Ricerche palebotaniche (palinologiche e paleocarpologiche) sulla successione 'Villafranchiana' del Rio Ca' Viettone (Torino, Italia)

Abstract

Near Levone Canavese (25 km north of Turin, NW Italy) the Ca' Viettone stream erodes a succession of alluvial fan sediments which include muds, sands, and gravels. The basal part of the succession (5 m) yielded abundant plant remains with a low-rank coalification, suitable for paleocarpological and palynological sampling, whereas in overlaying layers organic matter was oxidized. Aim of this study is to integrate paleocarpological and palynological results to develop a chronostratigraphic framework of vegetational succession. Fruits and seeds were collected through bulk and surface sampling of 21 outcrops located approx. 1 km along the stream bed, between sections 1 and 21 (Fig. 1). All recovered assemblages are interpreted as belonging to the same, extremely rich carpoflora, which includes 130 taxa (see Table 1), belonging to 41 exotic and 48 native genera. The high number of characteristic 'Tertiary" taxa, including subtropical elements such as: Symplocos, Cinnamomum, Eurya, Qcotea, Temstroemia, Toddalia, Visnea, etc. is particularly interesting. Palynological samples were collected from locally sandy mud layers from sampling sites 1, 4, 10 and 21 (see Fig. 1). Pollen enrichment methods were employed for all samples. Given the quantitatively and qualitatively rich pollen flora, analytical and synthetical pollen diagrams have been developed for site 10 (Figs. 1 & 2); these show apredominance of so called "Tertiary" floristic groups, represented by both the Taxodiaceae group and tropicalsubtropical elements such as Symplocos, Clethraceae/Cyrillaceae, Magnolia type, Rutaceae, archaic tricolporate Fagaceae (p. p. cf. Trigonobafanus), Rhoiptelea, etc. From a stratigraphical point of view, the latter group characterises the pollen sequence. Comparisons between paleocarpological and palynological data highlight similar paleofloristic features, which form the basis for the proposed reconstruction of a highly diversified forest community with outstanding south-eastern Asian affinities, in which evergreen (particularly shrubs), summergreen and coniferous taxa grew. From a biostratigraphical point of view, the occurrence of the same tropical-subtropical elements (at least at genus/family level) in the two datasets, assigns the Rio Ca' Viettone succession to the "Macrian" local vegetational zone (Fig. 7), which pre-dates the Villafranchian.