Concurrence of load-and-flame structures, balls-and-pillows, clastic injectites and shear deformation bands as indicator of seismicity in mixed siliciclastic-carbonate successions (Finale Ligure Basin, Italy)

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The present research explores a diverse suite of soft sediment deformation structures (SSDS) combined with clastic dykes and shear deformation bands distributed throughout a Early Miocene-age mixed carbonate-siliciclastic coastal wedge located on the Ligurian coast (NW Italy) of the Mediterranean region. The studied Aquitanian-Serravallian successions of the Finale Ligure Basin are typified by the presence of lithologically heterogeneous units, internally characterized by abrupt changes of lithofacies distributions and associated interpreted depositional environments. The interface between the terrigenous base and the carbonate wedge in many places features SSDS. Here, we report the discovery of a multiform deformational pattern, ranging from small size (cm-to dm-scale) load-and-flame structures, ball-and-pillows and mini-slumpings, to outsized (several meters in height) chaotic contorted and folded stratigraphic intervals, sand intrusions and diapir- or tree-like water escape structures. Many of the SSDS are related to fast injection of overpressurized fluids after sand liquefaction, and occur in close association with disaggregation and phyllosilicate shear bands, and sand dykelets. The stratigraphic distribution and cyclic repetitions of SSDS-prone intervals unambiguously indicates syn-depositional seismicity, thus suggesting that juxtaposition of SSDS, diapiric injectites, and shear deformation bands can serve as a suitable diagnostic criteria for seismite identification in the field. Since these particular assemblages of deformation structures reflect seismic activity in the area, they provide compelling evidence for previously undocumented tectonic episodes in Langhian-Serravallian times, associated with the latest stages of the Liguro-Provençal Basin opening.