Soft-sediment deformation structures in seismically affected deep-sea Miocene turbidites (Cilento Basin, southern Italy)

Abstract

Soft-sediment deformation structures (SSDS) are widespread in the upper part of the S. Mauro Formation (Cilento Group, Middle-Late Miocene). The succession is represented mainly by thick and very thick, massive, coarse-grained sandstones, deposited by rapid sedimentation of high-density turbidity currents. The most common SSDS are short pillars, dishes, sedimentary sills and convolutions. They occur mostly in the upper parts of sandstone beds. Vertical tubes of 4-5 cm in diameter and up to 50 cm long constitute the most striking structures. They begin in the middle part of sandstone beds, which are basically massive or contain faint dish structures. These tubes can bifurcate upwards and/ or pass into bedding-parallel veins or dikes. The vertical tubes sometimes form sand volcanoes on the then sedimentary surface. The SSDS are interpreted as the result of earthquake-triggered liquefaction and/or fluidisation of the turbidites that were affected by the seismic shocks. This implies that the deformed layers should be considered as seismites.

Interpreting syndepositional sediment remobilization and deformation beneath submarine gravity flows; a kinematic boundary layer approach

“…Deformation and dewatering of the sandstones in the Lago Santa Maria section that hosts this example have recently been used to infer synsedimentary seismic activity (Valente et al 2014). We suggest that such causal relationships be treated with caution given the protracted syndepositional shearing described above.…”

“…Mulder & Cochonat 1996) or may form by entrainment during dewatering, perhaps triggered by earthquake shaking (e.g. Ross et al 2013;Valente et al 2014). These forms of sediment remobilization can occur on scales from single beds to many kilometres of stratigraphic section.…”

Short length-scale variability of hybrid event beds and its applied significance

“…The sheet-like geometry applies only to the main system architecture, and not to the medium and thin-bedded bed stacks that can pinch-out or change in thickness between the measured sections. In the absence of multi-bed slumping and given the overall coherence of the palaeocurrent indicators, the succession is interpreted as a series of outer fan, lobe aggradation and retreat cycles (Cavuoto et al, 2007;Valente et al, 2014) punctuated by several outsize events that periodically were able to smooth the sea-floor.…”

The Mesoproterozoic ‘seismite’ at Laiyuan (Hebei Province, E China) re-interpreted

“…Van Loon & Pisarska-Jamroży, 2014), which commonly affects only the uppermost few decimetres of sediment, in exceptional cases a metre or somewhat more, which can result in 'giant' seismites (Alfaro et al, 2010). No such deformations are present in the sediments under study, however, although they are common in seismites (see, for instance, He, 2014;Perruca et al, 2014;Sarkar et al, 2014, Valente et al, 2014, Uner, 2014 all in this issue). It should be kept in mind, however, that Liang et al (2002a) apparently used the term 'seismite' not in the sense in which Seilacher (1969Seilacher ( , 1984 introduced it, viz.…”