Correlation of the Early Devonian Rhynie chert beds between three boreholes at Rhynie, Aberdeenshire, Scotland

Three sequences of the Early Devonian Rhynie Cherts Unit intersected by cored boreholes at the Rhyme SSSI (Site of Special Scientific Interest) are compared on the basis of both lithology and plant content. It is shown that over the 45-65 m distance covered by the boreholes general correlation can be made of the chert-bearing unit, and also sandstone and shale units. General thicknesses of units are similar in the three boreholes. However, there is no bed-to-bed correlation of the cherts or other lithologies, neither is there a correlation in the sequences of plants recorded. It is considered that the sequence was deposited in the distal regions of sinter deposition on a low-angle outwash apron from a hot spring, where the water table was close to the surface, and water was frequently ponded. Overbank flooding from a northward-flowing river system periodically inundated the outwash apron, depositing sand and mud and interrupting sinter deposition. On the basis of plant occurrence in the chert beds intersected in the boreholes it is estimated that on average 15% of the area was bare ground, 30% covered with plant litter, and 55% with growing plants. Similar environments are seen today on the distal marshy areas of outwash aprons from hot springs in Yellowstone National Park, USA.