Paleoecologic and taphonomic signature of mollusk assemblages in cores: implication for sequence stratigraphic interpretation

Late Quaternary siliciclastic shallow water sediments of the Romagna plain (Northern Italy) form strongly asymmetric deepening-shallowing cycles, which are interpreted as recording changes in relative sea level. These cycles correspond to two 4th order depositional sequences that accumulated in the Po Plain (Northern Italy) during the last and present interglacial cycles and bracket a time interval of approx 100 ky, as demonstrated by facies analysis and detailed stratigraphic correlations. Continental deposits of the low stand systems tract were scarcely fossiliferous (therefore not sampled), whereas shell concentrations were found in the retrogradational transgressive systems tracts (TSTs), at the maximum flooding surfaces (MFSs) and in the overlying progradational highstand systems tracts (HSTs) of marine deposits. In order to evaluate potential signatures exerted on mollusk assemblages by sea-level changes, several computer-based strategies (including rarefaction-based diversity analyses and multivariate methods) have been applied to 30 shell-bed samples recovered from four cored boreholes. According to their occurrences within depositional sequence, three types of shell concentration can be recognized that differ from each other in a number of ecological and taphonomic features, such as species diversity, preservation quality, percentage of articulation, and degree of biogenic alteration. The study provides an excellent example of non-random distribution of shell-bed styles across sedimentary cycles. Shell beds associated with TSTs are characterized by high degrees of disarticulation and fragmentation, high diversity, and a generally poor preservation quality. Shell beds associated with HSTs are characterized by lower degrees of diversity, high biogenic alteration (bioerosion + encrustation) and average-good preservation quality, and shell concentration associated with MFSs show high degree of biogenic alteration, good preservation quality, high diversity, and the highest percentage of articulated shells. As importantly, the study forcefully reaffirms previous studies in showing that shell beds offer a superb tool for delineating bases and tops of TSTs as well as for confidently recognizing other key surfaces within sequence-stratigraphic framewor