The area investigated is the marble deposit "Perlato di Orosei". It is an extensive zone of the upper Jurassic which outcrops near the coast a few kilometres from the village of Orosei (central-eastern Sardinia, Italy). Only in recent years have quarries been opened on the south-eastern slopes of Mount Tuttavista.
From a geological point of view, the terrain is characterized by outcrops having four main formations:
- –
various crystalline schists of the Paleozoic;
- –
granite: predominantly pink granite of the Hercynian;
- –
limestone of the middle and upper Jurassic;
- –
olivine basalts of the Pleistocene.
Wide alluvial cones branch out from the spurs of the mountain and extend towards the plane of the Orosei coast, partially covered by alluvial formations.
The three main fracture systems in the region strike on average NNW-SSE, N-S, NE-SW. The fracturing, especially in the rock mass of Mount Tuttavista, is so intense locally and extensive, that the bulk Limestone formations can be considered as a weak rock mass.
Geophysical surveys were carried out with the aim of identifying the fracture pattern with the degree of accuracy required by the programming of excavation works in marble quarries. Seismics and VLF electro-magnetics were employed in various test-sites of the deposit. In order to acquire a more detailed picture of the extracted material, laboratory tests were conducted to determine the main chemico-physical and mechanical characteristics of the rock. Industrial cutting and polishing tests were also carried out.
Mount Tuttavista is essentially composed of two limestone formations. The first, at the base of the rock mass, is a partially conglomeratic dolomite limestone. It outcrops along the north-western slope. The second, 'overlying the first for a thickness of about 600 m, is represented by light-coloured stratified limestone, or cliff limestone, in which fossils are present; Nerineae are particularly abundant.
Sixteen quarries have been opened throughout the latter formation (within the next few years it is anticipated that the number will be increased to twenty). Up to the present time only the superficial strata have been excavated. These range in thickness from 4 to 7 m and have an almost regular development (average strike N 250W-S 250E; dip approximately 56°E; slope between 20° and 25°).
Of the fractures surveyed those striking N-S are auasi-vertical with spacings ranging from 6 to 15 m, often increasing with depth. Fractures divide the rock mass into more or less regular blocks of various sizes. These fractures, from 10 to 50 cm wide, are usually filled with detritus and vegetal soil, rich in iron oxides. They present some lime-stone concretions to be attributed to karstic phenomena. The other fractures are transversal to the former and have a much more irregular spacing and development. They are usually indicated by zones of cataclasis and lamination. The fracture systems identified in the deposit coincide with the pattern of the regional structures.
(Figure in full paper)
