Nature, origin and significance of a clayey cave deposit: Relationship between karst and Oligocene/Miocene tephra (NW Dinarides)

Abstract

A limestone cave filled with montmorillonite clay was found in a hill above the karst plateau on the north-eastern edge of the Adriatic Sea. The cave fill shows no correlation with sediments found in the surrounding caves or other known caves of the Classical Karst, where montmorillonite content is negligible. In other caves, the primary deposited sediments originate from weathered Eocene flysch and were transported into caves by sinking streams. Powder X-ray diffraction analyses indicate a pure montmorillonite composition with a few coarser quartz grains and heavy minerals. Based on these results and comparisons with tephras from volcanic centres in the wider region, we conclude that the geochemical characteristics of the montmorillonite clay (rare earth element distribution, enriched patterns of light rare earth elements, LaN/YbN ratios and significantly negative Eu/Eu* ratio) are similar to weathered volcanoclastic material of the Smrekovec Volcanic Complex (Oligocene to Miocene eruptions in north-eastern Slovenia) and transported to the depositional centre by north-eastern winds. Dating the clay using apatite fission-track thermochronology yielded 22 ± 7 Ma, while K-Ar dating gave 23.4 ± 1.7 Ma. Both ages are consistent with the activity of the Smrekovec Volcanic Centre, previously dated to 28–23 Ma. The presence of weathered volcanic ash in the studied cave indicates considerable explosive activity of the stratovolcano, previously interpreted only as a submarine edifice, and confirms the existence of a contemporary karst landscape at the time of volcanic activity where volcanic ash was deposited and subsequently in situ weathered in a levelled corrosion-tectonic plane. Consequently, it can be concluded that a karst landscape in the north-western Dinarides has existed since the Late Oligocene, with the studied cave being the only known remnant identified so far. This study presents a rare sedimentary record and highlights the role of karst landscapes as valuable archives of past geological events and environmental changes.