GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 51 interpreted for the first time on a seismic reflection line (Fig. 1C.i), recorded in the offshore domain by the OGS (Finetti and Morelli, 1973). Seismic velocities (Fig. 1C.ii, Montadert et al. , 1978), allowed the definition of three seismostratigraphic units that were named, starting from the bottom, by Rehault et al. (1984) as: Lower Evaporites, Messinian Salt and Upper Evaporites (Fig. 1C.iii). These three seismic units have been identified together with the name of Messinian Trilogy. Lofi et al. (2011) proposed a new global terminology for Messinian surfaces and depositional units that is based on seismic facies and/or geometrical relationships: Mobile Unit (MU) is the intermediate salt layer (halite according to Ryan et al. , 1973), Lower Unit (LU) the layer beneath the MU, Upper Unit (UU) the one laying above the MU. The MSC units are hereafter referred to this terminology. The LU has a seismic facies corresponding to a group of continuous high amplitude and low frequency reflections, recognized only in the deep basin (Rehault et al. , 1984; Lofi et al. , 2011). Due to its position, immediately below the salt layer, the acoustic energy does not always penetrate deeply enough and thus the LU cannot be well imaged (Lofi et al. , 2011; Geletti et al. , 2014). According to Montadert et al. (1978), the interval velocity is uncertain, ranging between 3200 and 4200 m/s (Fig. 1C.ii). The LU, never calibrated, may be lithologically constituted of carbonates and evaporitic sediments (Ryan et al. , 1973; Rehault et al. , 1984), but it could also contain a large part of detrital sediments eroded from the continental slopes and shelves during the sea-level falling stage of MSC (Lofi et al. , 2005; Sage et al. , 2005). In the western Mediterranean, the MU is characterised by a transparent seismic facies and an associated ductile deformation that generates dome and diapiric structures (Rehault et al. , 1984; Sage et al. , 2005; Lofi et al. , 2011; Geletti et al. , 2014). Halokinesis is observed at lower slope of the western margin, where diapirs generally distort reflectors at the base of the post- MSC sediment cover only (Sage et al. , 2005). Growth faults demonstrate Pliocene activity (Lofi et al. , 2011), even though locally some growth strata in the UU indicate late Messinian salt tectonics (Geletti et al. , 2014). In some places of the deep basin the MU forms diapirs that disrupt the uppermost part of the overlying sediments (Sage et al. , 2005; Lofi et al. , 2011; Geletti et al. , 2014). In the western Mediterranean the UU has an interval velocity of 3500 m/s (Montadert et al. , 1978; Rehault et al. , 1984; Lofi et al. , 2011; Geletti et al. , 2014) and it is characterised by an alternation of high amplitude reflectors and transparent layers thicker in the deep basin than that in the lower slope. Where the thickness is greater, a lower less reflective seismic package is present. One of the uppermost transparent layers is particularly thick and it was interpreted as an autochthonous salt layer, labelled with “s” (Geletti et al. , 2014). The UU sampled within the western Mediterranean basin was described as composed of marly beds, dolomitic or gypsiferous layers (Ryan et al. , 1973; Rehault et al. , 1984). Five MSC surfaces were defined within the Mediterranean basin by Lofi et al. (2011), on the basis of their relationship with downslope Messinian units. These surfaces are labelled as: BS/ BES (Bottom Surface/ Bottom Erosion Surface), IES (Intermediate Erosion Surface) and TS/ TES (Top Surface /Top Erosion Surface). BS/BES correspond respectively to the bottom non- erosional/erosional surface at the base of the MSC units. IES is a term that identifies all those intermediate unconformities present within the MSC depositional units. TS/TES correspond respectively to the uppermost non-erosional/erosional boundary of the MSC deposits. In the upper continental slope the Messinian units are generally absent and the main seismic feature is a strong reflector which truncates the top-lapping pre-Messinian reflectors (Geletti et al. , 2014) evidencing an angular discordance between pre-MSC and Plio-Quaternary deposits (Lofi et al. , 2011). It is interpreted by Lofi et al. (2011) as a time equivalent of the entire MSC units and surfaces of the deep basin. In the West Sardinian margin this unconformity may be the result of the last erosional events linked to the MSC and superimposed on probable earlier erosional surfaces (Geletti et al. , 2014). This unconformity is known as Margin Erosion Surface, MES (Lofi et al. , 2011; Geletti et al. , 2014).