Preliminary quantitative data on a Middle Eocene Nummulites lyelli-dominated bank from Pederiva di Grancona (Veneto, Northern Italy): implications for the paleoenvironmental reconstruction.

The "nummulite bank" is an accumulation of nummulite tests, mainly monospecific, with both microspheric and megalospheric generations of the same species. Moreover, it usually contains more tests of larger B forms as compared with the normal nummulite-bearing limestones.Previous works on nummulite banks have resulted in the proposition of two models. The first, by Arni, suggests that nummulite test accumulations form autochthonous or in-situ banks acting as an effective barrier, analogous to present-day coral reefs. On the other hand Aigner, based upon several evidences of physical transportation shown in sedimentary structure and biofabrics of Nummulites deposits, considered them as allochthonous.So far there is no quantitative data available about the taxonomic composition of nummulite banks. Here we present the preliminary results of a quantitative examination on a case-study from the Middle Eocene of Northern Italy. The samples were collected in Pederiva di Grancona (Vicenza Province, Veneto area). The Nummulites lyelli from this well-known site were described and figured by Schaub in his monograph.The present work is based upon the preliminary examination of 50 grams from a washed sample of the nummulite bank. In this fraction, 509 individuals (A forms) of Nummulites have been counted. The dominant species is N. lyelli accounting for approximately 77.1%, followed by N. striatus with 10.5%. The rest are the accessory species including N. biarritzensis (5.9%), N. discorbinus (4.9%), and N. beaumonti (1.6%). According to the dominance of N. lyelli and the stratigraphic distribution of the other Nummulites species the sample is assigned to the SBZ 17 (early Bartonian).N. lyelli A in this sample has a mean diameter of 5.75 mm, whereas that of N. striatus Ais 4.20 mm. The two most abundant species display a clear difference in size: N. lyelli mean diameter exceeds about +37% in comparison with N. striatus. Given that N. lyelli is also more inflated than N. striatus, they probably had a quite different hydrodynamic behaviour.The taxonomic composition of the bank in this case study shows that it is not strictly monospecific, as in the models from the literature: even if there is a strong dominance of N. lyelli, N. striatus cannot be discarded as negligible. Moreover, there are three more species of the same genus which are present in significant percentages, pointing to an unexpectedly rich nummulite assemblage. This in turn casts serious doubts on the allochthonous model for nummulite bank accumulations.