Page 60 - MSc_thesis_R A Kil

major storms easily affected by erosional processes. Parts of a dune crest face a small degree of
erosion, likely creating a preferred path where the flow could be concentrated. Gravity processes
enhanced the erosive processes and created large scours of up to 10 to 20 metres deep. Because
of the storm-related nature of these processes energy supply suddenly dropped which filled the
deepest parts of the scours with structureless sediment, locally showing a high amount of rhodiliths
and other coarse bioclastic material at the base. Figure B.3 in appendix B shows part of the Scalo
Cavallo cliff with several scour-related features. The downstream part of the scours induced the
possibility to develop small scale trough cross-stratification. Interpretation of the field data led to
the hypothesis that the big scours created in the northwest — Scalo Cavallo and its surroundings
— provided a concentrated energy fanning out to the southeast. The small scale troughs that are
observed in Cala Rossa, Bue Marino and Cala Azzurra can possibly be explained with this lobe-like
geometry. The transition from tabular cross-stratified dunes to troughs is very well visible in the
Cala Rossa photo panel (figure B.2 in appendix B).

Some of the undulated beds are associated with upper flow regime structures related to the scouring
nature of parts of the stratigraphy. In particular the inland area of Bue Marino, western Cala Rossa
and the central part of the island show these structures. They are interpreted as an intermediate
step when moving towards the end of a scour fan — from massive infill to undulated beds and
finally to troughs. The undulations are in some cases associated with smaller scale scour and fill
systems (figure B.4). Therefore the nature of these features differs from the large scours. The high
energy and the gravity based flow can create a scour which has an orientation perpendicular to the
flow direction. Because of the hydraulic jump that is associated with such a system, large amounts
of sediment are instantly dropped into this structure, with the coarsest material at the base.

Summarized, the reconstruction of the eastern Favignana depositional framework consist of three phases.
The first two phases are general for the entire area, where a system builds up with low energy deposits at
the base, and large prograding dunes towards the top. Local erosive events created the deep scours and
the associated lobe structure. This explains the lateral variability of some of the sedimentary structures
related to this phase.

Figure 5.3 illustrates the resulting architecture based on the described stratigraphical evolution of the
Favignana calcarenite. The phases described above are schematically positioned next to the Monte Santa
Caterina structure. Sediment supply in this situation is from the northeast, indicated with the arrows.
Only one scour and its interpreted fan geometry is drawn to emphasise the conceptual nature of this
diagram. It is likely that in reality an interplay between different scours and their lobes exists. The cross
section parallel to the flow direction shows how the undulated scour and fill system is positioned in the
framework. The end of the fan — with its trough cross-stratification — is situated at the top left part of
the frame.

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