Dryden Flags Formation
The Rhynie cherts occur within the Dryden Flags Formation which forms the upper part of the of the sequence of the basin fill. It is a sequence of thinly bedded micaceous sandstones (traditionally termed 'flags') and shales, that were previously recognised as the Dryden Flags and Shales and in part may represent a lateral equivalent to the Quarry Hill Sandstone Formation (see Rice et al. (2002) for details of the stratigraphy described below).
The succession of the Dryden Flags Formation in the Rhynie area, following the drilling programmes by the University of Aberdeen and recent studies, can now be divided lithologically into four discrete units. The lowest unit, informally termed the "Shales and Muddy Sandstones Unit" grades up from the "White Sandstones Unit" below and comprises primarily thickly bedded (up to 1m thick), erosively based massive muddy sandstones interbedded with thin shales and siltstones which show abundant soft-sediment deformation features. This unit passes upwards into a thick sequence of dark shales with thin fluidised and disrupted sand and silt laminae, informally termed the "Lower Shales Unit" (see inset below left).
Above the shales lies the chert-bearing part of the sequence, informally termed the "Rhynie Chert Unit". Comparing borehole and trench data the chert beds (see inset above right) are laterally impersistant (Trewin & Wilson 2004). One borehole (19c) studied by Clare Powell exhibited 53 individual chert beds over a depth interval of 26m, many of which are plant-bearing (Powell et al. 2000b) (for a more detailed review of the chert morphologies and textures see the following section on Chert Textures). The chert horizons are variably interbedded with silicified compacted sandstones with organostylolites, together with thin shales and siltstones similar in lithology to the units below and above. By comparing the biota content and textures of the discontinuous plant-bearing chert beds of the Rhynie Chert Unit with Recent plant-bearing sinters and modern hot spring environments at Yellowstone National Park, USA, Trewin and Wilson (2004) suggest that much of the Rhynie chert succession represents sinter deposition on the distal, cooler reaches of a hot spring outwash apron.
The uppermost lithological unit in the Dryden Flags Formation is informally termed the "Upper Shales Unit" and is at least 13m thick. It comprises primarily green-coloured laminated shales (locally exhibiting desiccation cracks) variably interbedded with impersistant thin sandstone beds that often display sharp to erosive bases, mud rip-up clasts, current ripple laminae and graded bedding. Patchy calcite cementation is present in the sandstones. Similar lithologies host the Windyfield chert, some 700 m to the northeast of the Rhynie chert locality, and therefore it has been suggested the Windyfield chert probably occurs within this younger unit (Rice et al. 2002; Rice & Ashcroft in press).
The overall environment represented by the rocks of the Dryden Flags Formation appears to be an alluvial plain setting with mainly overbank deposition on floodplains and in ephemeral lakes with occasional sheet-flood events. The cherts and their interbedded sediments representing localised deposition of subaerial and subaqueous sinters from hot spring and geyser activity and the accumulation of soils on floodplains (see inset below).