Abstract

In the Tarquinia basin, two Pliocene sedimentary cycles are developed on the substratum consisting of the Late Cretaceous-Eocene Tolfa Formation (outer Ligurid unit), and are separated by a 0,3-0,4 Ma hiatus along an unconformity surface. The first cycle has an Early Pliocene-early Middle Pliocene age and consists of shelf mudstones; the later has a Middle Pliocene age and consists of the Tarquinia Calcarenite and its heteropic sands. The Tarquinia Calcarenite succession consists of biocalcarenites and biocalcirudites and intercalated hybrid arenites with a thickness of 30 m. These sediments are almost completely bioturbated and the preserved sedimentary structures include large scale low angle trough cross-bedding (migrating megaripples and/or dunes), low-angle dipping laminae and more rare hummocky cross-bedding (storm waves), suggesting a lower shoreface depositional environment. The coarsening-upward succession consisting of lower mudstones-Tarquinia Calcarenite and its heteropic sands has been referred to shelf shallow water bottoms grading to a shoreface. In this area were deposited sands and bioclastic materials eroded from a nearby intabasinal source. This type of sediments has been recognized in some Mediterranean bottoms (the so called "coralligene zone" of shelf) and is also recorded in the upper part of the lower mudstones. The general transport direction of sands and organogenic materials towards NNE suggests that the paleocoast was stretching WNW-ESE. Petrographic analyses carried out on the clastic lithofacies (biocalcarenites and biocalcirudites, hybrid arenites) showed a composition consisting of clearly predominant intrabasinal grains (NCI and CI) compared to the extrabasinal grains (NCE and CE). The biocalcarenites and biocalcirudites are skeletal detrital pack-stones and rudstones whose detrital organogenic fraction consists almost exclusively of fragments of shallow water benthic intrabasinal organisms. The hybrid arenites are characterized by a high content of micrite matrix and prevailing intrabasinal grains. Extrabasinal grains are in small amounts and of continental origin; they include, in order of decreasing amount, soils, angular quartz, K-feldspar, micas, Early Pliocene mudstone fragments, iron oxides, terrigenous carbonate fragments of the Tolfa Formation, rounded quartz. Intrabasinal grains consist of continental origin fragments (plant fragments, abundant fragments of calcareous pedogenic duricrusts) and of marine origin (chert, authigenic glauconite, fossils). Therefore the intrabasinal and extrabasinal grains of continental origin are more abundant than the intrabasinal grains of marine origin. The hybrid arenites are litharenites according to the QFR diagram. The comprehensive composition of biocalcarenites and hybrid arenites confirms that sedimentation of the Tarquinia Calcarenite succession was basically intrabasinal, with alternating bioclastic and hybrid arenitic episodes, whilst the extrabasinal supply was very scarce. The composition of the hybrid arenites intercalated within the Tarquinia Calcarenite, in particular the non carbonate intrabasinal grains, documents condensed arenite intervals that confirm a period of starvation of the Tarquinia basin connected to the Middle Pliocene transgression during sea level rise and in an extensional tectonic context.

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