Rhynie Chert - Conditions of Formation

Conditions of Formation

The chert was formed when silica-rich water from volcanic springs rose rapidly and petrified the early terrestrial ecosystem, in situ and almost instantaneously, in much the same fashion that organisms are petrified by hot springs today - although the astounding fidelity of preservation has not been found in recent deposits. Hot springs, with temperatures between 90 to 120 °C (194 to 248 °F), were active in a number of episodes; the water had probably cooled to under 30 °C (86 °F) before it reached the fossilised organisms. Their activity is preserved in 53 beds, 80 mm (3 in) thick on average, over a 35.41 m (116.17 ft) sequence, interbedded with sands, shales and tuffs - which speak of local volcanic activity. Deposition was very rapid. The fluids originated from a shallowly dipping extensional fault system to the west, which bounded an extensional half-graben.

Fossils were formed as silica formed in the hot springs themselves; when silica-rich water flooded the surrounding areas; and when it permeated into the surrounding soil. The texture of the sinter formed resemble those found today in freshwater streams at Yellowstone which are typically alkaline (pH 8.7) and tepid 20 to 28 °C (68 to 82 °F). The springs were periodically active, and flowed into an alluvial plain containing small lakes. By analogy with Yellowstone, the chert itself probably formed in a marshy area towards the latter end of the extent of outwash from the springs. Living vegetation covered around 55% of the land area, with litter covering 30% and the remaining 15% of the ground being bare. A braided river flowing to the north periodically deposited the sandy layers found in cores when it flooded its banks.

Sedimentary textures which appear to have formed in the hydrothermal vents themselves are preserved with a brecciated texture; "geyserite", a sediment with a botryoidal form reminiscent of modern vent margins, is also found. Spores collected from within surrounding rocks had been heated to different degrees, implying a complex history of local heating by volcanic processes.