Carbonate build-ups in continental settings have recently received new attention resulting from major hydrocarbon discoveries in non-marine carbonates in the South Atlantic. In order to develop actualistic models for these unique build-ups, this work aims to characterize the geometry, internal architectures and biotic and abiotic fabric types of a Pleistocene hydrothermal travertine mound outcropping in Central Italy. The outcrop comprises a 30-m-high cone-shaped mound with a 100-m-wide asymmetric circular planar section, which precipitated from a punctual thermal spring. A flat, irregularly shaped circular top and steep slopes (up to 70°) characterize the mound.
Five main fabric types, with characteristic porosity, are arranged into centimeter- to decimeter-thick carbonate beds and millimetre-thick laminea, which build the metre-scale mound. The different arrangement of these layers identifies three internal architectural patterns, with different spatial distributions in the mound: tabular sub-horizontal, clinoformal and less-extended stepped layers, respectively corresponding to pond, smooth slope and terraced slope depositional environments. Tabular sub-horizontal layers, formed by a vertical alternation of dendritic, laminated, raft boundstone and intraclast/extraclast grainstone/rudstone, can be traced laterally for over tens of meters until they abruptly show changes in stratal patterns, varying from clinoformal to stepped layers. The clinoformal architecture consists of packages of slightly undulated and parallel to each other, inclined crystalline dendritic and laminated boundstone. These may evolve into a terraced architecture. Terraced architectures are differentiated into different stratal orientations: vertical to overhanging morphologies, showing vertical alternations of crystalline dendritic and laminated boundstone, sub-horizontal to concave morphologies composed of dendritic, laminated and raft boundstone, as well as raised and convex morphologies, predominantly comprising crystalline dendritic boundstone.
Petrographic and XRD diffraction analyses show that the fabrics are predominantly calcite (92-100 wt.%) with minor amounts of gypsum, quartz, plagioclase, dolomite, muscovite and rock fragments. SEM-EDS magnification reveals the presence of organic matter remains and organomineral nano-globules. Diagenetic processes have produced modifications of the primary fabrics. Stable isotope data, obtained both from primary and secondary precipitates, show a positive field of distribution for δ13C values and a negative field for δ18O values, largely following the pattern of published data for hydrothermal continental carbonates.
Our investigations demonstrate that a combination of abiotic and biotic processes are fundamental for the precipitation of travertine deposits. The build-up of the mound geometry and relative depositional systems are controlled by environmental and hydrodynamic conditions (e.g., location, typology, water supply, hydrostatic pressure of the spring, and pre-existing topography along which hydrothermal water flows).
