Hypogene Caves in France

Abstract

Hypogene speleogenesis (HS) describes cave development by rising fluids independent of recharge from the overlying or adjacent areas. Dissolution includes deep-seated acidic sources (CO2/H2S), “hydrothermal” cooling, mixing corrosion, and Sulfuric Acid Speleogenesis (SAS) at and above the water table. HS localizes in basin at sites of upward flows and convergences, in deformed strata at structural highs and major faults. In disrupted basins, geothermal gradient “pumps” downward meteoric water. Volcanism and magmatism produce “hyperkarst” combining CO2, H2S, thermalism, and microbial activity. Hypogene caves in France are presented according to a conceptual model. Isolated geodes are lined by calcite spar. 3D multistory caves (Pigette, Adaouste) show CO2 degassing morphologies (bubble trails, folia, tower cones, coral towers) and “hydrothermal” minerals (Fe–Mn oxi-hydroxides, barite, celadonite), with condensation–corrosion cupolas above the water Table. 2D maze caves develop below less permeable strata. Giant ascending shafts (Salins shaft), collapse shafts, and breccia pipes (called “organs” in the N.-French coal basin) correspond to “hyperkarst” phenomena. Caves with ore deposits are associated with sulfurs (Fe, Pb, Zn) or oxidized Fe. They often record past positions of base level and harbor unique features (black tubes, ghost-rock) or rare minerals. Along the water table, thermal gradient and CO2 /H2S degassing make condensation–corrosion and HS above the water table. Chevalley Aven shows upward dendritic pattern. Eventually, cupolas expand to large isolated chamber. Water table sulfuric caves develop headward with typical morphologies (corrosion tables, condensation domes, sulfuric karrens). Vapor shafts form by warm air convections and condensation–corrosion.