5. Hypogene Imprints in Coastal Karst Caves from Mallorca Island (Western Mediterranean): Morphological Features and Speleogenetic Approach

The southern and eastern coasts of Mallorca Island (western Mediterranean) host abundant karst caves, some of them containing solutional features and deposits presumably related to hypogene basal recharge. The caves were formed in calcarenites whose ages range from Upper Miocene (reef deposits) to Middle Pleistocene (eolianites), which form a fringing postorogenic belt deposited over Mesozoic folded and thrusted carbonate deposits. The hydrogeological setting corresponds to an unconfined coastal aquifer in very porous eogenetic rocks, but showing important lateral and vertical permeability variations related to different sedimentary facies. Six caves containing hypogene features are distributed in three main coastal areas: the Llucmajor Upper Miocene platform, the Campos Plio-Pleistocene basin, and the Portocristo Upper Miocene littoral fringe. The first two areas are spatially coincident with low-grade geothermal anomalies reported in southern Mallorca, which are associated to important SW–NE faults. The observed cave features include a suite of solutional rising forms embracing, among others, subvertical feeder-like conduits and small ascending wall channels of variegated morphologies and dimensions. Sediments and black crusts enriched in Fe and Mn oxides are frequent along with some uncommon minerals, documenting a deep recharge rising into the unconfined littoral groundwater. Regarding their speleogenesis, the studied sites must be considered as complex littoral caves in which the imprints of hypogene processes are evident, but intermingled with other morphogenetic vectors like coastal mixing processes and a substantial meteoric recharge. The lithofacies variability of the carbonate rocks introduces additional complexity, affecting the hydrological behavior of the coastal aquifers as well as the pattern and morphology of the resulting caves.