Petrology and diagenesis of Palaeogene clays from Ølst and Ålbækhoved, Denmark

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The petrology and diagenesis of Paleocene and Eocene clays from 01st and Albrek­hoved in northern Jutland, Denmark has been investigated using backscattered electron microscopy and XRD techniques. The Holmehus, 01st, Rpsnres and Lillebrelt Forma- tions were sampled at both localities, whilst at 01st, the Spvind Marl was also sampled. These sediments are extremely fine grained, smectite-rich, vari-coloured claystones with ash horizons in several formations. Smectite and carbonate minerals are the principal authigenic phases. Smectite is inferred to have formed as a result of alteration of volcanic glass, though it is unclear how much of the smectite is detrital. Three manganese bearing carbonate cements have been identified in all Formations sampled at both localities except the Holmehus Formation. These cements are manganoan calcite, calcian rhodochrosite and manganoan siderite. All occur as equant to sub­equant rhombs, up to 15 µm across, but typically 2-5 µm. Calcian rhodochrosite is present as overgrowths on rounded clay-size particles of high Mg (20%) calcite, which are either detrital grains or corroded authigenic cement. It also occurs as overgrowths on equant rhombs of earlier formed manganoan calcite. The manganoan siderite occurs as relatively large crystals (20 µm) scattered through the clay matrix and as concre­tions. Calcite cement is present in the Lillebrelt and 01st Formations. Quantitatively it is not as important as the more unusual Mn-bearing calcium carbonates.