Since the introduction of the concept of a “nummulite bank” more than 50 years ago, the significance of these peculiar sedimentary structures has been debated, especially whether they are of autochthonous or allochthonous origin. At first considered as substantially autochthonous, an alternative interpretation as allochthonous deposits was suggested in the mid-1980s to explain the observation of sedimentary features indicating displacement of the nummulite tests. In more recent literature, we found examples of non-uniform use of the nummulite bank concept; sometimes the ‘banks’ are vaguely described as nummulite-rich beds, and illustrations may not allow recognition of the features characteristic of the real banks. In an attempt to achieve a scientifically robust model explaining the genesis of the nummulite banks, our objective was to test the original definition, based upon the most fundamental and useful characteristics that can be observed in the field, even in non-optimal outcrop conditions: the A/B ratio and the percentage composition (dominance) of the fossil assemblage. Both can be quantified using simple techniques on samples collected for this purpose.
In an extensive survey of nummulite banks and non-banks from Italy, Spain, Romania, and Germany, we determined A/B ratios and taxonomic composition of the nummulite assemblages. Recognized nummulite banks are characterized by A/B ratios <60 and dominance of one species accounting for at least 75% of the specimens of Nummulites. Non-bank samples show A/B ratios between 60–350 and dominance usually <50%. An unexpected result was the discovery of an ‘Assilina bank’, characterizing both conditions for recognition of a nummulite bank, but with Assilina cuvillieri as the dominant species representing >75% of the larger foraminiferal assemblage.