Page 41 - MSc_thesis_R A Kil

4.3 Sedimentary macrofacies

”A sedimentary facies is a body of rock with specified characteristics” (Reading, 1996).
This section describes the different macrofacies as observed in the fieldwork area. The facies are
distinguished by properties like lithology, sedimentary structure, and the presence of bioclastic material —
either in preserved or fragmented state. The Favignana bioclastic sands show sedimentary structures
of variable nature, which will be the most important characteristic in the macrofacies distinction. In
general, the Upper Pliocene to Lower Pleistocene deposits found on the eastern part of Favignana consist
of coarse grained fragments of bioclastic material, and internal differences are not very well pronounced
on the rock face.
Each facies has been interpreted as being formed under specific sedimentary conditions, from which
sediment supply and the amount of associated energy can be derived. Table C.1 in appendix C summarises
the facies as recognised in the field, of which the interpretation given will be used to establish the conceptual
geological model.

4.3.1 Massive calcarenite

Description

Well sorted calcarenite, generally showing a large amount of non-fragmented fossils like urchin shells,
molluscs, foraminifera and bryozoa. Figures 4.7a and 4.7b show examples from the field. Grain size
varies between 0.5 and 2 mm, with occasionally rhodoliths up to 4 centimetres in diameter floating in the
matrix. Sedimentary structures are generally absent in these deposits, however some faint lamination can
be seen. Such hardly visible lamination can be partly caused by the alignment of rhodoliths or other
fossils. The most typical location where this facies is observed is the northern end of Punta San Vituzzo,
where it can reach a visible thickness of over three metres.

Interpretation

The massive deposits must were most likely associated with a very fast sedimentation rate. This is the
only way of preventing the development of internal sedimentary structures. The occurrence of large
fossils like rhodoliths floating in the matrix is also consistent with a very fast sedimentation process. A
slower sedimentation would allow a better sorting. In order to achieve this amount of sedimentation, the
system must have been 1) facing a very high sediment supply; and therefore also be 2) subjected to a
high amount of energy to transport the particles.

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