Speleogenesis of Valdemino Cave (Borgio Verezzi, Liguria, Northern Italy) shows very slow uplift of this coast since Middle Pleistocene

Abstract

Valdemino show cave (Borgio Verezzi, Northern Italy) is located 600 m from the Ligurian coast. The cave, hosted in Triassic dolostones, is characterised by mixing corrosion morphologies and large collapses, some of which covered with thick speleothems, and lacks typical morphologies and sediments related to turbulent water flow. U/Th dating of speleothems allowed bracketing the timing of collapses, coastal mixing, and speleogenesis. The oldest speleothems indicate the first phases of speleogenesis to have occurred prior to ca. 600 thousand years ago (ka, before 1950 CE), whereas important calcite deposition occurred during both warmer and colder periods. Collapses repeatedly took place during cold stages, since at least 600 ka. Present drip waters are highly mineralised, causing speleothem growth and testifying to a long water-rock contact, despite the poor thickness (<10 m) of the rock above the cave. Speleogenesis is mainly related to coastal mixing corrosion which dissolved portions of the rock mass during various periods of the Middle- to Upper Pleistocene, and occurs still today in deeper parts of the cave. During cold stages, when the sea retreated and the water table lowered, loss of buoyancy caused cave rooms to collapse, creating the voids accessible today. The Valdemino cave system was positioned several times in the fresh-salt water mixing zone during different sea level highstands over the last 600 ka, the highest of which occurred during MIS 5e. This demonstrates that this coastal sector must have been subjected to a slow uplift since 600 ka, with rates of ca. 0.05 m/ka. Six-hundred thousand year old corroded speleothems, that were in the mixing zone during MIS 5e, are now found at +6 m above present mean sea level.