Process Interpretation of Flow-Substrate Interactions in the Late Oligocene Macigno Formation, Northern Appeninnes, Italy

Joris Eggenhuisen¹, Robert W.H. Butler¹, William D. McCaffrey¹, Peter D. W. Haughton², and Bill Hakes^3
1 University of Leeds, Leeds, United Kingdom
² University College Dublin, Dublin, Ireland
³ Britannia Operator Ltd, Aberdeen, United Kingdom

The Macigno Fm is interpreted as the oldest foredeep clastic wedge of the Northern Apennines late Oligocene collisional phase. Classically it is interpreted as a slope-attached, sand-rich, low efficiency turbidite system, deposited immediately outboard of the eastward-moving orogenic wedge. The sequence preserves turbidites of high- and low-density turbidite facies together with associated matrix-supported, muddy debrites. There is widespread evidence that the substrate was unstable during ongoing aggradation, with common examples of substrate deformation occurring synchronously with turbidite emplacement. Well-developed convolute lamination is very well exposed along bedding planes. The sense of displacement of these structures can thus be measured in 3D, and related to the main turbidite dispersal direction (determined from rare sole structures and abundant primary current lineations). Within the debrites, deformation of ductile intraclasts indicates ongoing substrate shear, associated with local loading of turbidite sand. Growth fabrics within the supradjacent sandstone in association with planar upper contacts confirm that deformation was synchronous with sand emplacement – thickness variation in the sand reflecting the creation of local accommodation due to loading. In each case, the dispersal direction within associated sediments constrains the local slope and allows the usefulness of the deformation fabrics as slope indicators to be assessed. Although processes of substrate deformation during sedimentation are relatively well-documented in muddy turbidite systems, they are under-represented in studies of sandy systems, such as the Macigno Fm, which can therefore serve as an analogue for sand-rich systems elsewhere.