Sedimentary facies, heterogeneity development and provenance of the sand-rich pre- collisional Bordighera turbidite system

The Upper Cretaceous San Remo Unit of the Western Ligurian subduction flysch complex represents a trench fill that comprises a basal complex (San Bartolomeo Fm.) and two thick turbiditic sequences: A low-efficiency siliciclastic turbidite system (Bordighera Sandstones) which interfingers with a calcareous turbiditic sequence (San Remo Helminthoid Flysch) that becomes increasingly abundant toward the lateral and distal domains. In order to test the outcrop analog potential of the coarse-grained, sand-rich axial fan, the Bordighera Sandstones have been investigated by detailed facies analysis. Main emphasis is being placed on the quantification of inter-sandbody heterogeneity due to hybrid event bed (HEB) development. In the context of complex tectonic deformation and partly limited lateral outcrop exposure, the utilization of sedimentological metrics (e.g. amalgamation ratios, sandstone-mudstone-ratios, grain size distribution trends, facies proportions) provides an effective tool for the determination of depositional environments and subsequent spatial allocation of facies heterogeneity distribution along the sand fairway. Three main depositional domains - marked by strikingly contrasting dominant lithofacies proportions along a downstream transect - can be recognized: (1) Proximal channel-fill successions, (2) a spatially limited transitional zone of ca. 5 km basin-ward expansion defining an abrupt transition to (3) extensive lobate sand sheets. Whereas the channelized proximal domain represents an array of comparably homogeneous sandbodies defined by the presence of mud-poor sand bed types which are distributed in thick, highly amalgamated intervals, the transitional zone records a modest increase of bed types proportions accountable for heterogeneity. By contrast, the more distal sheet-like succession is dominated by mudclast- rich sandstones and HEBs typically intrinsic to amalgamated packages. Remarkably, the widespread occurrence of such bed types within axial zones of the preserved elongated sand fairway contrasts models that predict HEB distribution to be characteristic of outer fan environments. The atypical nature of a low-efficiency (i.e. initially mud-poor) turbidite system being highly prone to HEB-related heterogeneity is interpreted as the result of enhanced availability of cohesive mud due to intercalations of the calcareous Helminthoid Flysch. Sediment provenance analysis of the Bordighera Sandstones provides new insights for understanding the complex pre-collisional paleogeological evolution of the Western Tethys. Modal framework grain analysis characterizes the sandstones as arkosic arenites (average modal composition: Q50F48L2; mean K/P-ratio: 0.68), suggesting that elevated bedrock (granitoid plutons and low-grade metamorphic geobodies) provided the provenance for the sand-rich turbidites. New geochronological data (U-Pb detrital zircon ages) reveal the strong affinity of the clastic detritus to the Paleo-European margin. Ongoing research comprises the integration of geochronological data of the underlying basal complex and the detailed confrontation of detrital age clusters against published datasets of peak magmatism age populations of candidate source areas (i.e., Calabria, Sardinia, Corsica, Dora-Maira Massif), with the aim of clarifying the geodynamic evolution of continental microterranes that constituted the reactivated hyperextended European continental margin.