Deepwater channel bypass and deepwater channel bypass and backfilling processes from the Eocene Ainsa Basin, South-Central Pyrenees, Spain

Deepwater channel outcrops in the Eocene Ainsa Basin have been previously well documented. The work presented here, however, focuses on evidence of turbidity current bypass and backfilling. Channel bases are characterized by a composite erosional surface with significant incision and scouring, and facies comprising scoured thin beds, fine-grained drapes, relatively coarse-grained bars and mud-rich debrites. Above this, channel fills are generally finer-grained thick-bedded sand facies. In the channel axes thick-bedded facies are commonly amalgamated, with dewatering and flame structures. Towards the margin these facies change laterally to thinner inter-bedded graded sandstones. These observations imply that numerous relatively high velocity turbidity currents were responsible for cutting the channels, with the majority of the sediment load bypassing down slope. Increases in flow velocity can either be related to changes in the staging area of the flows, or an increase in the channel floor gradient as channels attempt to establish equilibrium gradients on an irregular or dynamic slope. Seismic data from analogous subsurface systems suggests that the latter is a very common process in controlling channel architecture. In many channels, after the initial bypass phase, flows with a lower velocity backfill the channel resulting in rapid sand deposition. Debris flow deposits within channels are considered to be random events, but their common association with bypass facies may be related to the longevity of the bypass phase relative to the backfilling phase.