Hypogene Imprints in Coastal Karst Caves from Mallorca Island (Western Mediterranean): A Review of the Current Knowledge on their Morphological Features and Speleogenesis

Among the abundant caves existing in the southern and eastern coasts of Mallorca Island (western Mediterranean), a few of them exhibit solutional features and deposits presumably related to hypogene basal recharge. The caves were formed in calcarenites whose ages range from Upper Miocene (Tortonian reef deposits) to Middle Pleistocene (eolianites) which form a fringing postorogenic belt, mainly deposited over Mesozoic folded and thrusted carbonate deposits. The hydrogeological setting corresponds to an unconfined coastal aquifer in very porous eogenetic rocks, with important lateral and vertical permeability variations related to different facies affected by coastal karst processes. Six caves containing hypogene evidences are distributed in three different 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 geothermal anomalies reported in southern Mallorca, which are associated to important SW-NE faults. The observed cave features or speleogens 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 rich in Fe and Mn oxides are frequent along with some uncommon minerals (barite, celestine, chamosite, and strontianite) and multicolor alteration deposits, documenting a deep recharge rising into the unconfined and oxygenated littoral groundwater. No evidences of sulfuric acid speleogenesis are present in Mallorcan caves. From the speleogenetic point of view, the studied caves 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. This specific hydrogeological environment, where contrasting waters of very different geochemistry occur (meteoric input, coastal mixing, and deep basal recharge), has favored the development of extensive caves that show significant differences with respect to the conventional model of coastal karst speleogenesis. It is worth mentioning that the lithological variability of the involved eogenetic rocks introduces additional complexity to these phenomena, affecting both the hydrological behavior of the coastal aquifers and the planimetric pattern of the resulting caves