Facies Analysis and Architecture of the Fosado Turbidite Channel Complex

In the Ainsa basin, the Castissent sequence has been subdivided in two different composite depositional sequences: Castissent-1 (Cs-1) and Castissent-2 (Cs-2). This study focuses on the lower part of Cs-1, where the Fosado Channel Complex lies. Cs-1 was deposited synchronously with a high-deformation phase in the Pyrenean orogeny (Upper Ypresian to Lower Lutetian), with strong tectonic reactivation of the South-Central Pyrenean Unit; sediments were affected by the Atiart thrust (east) and later on by Los Molinos thrust (west). So far, the knowledge about Cs-1 was mainly its stratigraphic position within the Torla turbidite system, its physical correlation with the fluvio-deltaic feeding system of Castissent Fm. s.s., as well as its facies detail of the uppermost individual channel (the Fosado channel, s.s.), which has been used as a facies model for channel margin settings. Nevertheless, its high resolution stratigraphic study, its facies associations and architecture, still remains unknown in detail. Therefore, it has been possible to map and redefine the extension of the Atiart unconformity in the hanging wall of Los Molinos thrust. This 3rd order sequence boundary at the base of Cs-1 separates: (1) a lower mud-dominated unit, with a high content in debris flow deposits and fine-grained turbidites; and (2) the upper unit, mainly composed by a channel-overbank complex (channel-levee), forming sandy packages belonging to highly-heterogeneous channels. In the stacking pattern of the Fosado complex two main growing stages have been identified: Cs-1a and Cs-1b. Cs-1a displays a relatively high percentage of sand content in individual channels; on the contrary, Cs-1b is a mud-dominated unit, mainly composed by fine-grained channels and their related overbank deposits. This last unit would be the lowermost part of the slope fan related with the Castissent deltas. Consequently, the study has focused on the channel complex of Cs-1a unit, where a stacking pattern with unconformities is described and has led to organize it hierarchically in five 4th-order sequences (high resolution seismic scale). Each channel group shows a nested stacking pattern, with hyperconcentrated flow deposits at the base, and lateral and vertical accretion (up to 200x20 m), which is probably indicative of large-scale high-sinuosity channels.