Page 7 - Tondietall2015

Italian Journal of Geosciences Accepted manuscript

this study yielded an effective porosity ranging from 45 to 50 %. The results are presented by means
of a box and whisker diagram (fig. 3a).

The pore throat distribution shown in figure 3b (blue line) indicate that around 80% of the
measured pores have a size comprised between 10 and 100 microns, whereas the size of the residual
20% of the pores ranges between 100 and 400 microns.

The grainstones of the Orfento Formation are homogeneous (BAUD et alii, 2009 and CILONA et
alii, 2012), their porosity has been firstly measured by TONDI et alii (2006) on redrawn
photomicrographs. These authors measured values comprised between 15 and 28 %. More recent
papers measured the porosity of the same grainstones by means of the triple weight method and
their data are consistent with porosities ranging from 30 to 32% (BAUD et alii, 2009; ZHU et alii,
2010; CILONA et alii, 2012). Our helium pycnometer measurements yielded porosity from 31 to 32
%. The results are presented by means of a box and whisker diagram (fig. 3a).

Figure 3b (red line) displays the bimodal pore size distribution of the Orfento grainstones. ZHU
et alii (2010) documented that these grainstones are dominated by micropores, indeed more than
80% of pores are smaller than 20 µm in size. On top of this around 12.5% of the micropores have a
sub-micron size (fig. 3b), the presence of very tiny micropores is common is carbonates (BLUNT et
alii, 2013), it is believed that these pores localize within the micritic cement and single rudist
fragments (see ZHU et alii, 2010 and CILONA et alii, 2012 for more details).

FAULT ZONES CHARACTERISTICS
The carbonate grainstones exposed at Favignana and at the Majella Mountain are both affected
by the following structural elements: i) bedding-parallel compaction bands, ii) compactive shear
bands (occurring as single or in zonal patterns) and iii) fault zones. The latter structures represent
the latest evolutionary stage of former compactive shear bands (TONDI et alii, 2006, 2012). Fault
zone characteristics observed in the field and in thin section are described in the following section.