Gypsum Karst in the Southwest Outskirts of the Eastern European Platform (Western Ukraine): A Type Region of Artesian Transverse Speleogenesis

Abstract

The gypsum karst in the Western Ukraine occurs in the southwestern outskirt of the Eastern European Craton and is a model example of evolution of intrastratal karst and of artesian transverse speleogenesis in this lithology. Speleogenesis commenced in the regionally extensive confined Miocene aquifer system, in which the Middle Miocene (Badenian) gypsum bed is sandwiched between two “normal” aquifers with diffused permeability. Speleogenesis developed due to ascending leakage across the gypsum induced by valleys incising into the upper confining sequence. Large gypsum caves in the region (including the five longest gypsum caves in the world) are generally stratiform multi-story mazes of passages developed along vertical and steeply inclined fissures. Cave patterns in plan and the cross section are variable depending on fracture stratigraphy, patterns and connectivity, areal distribution of recharge (at the lower contact) and discharge (at the upper contact) points, and local hydrodynamic conditions and history. Patterns and meso-morphology of the caves clearly demonstrate functional organization that evolved in response to ascending hydraulic communication between the aquifers in the Miocene aquifer system, reflecting the complex pattern of transverse flow across the gypsum. This study demonstrates that artesian transverse speleogenesis is of fundamental importance to groundwater circulation in the region-wide Miocene aquifer system. It has important implications for regional and site-specific hydrogeology, the origin of large bioepigenetic sulfur deposits, and assessment of collapse/subsidence hazard. The style and principal characteristics of the large maze caves in the gypsum karst in the Western Ukraine are not specific to dissolution of sulfate rocks but exemplify general regularities of artesian transverse speleogenesis that are applicable to other lithologies including carbonates.