Cyclic stacking pattern, architecture and facies of the turbidite lobes in the Macigno Sandstones Formation (Chattian-Aquitanian, northern Apennines, Italy)

This study presents a detailed facies analysis of an 800 m-thick stratigraphic succession in the basinal turbidites of the Macigno Sandstones Formation (MSF) recording the first foredeep in the northern Apennines (Italy). This stratigraphic succession shows an evident stacking pattern that, besides the basal part (200 m thick) dominated by fine grained and thin beds, is characterized by seven magnificently exposed sandstone lobes (from 90 to 19 m thick) separated by well-developed fine-grained interlobe deposits. Thanks to the detailed facies analysis, not only a well-defined architectural hierarchy has been identified, but a facies scheme has also been proposed, whereby cyclic recurring of three facies associations has in turn been identified. Each facies association records a well-determined evolutionary stage of lobe development, where an early depositional stage of the lobe building phase is followed by a flow impact and bypass stage recording flow decelerations and hydraulic jumps induced by the morphology created by the underlying depositional lobe. The latter phase promotes a switching and channel feeder avulsion that heralds the third phase of abandonment with the deposition of fine-grained interlobes. The facies stacking pattern and the evolutionary model of the MSF - also supported by other similar field and experimental works - are intended as a contribution to the understanding of intrinsic autogenic mechanisms at the basis of sand-rich depositional lobe development.