The geology of the Rosas-Terreseo area (Sulcis, South Sardinia)

Cambrian sandstones, limestones and slates, unconformably overlain by Ordovician and Silurian slates, intruded by Hercynian granodiorite and dolerite, and Alpine andesite, occur. The traditional rock stratigraphic subdivisions of the sequences below the Ordovician unconformity are here proposed as formal units. Cabitza Formation Calcescisti Member Iglesiente Group Metallifero Formation Calcare Member Arenarie Formation (base not seen) Dolomia Member It is possible to distinguish Cambrian from Ordovician slates even where the basal conglomerate of the latter fails. As a consequence part of the official geological map (scale 1 : 100000, 1938) had to be revised. The Cambrian and the Ordovician-Silurian rocks have undergone respectively four and three deformation phases with the development of slaty, fracture and crenulation cleavages. The large E-W folds of the Cambrian Sardic-phase are partly concentric (Arenarie and Metallifero rocks) and partly similar (slaty cleavage in the Cabitza slates). The second phase, also E-W, caused a further tightening of the ”Sardic” structures and folded the Ordovician and Silurian rocks. The third deformation phase (N-S) was the Hercynian mainfolding, accompanied by fracture and crenulation cleavage. The fourth and last Hercynian phase (NW-SE and NE-SW) made a conjugate system of folds and cleavages. A specific structural pattern (domes, basins etc.) followed from the interference of these fold systems (”Schlingenbau”). Most folds are disharmonie as a result of differences in rock competencies of the Cambrian Formations. The Cambro-Ordovician unconformity has been partly obliterated by slip (”decollement”) due to disharmonic folding above and below this plane. Limestones have been partly converted into lime-silicate rocks by Hercynian hydrothermal and pneumatolytical action along irregular zones and an interesting skarn mineral paragenesis developed. The ore deposits (Pb, Zn, Cu etc.) are almost completely tied to the limestones of the Cambrian Metallifero Formation. The ”lead-modelages” of galena samples from Min. Giuenni (N-Sulcis) and Min. Monte Poni (S-Iglesiente), both deposits in the Metallifero Formation, are about 600 million years old; samples from Min. Monte Vecchio (N-Iglesiente) and from an outcrop along the road Siliqua-Acquacadda, deposits respectively in a Hercynian quartzdike and a Cambro-Ordovician lime-silicate rock, are about 400 million years old. Almost all deposits have been rejuvenated during the Hercynian orogeny.