Paleokarst as a target for modern karstification

Paleokarst phenomena are those that at present or for some duration in the past have been decoupled from all active hydrogeochemical systems. They are widely distributed geographically and are recognized from the Lower Proterozoic onwards in the geological record. Amongst the many fascinating features that they offer for study is their exploitation as “targets” for modern karstic groundwater circulation, cavern genesis and surface landform development. This topic appears not to have been surveyed in a general manner. As a first approach, exploitation by descending meteoric waters (theper descensum waters that quantitatively dominate karst genesis) may be differentiated from ascending waters and gases (per ascersum fluids that often are heated).Per descensum exploitation can vary greatly both within and between sites. For example, in the Mendip Hills, UK (a striking Permo-Triassic semi-arid paleokarst) it appears to be negligible despite high groundwater hydraulic gradients in both the paleo and the modern cycles. More commonly there is “hit or miss” utilisation of portions of the buried features that does not, however, determine the principal trends of the modern groundwater flow. There are many examples in Canada that were produced at times of glacier melt and crustal isostatic rebound. Near-perfect physical exhumation of the paleo-landforms and re-activation of the infilled cavern systems of a paleokarst is rare.Paleokarst plays a more generally important role in many ancient and modernper ascensum karsts, often appearing to be an essential pre-condition of their location. It applies to many active and relict hydrothermal groundwater systems and caves, of both CO2-dissolution-dominant and (supposedly) H2S-dissolution-dominant types; celebrated examples in carbonate rocks include most thermal caves of Hungary, and the Jewel, Wind, Lechuguilla systems of the USA. This greater significance to modernper ascensum karst reflects the focal attraction of paleokarst porosity in interstratal, low-porosity groundwater systems, plus the low intensity, long duration nature of dissolution in ascending fluids when compared to meteoric waters.