The “Nummulite bank” enigma: sedimentary or biological origin?

During Eocene times, larger foraminifera of the genus Nummulites produced accumulations named “nummulite banks” (ARNI, 1965), dominated by B-forms and usually monospecific. In his original model (Fig. 1), Arni compared the paleoecological conditions generating the nummulite banks with that of modern coral reefs. In his model the banks acted just as modern coral reefs do, producing “back-bank”, “fore-bank”, and “lagoonal” facies. The model, originally developed studying Eocene limestones from the Northern Africa, was successively extended by ARNI & LANTERNO (1972) and DECROUEZ & LANTERNO (1979) to the northern side of the Mediterranean. AIGNER (1982) questioned the validity of the model suggesting the accumulation of large tests was strongly influenced by water energy. He studied nummulite accumulations from Egypt, recognising sedimentary structures and depositional textures reflecting physical transport. Moreover, he underlined that the highly porous nummulite tests should be easily transported and storm events could originate episodic winnowing, responsible of the increased abundance of B- forms. Therefore, the bank facies does not reflect the original biocoenosis (Fig. 2), but could be a parautochtonous or residual assemblage (AIGNER, 1985). Due to this interpretation, he coined the term “nummulite tell” to replace “nummulite bank”. He initially recognized the paleoecological conditions could act to some extent to increase the potential of nummulite growth (AIGNER, 1983), but never considered the peculiar taxonomic composition of the bank itself. The model of Aigner gained increasing consensus during the last decades. GEEL (2000) included the nummulite bank in his ramp model, even if he did not consider it as an effective barrier. Some studies were carried out to confirm the Nummulites tests are easily displaced by water energy (e.g. BEAVINGTON-PENNEY et alii, 2005). JORRY et alii (2006) made a thorough synthesis of the main different models proposed in the last 40 years; they also presented experimental results about the possible transportation of Nummulites concluding that A- and B-forms could be transported and deposited simultaneously, having very similar hydrodynamic behaviour. Summing up, the two models interpret the bank either as a biologic reef-like accumulation (Arni) or as the result of transported tests passively accumulated (Aigner). With the aim to verify the models, a palaeontological study of nummulite banks from different countries has been started. In particular, the taxonomic composition of nummulite banks has been preliminarily examined for five localities, two from northern Italy, two from Romania and one from Spain, all from the Middle/Late Eocene. The larger foraminiferal assemblages are reported in Tab. 1.The species diversity is variable: only in Leghia and Igualada the bank could be considered as really monospecific (in both cases with N. perforatus). The other localities often contain accompanying species of Nummulites with different sizes. The most diverse is the bank of Pederiva di Grancona, where the bank is dominated by a couple of species (N. lyelli and N. biarritzensis), but several species of other larger foraminifera are also present (Tab. 1). The examined localities display different sedimentary features together with different taxonomic compositions of the assemblages, suggesting different depths of accumulation. The Leghia N. perforatus bank seems the shallowest, occupying the inner platform, whereas the Pederiva N. lyelli-N. biarritzensis bank (Fig. 3) could be the deepest, near the edge of an open platform; the N. fabianii banks of San Germano dei Berici and of Cluj probably developed near the transition from middle to outer platform (presence of Operculina, Assilina, and Spiroclypeus). In all banks there is a small amount of fine terrigenous sediment. The latter is even present in the Leghia bank (Fig. 4), where robust tests and intense bioerosion indicate very shallow water and probably high water energy. The presence of fine terrigenous sedimentation has never been evidenced in the past and deserves more investigation. However, it raises questions on the winnowing model of Aigner, since the water energy necessary to move the large microspheric tests is definitely higher than the one maintaining in suspension clay-sized particles. The presence of different species of Nummulites, with different size, shape, and weight, together with the simultaneous presence of A- and B-forms of the same species, but with very different size and shape, seems to contradict the hypothesis of selective winnowing. At the same time, the impressive dominance of one (maximum two) species of larger foraminifera is probably linked to some peculiar, but still unknown, paleoenvironmental conditions. The latter could in some way be linked to an increased rate of sexual reproduction, leading to an abnormally high production of microspheric tests. The preliminary observations, even if giving some hints, are not sufficient to solve this enigma. A more extensive, thorough investigation of the physical and palaeo-biological conditions is needed, with special regard to the growth of the giant B-forms and to the different sedimentary settings under which the banks developed.REFERENCES AIGNER T. 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