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Composition and origin of Lower Cretaceous clay-rich sediments from island of Mljet, Croatia

Grayish-green clay-rich sediment and marls found as thin, irregular layers in Lower Cretaceous shallow-water carbonate sediments on the island of Mljet (Dalmatia, Croatia) were analyzed in order to determine their composition and origin. Samples of limestones taken immediately below and above clay layer were analysed by micropalaeontological and microfacies methods while their insoluble residues were analysed by X-ray diffraction method. Minerals that were determined in the clay are illite, chlorite, kaolinite and calcite. Limestone sampled above the clay layer had 0.42 wt.% of insoluble residue composed of illite, chlorite and kaolinite, which is very similar in composition to clay samples. The mineral composition of insoluble residue (which was 1.6 wt.%) of brecciated limestone underlying the clay layer is muscovite, quartz and kaolinite, which differs from that of clay. The age of underlaying brecciated limestone is Barremian, as indicated by dasyclad algae. Microfacies analysis of the same layer shows possible remains of paleosol and mm to cm-sized cracks that could be produced by plant roots (rhizoliths). It is classified as algal wackestone-packstone. Limestone above clay layer, classified as mudstone, is micritic and has cracks with geopetal infilling. Succession of deposits is made of shallowing-upward cycles and the clays indicate emersion. The origin of clay can not be determined unambiguously. Different mineral composition of clay and insoluble residue of the underlying limestone indicate that most likely clay is not derived from these limestones. The source of the clay could be volcanic dust, which could be transported by wind from very distant areas.

clay minerals; Barremian; emersion; Mljet island; Croatia

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