CARBONATE DEPOSITION IN CONTINENTAL SETTINGS: SPATIAL DISTRIBUTION AND PETROGRAPHY OF LATE MESSINIAN TRAVERTINES IN ALBEGNA BASIN (SOUTHERN TUSCANY) AND LATE PLEISTOCENE TRAVERTINES IN AQUE ALBULE BASIN (TIVOLI, CENTRAL ITALY).

The hydrocarbon reservoir discoveries in continental Lower Cretaceous carbonates in the South Atlantic Pre-Salt (offshore Brazil and Angola) have renewed the scientific interest in recent and ancient carbonates developed in lacustrine, hydrothermal and fluvial environments. Outcrop analogues provide key information to improve the understanding of the stratigraphic, sedimentological, diagenetic and petrophysical subsurface reservoir properties and lithofacies spatial distribution. To improve the knowledge about continental carbonates lithofacies types, stratigraphic architecture and diagenesis, this project focuses on two case studies: the Messinian mixed terrigenous-travertine succession of the Albegna Basin (Southern Tuscany, Central Italy) and the Pleistocene Acque Albule Lapis Tiburtinus (Tivoli, Central Italy). In the both studied areas, travertines include facies types precipitated from thermal water through abiotic and biologically influenced processes, ranging from clotted peloidal micrite boundstone, to crystalline dendrites and coated reeds. The 70 m thick Messinian mixed terrigenous-travertine system, accumulated in the extensional continental Neogene Albegna Basin, was investigated in terms of lithofacies types, depositional environment and their spatial distribution, petrographic analysis, carbon and oxygen stable isotope geochemistry, diagenesis and porosity. The succession recorded three phases of evolution of the depositional system. 1) At the base, a 20 m thick northward prograding hydrothermal travertine terraced slope interfingered in the eastern part with an alluvial plain system prograding northward and westward (Phase I). 2) The continuous travertine succession was interrupted by the deposition of several metres thick alluvial fan breccias intercalated with 2-3 m thick travertine lenses (Phase II). 3) During Phase III, the basin evolved into an alluvial plain with ponds rich in coated reed travertines. Travertine stable isotope signatures during phase I and II confirm the geothermal origin of the precipitating water with 13C value averaging 1.46 ‰ V-PDB, while 18O is -7.50 ‰. Phase III travertines, enriched in coated plants, show lighter 13C values (mean 13C -0.36 ‰; 18O -7.22 ‰) indicative of influence of meteoric water with soil-derived CO2. The paragenetic history of the studied succession shows that it was affected by hydrothermal, meteoric and burial diagenesis. The studied succession was firstly affected by a burial phase, afterwards followed by uplift and exhumation to meteoric diagenesis. The 20 km2 and 50 m thick Pleistocene travertine succession accumulated in the Acque Albule Basin close to Tivoli village was investigated in terms of lithofacies types, its geometry and architecture, through the analyses and the correlation of six drilled borehole cores. The travertine unit consists of a wedge shape geometry thinning southwards and it is subdivided in a proximal, intermediate and distal part from North to South. The succession recorded four principal units separated by four main unconformities that consist of centimetre to few metres thick intraclastic/extraclastic wackestone to floatstone/rudstone, indicative of periods of non deposition and erosion, due to the temporary interruption of the thermal water out of the springs. The southern part of the wedge is intercalated with conglomerate and sandstone representing palaeo-river channels. This study shows the stratigraphic architecture and sedimentary evolution of these two decametre scale continental sedimentary successions in which hydrothermal activity and travertine precipitation were driven by the extensional and transtensive tectonic regimes, with faults acting as fluid paths for thermal water and creation of accommodation space for terrigenous and travertine deposition. Humid climate might have been instrumental for the aquifer recharge that fed the hydrothermal vents. This study proposes two different geological models useful for further comparison with other continental basin successions and hydrothermal travertine systems in outcrops and subsurface. It provides useful information for the petrographic, spatial and reservoir characterization of subsurface travertine analogues.