Well preserved late Precambrian paleosols from Northwest Scotland

Two paleosol profiles were developed side by side on gneiss (Sheigra clay paleosol) and amphibolite (Staca clay paleosol) along the same unconformable contact of Lewisian metamorphic rocks (2.7-1.7 Ga) overlain by Torridonian sandstones (0.8 Ga) in northwestern Scotland. Although altered during burial by compaction and illitization, these profiles are well preserved compared with most other Precamhrian paleusols that are metamorphosed to at least greenschist facies. In addition, deformation of pegmatite veins in a manner similar to soil creep can be taken as an indication that upper horizons of these paleusols have been preserved. Soil development in both profiles was characterized by the oxidation of iron in biotite and magnetite, and by hydrolysis of plagiodase, orthoclase, and hornblende. Clay skins, soil-like microfabrics, and pedogenic smectite have been preserved in the profiles. Alteration of silicate grains other than quartz, microcline, and biotite has been intense, but chemical analyses failed to show dramatic removal of alkalis and alkaline earths from the profiles. Microscopic concretions similar to pedogenic carbonate were found in subsurface horizons of the Staca clay. Rare-earth elements are more abundant within the paleosols than in their parent materials. These paleosols developed on a well drained landscape that was buried by Torridonian alluvial fans. Atmospheric oxidation was greater than revealed by studies of Archean paleosols, but probably still short of modern levels. Paleoclimate was subhumid and probably seasonal. Organic carbon and its light isotopic composition in the paleosol is evidence for photosynthetic microbial crusts. Paleosols of such great antiquity do not fit com. fortably within modern soil taxonomies, and are for the moment assigned to the informal category of "Green Clays".