Transition from deep-marine lower-slope erosional channels to proximal basin-floor stacked channel–levée–overbank deposits, and syn-sedimentary growth structures, Middle Eocene Banastón System, Ainsa Basin, Spanish Pyrenees

Abstract

We document a rare example of deep-marine siliciclastic environments associated with a transition from lower-slope erosional channel/canyon to proximal basin-floor channel–levée–overbank environments from the Middle Eocene deep-marine Banastón System (Upper Hecho Group), Ainsa Basin, Spanish Pyrenees. The base and top of the Banastón System are defined by angular unconformities; the margins show pinch-out onto hemipelagic slope mudstones or sidelap onto canyon walls. The Banastón System comprises six lower-slope erosional channel/canyon fills with an ~520m cumulative thickness that correlate with six linked proximal basin-floor, laterally offset-stacked low-sinuosity, coarse-grained, channel–levée–overbank complexes (BI–BVI) totalling ~700m cumulative thickness in the lower slope setting. The younger Banastón IV, V and VI sandbodies (BIV–VI) show less confinement and overspill the more aerially restricted older parts of the Banastón System. About 3km farther NW into the Ainsa Basin, the BI–VI sandbodies are less confined, and laterally offset-stacked towards the WSW in increments of ~1km; they are interpreted as channel–levée–overbank complexes in a proximal basin-floor setting. The base of each complex is defined by a MTD/MTC comprising coarse-grained deposits and characterised by pebbly mudstones. The Banastón System is an example of a deep-marine, sand-prone, lower-slope and proximal basin-floor transition that shows the importance of the interplay between depositional control on sand accumulation (e.g., MTDs/MTCs), erosional confinement within lower-slope channels, a tectonically-driven mobile slope and syn-sedimentary seafloor growth structures, in controlling the lateral migration of channels away from the deformation front. We document these 3-D relationships and discuss the transport, deposition and tectonic processes responsible for the thickest turbidite sandstone accumulation in the Ainsa Basin.