Coalesced 3D scours on the base of a hybrid event bed from the Gottero Sandstone, NW Italy

Erosion beneath sediment gravity flows can determine patterns of bed amalgamation and hence sand connectivity. Where significant clay is incorporated in the flow, this may impact on subsequent flow dynamics down-dip. Evidence for scouring beneath turbidites is commonly seen in outcrops, mainly in vertical profile or on limited bedding plane exposures where small-scale (cm to m-scale) sole structures are well documented. The presence of broader scour features is also often inferred on the basis of erosive steps although the 3D geometry of such features is generally unclear as it is rare to be able to inspect large-scale (10-100s m scale) bedding plane exposures. The Cretaceous-Palaeocene Gottero Sandstone on Mount Ramaceto, NW Italy provides one such example. Here an overturned syncline provides a spectacular inverted bed base on which a shallow (up to 15 cm) but extensive (100s m2) scour field is preserved. This was formed beneath a flow that emplaced a 2.3 m thick sheet-like hybrid event bed that can be correlated laterally for up to four kilometres in what was a relatively distal setting. The scour field comprises a composite erosional feature up to 150 m wide and at least 40 m long. It is made up of a terraced surface reflecting three levels of substrate erosion. The shallowest level is covered by small-scale grooves and flutes and is cut by larger elongate scours that coalesced to excavate the sea floor in patches down to a mid-level bed-parallel surface. The latter is then further incised by the deepest scours. Individual scours (40-150 cm across and 1-5m long) have distinctive asymmetric cross-sections with both inclined and undercut lateral margins. The undercut margins are associated with mudclast detachment. The extent of erosion given the distal setting is curious and implies hybrid flow development could be triggered or enhanced by clay entrainment in relatively distal settings. Distal erosion may be promoted by enhanced turbulence during the early stages of flow transformation, as suggested by some experimental work (see Baas et al. 2011, Sedimentology 58 1953-1987). As scour coalescence results in mainly bedding parallel terraces, it can easily be overlooked in vertical sections.