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

The Sant Llorenc del Munt fan-delta succession forms a elastic wedge (ca. 3 m.y. duration) that built out into the NE Ebro Basin in response to transp ression and uplift along the Catalan Coastal Range. The wedge contains a se ries of composite transgressive-regressive sequences. The transgressive par t of one of these sequences, the Vilomara composite sequence, has been exam ined 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 ar e partitioned within the transgressive and regressive tracts of the three f undamental sequences within this overall landward-stepping succession. In t ransgressive tracts, the marine shoreline clastics are well developed, but only in narrow (250 m wide) strike-parallel zones, which are closely associ ated with ravinement clusters. The time-equivalent alluvial strata are thic k 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 o ut completely by their overlying sequence boundary. In regressive intervals , there was significant sediment bypass in the alluvial reaches, with feedi ng to the shorezone and shelf; in transgressive intervals, in contrast, gre at volumes of sediment were stored in the coastal plains. This partitioning of sediment volumes commonly led to wedge-shaped, seaward-thickening regre ssive geometries, and wedge-shaped, landward-thickening transgressive geome tries. The unusual thickness of transgressive strata, the ag,aggradationally-stack ed nature of successive ravinement surfaces, and the thick development of n onmarine strata during times of transgression are symptomatic of an unusual ly 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 transg ressive and regressive components, is probably also symptomatic of the high sediment supply setting. (C) 2000 Elsevier Science B.V. All rights reserve d.