Eosen Nummulit Yığışımlarının irdelenmesi: Yeni Bir Bakış - Investigating in-depth the Eocene Nummulite Banks: A New Perspective

During the Eocene, the larger benthic foraminifera represented surely the main skeletal carbonate producers in tropical, oligotrophic shallow seas. The more conspicuous example of this production is the so-called “nummulite bank” (Arni, 1965), a peculiar facies characterized by very low taxonomic diversity of Nummulites and very high percentage of large B forms with respect to the smaller A forms.Originally, the nummulite bank has been regarded as in some way analogue of modern scleractinian coral reefs, even if later (Aigner, 1985) several doubts have been casted about the authochthonous character of the test accumulation. At present the question remains unresolved, but it seems the interpretation of the banks (also renamed “tells” by Aigner) as residual assemblages generated by hydrodynamic energy has increasingly gained consensus. The problem is fascinating, also because any interpretation is hindered by the lacking, in present-day seas, of equivalent sedimentary bodies built up by larger foraminifera.Refining the current palaeoenvironmental models involving the nummulite banks is pivotal for at least two reasons: 1) because the authochthonous/allochthonous character of the bank changes its significance with respect to the facies distribution in space, given that a passive accumulation is the product of the palaeoenvironment, whereas a reef-like body is the source of derived facies (fore-bank, back-bank, etc.); 2) because the confirmation (or refutation) of a reef-like origin has major consequences for the extenct of larger foraminiferal growth on the budget of global carbonate production during the Eocene.The detailed investigation of different case studies could help in providing new information about the structure and taxonomic composition of nummulite banks. The first results of such an investigation suggest a picture more complex and variable than expected.First of all, the taxonomic composition, even if always with low diversity, could see one or sometimes two prevailing species (with abnormally low A/B ratio), with a quite variable number of accompanying foraminiferal species. Moreover, the prevailing species could display either an inflated test (e.g. Nummulites perforatus, N. meneghinii, N. puigsecensis, etc.) or a very flat one (e.g. Nummulites lyelli, N. maximus, etc.), suggesting accumulation at different depths.There are some hints pointing to a really crowdy palaeoenvironment, with tests accumulating in situ. Among them, the presence of organic matter preserved among and inside the tests opens a promising field to explore.In short, it seems that the nummulite bank is not a single object everywhere with the same significance. Under this label are probably included different sedimentary settings and maybe different mechanisms of origination. Only with a careful analysis of several situations we can hope to solve this puzzle.ReferencesAigner T. (1985) - Biofabrics as dynamic indicators in nummulite accumulations. J. Sedim. Petrol., 55 (1): 131-134.Arni P. (1965) - L’évolution des Nummulitinae en tant que facteur de modification des dépôts littoraux. Mém. Bur. Rech. Géol. Min., 32: 7-20.