Alteration of paleosols after burial

Abstract

Part of the problem with unraveling the alteration of paleosols after burial is clarifying the terminology for these changes. Diagenesis is the term used in studies of sedimentary petrology to describe alteration after deposition. Diagenesis thus includes soil formation, which on Earth occurs at 1 bar pressure and temperatures from 84 to –88°C (Kimmins 1987). Additional changes after burial pass into what would be called metamorphic alteration at temperatures of 200°C or above (Fig. 7.1). Because of the natural gradient of increased temperature downward in the Earth’s crust, there is a practical pressure limit to diagenetic alteration of less than 7 kbar lithostatic pressure, which is found at depths of about 25 km within the crust. Metamorphic alteration proceeds at more extreme temperatures and pressures where new mineral assemblages form at the expense of the old ones and rock structure is reconstituted by the growth of new crystals. Destruction of primary features of soils and imposition of metamorphic foliation, schistosity, and crystalline texture limit the interpretation of paleosols. However, many diagnostic features of paleosols such as root traces, soil horizons, and soil structure survive such changes. These structures have been found in paleosols metamorphosed to zeolite, prehnite-pumpellyite, and lower greenschist facies (Retallack, 1979, 1985). With greater metamorphic alteration, the only remaining indication of a paleosol may be a highly aluminous bulk composition and a mineralogy dominated by kyanite, sillimanite, garnet, or corundum (Reimer 1986, Barrientos & Selverstone 1987).