Hypogene speleogenesis and paragenesis in the Dolomites

Abstract

The Dolomites of northern Italy feature some of the most intensively studied carbonate rocks worldwide. Yet, little is known about the long karst history of this mountain range dating back to the Miocene. This study scrutinizes three caves (F10, Milchloch and Cioccherloch) in the Fanes-Sennes-Prags Nature Park (Province of South Tyrol) and the adjacent Fosses area in the Natural Park of Ampezzo Dolomites (Province of Belluno, Veneto). Paleo-phreatic passages of these limestone caves mostly follow a NW-SE orientation, similar to the nearby Val Salata fault. A multi-method approach including cave morphological and isotopic analyses of wall-rock cores was applied to reconstruct the multi-stage speleogenetic history of these cavities. Clastic sediments partly cemented to the cave walls and paragenetic features such as ceiling channels and solutional ramps are present in all studied caves. F10 cave shows widespread laughöhle geometries, including inverted cone chambers and horizontal passages with trapezoid cross sections, characteristic of slow water convection in a hypogene regime. Drill cores in F10 cave exhibit a systematic depletion in both oxygen and carbon isotopes close to the cave wall. The thickness of this isotopically altered zone ranges from a few mm to 4 cm and alteration can sometimes also be macroscopically identified. The amplitude of the isotopic shift ranges from 2.0 to 5.0‰ for δ13C and from 1.9 to 4.9‰ for δ18O. One of the wall rock cores from Cioccherloch also shows a thin isotopic halo, while no evidence of isotopic alteration was found in Milchloch. Our results provide strong geochemical evidence of wall rock alteration driven by hypogene water-rock interaction in at least two of the caves. We propose an early hypogene speleogenetic phase, followed by uplift and denudation resulting in the opening of these cavities to the surface that allowed clastic sediment influx. The sediment infill in combination with the abundant paragenetic features records a second phase in the karst evolution. This paragenetic phase did not involve hypogene waters, as indicated by the lack of isotopic alteration in the wall rock of paragenetic features. The latest phase of speleogenesis is represented by vadose morphologies and vadose speleothems locally dating back to at least 650 ka. This study demonstrates that the combination of morphological analyses and geochemical fingerprinting represents a powerful approach to decipher the commonly complex speleogenetic history of limestone caves.