International Journal of Speleology (Jul 2005)
Conceptualisation of speleogenesis in multi-storey artesian systems: a model of transverse speleogenesis.
Abstract
Conceptual and respective quantitative models of speleogenesis/karstifi cation developed for unconfi ned aquifers do not adequately representspeleogenesis in confi ned settings. A conceptual model for speleogenesis in confi ned settings is suggested, based on views about hydrauliccontinuity in artesian basins and close cross-formation communication between aquifers in multi-storey artesian systems. Soluble unitssandwiched between insoluble porous/fi ssured formations (common aquifers) initially serve as low permeability beds separating aquifersin a confi ned system. Conduits evolve as result of vertical hydraulic communication between aquifers across the soluble bed ("transversespeleogenesis"). Recharge from the adjacent aquifer is dispersed and uniform, and fl ow paths across the soluble bed are rather short. There is a specific hydrogeologic mechanism inherent in artesian transverse speleogenesis (restricted input/output) that suppresses the positive flow-dissolution feedback and hence speleogenetic competition in fissure networks, and accounts for the development of more pervasive channelling in confi ned settings, of maze patterns where appropriate structural prerequisites exist. This is the fundamental cause for the distinctions between cave morphologies evolving in unconfi ned and confi ned aquifers and for eventual distinctions of karstic permeability, storage characteristics and flow system behaviour between the two types of aquifers.Passage network density (the ratio of the cave length to the area of the cave fi eld, km/km2) and cave porosity (a fraction of the volume of a cave block, occupied by mapped cavities) are roughly one order of magnitude greater in confi ned settings than in unconfined. Average areal coverage (a fraction of the area of the cave field occupied by passages in a plan view) is about 5 times greater in confi ned settings. Conduit permeability in unconfi ned settings tends to be highly heterogeneous, whereas it is more homogeneous in confi ned settings. The storage characteristics of confined karstifi ed aquifers are much greater.Recognition of the differences between origin, organisation and behaviour of karst systems evolved in unconfi ned and confined settings can improve efficiency of exploration and management of various resources in karst regions and adequacy of assessment of karst-related hazards.
