Morphology and significance of super-mature calcretes from the Upper Old Red Sandstone of Scotland

Thick calcrete profiles occur in the Late Devonian-early Carboniferous Kinnesswood Formation of the Midland Valley of Scotland. These profiles consist of highly brecciated peloidal and coated grain fabrics with laminar calcrete horizons and linings to fractures. The profiles resemble supermature calcretes associated with late Cenozoic surfaces in the southwestern United States and reflect pedogenesis over periods of several hundred thousand years or more. The possible origins of the highly complex fabrics seen are reviewed and the polygenetic nature of such profiles is stressed. It is suggested that brecciation takes place co-incident with subaerial exposure of the calcrete. Mature profiles may be predisposed to brecciation and surficial weathering because of their susceptibility to soil erosion as a result of changes in vegetation cover. The calcretes underwent phreatic cementation, burial dolomitization and silicification.