Candidate cave entrances in a planetary analogue evaporite karst (Cordillera de la Sal, Chile): A remote sensing approach and ground-truth reconnaissance

Abstract

The Cordillera de la Sal (CDS) is a NNE-SSW elongated fold-and-thrust belt several km wide and over 100 km long located in the hyper-arid climate of the Atacama Desert. This ridge contains important Oligocene-Miocene continental sediments including thick interbedded salt rock units which form extensive outcrops. Despite the rare occurrence of rain events, these salt rock beds host well-developed and scientifically interesting underground cave systems, perfectly adapted to the contemporary drainage network. The complete lack of vegetation makes this area a perfect analogue to extraterrestrial evaporite karst areas. A remote sensing analysis of 600 km2 of Pleiades images (acquired in 2018 by courtesy of European Space Agency) at a spatial resolution of 0.5 m (panchromatic) and 2 m (RGB and near-infrared bands) and DTMs extracted from stereographic couples has allowed to map the lithological units, the drainage network, and the candidate cave entrances (CCEs) of most of the Cordillera de la Sal.The study area has been divided in eight morpho-structural units, based on our geological and geomorphological mapping. An Analytic Hierarchy Process (AHP) was used to classify the CCE potential of these karst zones into four classes: low, medium, high, and very high potential of finding new caves. This remote-sensing derived CCEs inventory has been ground truthed with two testing datasets (101 points): i) confronting the cave register based on explorations carried out prior to this analysis, and ii) with a field-based validation in completely unexplored areas. These ground-truthing methods support the quality and reliability of our remote sensing-derived CCEs, with accuracies of 71% and 83%, respectively.With this integrated remote-sensing and ground-truthing approach, we highlight that CCEs identification by image analysis and GIS processing appears reliable for speleological explorations in the CDS and might be a valuable tool also for objective decision-making in the search of caves and potential areas susceptible to karst formation on other planetary bodies.