The TPB is a classical area for Messinian stratigraphy. The work of Sturani (1973) provided a fundamental contribution to the debate on the Messinian Salinity Crisis in the 70's. The "normal" succession that he described at Alba, was for long time considered as representative of the Piedmont Messinian stratigraphy. Further researches have shown that Piedmont Messinian sediments mainly consist of chaotic deposits and that the "Alba-type" succession is only a local exception. The application of a physical stratigraphic approach, on the sediments exposed along the northern edge of Langhe, has allowed to propose a new stratigraphic scheme for the Messinian succession of the TPB. Three mappable unconformity bounded stratigraphic units have been recognized. Unit 1 groups the marine pre-evaporite sediments and the primary evaporites (Gessoso-solfifera Fm. of Alba). Unit 2 is floored by an erosive surface, associated with an angular unconformity, correlated to the "intra- Messinian unconformity" (IMU), cutting into the primary evaporites and down to the marine pre-evaporite and pre-Tortonian sediments. This unit is entirely represented by chaotic sediments, that consist of resedimented blocks of evaporites and carbonates (including CH4-derived carbonates), floating within a poorly exposed fine-grained matrix. It forms an irregular body, several kms wide and with a maximum thickness of 300 m, and is referable to the lower part of the "post-evaporitic stage". The chaotic sediments are the proximal portion of coeval chaotic facies, imaged by seismic data (Mosca, 2006) north of the study area, where they are buried below a thick Plio-Quaternary cover. The genesis of these sediments has been referred to large scale gravity driven phenomena, even if a concomitant contribution of shale diapirism and CH4-rich fluid expulsion has been also hypothesized for their origin (Dela Pierre et al., 2002; Irace, 2004; Irace et al., 2005). Unit 3 consists of terrigenous sediments, correlatable to the "Lago-Mare" deposits, and rests unconformably on both Units 1 and 2. The main results of this contribution can be summarized as follows: - in large sectors of the TPB, the Messinian succession is made up of chaotic sediments. Their common occurrence reflects a large-scale sedimentary instability during the lower part of the "post-evaporitic stage"; - the chaotic deposits are bounded at the base by an erosive surface, correlatable to the IMU, that is associated with an angular unconformity, clearly suggesting its tectonic nature; - the triggering mechanism that favoured sediment failure must be looked for in the ongoing tectonic deformation. However, a not negligible role, in promoting sedimentary instability, could have been played by the upward rise of CH4-rich fluids.
Physical stratigraphic scheme of the Messinian succession of the Tertiary Piedmont Basin (TPB): new data from the Langhe region.
Irace A;Dela Pierre F;Festa A;Mosca P
2006
Abstract
The TPB is a classical area for Messinian stratigraphy. The work of Sturani (1973) provided a fundamental contribution to the debate on the Messinian Salinity Crisis in the 70's. The "normal" succession that he described at Alba, was for long time considered as representative of the Piedmont Messinian stratigraphy. Further researches have shown that Piedmont Messinian sediments mainly consist of chaotic deposits and that the "Alba-type" succession is only a local exception. The application of a physical stratigraphic approach, on the sediments exposed along the northern edge of Langhe, has allowed to propose a new stratigraphic scheme for the Messinian succession of the TPB. Three mappable unconformity bounded stratigraphic units have been recognized. Unit 1 groups the marine pre-evaporite sediments and the primary evaporites (Gessoso-solfifera Fm. of Alba). Unit 2 is floored by an erosive surface, associated with an angular unconformity, correlated to the "intra- Messinian unconformity" (IMU), cutting into the primary evaporites and down to the marine pre-evaporite and pre-Tortonian sediments. This unit is entirely represented by chaotic sediments, that consist of resedimented blocks of evaporites and carbonates (including CH4-derived carbonates), floating within a poorly exposed fine-grained matrix. It forms an irregular body, several kms wide and with a maximum thickness of 300 m, and is referable to the lower part of the "post-evaporitic stage". The chaotic sediments are the proximal portion of coeval chaotic facies, imaged by seismic data (Mosca, 2006) north of the study area, where they are buried below a thick Plio-Quaternary cover. The genesis of these sediments has been referred to large scale gravity driven phenomena, even if a concomitant contribution of shale diapirism and CH4-rich fluid expulsion has been also hypothesized for their origin (Dela Pierre et al., 2002; Irace, 2004; Irace et al., 2005). Unit 3 consists of terrigenous sediments, correlatable to the "Lago-Mare" deposits, and rests unconformably on both Units 1 and 2. The main results of this contribution can be summarized as follows: - in large sectors of the TPB, the Messinian succession is made up of chaotic sediments. Their common occurrence reflects a large-scale sedimentary instability during the lower part of the "post-evaporitic stage"; - the chaotic deposits are bounded at the base by an erosive surface, correlatable to the IMU, that is associated with an angular unconformity, clearly suggesting its tectonic nature; - the triggering mechanism that favoured sediment failure must be looked for in the ongoing tectonic deformation. However, a not negligible role, in promoting sedimentary instability, could have been played by the upward rise of CH4-rich fluids.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
