Anatomy of regressive tracts in a regressive sequence set: Vilomara unit, Sant Llorenc del Munt, Ebro Basin, NE Spain

The Sant Llorenc del Munt fan-delta succession forms a elastic wedge (ca. 3 My duration) that built out into the NE Ebro Basin in response to transpre ssion and uplift along the Paleogene Catalan Coastal Range. This fan-delta succession is arranged in a series of transgressive-regressive sequences of different scales. A large-scale composite megasequence (the Milany megaseq uence) includes the whole fan-delta succession. This megasequence is subdiv ided in seven composite sequences that, in turn, can be subdivided into fun damental sequences. The Vilomara composite sequence (the Vilomara Unit) is located in the regressive part of the Milany megasequence. The regressive p art of the Vilomara composite sequence, consisting of a seaward-stepping se t of fundamental sequences (the Vilomara unit), has been studied to underst and the facies relations, geometries, and the characteristics of key surfac es in progradational settings. The Vilomara regressive sequence set is formed by stacked alluvial fan, fan -delta plain, proximal and distal fan-delta front, fan-delta slope (within the regressive part of fundamental sequences) and carbonate platform (usual ly within the transgressive part of fundamental sequences) deposits. The Vilomara regressive unit shows a sigmoidal geometry in contrast to the underlying El Marcet transgressive unit, which thickens landwards. This sig moidal geometry for the regressive unit and landward thickening for the tra nsgressive unit is common for all scales of sequence (composite megasequenc es, composite sequences and fundamental sequences). There are evident facies arrangement differences between transgressive and regressive deposits within a fundamental sequence. Key surfaces have been r ecognised from vertical-trend changes (from transgressive to regressive and vice versa). The facies successions immediately above and below the surfac es bounding fundamental sequences may indicate, at different stratigraphic levels, a basinward or a landward shift of facies belts. (C) 2000 Elsevier Science B.V. All rights reserved.