Subaqueous silicification of the contents of small ponds in an Early Devonian hot spring complex, Rhynie, Scotland

The Early Devonian Rhynie and Windyfield cherts of northeast Scotland originated as siliceous sinters deposited by hot springs. Silicification affected both subaerial and subaqueous environments, preserving a diverse terrestrial and freshwater biota. Cherts originally deposited in small shallow pools can be recognised on the basis of both texture and fossil content. Textures comprise clotted and microcoprolitic textures, bacterial coatings on plant axes that can be stromatolitic, and a variety of bacterial and fungal meshworks. The crustacean Lepidocaris, the euthycarcinoid Heterocrania, the charophyte alga Palaeonitella, and chytrid fungi are typical elements of the aquatic biota. Observations of modern hot springs in Yellowstone National Park, Wyoming, U.S.A., demonstrate that shallow ponds and streams on low-angle outwash areas and dormant vent orifices provide suitable environmental analogues. Textures comparable to those described from Rhynie are recorded from Yellowstone sinters, but examples of the rapid and complete silicification of delicate organic structures as seen in a few of the Rhynie chert beds have not been noted. Petrographic textures comparable to those seen in the cherts of freshwater origin from Rhynie occur in modern stream sinters at Yellowstone, where they form from waters at 20-28degreesC and with a pH of 8.7. This similarity occurs despite differences in environment at Yellowstone, such as the oxidizing surface environment, water table fluctuations, complex modern vegetation, fixing of silica by diatoms, and climatic extremes. Thus there are close similarities between textures seen in the Rhynie cherts and Yellowstone sinters deposited in freshwater pools and streams by hot springs.