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

Abstract

Fioraso G. et al., Karst phenomena in Messinian succession of Monferrato and Torino Hill (North-western Italy): speleogenetic mechanism and interaction with Pliocenic to Quaternary evolution of Tertiary Piedmont Basin. IT ISSN 0394-3356, 2004.
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-structural arrangement of
Cenozoic succession resting above the gypsum interval.
Three-dimentional array of cave network, size and interconnection magnitude of hypogean conduits and thickness of gypsum volume
involved in dissolution processes are controlled by exumation rate of CTV and consequently by superficial fluvial erosion. From
Northern Monferrato to Torino Hill, the Messinian succession has a monoclinalic configuration dipping of 2÷5° to the depocentral areas
of Asti and Alessandria, so that it is possible to distinguish, from North to South, three types of intrastratal karst:
- "entrenched karst", with gypsum completely dissected by fluvial erosion (i.e. Murisengo and Moncucco T.se areas). Hydraulic gradient is greater than in other sectors of karst complex and dissolution operate across the whole thickness of the evaporite formation;
- "subjacent karst", where gypsum is only partially dissected by superficial drainage system (i.e. Grana M.to and Gessi di Moncalvo
areas). Dissolution phenomena are well developed inside gypsum rocks only above hydrological base-level;
- "deep-seated karst", where gypsum sequence is buried and hydrologically confined below a thick post-Messinian sedimentary
cover, represented by Pliocenic Argille Azzurre and Sabbie di Asti Formations and by Pliocenic to Quaternary Villafranchian Complex.
Due to the thickness of the overburden (> 40÷50 m), deep-seated karst is devoid of any surface expression. Locally dissolution phenomena are linked to the rising of deep water along fault zones.
Development of gypsum dissolution is controlled by lithology and thickness of the coverbeds, generally characterized by low permeability (particularly the clayey matrix of CTV and the Argille Azzurre Formation), that slow down the trasmission of seepage water across the overburden. Just in contact with gypsum, water circulation depends on the pattern distribution of at least 3 fault and fracture systems with E-W, NE-SW and N-S directions. Such discontiniuties assumed the major role in determining the initial flow path inside a gypsum, otherwise characterised by low percentage (4÷8%) of primary porosity. Moreover, fault systems intersect and displace the impermeable bituminous inter-beds (with tickness < 3 m) controlling water circulation between adjacent gypsum intervals.
Tipology and dimensions (cross-section diameter and length) of karst features combined with analisys of cave pattern prove that the
highest dissolution rate occur within the first 15÷20 m below gypsum-coverbed interface: saturation index of solution in respect to
CaSO4 · 2H2O rapidly increase after the entrance inside karst system. Moreover, the restricted number of pipes and horizontal caves
intercepted by underground mining exploitation below the base-level (i.e. Gessi di Moncalvo, Murisengo and Montiglio quarries) confirm that dissolution rates drastically reduce in proximity of the water table, the last one fitting with local base-level: flow velocities in phreatic conditions are lower in comparison with those of vadose zone, so that water is not allowed to have a rapid exchange and speleogenesis does not fully develop inside evapotitic rocks.
Spatial orientation of dissolution pipes diverging from gypsum bedding clearly demonstrate the sequentiality of dissolution phenomena in comparison to litho-structural array of CTV. Locally "flat ceilings" in horizontal caves are altimetrically correlable with terraced fluvial deposits (with highest color index of 5÷7,5 YR) preserved along valley sides of Monferrato and Torino Hill: such features materialize paleo-phreatic surfaces originally related to old fluvial base-levels. Available data point out a late Middle Pleistocene to Olocene age for the karst systems characterized by highest evolution, like in Murisengo and Moncucco T.se areas.
Gypsum karst and related deposits in TPB, representing the products of polyphasic and polychronologic dissolution phenomena,
developed in responce to morphological change induced by differential exumation of CTV. Uplifting of Messinian succession is linked
to the tectonic deformation due to the northern thrusting movements of the TPB succession over padanian foredeep: the ongoing exumation is accompained by progressive downward migration of a dissolution boundary (that gradually comes close to and finally intersects the base of evaporitic succession) and by an evolutionary sequence of karst types providing the transition from deep-seated to entrenched karst.