Sequence stratigraphy of an alluvial-nearshore transition depositional system: insight from the Eocene Montserrat conglomerates of Eastern Spain

Sequence-stratigraphic concepts are well established within deltaic, coastal and shallow-marine depositional systems, whereas their application becomes increasingly difficult landwards, where the effects of climate, tectonics and changes in sediment supply may have been overwhelming sea-level fluctuations. Despite severa! attempts have been promoted in order to apply the principles of sequence stratigraphy to non-marine (coastal-plain and fluvial) successions, at present there is no generai acceptance of any single model. The Eocene Montserrat conglomerates, a well-exposed fan-delta complex cropping out 40 km NW of Barcelona, represent a useful case history to investigate the stratigraphic relationships between alluvial and coeva! littoral deposits, for a number of reasons: i) the alluvial to nearshore transiti o n takes piace over distances of few kilometres only; ii) spectacular outcrop exposures enable physical correlation of major sedimentary bodies; iii) the regional and local stratigraphic framework have been described in detail by previous work. The alluvial system contains eight fourth-order (105 years), transgressive-regressive (T-R) cycles reflecting the interplay between subsidence and sediment supply. We focused on Cycle 6 of Anadon et al. (1985): two detailed stratigraphic sections, one dose to the monastry (S. Benet section) and the other in a more distai position (Calsina section), were studied in terms of facies distribution and sedimentary body geometries. Three prominent stratigraphic surfaces, from bottom to top, can be easily identified and laterally tracked across the studied interval: i) the lower boundary of the amalgamated, channel-fill conglomerates, which is interpreted as the sequence boundary, reflects significant erosion under low-accommodation conditions. Fluvial incision was about 30m at S. Benet, where the amalgamated conglomerate facies is thickest. The incised valley body thins out distally, where incision was reduced to 8 m. ii) the top of the amalgamated conglomerates is a conformable surface that displays the best correlation potential. Despite its occurrence within alluvial deposits, this surface can be easily recognized by the associated sharp facies change from high net-to-gross, laterally extensive fluvial bodies (lowstand systems tract) to the overlying, ribbonshaped fluvial bodies encased within floodplain facies (lower transgressive systems tract), and for this reason is interpreted as the maximum regressive surface. The abrupt decrease in the proportion and interconnectedness of the fluvial bodies strongly indicates rapid creation of accommodation, which is highly suggestive of "transgressive" conditions. iii) the surface of wave ravinement is an obvious stratigraphic marker that can be easily detected in the field at the transition from comparatively finer-grained, "transgressive" alluvial and coastal plain deposits to the overlying nearshore sandstones. The littoral sandstones are invariably overlain by shallow-marine marly deposits (upper transgressive systems tract), with upper transition to a prograding deltaic succession (highstand systems tract). In terms of sequence-stratigraphic interpretation, the sharp facies change interpreted here as the maximum regressive surface represents an easily mappable surface across the study area, which is much more readily identified than the sequence boundary, thus supporting stratigraphic subdivision of the Montserrat conglomerates into T-R sequences (Burns et al., 1977).