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(a) Schematic diagram of the geometry of a tabular (b) A dune with undulated crest developing trough
cross-stratified dune system. cross-stratification.

Figure 4.6 – Two main types of cross-lamination as observed on Favignana. Note the difference
in geometry visible on the cross section perpendicular to the main flow direction. See also figure
4.1 for flow regimes. Figure courtesy of Southard (1996).

burrowing, especially parts of Cala Azzurra, the south coast and Punta Lunga. The latter shows large
networks of burrows up to five centimetres in diameter (figure 4.9d)

Interpretation

Bioturbation is sediment reworking by any living organisms, either flora or fauna. They can create
extensive systems of burrows. Depending on the degree of bioturbation, the primary sedimentary
structures may be entirely destroyed. The effect of burrowing can on one side homogenise an initially
heterogeneous unit, mixing particles of different size and composition. On the other hand, an initially
homogeneous layer can be mixed up because burrowing reworks sediment and aligns them in a preferred
orientation. Regarding reservoir connectivity, bioturbation can potentially interrupt continuity of shale
layers, improving the overall vertical fluid communication. Bioturbation is an indicator for 1) a string
decrease in sedimentation; and 2) a relatively calm palaeo-environment. A too high energy does not allow
the organisms to develop a system of burrows, because the youngest layer of sediment — where most of
the nutritious material is present — will be constantly reworked or buried.

4.3.8 Clay

Description

A sharp contrast in grain size is observed between the calcarenite and the stratigraphically lower clays
in some outcrops (4.9e). The clay is a yellowish to grey structureless unit, of which the base is not
observed in the field. The contact between the clay and the calcarenite locally shows a dark layer which
is interpreted as an iron and manganese mineralisation band (visible in figure 4.9f).

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