Diagenesis and stable isotope signature of Miocene travertine deposits (Albegna Basin, Southern Tuscany, Central Italy)

The investigation of non-marine carbonates from lacustrine, hydrothermal and fluvial settings provides key information for the understanding of lithofacies character, spatial distribution and diagenetic and petrophysical... [ view full abstract ]

The investigation of non-marine carbonates from lacustrine, hydrothermal and fluvial settings provides key information for the understanding of lithofacies character, spatial distribution and diagenetic and petrophysical properties of subsurface reservoirs. Most of the published literature about non-marine carbonates concerns Pleistocene-Holocene case studies and limited information is available about early to burial diagenesis.
The 70 m-thick mixed terrigenous-travertine succession of the Albegna Basin (Southern Tuscany, Central Italy) accumulated in an extensional basin during the late Miocene. It includes travertine facies precipitated from thermal water, ranging from clotted peloidal micrite boundstone to crystalline dendrites and coated reeds, interfingering with colluvial fan detrital breccias and alluvial plain deposits.
The succession recorded three depositional phases. During phase I a travertine terraced slope apron prograded northward, interfingering with alluvial plain lithofacies eastward. Phase I travertine δ13C varies between 0.1 and 3.9 ‰ V-PDB (average 1.7 ‰), with average δ18O -7.5 ‰, confirming the geothermal origin of the travertine precipitating water. Phase II is marked by the accumulation of tens of metres thick colluvial fan breccias, intercalated with metre-thick travertine lenses representing terraced slope and flat pond environments draping the colluvial fans. δ13C varies between 0.1 and 2.6 ‰ (average 1.2 ‰), while δ18O ranges between -10.5 and -6.8 ‰ (average -7.8 ‰). Phase III includes the alternation of alluvial plain lithofacies and travertine flat ponds, characterized by average δ13C of 0.5 ‰ and average δ18O value of 7.8 ‰. This suggests the mixing between thermal and meteoric water with soil-derived CO2 in travertine ponds and marshes rich in carbonate encrusted reeds and peloidal mollusc packstone.
The studied travertines have primary porosity up to 20 %, affected by diagenetic features indicative of superimposed diagenetic environments: a) early hydrothermal diagenesis (equant microsparite to sparite and bladed calcite cementation with isotope signature similar to the travertines); b) vadose and phreatic meteoric diagenesis related to temporary subaerial exposures of the travertines due to interruptions of the thermal water flow out of the vent (dissolution, pore filling by Fe hydroxide and silt, recrystallization of primary micrite precipitates, non luminescent to luminescent blocky and scalenohedral cement with negative δ13C values indicative of meteoric water); c) burial diagenesis (sutured grain contacts, stylolites, fracturing, luminescent blocky sparite filling the remaining pore space and fractures) followed by tectonic tilting and uplift. This study on Neogene travertines demonstrates the complexity of diagenetic features that can affect non-marine carbonates and their primary porosity.