Sequence stratigraphy of alluvial-nearshore transition depositional systems: insights from the Eocene Montserrat conglomerates of eastern Spain

The Eocene Montserrat fan-delta system contains eight fourth-order, transgressive-regressive (T-R) cycles reflecting the complex interplay between sea-level changes, subsidence and sediment supply. Stratigraphic and sedimentological investigations of the conglomeratic “St. Benet level” and of the overlying T-R cycle were carried out at two locations, one close to the monastery (St. Benet section) and the other in a more distal position (La Calsina section), leading to genetic facies characterization, identification of sedimentary body geometries, and sequence-stratigraphic interpretation. Three prominent stratigraphic surfaces were identified, physically tracked across the studied interval and tentatively interpreted in terms of sequence stratigraphy. The lower boundary of amalgamated, channel-fill conglomerates (facies association 1), which likely reflects significant erosion under low-accommodation conditions (lowstand deposits), might represent the sequence boundary. The top of the “St. Benet level”, which is inferred to represent the maximum regressive surface, is a conformable surface that displays very high correlation potential across the study area, being recognizable by the sharp facies change from laterally extensive fluvial bodies (facies association 1) to overlying, ribbon-shaped fluvial channels encased within overbank fines (facies association 2). Facies association 2 shows an increasing marine influence from proximal to distal locations, and for this reason can confidently be interpreted as the lower transgressive systems tract. A surface of wave ravinement marks the erosional boundary to overlying littoral deposits (facies association 3). These include nearshore sandstones, overlain by shallow-marine mudstones (upper transgressive systems tract), with upper transition to a prograding succession of prodelta and delta-front deposits (highstand systems tract). The sharp facies change observed at the boundary between alluvial facies associations 1 and 2 is the most readily identifiable stratigraphic surface and represents on a basin scale an almost flat, easily mappable stratigraphic marker. This non-erosional boundary, which separates thick, laterally extensive gravel bodies from overlying isolated fluvial channels, could obviously reflect, per se, a simple autogenic control within the fan-delta systems. However, given its occurrence within a distinctive cyclic stratigraphic framework, it is suggested here to represent the landward equivalent of a transgressive surface, thus supporting the stratigraphic subdivision of Montserrat Conglomerates into T-R sequences.