engleski

Composition and origin of the Barremian clay-rich sediments from the Mljet Island, Croatia

Grayish-green clay and marl can be found in several localities on the island of Mljet (Dalmatia, Croatia) within the Lower Cretaceous shallow-water carbonate platform rocks. The clays and marls are usually present as thin irregular layers, couple of centimetres thick, but in some localities they build layers having thickness up to 2 metres. They were analyzed by X-ray diffraction, IR-spectroscopic, chemical and K-Ar dating methods in order to determine their composition and origin. Samples of limestones taken immediately below and above the clay layers were analysed by micropalaeontological and microfacies methods while their insoluble residues were analysed by X-ray diffraction method. This study focuses on major mineralogical observations. The age of underlying limestones is Barremian, as indicated by their microfossil content (mainly dasycladacean algae). Microfacies analysis of some layers, classified as algal or peloidal-bioclastic wackestone-packstones, indicate shallow sea environment with low to medium energy. Some of them show possible remains of paleosol and mm to cm-sized cracks that could be produced by plant roots (rhizoliths). Limestones above clay layer, deposited in interdital/supratidal environment, classified as mudstone-wackestones, are micritic with cracks and fenestrae showing geopetal infilling and sometimes with stromatolites. The succession of deposits is composed of shallowing-upward cycles and the clays indicate emersion. Minerals that were determined in the clay are illite, chlorite, kaolinite and calcite. Limestones sampled above the clay layer had 0.4-2.5wt.% of insoluble residue composed of illite, chlorite and kaolinite, which is very similar in composition to the clay samples. In addition some of them contain authigenic potassium feldspar. The mineral composition of insoluble residue, which accounts for 1.6 wt.% of the brecciated limestone underlying the clay layer, is dominated by muscovite, quartz and kaolinite, which is significantly different from that of clay. The origin of clay cannot be determined unambiguously. Different mineral composition of clay and of the insoluble residue from the underlying limestone indicates that most likely clay is not derived from these limestones. It can be presumed that volcanic dust, possibly transported by wind from very distant areas, can be the source of the clay material, which is a result of volcanic dust weathering and subsequent diagenetic processes.

clay minerals; Barremian; K-Ar ages; emersion; Mljet island; Croatia

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