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

Decoding Last Interglacial sea-level variations in the Western Mediterranean using speleothem encrustations from coastal caves in Mallorca and Sardinia: A field data - model comparison

U-Th ages of phreatic overgrowths on speleothems in coastal caves of the Western Mediterranean record high sea level positions during marine isotope stage (MIS) 5e of the last interglacial.While relative sea level (RSL) on the southeastern coast of Mallorca stood w2.6 m above present sea level (apsl) during MIS 5e, it stood w4.3 m apsl at this time in northwestern Sardinia. The difference between the two sites during MIS 5e and the deviation from the eustatic sea level trend during the Holocene was investigated using the numerical code SELEN. The offset between the eustatic curve and those pertaining to the two sites principally reflect the vertical deformations and geoidal variations in response to meltwater loading. As Sardinia is closer to the center of the Mediterranean basin, the hydro-isostatic component of RSL is enhanced here compared to Mallorca, which results in a modeled offset ofw60 cm between the two sites. This result is qualitatively useful as it provides a partial reconciliation for the site differences. However, the need for refinement in the model is also recognized, as it does not match the observation based on UeTh dating of speleothem overgrowths in Mallorca that sea level there has remained stable for the past 2800 years. Overall, the results of the study first suggest that both sites largely track the eustatic sea level curve, and second suggest that glacial isostatic adjustment is a viable mechanism to reconcile some, if not most, of the relatively small elevation difference of MIS 5e sea level observed at Mallorca and Sardinia, although minor tectonic adjustments cannot be ruled out in explaining some low-amplitude local variation