Proposta di classificazione delle “Pseudocarniole” dell’Alta Valle di Susa (Alpi Occidentali)

Nelle Alpi occidentali, tra le rocce indicate in letteratura come evaporiti triassiche sono presenti litofacies che mostrano marcate differenze di tessitura, composizione, geometria, posizione stratigrafica e strutturale evidenziando quindi probabili differenze di genesi ed età. Tali rocce, definite in via provvisoria “pseudocarniole” a causa dell’apparente convergenza di facies con le carniole autentiche, sono l’oggetto del presente studio.
La ricerca si è sviluppata principalmente sul terreno ed è stata successivamente approfondita avvalendosi di microanalisi (catodoluminescenza e SEM).
In base ai caratteri litologici, tessiturali, strutturali, ai rapporti con altre rocce, alle relazioni con la deformazione, il carsismo e l’alterazione viene proposta una classificazione distinta in diverse facies:
- pseudocarniole “autoclastiche” (termine provvisorio): brecce monogeniche dolomitiche o calcaree, in cui è ancora ben distinguibile
l’originaria stratificazione della roccia d’origine;
- pseudocarniole residuali: brecce monogeniche dolomitiche o calcaree, caotiche e talvolta ricche di matrice;
- pseudocarniole detritiche: brecce e conglomerati carbonatici poligenici;
- pseudocarniole incrostanti: crostoni superficiali di cemento carbonatico;
- pseudocarniole tettoniche: brecce carbonatiche caratterizzate da tessiture variabili ma localizzate in corrispondenza di contatti tettonici.
In base ai dati emersi dalla recente letteratura e dai risultati delle ricerche condotte nell’ambito del presente studio, la genesi delle pseudocarniole risulta legata all’interazione e alla sovrapposizione di numerosi processi tra i quali il meccanismo-guida è rappresentato dalla dissoluzione di rocce carbonatiche e gessoso-anidritiche indotta dalla circolazione di fluidi. La diffusa dissoluzione ha instaurato una situazione di generale instabilità dell’ammasso roccioso con conseguente assestamento gravitativo i cui riflessi superficiali hanno attivato processi di deformazione gravitativa di versante.
Alla dissoluzione si accompagnano generalmente altri processi (tettonica distensiva e trascorrente, tettonica gravitativa, idratazione delle anidriti, carsismo, ecc.) che possono agire in momenti diversi e con intensità molto variabile. Ne risultano quindi diversi tipi di pseudocarniole che costituiscono corpi di dimensioni, geometria, associazioni assai variabili.
Infine i dati cronologici indiretti ricavati da analisi polliniche e da osservazioni dei rapporti intercorrenti tra le pseudocarniole e gli altri termini geologico-strutturali (es. assenza di evidenze del coinvolgimento di tali rocce da parte della deformazione e del metamorfismo alpino), concorrono nell’indicare che il processo evolutivo di tali rocce appare svolgersi in un intervallo di tempo in genere ampio, diverso a seconda delle facies considerate ed inquadrabile tra un limite massimo rappresentato dal post-Oligocene ed il Presente, in cui la rielaborazione carsica pleistocenica rappresenta molto spesso il processo maggiormente responsabile dell’attuale aspetto.

In the Italian, French, Swiss and German Alpine geological literature, among the lithological terms used to define Triassic formations, it is possible to find a group of rocks called "carniole", "cornieules", "cargneules", "Rauhwacken". Normally, they are interpreted as evaporitic Triassic rocks (further on described as "true" carniole) but they can also be described as rocks similar to the evaporites with some differences in their texture, composition, geometry, stratigraphical and structural distribution as well as in their genesis and age (such lithofacies are provisionally known as "pseudocarniole").
Particularly, in the sheets of the Geological Map of Italy, 1:100.000 in scale where the Upper Susa Valley is included (Map 66 "Cesana Torinese"), the above rocks are all interpreted as "Triassic evaporites".
