The geology of the northern half of the Rhynie Basin, Aberdeenshire, Scotland

A new geological map of the Early Devonian Rhynie Basin has been produced by traditional methods supplemented by trenching to bedrock and a ground magnetic survey. This shows that the basin margins are mostly fault-controlled and three trends are recognised: NE-SW, NNE-SSW and N-S. Three sets of open folds are distinguished with axial traces trending NE-SW, E-W and roughly NW-SE. The faults defining the basin margin and the folding may be related to basin formation within a regional strike-slip system of Early Devonian age. The stratigraphic succession comprises three mappable units: a lower mixed unit of sandstones, shales, conglomerates and andesitic lava (>700 m), a middle unit of laminated grey shale and siltstone (>300 m); and an upper unit of laminated sandstones and shales (>300 m). These correlate with the Tillybrachty Sandstone and Quarry Hill Sandstone Formations (lower unit), and the Dryden Flags Formation (middle and upper units). Small areas containing abundant chert float found outside the Rhyme SSSI may represent the surface expression of chert pods within the middle unit of laminated grey shale and siltstone, which also hosts the Rhyme cherts. The Windyfield cherts occur within the upper unit. No further centres of hydrothermal activity have been found in the northern half of the basin.