TO BE OR NOT TO BE (KARST) THAT IS THE QUESTION – ERR... PALEOKARST ANYONE?

Many karst terrains have been investigated and their geomorphology understood, but with hydrogeology there are contrasting definitions of karst aquifers. In the last few decades increasing sophistication in dating of cave deposits has often increased the relative ages of caves at many locations. In terms of the delineation of the extent of groundwater basins and proper protection of the groundwater resources and the environment, collecting regional geological data is essential, and often quite revealing. Geology has a habit of turning tables over with passing time when original interpretations change. In South Wales, UK, there is a rather “traditional” interstratal karst landscape where the subsurface includes > 50 km of mapped caves mostly containing large (several meters in diameter) passages. Pleistocene valley glaciation has truncated these large passages that are connected to the surface by much smaller tributary passages. An in-situ galena vein and structural features suggest that the larger passages are a modified paleokarst related to Pb-Zn ore migration formed along both Caledonian and Hercynian-related folds and faults dating to ~ 250 Ma. This follows the nature of ore deposition in similar age rocks in nearby Ireland. Elsewhere these same Carboniferous/ Mississippian rocks have Triassic fills in dissolution voids suggesting this is at least a > 210 MY old paleokarst. These and other Triassic sediments were remapped on the surface from what were previously often interpreted as Pliocene or Pleistocene deposits. There are some Paleogene and Neogene outliers that remain also. Another example of a subaerially-exposed paleokarst today is at the Gray Fossil Site in NE Tennessee in Cambrian/Ordovician Knox Group sediments, where a prolific megafauna fossil assemblage occurs in clastic fill sediments in dissolution pockets, that when found, were thought to be Pleistocene and later realized to be Miocene (5 Ma). Later the deepest sediments in the pockets were found to be Paleogene ~55 Ma. Parts of the Knox have been designated a Paleokarst Aquifer in many petroleum exploration-related publications. In terms of size, the scope of the Knox Paleoaquifer is quite staggering, is of continental scale with groundwater circulating to many km depth along pathways of > 1,000 km.