The main object of the present paper consisted in the acquisition of more data concerning the different classification problems for the pseudocarnioles. Throughout a detailed survey and a precise interpretation, integrated by microanalysis, it was possible to elaborate a genetic model for the pseudocarnioles of the Upper Susa Valley. The study has been formulated modularly so that it could be used in other study-fields.
The data collection was mainly developed on the field and subsequently studied by thin section microanalysis, some of which were observed by cathode-luminescence (CL) and by scanning electron microscope (SEM). These close examinations have permitted to specify the relationships among the different types of pseudocarnioles depending on the relationships between the different carbonatic cements found in them. It has also allowed the definition of the fragments nature, the reconstruction of the sedimentary processes that have determined or conditioned their origin, as well as the influences of the deformation processes on their texture and structure.
The field data collection, their modular and normalized fill-in forms (information cards) and the analysis of some pre-established situations has led to the individualization of five principal facies to which correspond many other genetic mechanisms:
- “Autoclastic Pseudocarniole”, constituted by monogenetic breccias (calcareous or dolomitic); the fragments are always angular, and
do not show any kind of rotation signs, they have varying dimensions from less than a centimetre to more than some meters and a chaotic distribution. Sometimes they preserve stratification traces, more or less disguised by the fractures, inherited by the original "protolite". The possible mud matrix is locally associated to more generations of cement. In some cases the spaces between the fragments are empty.
- Residual Pseudocarniole: they are constituted by monogenetic breccias (calcareous or dolomitic), more rarely polygenetic, with angular not selected fragments always with matrix. Locally they show a weak stratification.
- Detrital Pseudocarniole: constituted both by poligenetic breccias, with varying dimensions, matrix always present and clast- or matrix-supported, or by thin sand-silt sediments. Depending on the texture variability, it is possible to make a further subdivision in sub-facies with different genetic mechanisms (detrital sub-facies without transport, detrital sub-facies with transport, this last one distinguished in detrital sub-facies by channelled water transport, detrital sub-facies by stream water transport, detrital sub-facies by stream water from fill-in epigeous karst cavities).
- Incrustation Pseudocarniole: composed by abundant quantities of carbonatic poligenetic cement, polychrome, with small angular fragments of a different nature; typical vacuolar aspect.
- “Tectonic” Pseudocarniole: characteristic varying textures, but all with tabular geometry and distribution along movement planes and shear zones. Inside it is possible to recognize some deformation signs (boudin lithons, movement surfaces) as well as non-deformed portions.
Among the different processes of interaction and/or overlap of the pseudocarnioles, some indicate a predisposing role, some a determinant one and many others a conditioning one.
The main formation mechanism of the pseudocarnioles is represented by chalk and anhydrite dissolution with sulphate-ion rich fluids, that were even able to attack the dolomitic rocks (Fig. 5). The general dissolution and the presence of deep cavities from chalk dissolution have determined the decline of the geo-mechanical characteristics of the rock with the consequent collapse and formation of the "autoclastic pseudocarnioles". Locally, gravity has offered real collapses with the formation of the residual pseudocarnioles. All of this can be followed by one or more cementation phases.
Dissolution can be accompanied by other processes (transcurrent and stretching tectonic, gravitational tectonic, anhydrite hydration, karst, etc.) that can act independently, repeatedly and in different moments, with varying intensity. The rocks resulting from these processes are known as “tectonic” pseudocarnioles and detrital pseudocarnioles, and can also act independently and repeatedly from the processes.
The different types of pseudocarnioles constitute bodies with different dimensions, geometries and associations, each of which with the signs left behind by the conditioning processes.
Along the outcrops, the Ca-carbonate precipitation from the circulating waters and the surface streams has induced the travertines formation with the development of decimetric cemented crusts: the incrustation pseudocarnioles.
The results furnished by the preliminary pollen analysis together with the relationships found among the detrital sub-facies by stream waters and some manufactured objects, indicate that the genetic process of the pseudocarnioles takes place between the post- Oligocene and Today. Thus, it is possible to ascertain that the genesis of the pseudocarnioles does not correspond to the Triassic, but it might start at the very beginnig of the recent evolution.