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

In the Sassari-Muros area, the analysis of extensive exposures of the Porto Torres sub-basin fill sequences have enabled a detailed reconstruction of a complex mixed carbonate-siliciclastic channel system. The exposed sequences, dated as Burdigalian-Serravalian, comprise a few kilometres wide and up to 200m thick channel complexes 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 coble sized gravity flow deposits which are locally capped by thin-bedded, intensely bioturbated, hemipelagic marls which are related with temporary abandon of the channel, possibly in relation with sea level rise. Architectural elements recognised at outcrop comprehend distinct mid-channel, margin-levee and overbank complexes as well as sheets and drapes of marly basinal deposits. Channel-margin which are locally characterised by very complex depositional architectures which include also up to 15-20 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 wide displaced and\or tilted blocks. Megabreccias are commonly associated to 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 include both carbonate- and siliciclastic-dominated deposits. Carbonate deposits are made up of rhodalgal calcirudites to calcarenites and include 5-20m thick well sorted rhodolith-rich pebbly bed packages as well as sets of early hardened strata. Siliciclastic deposits ranges from quarts- rich coarse pebbly\sandy to silty turbidites. These are well bedded, laminated or massive, and display different degrees of cementation. Carbonate fractions can be locally significant but the passage between the siliciclastic- to the carbonate-dominated deposits are commonly abrupt and marked by sharp erosive surfaces.