Genesis of calcic and petrocalcic horizons from Coimbatore, Tamil Nadu: Micromorphology and geochemical studies

摘要

Understanding the development of calcic and petrocalcic horizons is important for palaeoclimate reconstruction. The micromorphology and geochemistry of Quaternary calcic and petrocalcic horizons formed in the area around Coimbatore, Tamil Nadu are discussed. The calcic horizons represent the Bk horizons that occur as thick complex profiles (～300 cm thick) in the foothill regions while the laminar petrocalcic horizons representing the K horizon (80-100 cm thick) formed on hard rock in the topographic low-lying area. Calcic horizons are represented by powdery, nodular calcrete nodules and root casts with 95.2-64.5% of CaCO_3. Laminar petrocalcic horizons are compact with well rounded coalesced nodules and contain 56-64% CaCO_3. Micromorphological study of the calcic horizons show the occurrence of alveolar septal structures, calcified filaments, coated grains, spherulites, calcified root cells and calcispheres that indicate biogenic origins, mainly induced by plant root related microbial activity. The calcic nodules within the calcic horizons consist of quartz sand grains cemented by finely crystalline, grain-coating, often glaebular and pore-filling micrite. This development has taken place in phases of soil formation, erosion and reworking. The inter-relationships between these processes have caused variations in the phases of accretion of soil profiles developed in the foothill region. Microfabrics of laminar petrocalcic horizons with detritus hosts show replacement, corrosion, displacement and shrinkage indicating that the laminar petrocalcic horizon formed under relatively semi arid conditions. These characteristics indicate that the laminar petrocalcic horizons formed in a moderately near-surface environment with relatively high rates of evaporation and ground water action and are polygenetic in origin.