Lateral relations of deltaic palaeosols from the Lower Jurassic Rønne formation on the island of Bornholm, Denmark
Abstract The Ronne Formation, which crops out on the southern coast of the Danish island of Bornholm, consists of coal-bearing siliciclastic strata. A major feature in these sediments is successions of different types of stacked palaeosol profiles beneath coal seams of varying thickness. The most obvious and diagnostic properties of these palaeosols are the occurrence of fossil roots and the colour of the horizons penetrated by roots. Three types of palaeosol profiles represent an association of soils that developed on the ancient, tidally influenced lower deltaic plains. Low relief areas were characterized by hydromorphic conditions due to poor drainage. This resulted in the development of waterlogged Gleysols in backswamp areas, and less acid Histosols in closed bays of coastal swamps. In the latter case peat-bogs were formed. In topographically higher areas, as levee banks and crevasse splays, freely drained Podzols developed. This soil-type was dominated by kaolinite in the clay fraction, promoted by acid leaching. The morphology and geochemistry of all palaeosols were controlled by the position of the ground-water table, topography, climate, and rate of subsidence. In the Podzol profiles occasional flooding resulted in the formation of spodic B horizons, accumulated close to the contemporaneous water-table of that time. The organic-rich top horizons of the Podzol and Histosols profiles acted as a geochemical barrier and trapped various elements. Litter decomposition by microorganisms in these soils resulted in a concentration of potassium at the surface, thus giving an opportunity to form illite. The presence of siderite in the peat-bogs suggests that reducing conditions prevailed here at the time of their formation. The soils were formed in a humid tropical climate. This is indicated by the overall leaching pattern on cations in the Podzols and Histosols, the accumulation of peat in the Histosols, and the fossil flora.