Sedimentological characterization of the Bordighera deep-sea fan: deciphering facies distribution within a sand-rich submarine fan complex deposited in a convergent setting

The Campanian-Maastrichtian Bordighera Sandstone represents a passive margin sourced, sand-rich medium-scale turbidite system deposited in a trench, with estimated depositional dimensions in the range of c. 15 x 45 km (across... [ view full abstract ]

The Campanian-Maastrichtian Bordighera Sandstone represents a passive margin sourced, sand-rich medium-scale turbidite system deposited in a trench, with estimated depositional dimensions in the range of c. 15 x 45 km (across and along paleoflow, respectively) and roughly 200 m in thickness. Basin-scale quantification of lateral and vertical facies heterogeneities within this basin-floor submarine fan complex offers a case study for predictive analyses of spatial distributions of sedimentary facies in reservoir-analogous sand-bodies. As a consequence of tectonic deformation, limited lateral outcrop accessibility and high degrees of horizontal and vertical facies variations, the integration of stochastic process analyses of facies transitions and cyclical stacking patterns provides a substantial aid in deciphering spatial facies patterns and facies associations distribution.

High-resolution facies analysis denotes the deep-sea fan as originating from various types of sediment gravity flows. Proximal depositional sub-environments are constituted by cyclically stacked, amalgamated coarse-grained successions involving bed type proportions dominated by “clean” microconglomerates and sandstones, often organized in lenticular bodies. Unidirectional paleocurrent orientations and low proportions of contiguous intercalations of heterolithic thin-bedded intervals suggest an attribution to low-sinuousity amalgamated channel complexes. Medial successions exhibit "mixed" bed type proportion distributions, defined by a decline in clean bed type proportions and moderate increase in mud-enriched bed types and hybrid event beds (HEBs), implying a less channelized but still proximal environment. More distally, a sudden change in the system character occurring over approximately 5 kilometres marks the onset of deposits dominated by HEBs and which appear to be composed of sheet-like sandstones. This realm can be differentiated in an axial domain prone to HEB development accompanied by less frequent proportions of heterolithic intervals (attributed to frontal fringes of depositional lobes), and marginal depositional sub-environments dominated by alternating mixed clastic and calcareous fine-grained laterally continuous thin layers (representing lobe-fringes / intralobe deposits). These observations illustrate the down-fan evolution from a pronouncedly channelized proximal sand-prone setting into an unconfined, distal zone characterized by HEB development. The complex interplay between siliciclastic sedimentation and Helminthoid-type calcareous San Remo Flysch turbidites is thought to be linked to the system’s rapid down-dip transformation. Influences of pre-existing basin floor topography probably resulting in isolated sub-depocenters and fluctuating clastic sediment supply rates are considered as possible causes for enhanced local- to basin-scale heterogeneities in facies distribution and stratigraphic architecture.