Hypogenic speleogenesis, late stage epigenic overprinting and condensation-corrosion in a complex cave system in relation to landscape evolution (Toirano, Liguria, Italy)

The Toirano karst system is located in the Ligurian Alps (north Italy), around 4.5 km inland from the coastline and carved in Middle Triassic dolostone. It comprises five cave levels over a 154 m altitudinal range, specifically Ulivo (340 m a.s.l.), Colombo (247 m a.s.l.), Upper Santa Lucia (215 m a.s.l.), Lower Santa Lucia (201 m a.s.l.) and Bàsura (186 m a.s.l.) caves. The system is active at lower altitudes, as testified by the thermal spring currently located at 70 m a.s.l. along the Varatella valley. Speleogenesis was attributed to the action of epigenic processes by other authors. However, the extraordinary geodiversity of the underground morphologies and deposits are at odds with this interpretation. Accordingly, this work investigates the genesis of the Toirano karst system, in relation to the landscape evolution of the surrounding area. A detailed morphological and mineralogical investigation of cave geoforms and deposits, together with the presence of an active low thermal sulphide spring on the Varatella valley and only ~100 m below the Bàsura Cave, sustain the hypothesis of a hypogene origin of the caves. This work shows that most of the caves formed close to the former water table (base) level, in turn determined by the mean sea level. Geochronological analyses, including U/Th (n = 13) and cosmogenic burial (n = 1) dating, together with an estimated incision rate of the Varatella valley of around 0.1 mm y‐1, have allowed to assess the age of the highest cave (Ulivo) at around 2.7 Ma, Colombo at ~1.8 Ma, Upper Santa Lucia at ~1.5 Ma, Lower Santa Lucia ~1.3 Ma and Bàsura at ~1.2 Ma. Estimated palaeotemperature attained through isotope analyses and fluid inclusions on speleothems suggest that the temperature of rising waters was lower than 50 °C and possibly ranging between ~12 and ~20 °C, indicating that hydrothermal fluids were not the main driver of speleogenesis, at least in the late speleogenic phases. Additionally, sulphuric acid speleogenesis by-products were not identified. Accordingly, hypogenic speleogenesis occurred because of the action of low temperature CO2-rich rising fluids. A late stage of epigenic speleogenesis has been detected because of the clear evidences of condensation-corrosion morphologies, occurred since ~150 ka (possibly earlier) when caves were finally connected to the surface because of valley enlargement. Besides uncovering the genesis of the Toirano karst system, this study demonstrates that the combination of local geology, surface vs underground geomorphological observations, climate change vs landscape evolution evaluation and geochemical data is of key importance for interpreting subsurface land-shaping processes.