedge (ca. 3 m.y. duration) that built out into the NE Ebro Basin in response to transpression and uplift along the Catalan Coastal Range. The wedge contains a series of composite transgressive–regressive sequences. The transgressive part of one of these sequences, the Vilomara composite sequence, has been examined to highlight the character of ‘transgressive’ successions in a highly aggradational setting. The analysis focuses on the nature of the component high-frequency (fundamental) sequences. There is a marked difference in how alluvial and marine sediment volumes are partitioned within the transgressive and regressive tracts of the three fundamental sequences within this overall landward-stepping succession. In transgressive tracts, the marine shoreline clastics are well developed, but only in narrow (250 m wide) strike-parallel zones, which are closely associated with ravinement clusters. The time-equivalent alluvial strata are thick and extensive. In regressive tracts, the marine clastics are extensively developed along both dip and strike as prograding shoreline sheets, whereas the interfingering alluvial deposits are only thinly developed or eroded out completely by their overlying sequence boundary. In regressive intervals, there was significant sediment bypass in the alluvial reaches, with feeding to the shorezone and shelf; in transgressive intervals, in contrast, great volumes of sediment were stored in the coastal plains. This partitioning of sediment volumes commonly led to wedge-shaped, seaward-thickening regressive geometries, and wedge-shaped, landward-thickening transgressive geometries. The unusual thickness of transgressive strata, the aggradationally-stacked nature of successive ravinement surfaces, and the thick development of nonmarine strata during times of transgression are symptomatic of an unusually high supply of sediment to the system during high rates of accommodation creation and a near-continuous rise of relative sea level. An unusual type of parasequence in the transgressive tracts, showing well-developed transgressive and regressive components, is probably also symptomatic of the high sediment supply setting.