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stratigraphic succession of paleogeographic Corniano and assumptions (Trentino). Genesis of

di Paolo Balocchi

Introduzione
Scopo dello studio è quello di misurare una successione stratigrafica analizzando le diverse facies presenti e descriverne le caratteristiche macroscopiche e microscopiche.
Si vogliono ricavare le caratteristiche stratigrafiche, sedimentologiche e paleontologiche della serie, per definire un modello paleogeografico inerente alle aree dove affiorane le medesime facies.
La successione è costituita da tre principali formazioni: Scaglia Variegata (membro superiore della Formazione del Biancone); Scaglia Rossa; Calcare di Torbole. La formazione al tetto della successione è ben differenziabile in affioramento grazie alla presenza di un hard – ground che mette a contatto la Scaglia Rossa e il Calcare di Torbole, mentre più difficile è stato definire il limite tra Scaglia Variegata e Scaglia Rossa, causa le loro similitudini litologiche e sedimentologiche. Lo studio delle sezioni sottili ha consentito di ricavare il contenuto paleontologico e di definire il limite biostratigrafico tra le due formazioni.

Inquadramento geografico
La successione è affiorante a nord del paese di Corniano lungo the trail across the road passing through the country and continues to the outside.
The country is geographically situated in the region of Trentino - Alto Adige, between Mori (south of Rovereto) and Torbole. To reach the estate is able to traverse the road that leads to Loppio Mori and Torbole, where you go in Agriculture Directorate Manzano; Nomesino and finally Corniano (Fig. 1).

Geological
The general stratigraphic framework of the Adige Valley consists of several formations characteristics of different depositional environments. The area is part of the Alpine chain, formed from the Late Cretaceous as a result of the collision between the African plate and Eurasian plate, causing the closure of the Tethys Sea. These tectonic movements, also associated with subsidence and rising relative sea-level in ancient times (pre-collisional), have led to different conformations Paleogeographic.

As for the series of coin, is represented by a stratigraphic succession outcropping and prominently includes the following formations (Fig. 2; Castellarin, 1971):

• Scaglia Variegata: The base is composed of calcilutiti layered ivory white or gray (rarely pink) with the presence of bioturbation and intercalated with marly gray levels - blackish (BOSELLINI Et al., 1978) with thicknesses ranging from a few cm to 20-30cm. Are visible nodules or beds of flint with red or gray. There are radiolaria and foraminifera (BOSELLINI et al., 1978) and at the base by tintinnidi (Castellarin, 1971). In the geological map of the latter, the Scaglia Variegata is a member of the higher education of Biancone. Age: Neocomiano - Albian.
• Scaglia Rossa : more or less marly micritic limestones (BOSELLINI et al., 1978) or laminated limestones (Castellarin, 1971) red or white. Are visible layers or nodules of flint and marl intrastrati. Planktonic foraminifera ( Rotallipore, Globiotruncane ) are abundant. The limonite this paste in the fund is responsible for the reddish color (BOSELLINI et al., 1978). The roof is a hard - ground with a temporal hiatus variable from location to location (Eocene - Paleocene). Age: Albian - Maastrichtian.
• Limestone Torbole : it consists of massive stone blocks and poorly stratified calcarenitic with a high content of fossil type macroforaminiferi, coralline algae neritic environment (Luciana, 1989). Age: Lutetian.

facies analysis of the sequence of Corniano

Corniano series (see end of article) shows, the macroscopic scale, as a concordant succession from the base to the roof, and a discontinuity of type paraconformity (BOSELLINI et al., 1997) is marked in outcrop by a hard-ground structure, is the study of thin sections showing a discontinuity paleontological .
The methods used during the in situ detection, are classic geology of the country. During the measurement sequence with dipstick, have addressed issues primarily due to intense tectonic deformation in the area, which has produced a succession of tiltaggio west, and in the central part of the series is a sporadic tree cover in patches that in some cases prevented detection. It has adopted a system of discontinuous sampling, collecting those samples of the different facies to perform microscopic investigations are more detailed than the simple macroscopic description. For the microscopic study of carbonate rocks have been referred to the classification of DUNHAM (1962).

The study carried out in the country macroscopic and microscopic study, have the following distinct facies within the lithological formations outcropping:

1. Scaglia Variegata Formation

This training has a measured thickness of 4.2 m of age Turroni - Santonian. The contact to the roof is not visible, but microscopic investigations it was possible to put a limit whereas the loss from the radiolaria which determine the transition to the overlying formation.

Facies of Limestone - Limestone marl a Radiolaria
marly to fine texture (wakestone) from gray to red, the color that lightens the base to the roof. The tabular facies is divided into layers, the thickness varies from 10 cm to 15 cm, sometimes due to the deposition of laminated shells of planktonic foraminifera in thin layers. Are contained within tabular layers and layers of black flint nodules or red, and rare type structures mineralized manganese dendrites. Interlayers levels thin (from millimeters to centimeters) of Marne for gray (Fig. 3).

microfossils: radiolaria, planktonic foraminifera (type Dicarinella Turonian - Santonian, Whiterinella Turonian - Santonian) which appear to be silicified. Washed from the interlayers was found a fossil content consists of poorly preserved and silicified foraminifera planton classification unclear.

