The Lower Cambrian of SW-Sardinia: From a clastic tidal shelf to an isolated carbonate platform

Abstract

Much of the Lower Cambrian sediments of SW-Sardinia were deposited on a clastic tidal shelf, which changed into an isolated carbonate platform. Terrigenous clastics with carbonate intercalations (Nebida Fm.) grade into the pure carbonates of the Gonnesa Fm. Algal-archaeocyathan carbonates of Atdabanian to Lenian age occur within the clastics of the Lower Cambrian Nebida Fm. Smaller algal-archaeocyathan mounds evolve with time into a more complex facies-association of stacked mounds. The last stage produces flat-topped algal-archaeocyathan biostromes, several hundred meters long and locally more than ten meters thick. They consist mainly of algal bindstones (Girvanella especially) and also contain (sometimes branched) archaeocyathans. These buildups are interfingered and interbedded with coated grains and are underlain and overlain by terrigenous clastics. This association is similar in age and facies to the Poleta Fm. of western Nevada and eastern California, from which ROWLAND (1984) has reported the first appearance in the history of the earth of a wave-resistant reef-framework. We cannot assume that the algal-archaeocyathan community had a considerable resistance to waves and that it was environmentally dominant in SW-Sardinia. This association of algal-archaeocyathan carbonates and coated grains is to be found in the W and N of Iglesiente-Sulcis, apparently marking the rim of a platform situated further to the E. In the westernmost part of Iglesiente this platform-margin facies of the Nebida Fm. is overlain by a slope facies of the Gonnesa Fm., unknown so far. The slope is the lateral equivalent of tidal and sabkha deposits of the Dolomia Rigata Member (laminated dolomite member) located further to the E. This slope facies is called the Planu Sartu Member, a new member of the Gonnesa Fm. The Planu Sartu consists especially of varve-like laminated limestones and dolomites and up to m-thick calcareous mudstones with cherts. Slumping features are common in this facies. The Planu Sartu also occurs E of the platform, in the eastern part of Sulcis, forming another slope characterized again by slumping features. Because of these two slopes the size of the platform in between can be estimated. It extended approximately 50–60 km in the present E-W direction (taking into account a shortening to half its original size). The slopes were formed by tensional tectonics, active throughout Gonnesa and lower Cabitza time. In contrast to the slope in the W, the eastern slope has no predecessor in Nebida time. Because of the newly developed basin in the E, the clastic supply to today's SW-Sardinia was interrupted. This lasted until the breakdown of the platform in Cabitza time, when clastic delivery was reinstated. At least during the lower Gonnesa time, the platform wasnot rimmed by a barrier which consisted of ooids and an algal-archaeocyathan community. Ooids occur only locally. This lack of a special marginal facies, together with the slope-facies mentioned above, indicates poor water circulation in the two basinal areas during Gonnesa time, and possibly an increase in salinity. The tensional tectonics in the Cambrian might have initiated the separation of northern parts of Gondwana, which subsequently drifted to the N. Microcontinental collision might have been responsible for compressional tectonics (“Sardic phase”), which was active most probably in the lower or middle Ordovician.