The concept of "nummulite bank" is well-established in the literature since mid '60s-'70s, from papers of Arni (1965), Arni & Lanterno (1972), Decrouez & Lanterno (1979). In 1982 (and later in 1983, 1985, 1986) Aigner pointed out the possible influence of water energy on the accumulation of the tests and preferred the term "nummulite tell" instead of "nummulite bank". He studied nummulite accumulations from Egypt, coming to the conclusion that the abnormally low A/B ratio there recorded is mostly due to selective winnowing of an originally "normal" assemblage. However, he did not take into account the detailed taxonomic composition of the bank itself.In this work some examples of nummulite "banks" from different countries on the northern side of Mediterranean are studied paying attention to the larger foraminiferal specific diversity. Two "banks" come from the middle and upper Eocene of northern Italy (Pederiva di Grancona and San Germano dei Berici, Berici Mts.), two from the middle and upper Eocene of Romania (Leghia and Cluj, Transylvania) and one from the middle Eocene of Spain (Igualada, Ebro basin). The species diversity is by definition never high, but only in Leghia the bank could be considered as really monospecific. In all other cases the diversity is low to medium, and the species involved are of different sizes. This suggests that, at least in the banks under study, water energy cannot be the only responsible of the accumulation of nummulite tests. To explain the abnormal A/B ratio recorded, it is suggested that particular paleoecological conditions should be involved in the increased production of microspheric tests. The long-living, highly specialized B-forms could be the result of oligo-(or meso-oligo-) trophic conditions in medium to high water-energy conditions.Moreover, it seems possible to distinguish, relying mainly on the taxonomic composition of the bank, different depths of accumulation. The Leghia N. perforatus bank seems the shallowest of all, whereas the Pederiva N. lyelli-N. biarritzensis bank could be the deepest.REFERENCESAigner, T. (1982) - Event-stratification in nummulite accumulations and in shell beds from the Eocene of Egypt. In: Einsele G., & Seilacher A. (eds.): Cyclic and Event Stratification, 248-262, 7 figs. Springer, Berlin.Aigner, T. (1983) - Facies and origin of nummulitic buildups: an example from the Giza Pyramids Plateau (Middle Eocene, Egypt). Abh. N. Jb. Geol. Paläontol., 166(3), 347-368, 12 figs. Stuttgart.Aigner, T. (1985) - Biofabrics as dynamic indicators in nummulite accumulations. J. Sedim. Petrol., 55(1), 131-134, 5 figs. Lawrence, Ks.Aigner, T. (1986) - Biofabrics as dynamic indicators in nummulite accumulations - Reply. J. Sedim. Petrol., 56(2), p. 320. Lawrence, Ks.Arni, P. (1965) - L'évolution des Nummulitinae en tant que facteur de modification des dépôts littoraux. Mém. Bur. Rech. Géol. Min., 32, 7-20, 2 figs. Paris.Arni, P. & Lanterno, E. (1972) - Considérations paléoécologiques et interprétation des calcaires de l'Eocène du Véronais. Arch. Sci., 25(2), 251-283, 9 figs., 2 pls. Genève.Decrouez, D. & Lanterno, E. (1979) - Les "bancs à Nummulites" de l'Eocène mésogéen et leurs implications. Arch. Sci., 32(1), 67-94, 11 figs. Genève.
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|Data di pubblicazione:||1999|
|Titolo:||Some examples of "nummulite bank" from Italy, Spain, and Romania: real "banks" or "tells"?|
|Appare nelle tipologie:||Abstract in Atti di Convegno|
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