STRATIGRAPHY AND SEDIMENTOLOGY OF THE DEVONIAN RHYNIE CHERT LOCALITY

Recent drilling in the vicinity of the Rhynie chert locality in the Rhynie outlier of Lower Old Red Sandstone allows a local stratigraphic succession to be established. A basal lithic sandstone and pebbly sandstone unit of local derivation rests unconformably on basement, and was deposited in an alluvial fan environment. An andesitic lava flow, now highly altered, overlies the pre-lava sandstone, but is of limited lateral extent. The lava is succeeded by tuffaceous sandstones and shales which grade upwards into a shale-dominated sequence, with greatly diminished volcanic input. The volcanic debris is of local origin, probably derived from eroding tuff cones, but the general depositional environment was an alluvial plain with ephemeral lakes. The Rhynie cherts are siliceous sinters, comprising more than 10 plant-bearing beds and 2 non-plant-bearing cherts and are equivalent to part of the post-lava tuffaceous sandstone and shale sequence. A second plant-bearing chert, together with evidence of a hot-spring vent, has been discovered 700 m from the original locality. The borehole evidence casts some doubt on the validity of the previously accepted stratigraphy of the Rhynie outlier, particularly with regard to the stratigraphic position of the Dryden Flags and Shales.