The end-signature of a deep-marine basin fill: The middle Eocene Guaso system, southcentral Spanish Pyrenees

The ~300-m-thick deep marine Guaso system is the youngest deposit of the deep marine fill of the Mid Eocene Ainsa basin, Spanish Pyrenees. It is overlain by ~150-200 m of fine-grained slope and deltaic sediments. Unlike the older turbidite systems in the Ainsa basin (canyon and channelised systems) the Guaso sandbodies are laterally extensive, built by laterally-switching 3-10-m-deep erosional channels, and confined only by basin structure during deposition. The oldest system, Guaso I, displays cyclic sedimentation packages and up slope thinning suggesting a sediment source greatly influenced by fourth order controls and not directly linked to the feeder system. Total Organic Carbon values in the fine grained sediment above Guaso I records suspected climatic controls in the order of 41kyr. The youngest system, Guaso II, is generally coarser with no cyclic sedimentation packages, suggesting a sediment source connected to the feeder system and not as effected by subtle climatic changes. The Guaso II system spans deep marine to slope environments. The first-order control on basin-scale accommodation was tectonicallydriven subsidence with eustasy the most likely driving factor for sand deposition (probably the ~400 kyr Milankovitch beat). The overlying Sobrarbe Deltaic Complex is characterised by alternating thin, sheet like sandstones and heavily bioturbated sandstone on a marlstone prone slope up to the delta front sandstones. The critical end-signature of deep-marine Guaso deposition was a phase of net tectonic uplift creating a narrower and shallower basin morphology, allowing the feeder delta to prograde to at or near the shelf edge. At the next eustatic sealevel fall sediment input patterns were not favourable to the cutting of canyons or deeply-incised slope channels, creating an unconfined turbidite system. Such clastic slopes often characterise the end-signature for the infill of other shallowing-up deep-marine basins where sediment supply is high and shelf edge deltas are present