Sequence stratigraphy and ichnology: Drilling predation patterns recorded ini Holocene depositional succession of the Po Plain, Italy

The relation between sequence stratigraphic (sea-level driven) processes and paleoecological patterns is receiving increasing attention. Here, we examine trends in drilling predation traces (Oichnus) across a transgressive-regressive (interglacial) cycle recorded by Holocene deposits of the Po Plain (Italy). To document predation patterns along a depositional sequence, five cores, aligned roughly parallel to the present-day shoreline, were sampled extensively. Facies analysis and detailed stratigraphic correlations both suggest that these sediments represent a 4th-order depositional sequence deposited during the present interglacial. The basal part of the sequence (non-marine deposits of the lowstand systems tract) was excluded from this analysis. However, the overlying marine deposits of the retrogradational transgressive systems tracts (TST) and the progradational highstand systems tract (HST) yielded numerous fossiliferous horizons rich in marine molluscan benthos suitable for quantitative sampling. The resulting samples, dominated by extant forms with well-understood ecologies, offer an empirical testing ground for exploring how climate-driven sea level changes affect predation patterns (1) at sample level; (2) within and across systems tracts; (3) within comparable facies across different systems tracts (i.e., TST shoreface sandy sediments vs. HST shoreface sandy sediments); and (4) across different facies within the same systems tract (i.e., HST muddy deposits vs. HST sandy deposits). For Holocene sediments, the intensity of drilling predation at sample level varies markedly (from <2% to almost 15%), with samples from TST silty-sand facies yielding the highest estimates of drilling frequencies. Also, sample-level frequencies decrease upward in all five examined cores. When drilling intensity patterns are examined at systems tract scale, TST samples display the highest estimates of predation intensity (in particular, the late TST shoreface sediments record the most elevated frequencies), HST shoreface deposits record intermediate values, and HST prodelta sediment samples yield the lowest drilling percentages . Drilling patterns in the studied deposits seem to reflect a combination of ecological causes (i.e., less diverse associations of HST vs. more diverse associations of TST) and sequence stratigraphic artefacts (as predicted by the Holland’s model). These results confirm that the biasing role of sequence stratigraphic framework has to be evaluated before evaluating the fossil record of ecological and behavioural interactions recorded by trace fossils such as drill holes.