Linking shelfal 'turbidites' to their feeding system: the Monastero Fm. (eastern Tertiary Piedmont Basin)

Though underappreciated in the sedimentary record, there is growing evidence that water- sediment mixtures generated at river mouths consequently to high-discharge events can transfer significant sediment volumes to deep-water via a range of flow types, referred to as hypo-homo-hyperpycnal flows. Identification of these deposits is difficult due to their resemblance to some classical turbidite facies, thus requiring accurate facies analysis.This work focuses on the facies analysis of a ca. 1100 m-thick pile of turbidites belonging to the Monastero Fm. (MF), which were deposited adjacent to coeval Gilbert-type fan-delta conglomerates (Savignone Conglomerates Fm.; SCF) as a part of the Rupelian-early Chattian fill of the eastern Tertiary Piedmont Basin (TPB; Northwest Italy).The MF consists of an apparently monotonous succession of thin-bedded sandstone-mudstone couplets with a sand/mud ratio >1 intercalated by erosive-based amalgamated beds-sets of sandstones and conglomerates a few-tens of metres-thick. The thin-bedded component of MF is dominant, representing about 70% of the total thickness of the studied section, and show a range of features which hardly fit into a turbidite depositional model. The majority of sandstone beds show erosional wavy bases and begins with either coarser basal lags or a thin inversely graded division, passing upward to two or more inversely-normally graded lamina sets separated each other by mud-drapes containing abundant phytoclasts. Typically, the top of these sandstone beds shows a variety of sedimentary structures (cross, trough-cross and hummocky lamination) and is capped by cm-thick heterolithics characterized by a range of flaser to lenticular bedding. In the lower part of MF, presence of symmetric ripple-marks ornamenting sandstone bed tops and articulated lamellibranch valves suggests deposition above the wave base level and short sediment routing.On the other hand, the thicker-bedded component of MF consists of amalgamated coarse-grained sandstones in which mud-draped scours, traction carpets, mud-chips and conglomeratic lags are suggestive of deposition from a range of hyper-concentrated to high-density flows transferring most of their load down-dip.The stratigraphic and lateral relationships of these two contrasting facies associations, along with their characters, suggest that the MF can be interpreted to reflect deposition in the pro-delta of the adjacent SCF system. In this view, the thin-bedded component of MF would represent the product of ‘background’ deposition by a range of hypo-homo-hyperpycnal flows generated at river mouths, whereas the coarser amalgamated counterpart likely represents pro-delta lobes.