The role of microbial organic matter in “nummulite banks”: an example from the Monte Saraceno (Southern Italy).

During the Eocene, the spectacular growth in size and diversity of larger foraminifera on shallow carbonate platforms gave rise to some peculiar biosedimentary bodies known as “nummulite banks”. The interpretation of such bodies is puzzling, because modern examples are missing. During the 45 years after the original description by Arni (1965) students debated the evidences for autochthony vs. allochthony of nummulite banks, underlining alternatively the influence of paleoecological conditions on the production of the large microspheric (B) forms or the effects of hydrodynamic energy winnowing the original assemblage and removing the smaller, lighter megalospheric (A) forms.Some preliminary, semi-quantitative observations on the taxonomic composition of different nummulite banks supported the idea of authochthonous accumulations (Papazzoni, 2008).The observation of fine-grained sediment preserved among the foraminiferal tests suggested to investigate it to determine the possible presence of still-preserved organic matter. A nummulite bank from the Monte Saraceno (Gargano, southern Italy) was examined in this respect. It is dominated by large B forms of Nummulites gizehensis and N. puigsecensis (SBZ 16- 17, upper Lutetian-lower Bartonian, Middle Eocene). Adams et al. (2002) accurately measured the original depositional surface, estimating it with an angle of about 20°, very close to the angle of repose of the loose sediment.We examined samples from both the nummulite bank facies and the underlying skeletal calcarenite beds. The interparticle micrite matrix was carefuly observed; we found that usually the original micrite is recrystallised into aggrading microsparite.The observation with the optical microscope revealed in the bank facies some peloidal, highly-epifluorescent micrite, with a relatively high organic matter content. The peloidal micrite shows also smaller-sized crystals in comparison with the remaining non-peloidal micrite.The EDS analyses of the Si content allow to distinguish the detritic micrite from the automicrite. The latter is present especially in the bank facies whereas the former is prevailing in the skeletal calcarenites.The FT-IR functional group analyses of the extracted organic matter allowed a geochemical characterization of it. The aromatic fraction prevails over the aliphatics and carboxylic groups, witnessing a relatively high degree of thermal maturation of the organic matter. Therefore, the biomarkers are probably not well preserved, even if the general pattern of the chromatogram is consistent with the presence of carbonatogenic microbes.These first results on the nummulite bank micrite allow to interpret it as automicrite, probably generated by early bacterial activity which could affect the mechanical strength of the nummulite bank, linking together to some extent the nummulite tests and possibly preventing their remobilization.REFERENCES Adams E.W., Morsilli M., Schlager W., Keim L. & van Hoeka T. (2002). Quantifying the geometry andsediment fabric of linear slopes: examples from the Tertiary of Italy (Southern Alps and Gargano Promontory). Sedimentary Geology, 154: 11-30.Arni P. (1965). L’évolution des Nummulitinae en tant que facteur de modification des dépôts littoraux. Mémoires du Bureau de Recherches Géologiques et Minières, 32: 7-20.Papazzoni C.A. (2008). Preliminary palaeontological observations on some examples of “nummulite banks”: sedimentary or biological origin? In Ciarapica G., Masetti D., Nardon S. & Ronchi P. (eds.), 3° Meeting Annuale del Gruppo Italiano Carbonati (San Donato Milanese, 28-30 Aprile 2008). Rendiconti online della Società Geologica Italiana, 2: 135-138.