Use of trace fossils in delineating sequence stratigraphic surfaces (Tertiary Venetian Basin, northeastern Italy)

The lowermost third-order sequence of the Venetian Molasse Basin (Upper Chattian-Lower Aquitanian) in the Belluno syncline is composed of a transgressive systems tract, comprising a basal, condensed, glauconitic sand sheet deepening upwards into finer-grained offshore deposits, and a highstand systems tract mainly consisting of prograding, offshore mudstones capped by very fine sandstones representing offshore-transition deposits. An integrated sedimentological, ichnological and micropaleontological approach has allowed the recognition of the internal organization of the sequence, the subdivision into parasequences in absence of an obvious physical expression, and the characterization of significant discontinuity surfaces at both sequence and parasequence scales. The transgressive surface shows a complex geometry of bored and encrusted cavities produced by erosional undercutting and a system of neptunian dykes and sills, with hiatal shell concentrations as infill. Component parasequences in the texturally homogeneous transgressive sand sheet can be identified on the basis of the preservational state of trace fossil assemblages (softground versus firmground conditions) and subtle changes in grain-size and glauconite content. Omission surfaces bounding the parasequences are marked by increased glauconite content, and densely crowded, predominantly vertical or oblique, relatively large, very distinct, thin-walled or unwalled, and uncompacted burrows. The condensed section coincides with highest values in relative abundance of planktic foraminifers and in diversity of benthic foraminifers. Parasequence-bounding flooding surfaces in the muddy offshore portion of the highstand systems tract are recognized as firmground surfaces mantled by thin shell concentrations in a matrix of silty, highly burrowed, glauconitic sand.