The palaeoenvironments of the Rhynie Cherts
With a radiometric age of 396 ± 12 ma, the Rhynie Cherts, Grampian Region, Scotland, are the oldest unequivocal surface expression of an epithermal system in the world. Data is presented from 8 cored boreholes drilled within 100m of the Rhynie Cherts subcrop. The cherts are present in the upper part of the Lower Devonian basin infill which forms the Rhynie outlier. The basin is a half-graben structure, with a northeast/southwest trending western boundary, following the regional stress trend. In the area of the Rhynie Cherts, the western boundary is complicated by a series of cross faults. The eastern boundary is unconformable. The basin infill, in the area of Rhynie village, is a fining upwards sequence, produced locally from the newly formed Caledonide mountains. The basal Pre-Lava Sandstones Unit is an alluvial fan deposit. This is overlain by a series of basaltic andesite lavas with associated agglomerates and lapillistones. The Tuffaceous Sandstones Unit, containing both airfall and fluvially reworked tuffaceous material was deposited at the close of volcanic activity. The next unit in the series is the Shales with Thin Sandstones Unit, which contains the Rhynie and Windyfield Cherts. These two units of the post-lava sequence represent alluvial plain deposition, with evidence for sub-aerial exposure. The uppermost unit in the outlier is the Quarry Hill Sandstones unit, comprising fluvial channel sandstones. Hot spring activity occurred during the deposition of the Shales with Thin Sandstones Unit, resulting in the deposition of the Rhynie and Windyfield Cherts. A 35m cored borehole permits study of a vertical section through the chert bearing strata.