TAPHONOMY OF CONDENSED INTERVALS ALONG A DEPOSITIONAL PROFILE: INSIGHTS FROM HOLOCENE DEPOSITS OF THE PO COASTAL PLAIN (ITALY)

In the marine realm, condensed deposits form in a variety of settings affected by sediment bypassing or sediment starvation. In siliciclastic marine settings skeletal concentrations are a diagnostic feature of condensed intervals and are of high relevance to sequence stratigraphy and paleobiology.

Here, using three fossiliferous cores, we investigate taphonomy of condensed skeletal deposits (Holocene) along a 30km-long depositional profile across the Po Coastal Plain. This succession records great part of a wedge-shaped transgressive-regressive sequence developed in a marginal marine setting of a subsiding basin.

When compared to the up-dip (most proximal) core, the most distal core (down-dip) is distinguished by shell-rich lithosome consisting of thicker polytaxic skeletal concentrations, presence of ecological condensation (co-occurrence of environmentally non-overlapping taxa), and a distinctive internal stratigraphy. Also, the skeletal biosome in the most distal core shows higher taphonomic alteration and more extensive immuration. These results point to a major role of sea-level (base level changes) in controlling both sedimentary settings and marine biota. A detectable starvation phase developed within a few kilometres of the coastline and resulted in a taphonomic feedback that increases in strength downdip.From the distal core to the proximal core, taphonomic alterations (encrustation, immuration, stratigraphic thickness and fragmentation) decrease, likely reflecting a sediment-supply gradient along the depositional profile of the Po basin. However, in the skeletal biosome of the proximal core, which was located coastally at the time of maximum transgression, the absence of immuration, and higher variability in other taphonomic variables both indicate a relatively less homogeneous structure of the skeletal biosome, suggesting minor phases of stillstand (or slow decreasing) of sea level during the late early-mid Holocene.

In summary, the results document multivariate taphonomic trends that clearly delineate gradients in sediment starvation/bypassing along the analyzed depositional profile.