ABSTRACT

The Upper Miocene Sulphur Series of Sicily comprises four main formations, which are, in ascending order, the diatomitic Tripoli Formation, the evaporitic Basal Limestone, the Gypsum Formation, and finally the Trubi Formation, an ancient Globigerina ooze. Other less continuous units are also present. Peculiar sedimentational conditions are described in detail in a larger Italian paper.

In the Gypsum formation primary anhydrite and primary gypsum are varved rocks with inverse graded bedding of the laminae, due to chemical deposition. Secondary gypsum derives from anhydrite by paradiagenetic transformation and shows two different petrologic types. In the "sulphiferous facies" of the Gypsum Formation selenitic secondary gypsum is developed, with large size crystals and a peculiar fabric of almost free growth. In the "saliferous facies" alabastrine secondary gypsum shows structures caused by hindered growth under considerable load.

A sequence of stages in the anhydrite-gypsum transformation seem to be the following: (1) building of limpid selenite crystals; (2) ruptural deformation of their peripheral zones, giving rise to super-individual polarisation; (3) production of intergranular fine-size gypsum aggregate of mylonitic character. Ruptural deformation is originated by volume increase of the solid phase during the transformation. Residual anhydrite inclusions are present in the first two stages, absent in the last one.

The above sequence is just the reverse of that described in the studies on American and German salt domes. Goldman (1952) gives the name "integration" to a process of progressive adjustment of randomly oriented fine-size gypsum pieces to superindividuals with less dispersed optical orientation and finally to limpid selenite crystals. This process is very different from the well-known "Sammelkristallisation" and does not seem to be feasible in view of the physico-chemical forces known thus far. The little disturbed Upper Miocene Gypsum Formation is likely to give a clearer picture than the severely disturbed Paleozoic salt domes.