SUBAERIALLY-EXPOSED PALEOZOIC CARBONATES: KARST, OR PALEOKARST?

Certain karst surfaces in Paleozoic carbonate rocks suggest that the karst landscape is today (again) a subaerially-exposed - paleokarst. One in particular is the Gray Fossil Site at Gray, Tennessee. This site is a pond/sinkhole currently on top of a hill and is now one of the richest fossil sites in the US that was discovered by accident while widening a road. The initial fossils found appeared to be of Pleistocene age, however, things changed when, what turned out to be Miocene fossils, were found. This made this karst surface age > 5.5 Ma. Later drilling in ~30-m deep pockets found 55 M.Y. old pollen at depth. Two interesting issues result: (1) the thickness and age range of the sediment blanket (which may have been a continental-scale? event; “pocket deposits” in Derbyshire maybe the same age as the Gray Fossil site sediments) and, of course, (2) the great age of the karst surface.

In the Tennessee example, the rocks are Cambrian-Ordovician Knox Group, with caves present in a few locations but are a small percentage of the total (in Tennessee). However, the Knox Group is known to be a prolific paleoaquifer, with hundreds of meters of karstic permeability thickness and that extends across the US Midcontinent. Fresh water has been encountered during drilling at - 1,500 m just above the basement, and other data from residential wells have geochemical signatures of deep saline basement-related water mixing with deep circulating meteoric water.

On the south coast of Wales, Triassic rocks have been mapped in/on the Carboniferous Limestone at Bullslaughter Bay, Pembrokeshire, and at Port Eynon on the Gower Peninsular. These carbonate rocks are currently subaerially exposed and contain some of the longer and deeper cave systems in the UK. Supporting an idea that this karst terrain relates to > 200 Ma, include galena veins in cave walls (Pb-Zn ore formation 290 Ma?) and, an exhumed Triassic landscape (Triassic sediments in valleys, fissures/ gash breccias and sinkholes?) on the Gower Peninsular, that may also have remaining, aspects of the former Triassic desert landscape.