Sand-rich turbidite system of the Late Oligocene Northern Apennines foredeep: physical stratigraphy and architecture of the ‘Macigno costiero’ (coastal Tuscany, Italy)

Abstract The ‘Macigno costiero’ turbidite system characterized the oldest foredeep clastic wedge of the Northern Apennines during the Late Oligocene collisional phase. The cropping-out thickness is about 500 m. The features of the ‘Macigno costiero’ indicate a sand-rich, low-efficiency turbidite system. The system developed within a partially confined basin, which was part of a complex foredeep system. The stacking pattern of the turbidite system was determined through the analysis of facies and physical stratigraphy. It consists of a succession organized in sedimentary units, which are characterized by particular associations of facies linked to distinct depositional environments. Several architectural elements are seen: (1) unchannelized and channelized lobes; (2) distributary channels with channel-fill, overbank and channel-margin deposits; (3) main channel with channel-fill, channel-margin and interchannel deposits. Five turbidite stages were identified. From the bottom up they consist of four lobe stages and one proximal channel stage. The lobe stages are characterized by thickening-coarsening upward trends, from distal lobes to proximal lobes up to the channel-lobe transition zone. The uppermost, fifth stage is linked to a main channel complex with stacked channel-fill, channel-margin and interchannel deposits. This final stage also marks the maximum progradation of the system up to its closure due to the synsedimentary overthrusting of the orogenic wedge.