Relationships between trace-fossil communities and substrate characteristics in some Jurassic pelagic deposits in the Umbria-Marche basin, Central Italy

The study of trace fossil assemblages is a very useful tool for defining depositional palaeoenvironments, such asthose of some early Jurassic pelagic sequences in the Umbria-Marche basin (late Carixian to early Aalenian). These sequences - Valdorbia and Fiuminata Colle Corno- are represented by thick bioturbated marls and limestones which alternate locally with poorly-bioturbated shales. Clay and silty-sandy deposits representing turbidites and storm beds (HCS = hummocky cross-stratified deposits) are very common, especially in the Toarcian interval. A comparison between trace fossil assemblages and sedimentologic/stratigraphic features permits an accurate paleoecologic and palaeoenvironmental analysis of sediments deposited in regressive conditions such as, for instance, the sequences studied. Trace fossil communities provide evidence of periods of non sedimentation or erosion during the Jurassic, when firm- or hardgrounds were commonly developed on muddy or sandy substrates. Penetration depth, burrow diameter, trace fossil density and species diversity emphasize the relationships with substrate characteristics and oxygen variations on the seafloor or in the sediment column. Consequently, analysis of different types of substrate and of substrate-controlled ichnofacies permits an evaluation of the paleoecologic strategies of the endobenthos in low to high-stress environmental conditions.