Cryptic delamination beneath distal hybrid event beds: evidence for local substrate entrainment

Hybrid event beds (HEBS) are increasingly recognised as an important component of the stratigraphy of many deepsea fan and sheet systems. Previous models have attributed their origin to vertical and/or longitudinal segregation of co-genetic turbidite and debritic flows in a downcurrent direction, triggered by mud acquisition in up-dip channels or channel-lobe transition zones. Sedimentological analysis of the deep-water Cretaceous-Paleocene Gottero Sandstone (NW Apennines, Italy) has revealed large substrate delamination features at the bases of mudclast-rich HEBs in the distal sector of the basin. The delamination features are generally bedding-parallel and can be shallow (15–20 cm deep and few 10s of meters wide) or deep (c. 1–2 m deep and several hundred meters wide). Such features can remain completely undetected without a careful sedimentological analysis and bed by bed correlations, and the minimum amount of stratigraphy removed can be estimated only in a few cases where at least part of the original stratigraphic succession is preserved laterally. The deepest features are overlain by thick hybrid event beds containing large rafts derived from the underlying substrate and composed of mudstone and intervening thin sandstone beds, passing rapidly along both downstream and lateral facies tracts into chaotic or mud-clast-rich HEB debrites. Similar bed types are present in other deep-water systems (Castagnola, Marnoso Arenacea, Ventimiglia flysch, Ross Formation), usually associated with more conventional HEB types. However, their significance and association with substrate delamination has not been demonstrated before. The field observations suggest that in the Gottero system incorporation of muddy substrate occurred when dense sandy flows were able to extend sand injections into the shallow substrate and detach large slabs, carrying them for short distances before they broke up due to sand injection and internal shearing. The substrate entrainment was therefore not due to turbulent processes that could not have picked up and carried such large rafts. Remnants of undetached sea floor ‘flaps’ are only occasionally preserved. Similar process at smaller scale may have accompanied the widening of tiered down-cutting surfaces seen elsewhere in the Gottero and these may have contributed smaller mudclasts to down-dip HEBs. An angular gradient change between the proximal fan-lobe area and the distal basin plain sectors, possibly accentuated by distal basin confinement, could have promoted substrate delamination and the development of raft-bearing and mudclast-rich hybrid event beds in this and other basins.