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.