Osservazioni geologiche e paleontologiche su di un orizzonte a piante fossili del Cenomaniano di Monte Chianello (Appennino meridionale)

A new calcareous-dolomitic horizon of a platy-dolomite type (sensu BERNIER, 1994), containing the remains of a Cenomanian continental palaeoflora, is reported. The platy-dolomite crops out in the Mount Chianello carbonate ridge (sheet tav. I.G.M. 1:25.000, 198 IT S.W. - Castel San Lorenzo), dominating the village of Magliano Vetere (SA). This ridge is a deformed part of a Mesozoic-Cenozoic platform (campano-lucana platform in D'ARGENIO et alii, 1973; apenninic platform in MOSTARDINI & MERLINI, 1986). A 200 m thick interval of the sequence has been sampled in order to date this platy doloinite. On the basis of facies, it has been divided into four intervals which are, from bottom to top: a) 50 m stromatolitic dolomite and limestones below the platy dolomite, showing the sedimentological features of a restricted lagoonal environment in an inner carbonate platform. Among the microfossils, Sellialveolina viallii COLALONGO, Nezza-zata simplex OMARA, Pseudolituonella reicheli MARIE, Biplanata peneropliformis HAMAOUI & SAINT-MARc and Pseudorhapydionina dubia (DE CASTRO) are present. This fossil assemblage indicates in age ranging from the base of the Cenomanian up to the middle Cenomanian (DE CASTRO, 1991). b) 70 m thin-bedded dolomite and dolomitic limestones of which the typical platy dolomite facies constitutes 40 m in which sets of millimetre-scale laminae are frequently present and often contain terrestrial floral remains on the laminae surfaces. The fossi-liferous horizon is ascribed to the middle-upper Cenomanian by, the presence of Nezzazata simply OMARA, Biconcava bemori HAMAOUI & SAINT-MARC, Biplanata peneropliformis HAMAOUI & SAINT-MARC Chrysalidina gradata D'ORBIGNY, Pseudorhapydionuna dubia (DE CASTRO), Pseudorhapydionina laurinensis (DE CASTRO). The sedimentological features of the platy-dolomite and fossil flora content suggest deposition in a tidal flat under upper- intertidal-supratidal conditions and, in particular, in an algal-marsh environment often exposed to dessication and storms, located close to a coastal area with primeval Angiosperms and Conifers. The first palaeontolological studies on this continental flora, allow its to ascribe some fossils to Sapindopsis sp.. c) 42 m limestones and bioturbated dolomite which, marks a drastic change in the environmental and sedimentological conditions on the carbonate platform, with heavily bioturbated bioderrital material (namely fragments of Rudists), often in massive lavers, proving a swift transition to deeper open marine conditions. In particular, the first 18 metres of this interval have many traces of Thalassinoid bioturbation. The tracks and burrows give an unmistakable appearance to the rocks and make this an important marker horizon in the whole ridge of Mount Chianello-Mount Vesole. Because of the presence in thin section of Dicyclina sp., Cisalveolina fraasi (GLMBEL) and Pseudorhapydionida casertana (DE CASTRO), a late Cenomanian age is assigned to this interval. d) 43 m of limestones and dolomite. The sedimentological features indicate an open carbonate platform emironment often with coarse biodetrital elements and in the upper part, a tendency to the emersion. This interval has been assigned to the Turonian by the presence of Murgeina apula (LUPERTO-SINNI). Biconcava bentori HAMAOUI & SAINT MARC and small Moncharmontia apenninica (DE CASTRO). In conclusion, the above observations show that the sequence including the platy dolomite of Magliano Vetere, ranges in age from the Lower Cenomanian to the Turonian. The flora of the platy dolomite is Middle Cenomanian in age and reflects an upper intertidal-supratidal environment. The plant remains ascribed to the genus Sapindopsis constitute the first Italian finding of this type and fit well with a shallow marine environment close to a coast line. The studied sequence documents peritidal or paralic environments with a restricted water circulation in the Lower Cenomanian, with a strong tendency to stagnation in the Middle Cenomanian (platy dolomite). The restricted conditions change drastically in the Upper Cenomanian with its bioturbated rudist fragment-rich horizons. As other similar horizons in the Mesozoic carbonatic sequences of the Southern Apennine (BRAVI, 1995), the platy dolomite of Magliano Vetere seems to fit well, due to its age, with a marked eustatic sea level minimum in the Middle Cenomanian; tis minimum is recognizable in the 3degrees order eustatic curve of HAO el alli (1987). As a final considerations, it could be said that the studied sequence is relevant to two important currents of research: The analysed stratigraphic imerval, because of its quality and thickness, is particularly appropriate for studies of the enironmental and ecological changes in carbonate shallow water platforms during the global anoxic event at the Cenomanian-Turonian (C/T) boundary, at present the object of important research (ERBA et alii; 2001; FORSTER et alii, 2001: KOLONIC et alii, 2001, WALSWORTH-BELL et alii, 2001). In the succession studied the Cenomanian Turonian boundary is located between the intervals c and d (see fig. 3). This transition represents a short period during which, in the neritic realm, strong environmental change and an important evolutionary overturn are associated. In the hydrocarbon context, the main source rocks in the Italian sedimentary sequences are constituted by organic-rich intervals deposited in shallow carbonate platform environments or in deep water interplatform basinal settings (KATZ et alii, 2000). Although similar rocks are present in a number of different stratigraphic levels, only those in the Upper Triassic-Lower Triassic (accumulations of Malossa, Rospo and Gela), Middle Triassic (accumulation of Villafortuna) and Upper Cretaceous (Monte Alpi) are at present known to have generated oil. Detailed geochemical studies (KATZ et alii, 2000) indicate that there is a sharp contrast in the stable isotope composition of the organic matter contained in the different Italian carbonate source rocks. The Triassic and Toarcian organic matter is isotopically light (delta C-13(bitumen) <-26%), while that of Cretaceous and Oligo-Miocene age is isotopically heavy (delta C-13(bitumen) heavier than 26%). From this distinction two different oil populations are derived, associated with carbonate source rocks. These are an isotopically light population which includes the oils at Rospo Mare, Malossa and the oil at Maiella; and an isotopically heavier Population, including the oils at Monte Alpi, Bagnolo and the Trarnutola seep (KATZ et alii, 2000). In this perspective, the deposit studied could constitute a source rock for oil plays in the Southern Apennines, for similar anoxic Middle Cenornanian thin-laininated facies have been frequently observed by one of us (S. Bravi, Carg Project) in other sectors of the chain (eg.: M.te Toberma, province of Salerno).