Base-of-Flow Processes During Turbidity Currents, Observations from The Late Oligocene Sand-Rich Macigno Formation in Italy

Eggenhuisen, Joris¹, William McCaffrey¹, Rob Butler¹, Peter Haughton² (1) University of Leeds, Leeds, United Kingdom (2) University College Dublin, Dublin, Ireland

The easternmost outcrops of the Late Oligoceen Macigno Formation in Italy are the oldest exposed representation of the foredeep associated with the Apenninic orogen. The outcrops from which data is presented here form the shoreline of a headland north of the Gulf of Baratti. The three-dimensional detail of features in the rocks makes them especially suited to study mm-m scale structures. A sturdy spatial framework is provided by the headlands two main sections and one strike section. Thus this field locality provides an excellent opportunity to study sedimetological processes in a high net to gross turbidite succession.

Evidence for two types of processes that illustrate the importance of substrate-turbidite relationships are particularly abundant in the outcrops and presented here.

A range of models for mechanisms of substrate entrainment by base of flow erosion during turbidity current events is recognised. On one side of the scale, cohesive substrate blocks, dm's thick and m's long are detached from subvertical scour steps. This detachment mechanism is an important cause of vertical connectivity in sheet systems and inferences are made about the geometry of amalgamation zones. Other, less cohesive substrate entrainment can be shown to be of importance for the initiation of co-genetic turbidite-debrite couplets.

A second focal point involves syn-event remobilisation of aggrading substrate as inferred from convolute lamination. The systematic relation between the trends in fold trains and paleoflow direction obtained from flutes, grooves and lineations is explored. Linkage between the predominant strike of folds in convoluted lamination and paleoflow direction might provide a useful field tool for determination of flow direction in successions relatively barren in conventional indicators.