Manifestazioni carsiche nelle evaporiti messiniane del Monferrato e della Collina di Torino (Italia nord-occidentale): analisi dei meccanismi genetici nel quadro dell'evoluzione pliocenico-quaternaria del Bacino Terziario Piemontese.

In Tertiary Piedmont Basin (TPB) widespread karst phenomena involved Messinian gypsum blocks (hectometric to kilometric in extention) cropping out within the clayey matrix of the Valle Versa Chaotic Complex (CTV). The presence of some gypsum quarries in Northern Monferrato area and in Torino Hill (whether opencast or underground) allowed the investigation of speleogenetic phenomena in different structural and stratigraphic contexts. This study has been carried out in order to detect three-dimensional array of karst systems, morphological characteristics of dissolution cavities and sedimentological analysis of karst infill deposits. Moreover, it has been possible to reconstruct some of karst evolutionary stages related to the Pliocenic to Quaternary exumation of TPB. Cilindrical or cone-shaped subvertical pipes less than 5÷6 m in diameter which characterize epigean dissolution morphology are located within upper 10 m of evaporitic rocks, below an impermeable sedimentary cover with thickness ranging between 4÷5 m and 50 m. In depth, dissolution make up subvertical pipes (less than 3÷4 m in diameter) connected with horizontal caves with typical "flat ceilings" and inclined (15÷45°) lateral facets. These forms are caused by a standing of water table combined with water chemical stratification phenomena: local small differences in water density promote natural convection currents inside gypsum caves which enable most pronunciated marginal dissolution by uppermost aggressive layers of water. Orientation of epigean and hypogean morphology is geometrically unrelated in respect to the bedding of CTV gypsum layers. Gypsum dissolution combine with suffosion processes so that transfer of insoluble sediments of the overlying clayey sequence inside empty karst cavities is enabled. This result in a gradual subsidence of the topographic surface or in a sudden collapse of the overburden, with development of cover-subsidence and cover-collapse sinkhole respectively, depending on granular composition and thickness of the coverbeds. Such features are well developed all over the outcropping area of CTV. Epikarst depressions and endokarst caves are filled with intricate sequences of deposits, made up of at least 4 sedimentary facies: a) - "collapse breccia", developed by gravitational sinking of sedimentary coverbed; b) - "massive or laminated silt and clay", due to sediments settling in water-filled karst features; c) - "massive or laminated sands", transported by percolating water inside karst system; d) - "extrusion breccia", generated by injection and squeezing of water-saturated clayey sediments along deepen free portions of caves. Distribution of sedimentary facies within karst network is highly variable in both vertical and horizontal directions. Biostratigraphical analysis carried out on 9 samples collected inside few hypogean cavities allows the detection of rich faunal assemblages covering a wide chronological interval: - Burdigalian (N5/6 Zone) - Zanclean (MPl4a Zone) for samples collected in Montiglio quarry, in which are present Globorotalia puncticulata in MNT-1 and Catapsydrax dissimilis, Catapsydrax unicavus and Globoquadrina dehischens in MNT-2; - Tortonian (N17 Zone) - Zanclean (MPl2 Zone) for samples collected in Moncucco T.se quarry, age confirmed by the presence of Globigerina quinqueloba in MC-13 and of Globorotalia margaritae in MC-23; - Zanclean (MPl3 Zone) for samples GRN-1 collected in Grana M.to quarry, confirmed by the presence of Globorotalia margaritae and Globorotalia puncticulata. On the contrary, sediments collected on Moncalvo quarry (samples MNC-1, MNC-2 and MNC-3) are completely devoid of microfaunal remains. Differences in faunal assemblages from one site to another are due to the specific and local litho-struc