Analisi di facies della successione triassico-giurassica di Porto Pino (Sardegna sud-occidentale)

The Mesozoic of Porto Pino (SW Sardinia) has been investigated in detail. The presence of two carbonate successions, belonging to two different stratigraphic-structural units (“Cala Su Trigu Unit” and “Guardia sa Perda Unit”, BARCA & COSTAMAGNA, 1997a) has been confirmed. Detailed sedimentological analysis has improved the recently established stratigraphy (BARCA & COSTAMAGNA, 1997a) and defined accurately the depositional environments of the successions. The whole thickness of the “Cala Su Trigu Unit” is about 120-130 m. The Unit starts at the base by the “Dolomie di Scollieddu formation”, consisting of grey, locally pinkish, dolostones passing upwards to grey calcareous dolostones; rare marly red layers also occur. This formation is characterized by several depositional textures and structures, as ooids, peloids, bioclasts, calcitic pseudomorphs after sulphates, cross- and parallel laminations and loferitic structures are clearly visible. In the lower part of the “Dolomie di Scollieddu formation” storm layers are often wellrepresented. Fossil organism (algae, brachiopods, bivalves, corals, gasteropods) are usually concentrated in few specific horizons. The age of this formation is comprised between Middle?-Upper Triassic (Megalodontidae, Thecosmilia) at the base and Liassic (defined by foraminifera) at the top. The Medau Mereu formation follows upwards; this unit is made up by green-yellow marls passing to well-stratified grey to blue-grey marly limestones. Fossils (radiolaria?, foraminifera, sponge spicules, worm tubes and rare pelagic bivalves, brachiopoda and bryozoan fragments) and nonskeletal grains (rare, reworked ooids and peloids) occur in the whole formation. The foraminifera association gives to the Medau Mereu fm. an Aalenian age. The deposition of the “Cala Su Trigu Unit” succession marks a carbonatic depositional environment ranging from a peritidal setting of inner ramp at the base (Dolomie di Scollieddu formation) to outer ramp close to the basin at the top (Medau Mereu fm.). The “Guardia Sa Perda Unit” consists of four formations; at the base the “Punta Tonnara formation” (BARCA & COSTAMAGNA, 1997a) similar to the German “Muschelkalk” facies, is made up by well-stratified, blue-grey limestones with marly limestones and marls intercalations, usually arranged in decametric “shoaling upward” sequences. In the upper part of these sequences storm layers are very common; bioclasts (bivalves, gasteropods, echinoids spines, algal fragments, crinoids, ophiuroids, cephalopods?) such as pseudomorphs after sulphates, represent the basal lag of the tempestites. The age of this formation, about 25 m thick, can be referred to Middle-?Upper Triassic on the base of presence of Agathammina sp. (MARTINI et al, 1987). A comparison made with other similar succession of Sardinia (Punta del Lavatoio; GANDIN, 1978a; Scivu-Is Arenas, BARCA et al., 1995; Monte Maiore, COSTAMAGNA et al., 1999) allows to assign to this formation a Ladinian age. The depositional environment of this formation varies from the intertidal? - shallow subtidal conditions of inner, shallow ramp to the deep subtidal ones of outer, deep ramp. The “Monte Zari formation”, corresponding to the German “Keuper” facies, follows upwards; the transition from the “Punta Tonnara formation” is marked by a thick horizon of dissolution-collapse breccias. This formation consists of grey dolostones containing rare layers of polychrome marls. The dolostones are usually well stratified and present a large spectrum of textural arrangement (loferitic bindstones, dissolution breccias, massive dolostones with evaporite casts, oolitic, more rarely peloidal, grainstones interested by large cross lamination structures). Fossils and bioclasts, with the only exception of algal mats and their derived fragments, are totally lacking. The whole thickness is about 120-130 m. The Upper Triassic - Lower Liassic age of this formation can be deduced from the paleontological data recorded in the underlying Punta Tonnara and overlying Guardia Sa Perda formations. The depositional environment is referred to a inner, shallow carbonatic ramp subjected to subtidal, lagoonal conditions, often restricted. Rarely intertidal environments have been also recorded. The limestones of the “Guardia sa Barracca Formation” sharply overlay the “Monte Zari fm.”. They consists of bioclastic-oolitic packstones to grainstones, with a minor amount of peloids. Bioclasts (crinoids, brachiopods, foraminifera, bryozoa, bivalves, green algae) are diffuse and clearly visible; the non-skeletal grains (ooids, peloids, coated grains) are also abundant. Chert nodules occur too. Some horizons are rich of macrofossils as belemnites, brachiopods and crinoids. The age of this formation is Upper Liassic (Domerian? - Toarcian). It is gradually followed by the Aalenian “Medau Mereu formation”, here made by the same alternances of marls and limestones just described in the “Cala Su Trigu Unit”. The thickness of the Guardia Sa Barracca and Medau Mereu fms. together is about 150 m. The depositional environments of Guardia Sa Barracca and Medau Mereu fm. are strictly connected, so representing the gradual evolution from an oolitic shoal of a shallow ramp (rarely interfingering with the lagoon) environment toward the outer, deep ramp close to the basin. Comparing the typical successions of the German Triassic with the lithologies described in the succession of Porto Pino (in particular those of the “Guardia Sa Perda Unit”) the latter might be resulting by the deposition during Upper Triassic times in a transitional setting between the environments of the Germanic Realm and those of the Alpine one. In addition, setting of the Triassic part of this successions is redefined referring to classic “Muschelkalk” and “Keuper” stratigraphical units, utilized as sedimentary facies associations.