USE OF TRACE FOSSILS IN DELINEATING SEQUENCE STRATIGRAPHIC SURFACES (TERTIARY VENETIAN BASIN, NORTHEASTERN ITALY)

The lowermost third-order sequence of the Venetian Molasse Basin (Uppe r Chattian-Lower Aquitanian) in the Belluno syncline is composed of a transgressive systems tract, comprising a basal, condensed, glauconiti cs and sheet deepening upwards into finer-grained offshore deposits, a nd a highstand systems tract mainly consisting of prograding, offshore mudstones capped by very fine sandstones representing offshore-transi tion deposits. An integrated sedimentological, ichnological and microp aleontological approach has allowed the recognition of the internal or ganization of the sequence, the subdivision into parasequences in abse nce of an obvious physical expression, and the characterization of sig nificant discontinuity surfaces at both sequence and parasequence scal es. The transgressive surface shows a complex geometry of bored and en crusted cavities produced by erosional undercutting and a system of ne ptunian dykes and sills, with hiatal shell concentrations as infill. C omponent parasequences in the texturally homogeneous transgressive san d sheet can be identified on the basis of the preservational stale of trace fossil assemblages (softground versus firmground conditions) and subtle changes in grain-size and glauconite content. Omission surface s bounding the parasequences are marked by increased glauconite conten t, 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 abun dance of planktic foraminifers and in diversity of benthic foraminifer s. Parasequence-bounding flooding surfaces in the muddy offshore porti on of the highstand systems tract are recognized as firmground surface s mantled by thin shell concentrations in a matrix of silty, highly bu rrowed, glauconitic sand.