What, If Anything, Is A Nummulite Bank?

Since the introduction of the concept of a “nummulite bank” more than 50 years ago, the significance of these peculiar sed- imentary structures has been debated, especially whether they are of autochthonous or allochthonous origin. At first consid- ered as substantially autochthonous, an alternative interpreta- tion 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 litera- ture, 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 recog- nition 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 use- ful 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 as- semblages. 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 sam- ples show A/B ratios between 60–350 and dominance usu- ally <50%. An unexpected result was the discovery of an ‘As- silina bank’, characterizing both conditions for recognition of a nummulite bank, but with Assilina cuvillieri as the dom- inant species representing >75% of the larger foraminiferal assemblage.