Automicrite in a “nummulite bank” from the Monte Saraceno (southern Italy): evidence for synsedimentary cementation

Following introduction of the term “Nummulite Bank”, there has been debate regarding interpretation of these deposits as autochthonous or allochthonous. We analyzed the nanomorphology, geochemistry, and organic matter remains in micrites of a Nummulite Bank in the Middle Eocene of Monte Saraceno (Gargano, southern Italy). The presence of automicrites has implications for synsedimentary cementation and stabilization of the bank. Optical and SEM investigations showed that the micrite has been recrystallized to aggrading microsparite. Epifluorescence observations on selected micrite/microsparite areas with peloidal texture revealed the presence of organic matter. SEM observations on epifluorescent micrites showed that the microbial peloids have smaller crystal sizes than in organic matter-depleted areas. The geochemical characterization of extracted organic matter, performed through the functional group analyses by FT-IR spectroscopy, shows strong prevalence of the aromatic fraction over the aliphatic and carboxylic ones. These characteristics of organic compounds indicate both their thermal maturation and their likely derivation from degradation of bacterial communities. The local presence of peloidal antigravitative textures, bright epifluorescence and organic molecules, in micritic areas suggest that the metabolic activity of microbial communities could have induce precipitation of these micrites and consequently the syndepositional cementation of the nummulite bank. This type of cementation can rapidly stabilize sediments and promote the depositional bank geometry.