Architectural patterns in a multistorey mixed carbonate–siliciclastic submarine channel, Porto Torres Basin, Miocene, Sardinia, Italy

In the Sassari area, the analysis of extensive exposures of the Porto Torres sub-basin fill sequences have facilitated a detailed reconstruction of a complex multistorey mixed carbonate–siliciclastic submarine channel, the Sassari Channel. The exposed sequences, dated as Burdigalian–Serravallian, comprise channel complexes up to 4 km wide and 250–300 m thick fringed by marly sheet deposits. Channels are erosional or mixed erosional–depositional type and show multiple, stacked partly nested channel-fill sequences which relate to different filling phases.Individual channel-fills include sand- to cobble-sized deposits which are locally capped by thin-bedded, intensely bioturbated, hemipelagic marls which are related to temporary abandon of the channel, possibly in relation to sea level rise. Architectural elements recognised at outcrop include distinct mid-channel, margin–levee and overbank complexes as well as sheets and drapes of marly basinal deposits. Channel margins, which are locally characterised by very complex depositional architectures, also include up to 15- to 20-m-high lateral bars. Mid-channel complexes are commonly parallel to concave up stratified and locally exhibit minor order nested channel bodies. These erode, overlap or lie alongside each other and suggest repeated channel thalweg digressions and avulsions. Channel-fill architectures are locally complicated by the presence of megabreccias which include up to a few tens of metres high and hundreds of metres wide displaced and/or tilted blocks. Megabreccias are commonly associated with sand injections and/or minor faults and resulted from channel–margin collapses boosted by tectonics and/or by pore water overpressures generated in horizons hydrologically confined between early-cemented bed packages. Channel-fill sequences consist of carbonate- and siliciclastic-dominated deposits. Carbonate deposits are made up of cobbly to sandy rhodalgal debris and also exhibit 5- to 20-m-thick well-sorted rhodolith-rich gravelly clinostratified bed packages as well as sets of early hardened strata. Siliciclastic deposits range from quartz-rich coarse pebbly/sandy materials to silty turbidites. These are well bedded, laminated or massive, and display different degrees of cementation. Carbonate fractions in these turbidites can be locally significant but the passages between the siliciclastic- to the carbonate-dominated deposits are commonly abrupt and marked by sharp erosive surfaces.