Summary

Calcretes are abundant in continental Devonian (Old Red Sandstone) rocks, occurring both within the sequence and at the basal unconformity. Basal limestones in the Orcadian Basin also include algal deposits and fissure linings, all of which can be considered as a single phase of carbonate precipitation and growth. A prominent silcrete in Orkney replaces both rudites and chert-bearing lacustrine laminites. Silcrete and calcrete are examined in a wider context of chemical sedimentation in lacustrine rocks. The effects of weathering processes in these rocks may be combined or confused with those of algal processes. A silicified plant-bearing sequence at Rhynie displays the same diagenetic features as the Orkney silcrete. Limited metalliferous and bituminous enrichments are recorded in the largest ORS duricrusts. A basal sequence at Dalroy exemplifies the possible confusion between weathering concentrations and mineral deposits trapped at an unconformity surface. Barite is a common component of some duricrusts. Duricrust formation is part of a range of sedimentary replacement and cementation phenomena which may be difficult to distinguish in ancient rocks.