2. Formation of the Scaglia Rossa
The Scaglia Rossa has a thickness of 59.0 m and a stratigraphic distribution from the Turonian to Maastrichtian. This formation is characterized by non-cyclic alternations of three main facies described below. The transition to the roof with overhanging formation appears to be clearly visible through a clear and hard - ground (hardened crust more compact) of Maastrichtian age. The microscopic analysis shows an increase in fossil content to the roof and a sudden change of faunal associations between the Scaglia Rossa Formation of Limestone Torbole. The facies are:

facies of limestones -
marly marly limestones (Fig. 4) layers divided into tabular mostly regular, the thickness varies from 5 cm to 15 cm. They have variable color from white to red, to red and pale gray, depending on stratigraphic position. The roof of the training in the last meters There is a hardened crust coarser texture where the stratification is lost (hard - ground).

E 'possible to further differentiate them according to texture:

• facies of limestones - marly fine texture : mudstone to fine texture at the base of the formation, with nodules and layers of red chert horizons tabular .
• facies of limestone - marl woven medium - fine : wakestone woven medium - fine and sometimes coarse, placed in the roof of the training, with poor or nearly absent levels of flint nodules.

These two divisions are quite recognizable even in the country, being one of them separated by a thickness of 18.2 m of nodular limestone facies. The contact to the roof and the base of this facies is always unclear and poorly identifiable, through a gradual transition.

microfossils: a second you can find the stratigraphic position of planktonic foraminifera (type Marginotruncana Turonian - Santonian in the basal part; Globotruncane Campanian - Maastrichtian at the summit).

nodular limestone facies of nodular limestones
coarse texture (packstone - wackestone), divided in piccoli straterelli composti da matrice fangosa fine di colore rosso – grigio e talune volte assume delle sfumature di colore bianco. Nelle facies alla base della formazione assumono colorazioni più rossastre, mentre al tetto si hanno colorazioni più biancastre.

Microfossili: foraminiferi planctonici tipo Marginotruncana Turoniano – Santoniano).

3. Formazione del Calcare di Torbole
Tale formazione si presenta con uno spessore di 5.20 m di età Eocenica e caratterizzata da una sola facies:

Facies dei Calcari Massicci
Calcari (pakstone) grigi talvolta anche giallastri mal stratificati con tessitura medio – fine. Risulta essere compatto e resistente.

Microfossili: Macroforaminiferi (tipo Nummulites Eocene, Discocicline Eocene), piccoli foraminiferi planctonici, rodoliti, alghe.

Analisi Stratigrafica deposizionale della serie di Corniano
Nelle facies appena descritte non si è rilevata alcuna ciclicità nel processo di sedimentazione; le caratteristiche più salienti sono la tessitura e la colorazione che variano procedendo dalla base al tetto. Anche il contenuto paleontologico risulta differente, con morfologie più antiche alla base della serie e più recenti verso il tetto, questo a indicare un’assenza di disturbi tettonici fragili (Faults or diaclasi) to displace the stratigraphic sequence.
stratigraphically the series appears to be consistent from base to roof, with visible layering formed by banks of more competent material ( facies of limestones - marl, marly limestone of Facies in radiolarians, the nodular limestone facies ).
analysis of facies and microfacies have been able to set limits and determine the various formational environments, in order to obtain the evolution - stratigraphic depositional series Corniano. The presence of
Dicarinella , Whiterinella Marginotruncana and put the first number in the range of 32.0 m Turonian - Santonian. In this interval are present, in continuity of sedimentation, the two formations of the Scaglia Variegata and the Scaglia Rossa above. To determine the boundary between the two formations is considered the disappearance of the forms in place radiolarians to 4.2 m (Fig. 6). In the first few meters of the Scaglia Variegata, there is a deposition of a marly limestone of Facies radiolarian characteristics of a closed environment of the deep sea, little oxygen (BOSELLINI et al., 1978). Just the appearance of
Marginotruncana for the start of sedimentation of the Scaglia Rossa (fig.10) with marly limestone facies of fine texture and nodular limestones . In this case there is a deposition of facies, sometimes with a pinkish red hue, due to the presence of limonite (2Fe 2 O 3 • 3H 2 O), a mineral of iron oxide hydrate which indicates a oxygen-rich environment. The texture varies from one end of the marly limestone facies that grossly nodular limestones, may indicate different environments. While the first facies is a not confined to the open sea, deep and rich in oxygen, the second seems facies deposited in a similar environment but with a hydrodynamic higher than the previous.

