The Flandrian (Quaternary) Caerwys Tufa, North Wales: an ancient barrage tufa deposit

SUMMARY An extensive deposit of tufa, including organic build-ups, oncoid facies and micrite facies, extends as a lobate deposit downslope from a Carboniferous Limestone scarp near Caerwys, North Wales. The deposit contains terrestrial and freshwater molluscs, ostracods and plant remains, the latter now preserved as moulds and casts. Collectively, the carbonates caused blockage of a pre-existing valley, by a combination of CaCO 3 precipitation associated with oscillatoriacaen “algae” and the growth of higher plants. Oncoid and detrital facies accumulated upstream of the tufa barriers in small standing pools and marshes. Carbonate precipitation, brought about by both physical and organic processes, produced freshwater isopachous fringe cement and micrite; peliods associated with the tufa appear to be related genetically to these. The tufa build-ups (phytoherms) within the Caerwys deposit are associated with contemporaneous living vegetation such as reed and willow, although mosses and liverworts are most common. The plants were bonded during life by freshwater isopachous fringe cements and were colonized by cyanobacteria and animals. The phytoherms had positive relief and effected local changes in water flow, sedimentation rates and ecology. Collectively, the deposit illustrates many of the typical features of barrage tufa deposits and serves as a useful model for ancient deposits.