Content area
Full Text
Abstract: The drilling of nine cored boreholes, including a deep hole (233 m) to a gabbroic basement, in the vicinity of the Rhyme 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 Rhyme 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 Rhyme deposit has survived due to a fortuitous combination of circumstances.
Keywords: Rhyme, Devonian, geothermal systems, structure, stratigraphy.
The Rhynie chert SSSI (Site of Special Scientific Interest) is of world-wide importance in that it is one of the earliest undoubtedly terrestrial ecosystems with a well preserved biota yet discovered. The cherts occur in a succession of Early Devonian sedimentary and volcanic rocks near Aberdeen, Scotland (Fig. 1) and represent the surface manifestation of one of the earliest known (396 Ma) subaerial hot spring systems (Rice et al. 1995). Hot spring activity (Trewin 1994, 1996) produced siliceous sinters, now cherts, that contain the exceptionally well-preserved remains of early vascular land plants (Kidston & Lang 1916-21; Remy & Remy 1980; Hass 1991), lichens (Taylor et al. 1995, 1997) fungi (Hass et al. 1994), algae (Edwards & Lyon 1983),...