Calcrete paleosols in the upper buntsandstein from the Iskur River gorge, northwestern Bulgaria

摘要

The Lower Triassic calcrete paleosols from the Iskur River gorge resemble numerous paleopedogenic occurrences from Upper Buntsandstein sequences in Europe. Their predominant autochthonous varieties (mottles, nodules, rhizocretions) occur in fluvial sandstones and shales, whereas reworked calcrete fragments are present in rare conglomerate layers. The paleosols developed through early diagenetic replacement and displacement (but not cementation) by calcite resulting in both alpha- and beta-type fabrics. Crystal enlargement of the presumably original pedogenic micrite preceded local dolomitization at shallow burial. The intermittent formation of calcrete paleosols was favored by semi-arid climatic conditions, with rainwater as the probable source of calcium. Mainly submature pedogenesis took place, but mature stage was occasionally reached as well. Evaporation was the major driving mechanism of soil formation while evapotranspiration, plant uptake and root decay were less significant. Despite the convincing pedogenic nature of most calcretes, the shape and orientation of some nodules suggest possible influence by a fluctuating water table that periodically invaded the lower vadose zone.