Anatomy of a submarine channel system and related fan in a foramol/rhodalgal carbonate sedimentary setting: a case history from the Miocene syn-rift Sardinia Basin, Italy

During Aquitanian-Burdigalian times, thick mixed carbonate-siliciclastic successions were deposited in basins located on the grabens and half–grabens along the Oligo-Miocene Sardinia Rift Basin. Locally active tectonics, sea level variations and ecological factors combined to control the development and distribution of foramol carbonate factories as well as the remobilisation and the redeposition of carbonate sediments into the adjacent deeper areas. In the Isili Basin, foramol\rhodalgal carbonate factories developed on submerged structural highs which resulted from pre- and syn-sedimentary tectonics. These carbonate factories were periodically shaved mainly during negative sea level oscillations and the sediments removed were funnelled towards the basin through a complex submarine channel network which included a tributary belt, one main channel (Isili Channel) and the related fan. The Isili Channel is up to 1 km wide, 60–100 m deep and includes two stacked channel complexes each built up by several minor order channel-units. Complex strata geometries characterise the Isili Channel and its related architectural elements (e.g., overbank, levee, margin and channel thalweg) which also include up to 15 m high bedforms. Individual channel complexes were temporally related to individual fan systems whose spatial distribution and internal geometry were strongly controlled by the type and rate of sediment accumulation and in turn by relative sea-level oscillations. Facies associations include sandy to cobble-sized gravity flow and bottom current deposits as well as megabreccias characterised by impressive displaced and tilted blocks which resulted from major channel margin collapses. Detailed analysis has led to the reconstruction of the internal geometry and depositional architecture of these carbonate bodies and to the determination of the main controlling factors. The dimension and distribution of channel and channel-related depositional bodies have been accurately determined. This information provides a useful tool to analyse less extensively exposed analogues and to model foramol shelf to basin transitions and related channel and fan systems.