Lithology and paleosols of the Klikov Formation in the borehole 2140 01T1 Dunajovice, Třeboň Basin
Authors: Lojka Richard 
Published: June 2023

The 2140 01T1 borehole sampling the Late Cretaceous non-marine strata of the Klikov Formation in the central part of the Třeboň Basin in southern Bohemia provided ~ 70 m thick alluvial succession composed of alternating sandstone and mudstone intervals that overlays deeply weathered crystalline basement. The mudstone interval in the lower part of the Klikov Formation is separated by sandy layers into several 2–4 m thick fining-upward units. The sharp-based sandy layers with fusain, carbonized stems and rip-up clasts of underlying mudstone are interpreted as avulsion deposits. They grade upwards into heterolithic sand-silt sequence and mottled massive mudstone with burrows, rhizohalos and variably developed pedogenic features. Such cyclic arrangement probably reflects autogenic processes on distal floodplain with episodic deposition and upward increasing subaerial exposition and pedogenic overprint. In the middle to upper parts of the Klikov Formation up to 6 m thick channel sandbodies with sharp erosional bases cutting into the underlying laminated sand-silt heteroliths of channel levee deposits occur. The thickness of the single-storey sandbodies corresponds usually to c. 2 m which suggests up to 3 m deep channels during bankfull discharge. The thicker multi-storey sandbodies record channel amalgamation on the alluvial ridge or shallow paleovalley fillings. The sandbodies grade upwards to massive mudstones of abandoned channel fills and floodplain deposits. These floodplain units are up to 8 m thick and consist of variegated massive mudstones interrupted by discrete up to 1 m thick fining upward layers of rippled sandstone representing crevasse splay deposits. Depositional context including preserved levee deposits, abandoned channel fillings and floodplain finegrained deposits suggests rather higher sinuosity of the formative rivers. Palaeosol profiles represent pedogenically modified floodplain fines with gradational bases and sharp erosive top surfaces. At least 10 palaeosols were identified in the measured section, forming mostly discrete profiles up to 2 m thick consisting of several B-horizons that differ in the degree of the soil structure development, colour, mottling, grain size, presence of clay hypocoatings and bioturbation index. The most common Bw horizons form lower parts of the profiles and have no or only a weakly developed angular blocky structure (ABS). The typical mottling is caused by a hierarchical system of vermicular greenish-grey reduction rhizohalos, that may be filled by silt to sand or illuviated clay hypocoatings and probably follow the original plant root system. Some patches have diffuse orange rims of iron oxides, probably represented by goethite. Mudstone of Bsst horizons displays well-developed ABS with rare wedge-shaped peds and contains a number of illuviated clay skins and redox depletions. Vertic features like fine cracks filled by sand are common, and slickensides with clay hypocoatings occur rarely. Mudstones of Bt horizons contain a number of illuviated clay hypocoatings filling root channels or interpedal cracks, while the Bss horizons exhibit well-developed ABS with slickensides and contain a number of vertical cracks filled by sandy grains. Palaeosol profiles represent mainly argillisols and vertic argillisols with transitions to argillic vertisols. Poorly developed argillic to vertic protosols are also common. Palaeosol profiles differ mainly in the degree of structural development/maturity, which is given by various lengths of exposition to similar climatic conditions. Vertical distribution of palaeosols reflects their position in the alluvial landscape with locally specific hydromorphic conditions rather than a distinct climatic trend. Most of palaeosols exhibits downprofile translocation of clay (illuviation) leading to the formation of argillic horizon, which typically develop on stabilized landscape in humid climate with seasonal moisture deficit.