Above the Campanian - Maastrichtian, determined by the appearance of the genus
Globotruncana (34.0 m; fig.7), we have the marly limestone facies of a medium texture - fine. Again the red color and the texture variable reveal a non-secure environment of shallow sea oxygenated. The variability of textural changes from medium - an end to 34.00 m remains nearly unchanged up to 54.00 m, where he began a coarser sediment. Up to shade the roof in a very compact hard crust which is lost in the last meters layering and texture is more coarse, very similar to the nodular limestones of Facies . This hard - ground (fig.7, fig.8) is in Upper Maastrichtian (disappearance of Heterohelidae ) and represents an erosional surface that shows a hiatus around the Paleocene. The similarity of facies between hard - and ground nodular limestones may indicate the same origin and same environment. In fact, several authors (FUGANTI, 1964; Castellarin, 1971, 1972; BOSELLINI & Luciana, 1985) have suggested tectonic rhythmic pulse that led to the emergence of the area with the deposition of the Scaglia Rossa conglomeratic levels, demonstrating that these events are gaps produced by stripping and underwater portions conglomerate deposits are the result of gravitation.

Eocene (appearance of Nummulites and Discocicline ) is a marine carbonate platform sedimentation (shallow sea) facies represented by a massive Limestone (Limestone Formation of Torbole). These new conditions are due to tectonic processes of collision between the Eurasian and African plate that led to the lifting of the Trento Plateau (with deposition of deep sea), and the formation of a new configuration represented by paleogeographic Lessini - Shelf (BOSELLINI, 1989 ).

paleogeographic model air coin
The area surrounding the succession of Corniano where I found the same lithological formations, from the Cretaceous can be considered an environment of deep sea open place below the minimum oxygen (from-700m to -100/-1500m now, but in the Cretaceous was extended to -2000 /-3000m depth) where it was the deposition of the Scaglia Variegata, under anoxic conditions and protected environment, confinement, still below the low oxygen, were deposited in the Black - Shale. Above the minimum oxygen was an increasingly deep and oxygenated but this time not limited where you filed the Scaglia Rossa (Fig. 9).
According to the model proposed the Black - Shale and the Scaglia Deporsi were varied in sheltered conditions, with limited amounts of oxygen. These conditions could be possible if there were ups and downs of the structural support the diversity statement. If we consider now the active tectonics in that particular moment in geological history (BOSELLINI et al. 1978; Cassinis & Castellarin, 1988; DOGLIONE & BOSELLINI, 1987), it is possible to envisage a more complete model than the previous.

The tectonic conditions of the late Cretaceous (Dogger - Malm) provided a paleogeographic conformation formed by a platform of Trent (AUBOUIN 1964; Castellarin, 1972) Drowning (for soothing) and becomes a sblocchettato plateau (Fig. 10; BOSELLINI & Winterer, 1981; DOGLIONE & BOSELINI, 1978). Under these conditions, structural and topographic high formed by topographic low, you can define the different environments that we have obtained previously (Fig. 11):

1. Euxinico Environment: the deposition of the Black - Shale in protected conditions deep sea in the absence of oxygen and placed in low-current deep structural
2. Environment deep in the zone of minimum oxygen : the deposition of the Scaglia Variegata conditions in the deep sea (below-700m depth )
3. Environment deep zone above the minimum oxygen : the deposition of the Scaglia Rossa in open sea conditions.

the Scaglia Rossa are several intervals variable in texture, with coarse facies as Facies of nodular limestone, deposited on structural highs in the open sea and under conditions of ocean currents due to denudation ( BOSELLINI & Luciana, 1985) are formed of hard - ground.

A different pattern is Eocene (Fig. 12), where the deposition of facies belonging to the Limestone indicates a Torbole platform environment. This may be related to the lifting of the Trento Plateau due to the compression between the African and Eurasian plates. We are forming new environmental conditions, shallow carbonate platform and strong hydrodynamics Lessini - Shelf (BOSELLINI, 1989, 2004), with the deposition of Tertiary limestones.

conclusions from the study - stratigraphic sedimentological detail the sequence of coin was possible

• microscopic analysis has defined the beginning of sedimentation of the Scaglia Rossa (appearance of Marginotruncana) placing the stratigraphic boundary between the Scaglia Variegata e la sovrastante Scaglia Rossa;
• determinare le differenti facies e descrivere l’evoluzione stratigrafico – deposizionale delle Formazioni affioranti;
• definire un modello paleogeografico dell’area di Coniano e delle zone limitrofe dove sono presenti le medesime formazioni.

Dal modello paleogeografico ipotizzato nel presente studio, è possibile determinare alla scala regionale il medesimo assetto paleogeografico, con la deposizione delle differenti facies in corrispondenza di zone differenti. Tale modello è influenzato notevolmente dalle strutture tettoniche presenti nell’area di studio. La linea Giudicariense con la sua struttura a fiore eoalpino (E Bosellini AL. 1978; Bosellini & Doglioni, 1987) determines a di alti e bassi Assetto struttura determinando ambienti deposizionali different.

Bibliografia

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