Anatomy of an upper Zanclean mixed silici-bioclastic transgressive system: The role of high-frequency eustasy and tectonic subsidence on the stratal architecture of the Spungone Sandstone, eastern central Italy

The upper Zanclean Spungone Sandstone records the onset of a major transgression along the western margin of the Periadriatic Basin, eastern central Italy, and comprises a 130 m-thick succession of shallow-marine strata characterized by a highly regular cyclical stratigraphy. Integrated analysis of both physical and biogenic structures, together with the identification of major stratal surfaces and the reconstruction of the stratigraphic framework, proved essential for understanding the erosional and depositional processes that occurred during sediment deposition. Eight high-frequency de-positional sequences, typically a few to tens of meters thick, have been identified based on the repetitive vertical stacking of facies associations. Sequences are bounded by low-relief erosional discontinuities recording abrupt shallowing and modest basinward shifts of facies, interpreted as amalgamated sequence boundaries formed by subaerial exposure during relative sea-level lowering and marine planation during the subsequent relative rise.Each sequence is characterized by remarkably similar facies successions organized in a deepening-upward pattern and interpreted as transgressive systems tracts, partially truncated by erosional beveling at the overlying sequence boundary. Deposits attributable to the highstand and falling-stage systems tracts are absent, an architectural style typical of depositional sequences formed in low-accommodation basin-margin settings. Patterns in facies composition are suggestive of deposi-tion in shoreface settings under storm conditions of varying intensity, revealing two fundamental types of sedimentary motif: weakly storm-affected and strongly storm-dominated shorefaces. The overall sequence-stratigraphic organization of the Spungone Sandstone exhibits an aggradational stacking pattern of component sequences, suggesting that they were almost certainly generated by high-rate, small-amplitude glacio-eustatic sea-level fluctuations superimposed on lower rate, tectonically-induced subsidence.Petrographic analysis of these well-sorted mixed siliciclastic-carbonate sandstones indicates that the bioclastic com-ponent mainly consists of skeletal debris derived from a contemporaneous cool-water carbonate factory. The terrigenous fraction is instead dominated by limestone lithic fragments, chert, and metamorphic quartz that were sourced from the pro-gressive unroofing of the Mesozoic-Cenozoic cherty limestones of the Umbria-Marche domain and the Messinian turbiditic sandstones of the Laga Formation exposed in the uplifted Apennine sectors west of the study area.