Deep-marine structurally confined channelised sandy fans: Middle Eocene Morillo System, Ainsa Basin, Spanish Pyrenees

Abstract

The middle Eocene deep-marine Morillo System is up to ~260m thick and is the penultimate deep-marine sand-prone system in the Ainsa Basin, Spanish Pyrenees. It comprises the proximal parts of three structurally confined, coarse-grained and channelised mid-slope/canyon to lower-slope submarine fans (designated the Morillo I–III fans). Their constituent channels appear to be low- to moderate-sinuosity channels with widths estimated in the range of 600m to the order of 1km, and depths typically several tens of metres or more. Locally, the presence of metre-scale gravel-rich and sandy barforms is consistent with either relatively sinuous thalweg channels or side- and mid-channel bars within a more braided-like channel complex. The abundance of pebble-rich deposits throughout the Morillo System is interpreted to reflect a response to increased gradients in the staging area for sediment gravity-flows and mass wastage, which also may have been linked to a fall in relative base level as the Ainsa thrust-top (piggyback) basin narrowed and was uplifted between the tightening Boltaña, Añisclo and Mediano anticlines (submarine growth structures). As the shallow-marine and non-marine source areas were degraded and the sandy fans were abandoned, up to several hundred metres of very fine- and fine-grained, thin- and very thin-bedded turbidites and hemipelagic sediments mantled the coarser-grained Morillo deposits. This change in depositional style is interpreted as a response to a reduction in seafloor gradients during a phase of increased tectonic shortening across the Ainsa Basin, thereby permitting the growth of the final structurally confined system, the Guaso System, as low-gradient submarine fans in the Ainsa Basin.