SEDIMENTOLOGY AND STRATIGRAPHY OF TURBIDITE SYSTEMS WITH CONTRASTING ARCHITECTURES: EXAMPLES FROM THE TERTIARY PIEDMONT BASIN (NW ITALY) AND THE TAZA-GUERCIF BASIN (NE MOROCCO)

Architectures of submarine fans and turbidite systems are endowed by a high grade of complexity, inherited by the large variability of depositional flows characterising them. This complexity is reflected upon the superimposition of depositional elements (e.g., channels, levées, splays, etc.) giving rise to contrasting sedimentary facies and geometries at different scales. Thus, the understanding and prediction of their heterogeneity over space is essential to obtain reliable depositional models, which can be applied in the field of natural resources exploration. Despite heterogeneities of facies and architectures in turbidite deposits are largely studied, several questions regarding evolutionary controls still remain open due to the intricate interplay between autocyclic and allocyclic factors and the lack of roboust chronostratigraphic (i.e., time-related) constraints to be applied to turbidite systems evolution. This doctoral work focuses on two turbidite units, different for facies, architectures, age, and depositional setting. The work pursues the goal of complementing sedimentological data (collected with cm-scale resolution) with bio-chronostratigraphic data (both novel and collected from the literature) in order to provide a trustworthy estimation of the geological time in turbidite systems depositional models. In the first two chapters an overview on sediment gravity flows and their evolutionary models is provided, emphasising classical facies and depositional geometries. In chapter three, the sedimentary architecture and the controls on the evolution of a channel-levée complex from the Tachrift Turbidite System (Upper Miocene, Melloulou Fm., TazaGuercif Basin, NE Morocco) is addressed. The studied complex is spectacularly well exposed and then suitable for detailed sedimentological logging with cm-scale resolution. Results show a tripartite stratigraphic organisation of the complex, including: (i) a lowermost mud-prone interval with relatively small (a few hundred of metres across and metre-thick) and vertically stacked channels fills, (ii) a middle interval (ca. 4 m-thick and >1 km-wide) made of dominantly amalgamated sandstones with eastward-directed lateral accretion packages (LAPs), and (iii) an uppermost interval made of vertically aggraded channel fills with variously directed LAPs and well-developed levées. This organisation suggests that, after a relatively short phase of inception (lowermost interval), the channel underwent a prolonged lateral migration, prior to become aggradational (uppermost interval). Proportions of turbidite vs. hemipelagic deposits suggest that the migrational and aggradational phases straddle a lapse of reduced turbidite input to the studied section of the slope. It is suggested that the observed architectural style turnaround reflects the feedback of channel morphodynamics, sediment input, and along-dip channel depth profile. In chapter four, the Rupelian Monastero Fm. turbidite system (Tertiary Piedmont Basin, NW Italy) is addressed. This 1100 m-thick turbidite system was deposited adjacent to a coeval and heteropic fan delta system (i.e., the Savignone Conglomerates), upon a south-eastward dipping clastic ramp, and consists of an apparently monotonous succession of thin-bedded sandstone-mudstone couplets intercalated by erosive-based amalgamated bedsets made of sandstones and conglomerates. The unit was studied along the best exposed and most continuous section (Val di Grue-San Gaudenzio section) by cm-scale resolution sedimentological logging. Thesedimentological data were subsequently complemented with bio-magnetostratigraphic data (novel and from the literature) to refine the depositional model. In addition, seismic lines interpretation (in professional partnership with Eni S.p.A.) was used to constrain the Monastero Fm. depositional setting in the host basin. Results provide information about facies and architecture of the Monastero Fm. and insights for analogue systems: (i) the investigated stratigraphy is dominated for the 60% of section thickness by a muddy heterolithic background (with a density of 3 beds/m on average) alternated with conglomeratic channel fills and sandy lobes, (ii) the 65% of the studied section is composed of sedimentary bed repetitions interpreted as hyperpycnites, (iii) these repetitions cohexist with ‘classical’ turbidite facies indicating that the latter may represent the transformation of the former, (iv) bio-magnetostratigraphic data suggest an average accumulation rate of 400 m/Ma for the Monastero Fm., with an estimated return period of sedimentary gravity flows lower than a few thousand years, (v) seismic interpretations confirm the confined nature of the Monastero Fm. turbidite system. In conclusion, the new results of this doctoral work suggest that the use of magneto-biochronology can provide useful constraints for depositional age models, thus contributing to enhance our understanding of the time-scale and control factors of changes in sediment delivery to deep-water.