|Titolo:||Analisi di facies e stratigrafia della successione permo?-triassica di Campumari-Coremò (Iglesiente, Sardegna SW)|
|Data di pubblicazione:||2003|
|Abstract:||The Campumari-Coremò succession, at least 50 m thick, starts with the «Rio Is Corras Formation», a lithostratigraphic unit consisting of alternations of conglomerates and dolomites and rare sandstones with caliches, reddish siltites and argillaceous siltites. These deposits are transgressive and discordant over the folded Hercynian basement, which is irregularly cut by erosion channels. The lithotypes described show abrupt lateral facies changes. The abundance of the carbonate sediments increases towards the south, thus suggesting the provenance direction of the transgressive sediments. Sediment deposition probably occurred in variable environments, ranging alternatively from alluvial fan-delta to restricted carbonate (lagoonal?-coastal?) lake, with frequent emersions related to fluctuations of the base level. The consequent oscillation of the water table may be one of the possible causes of dolomitization and calcretization of carbonate sediments, together with variations of temperature and salinity under a dry-hot climate with sporadic hard rainfall episodes. In some of the calcretes, Characeae remains have been found, testifying to the former subaqueous (brackish?) depositional environment of the calcretized sediments. In its lower part, the Riu Is Corras Formation is also crossed by thin veins containing barite and sulfides. These features, together with sedimentological characters and facies analogues, suggest an age older than previously supposed and connected with the late-post Hercynian (Permian) hydrothermal circulation. The Rio Is Corras Formation shows a maximum thickness of 23-24 m at the west side of the Riu Is Corras valley: it can be dated as Permo?-Triassic, but its age cannot yet be better defined. Upwards, the carbonate-bearing «Campumari Fm.» follows after an unconformity. The «Campumari Formation» begins with the «Su Passu Malu Member», consisting initially of thin marly-clayey greenish sediments, locally rich in plant debris and containing an Upper Anisian (Pelsonian/Illyrian) palynomorph association. A thin horizon of dark, fetid dolostones rich in former sulfate nodules with chickenwire structure follows and is abruptly overlain by a succession consisting of about 15 m of yellowish, finely stratified, marly dolostones to dolostones, also rich in sulfate pseudomorphs with chert nodules. The main feature of the latter deposit is the gradually increased folding towards the top, caused probably by diagenetic deformation of the evaporite layers. Close to the upper boundary of the laminated dolomites, the folded structures pass to chaotic breccias, some of which appear very similar to typical «tepee» deposits. Mud-cracks and ripple marks have also been locally observed. The «Su Grifoneddu de S’Acqua Member» is the last member of the Campumari Fm. It starts with a thick collapse breccia made of angular clasts of dark limestone embedded in a carbonate matrix, locally containing pseudomorphs after sulfates. These breccias are overlain by grey dolostones, at first thick but subsequently well-stratified. Some horizons are characterized by pseudomorphs either after either gypsum or anhydrite, while others contain unclassifiable bioclasts and are bioturbated, this occurring mostly at the top of the dolomitic succession. The Campumari Fm. is transgressive over a substrate constituted by the Is Corras Formation: at Su Grifoneddu de S’Acqua it has a maximum thickness of 26 m. This formation was deposited initially in an ephemeral, low-energy, reducing lagoon with local clastic supply, that quickly evolved first to a restricted, frequently hypersaline carbonate environment (with subordinated sabkha episodes), and later to a carbonate platform environment fluctuating from strongly to weakly hypersaline. Both the absence of continuous, major barriers and the lack of significant slump deposits, as well as the gradual passage from shallow to deeper deposits, point to a ramp depositional model. In more detail, the Campumari-Coremò succession can have been deposited in a carbonate ramp environment, passing from a lagoonal substage, with restricted episodes at the beginning, to an inner ramp and then to shallow ramp subenvironments.|
|Tipologia:||1.1 Articolo in rivista|
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