Soft-sediment deformations in post-collisional calcarenites: a multi-scale descriptive approach
Authors: Tamburelli Silvia , Mueller Pierre , Amadori Chiara , Crispini Laura , Maino Matteo , Menegoni Niccolò 
Published: March 2022

&lt;p&gt;In the time frame after initial deposition but prior to lithification, sediments are frequently prone to physical, chemical, or biological disturbance. The resultant structures &amp;#8211; 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.&lt;/p&gt;&lt;p&gt;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 &amp;#8220;Pietra di Finale&amp;#8221;, 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 &quot;Pietra di Finale&quot; is considered as a low strain region due to the lack of any deformation evidence, including seismic&amp;#160;record, suggesting a Miocene tectonic quiescence in the southernmost&amp;#160;part of the Alps.&amp;#160;The &amp;#8220;Pietra di Finale&amp;#8221; 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.&amp;#160;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 &amp;#160;uncommon mineralization and intrastratal sediment mobilization.&lt;/p&gt;