Soft-sediment deformations in post-collisional calcarenites: a multi-scale descriptive approach

In the time frame after initial deposition but prior to lithification, sediments are frequently prone to physical, chemical, or biological disturbance. The resultant structures – commonly referred to as soft sediment deformation (SSD) - can be explained by a variety of mechanisms, each defined by a distinct set of parameters. Among the factors responsible for upward-oriented, physically-induced disturbance, two main triggering mechanisms are distinguished: (i) Fluidization of sediment, where SSD occurs as a fluid (typically saline water) passes through a layer of solid particles via areas of available pore space, and (ii) Liquefaction of unconsolidated sands, a process that commonly occurs in response to sudden loading on a bed which forces the sediment to transition from a solid to a liquefied state. Liquefaction can moreover be caused by seismic shocks. When subjected to seismic shocks, unconsolidated sand-size sediments tend to decrease in volume, which in turn produces an increase in pore-water pressure and a decrease in shear strength. A contrasting mechanism responsible for SSD is chemical disturbance which is thought to be the result of desiccation, cementation and crystal growth, thermal expansion, and contraction of partially lithified sediment during a continuous spectrum of diagenetic stages.

The origin of SSD remains a disputed topic in clastic sedimentology and a challenging task in outcrop studies. We present the first report of disturbed calcarenites in the “Pietra di Finale”, which crops out along the Ligurian coast, bordering the Ligurian Alps transect of the Western Alps. It represents an Early to Late Miocene mixed carbonate-siliciclastic coastal wedge that unconformably superimposes the Alpine metamorphic units. The "Pietra di Finale" is considered as a low strain region due to the lack of any deformation evidence, including seismic record, suggesting a Miocene tectonic quiescence in the southernmost part of the Alps. The “Pietra di Finale” can be subdivided in two formations: a basal terrigenous sequence resting below calcarenites making up the top of the formation. The calcarenitic formation displays a uniquely well-exposed assemblage of SSD features. These features comprise (i) vertical sediment expulsions recognizable by gross changes in granulometry with respect to that of the hosting sediments, (ii) carbonatic fluid-expulsion veins, (iii) lateral continuity of SSD-prone layers and (iv) sequential vertical and lateral organization of SSDs. The main aim of this study is to unravel the origin of untypically large coarse-clastic injections into the hosting calcarenites, with emphasis put on distinguishing the role of discriminating seismically from diagenetically induced sediment disturbances. Results from a multi-proxy approach: comprising a detailed study of the sedimentological characteristics at the outcrop-scale and photogrammetric investigations of the geometry of the structures and their stratigraphic occurrence; petrographic investigations of both grain and intergranular features (i.e. clasts and cement); as well as compositional and microthermometry analyses of the vein-filling cements, can yield insights into the pivotal role of the fluids as driver of seismicity-induced liquefaction and  uncommon mineralization and intrastratal sediment mobilization.