engleski

Late Aptian and Early Albian Regional Emersion Phase in Istria, Croatia

Istrian part of the Adriatic carbonate platform emerged in the Middle/Late Aptian for the second time and was exposed for 11-19 MY. Exposure related features, which mark type 1 sequence boundary, are represented by greenish-grey clays, marls and lime breccias. Mineralogical, micromorphological and geochemical analysis were performed on clay layers, ranging in thickness from several centimetres up to 1 meter, and the insoluble residues of limestones situated immediately bellow clay layers. The main mineral components of clays are illitic material and illite/smectite mixed layer minerals. Occasionally, pyrite, gypsum and chlorite are present. In contrast, insoluble residue is dominated by smectite and illitic material, and occasionally kaolinite. The clay mineral distribution of one investigated profile show a clear trend indicating the influence of both pedogenic and diagenetic processes. Micromorphological data support consideration of pedogenetic alteration. Distribution of vanadium, uranium, molibdenum and REE, as colour of clay and presence of abundance of pyrite framboids indicate unoxic environment. The current investigation should give answers to provenance of the material from which this clays are formed. We consider that the clays are remnants of seasonally marshy soils to permanently watterloged soil, which formed from the erosional remains of surficial soils and sediments, which were accumulated in palaeokarst pits following an oscillating marine transgression that terminated emergence. The Lower Cretaceous shallow-marine deposition sporadically interrupted by periods of emersion are sedimentologically and palaeogeographically very similar to Purbeckian sediments of NW Europe.

Aptian/Albian; emersion; paleosols; clay

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