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Sedimentology of Miocene fresh-water lacustrine deposits of the Sinj Basin (Dinarides, Croatia): Preliminary results

The Sinj Basin, in SW Croatia, comprises about 300 m thick lacustrine infill of Miocene deposits discordantly overlaying basement rocks. A detailed logging along the Sutina Brook at its SW margin brought information on its topmost basinal infill. The 139 m thick succession is portioned into two large scale cycles of upward organic matter increase, ending each by massive coal layers on top. The succession comprises five distinct facies of different frequency. Hence whereas the limestone strongly predominate the section and the biogenic deposits such as lignite intercalations and mollusk coquinas are very frequent, the occurrence of clastics such as sandstones, marls as well as pyroclastics is only subordinate. The limestones include few cm to 11 m thick, usually well bedded and horizontally laminated units ; the massive beds are rare. They show transition into lignite beds or lignite laminae, and in that way form small rhythmic cycles 40-10 cm thick. The lignite concentration is highest in the middle and at the top of the studied succession intercalating almost with all other facies, but mostly with limestones. In coquinas the most conspicuous are articulated shells of large, thin walled Mytilopsis aletici. Among gastropods thick walled, littoral elements like Melanopsis and Theodoxus dominate the assemblage. The paleoenvironmental analysis implies the deposition within the lake littoral during rhythmically alternating carbonate accumulation and swamp formation. That process documents frequent subordinate lake level fluctuations although the relative balance between the rate of water supply and the rate of sedimentation was given throughout the duration of the deposition.

Sedimentology; Miocene; lacustrine deposits; Sinj Basin; Dinarides

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