Vertical facies evolution of the Tufiti di Tusa Formation cropping out in the Lucanian Apennines (Southern Italy)

A bed by bed facies analysis of the late Oligocene-early Miocene Tufite di Tusa Formation, cropping out along the Rotondella Ridge (Lucanian Apennines external wedge), allows us to point out how its turbidite facies and related processes change with time. In this area, the aforementioned formation, belonging to the Rocca Imperiale Sicilide Unit, lies stratigraphically on the Argille Varicolori Formation; it is about 230 metres thick with a lower portion (about 160 meters) characterized by calciclastic and siliciclastic turbidites, and an upper portion (about 70 metres) dominated by volcanoclastic sediment gravity flow deposits. The most peculiar facies of the studied stratigraphic succession are: Bed types 1) thin to very thick fine-grained normally graded beds with plane-parallel to slightly undulated lamination, ripples and convolute lamination; this facies can be related to low-density fully-turbulent flows. Bed types 2: thin to very-thick fine-grained normally graded tabular beds with vergent convolute lamination, hummocky-type structures, biconvex ripples and megaripples usually with sigmoidal laminae; the paleocurrent directions indicated by the sedimentary structures are often different from one another; this facies can be interpreted as deposited by low-density fully turbulent flows, which experienced ponding and rebound processes. Bed types 3: very-thick coarse to fine-grained tabular beds composed of a basal coarse-grained sandstone, an intermediate fine to medium-grained sandstone with structures similar to those of bed types 2, and an upper very-thick mudstone unit; the paleocurrent directions indicated by the sedimentary structures are different from one another; these beds can be interpreted as deposited by large-volume bipartite sediment gravity flows, which underwent ponding processes. Bed types 4: very-thick, fine to coarse-grained tabular tripartite beds with an intermediate slurry muddy sandstone often including mudstone clasts and pseudonodules, organic matter and liquefaction structures; this facies is generally associated with hybrid sediment gravity flows previously enriched in mud through erosive processes favoured by slope changes or intrabasinal highs. Bed types 5: very-thick coarse to fine-grained tabular and normally-graded beds characterized by a lower coarse to medium-grained massive sandstone, overlain by a thin fine-grained unit; this facies can be interpreted as related to large-volume bipartite sediment gravity flows with an upper low-density turbulent flow able to bypass the depositional areas of the lower high-density flow. Bed types 6: thin to thick slightly calcareous mudstone to calcareous marl beds. In accordance with the vertical facies distribution, the stratigraphic log can be subdivided into three main intervals: a basal one composed of bed types 1, 2, 3 and 6 (between 0 and about 54 meters); an intermediate interval characterized by the occurrence of bed types 4, which are associated with beds 1, 2, 3 and 6 (between about 54 and 190 meters); a top interval characterized by the occurrence of bed types 5, which are associated with facies 1, 2, 4 and 6 (between about 190 meters and the top of the section). The vertical facies changes give informations about the turbidite system evolution where morphologic confinement played a major role in determining facies characteristics.