Axial hybrid event bed development in a pre-collisional low-efficiency turbidite system: the Bordighera Sandstones (NW Italy)

Increasing studies document that deposits of sediment gravity flows which record evidence of co-genetic laminar and turbulent flow regimes represent an integral part of deep-water clastic sedimentary systems. Such transitional flow behaviour can be explained by turbulence suppression due to modulated detrital clay concentrations arising from erosional entrainment of underlying muddy substrate at both up-dip locations and local distal positions. The resulting facies motif displays a bi- or tripartite arrangement defined by a lower mudpoor sandstone portion and a middle or upper clay-rich, debritic sandstone interval. These deposits (hybrid event beds; HEBs) negatively impact hydrocarbon reservoir properties, as the presence of argillaceous intervals baffles fluid migration. Studies from a wide range of turbidite systems suggest their principal spatial distribution in distal/marginal lobe sub-environments. A high-resolution stratigraphic dataset has been acquired from the Upper Cretaceous Bordighera Sandstones. The deep-water succession represents a sand-rich elongated turbidite system which – essentially towards its medial and distal domains - becomes heteropic with muddy calcareous turbidites of the Helminthoid Flysch family. Main emphasis was put on the quantification and spatial allocation of inter-sandbody heterogeneity due to HEB development and to contribute to predictive models of HEB distribution. Three main facies groups have been distinguished: (1) clean (i.e. mud-poor) sandstones and microconglomerates, (2) a suite of mudclast-rich sandstones and HEBs and (3) fine-grained sediments. Three main depositional domains - marked by strikingly contrasting dominant lithofacies proportions along a downstream transect - can be recognized. Proximal channel-fill successions are divided from extensive lobate sand sheets by a spatially limited transitional zone of ca. 5 km basin-ward expansion. Whereas the channelized proximal domain represents an array of homogeneous sandstone bodies defined by the dominance of mud-poor facies types, the transitional zone in which the sand fairway lost lateral confinement 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 distributed in high net-sand packages. A close environmental relationship between meso-scale scouring and enhanced availability of cohesive substrate in the transitional zone is interpreted to control the observed down-dip shift in dominant facies types. Remarkably, the widespread occurrence of HEBs within the high-energy axial zones featuring high sandstone/mudstone ratios and high degrees of amalgamation contrasts models that document HEB distribution to be characteristic of lateral outer fan environments.