Geological setting of the Early Devonian Rhynie cherts, Aberdeenshire, Scotland: an early terrestrial hot spring system

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Abstract
The drilling of nine cored boreholes, including a deep hole (233 m) to a gabbroic basement, in the vicinity of the Rhynie chert locality has resulted in a major revision of the structure and stratigraphy of the area. The main new structural element recognized is a low angle extensional fault system which defines the western edge of a half graben containing the Early Devonian succession. The fault system was the main conduit for the fluids that fed the hot springs, and a heat source to the southeast of Rhynie is indicated. Rapid and unexpected lateral and vertical variations in lithology, together with new lithological units, have been identified. The latter include a thick unit of intensely altered lapilli tuffs which are unique to the Rhynie basin and to other Devonian basins in NE Scotland. Analysis of these lithologies in conjunction with the new structural model allows the succession at Rhynie to be correlated with the succession in the rest of the basin and a model of basin evolution and hot spring development to be constructed. Siliceous sinters were deposited by multiple episodes of hot spring activity and are variably interbedded with shales, sandstones and minor tuffs in a unit about 35 m thick. Sinter deposition was ultimately controlled by repeated subsidence along the basin margin. Geothermal activity was probably widespread in Northern Britain in the Early Devonian but the surface deposits are unlikely to be preserved. The Rhynie deposit has survived due to a fortuitous combination of circumstances.

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