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Depositional model of freshwater lacustrine/palustrine carbonates (Early-Middle Miocene, Sinj Basin, Croatia)

During the Neogene due to the ongoing collision of Africa/Arabia with Europe a Dinarian-Anatolian island was formed. It represented a geographic barrier between the Paratethys and the Mediterranean (HARZHAUSER & MANDIC, 2008a). In the northern part of the island due to a compressional regime, a series of tectonic depressions were occupied by numerous synchronous lakes called the Dinaride Lake System (KRSTIĆ et al., 2003). The Sinj basin occurs as one of the southernmost basins of this Miocene system. Today the Sinj basin is located on the Croatian Karst (External) Dinarides. The pre-Neogene basement is dominated by the Adriatic Carbonate Platform (AdCP) carbonates along the basin margins, ranging in age from the Middle Triassic to the Eocene (VLAHOVIĆ et al. 2005). Extensive Permo-Triassic evaporites occur immediately beneath the basin and are exposed in the middle of the basin probably as a result of post-Miocene diapirism. In the Sinj Basin two overlapping composite sections were logged in detail, representing together a continuous, approximately 500 m-thick record of lacustrine/palustrine deposits. The Lučane section (CSL) was logged in and around Sutina creek, near the village of Lučane at the NW margin of the basin. The Glavice section (CSG) is also a composite section from the central part of the Sinj basin, about five km east of the Lučane section. Correlation of sections was established by using tephrostratigraphy (40Ar/39Ar dating) and magnetostratigraphy (de LEEUW et al, 2010). The Sinj basin infill was deposited from 18.0 to 15.0 Ma, correlating with to the upper Burdigalian and lower Langhian Mediterranean stages and the Ottnangian, Karpatian and lowermost Badenian Paratethys stages. The lake sediments are dominated by freshwater carbonates. The CaCO3 content varies between 82% and 99% ; the siliciclastic input was insignificant. The uppermost 100 m of the section include up to 3-m thick, coal seam intercalations, whereas the rest of the infill is exclusively carbonate rocks. The whole succession is intercalated rarely by volcanic ash layers. The coal deposition at the top of the section shows a straightforward correlation with the Middle Miocene climate optimum. Sedimentological analysis and petrography identified nine facies types. Seven facies are carbonate in origin in addition to the coal and tephra facies: A - Lime mudstone, B – Calcisiltite, C - Charophytic micrite, D – Laminated micrite, E – Coquina/rudstone, F – Tuff/clays, G – Extrabasinal breccia, H – Conglomerates and I – Coal. The infill is divided into three main units (Units 1, 2, and 3). Unit 1 is comprised of alternations of Facies A, B, and C with minor contributions by Facies D, E, F and G. Unit 2 contains only Facies C with some Facies D and E. Unit 3 is composed of Facies C and I (coal) with minor interbedding of Facies D, E, F and H. Carbonate sediments in the lacustrine-paludal paleoenvironments of the Sinj Basin formed broad, shallow benches around lake margins which prograded basinward with time (MURPHY & WILKINSON, 1980 ; TREESE & WILKINSON, 1982). A detailed time frame (de LEEUW et al, 2010) made it possible to see changes of three major factors controlling carbonate deposition – hydrology, sediment input and temperature changes (GIERLOWSKI-KORDESCH, 2010). From Unit 1 thru transitional Unit 2 and then to Unit 3, carbonate textures indicate a change from intermediate to humid climate/hydrology conditions (PLATT & WRIGHT, 1992). Concomitant with these changes up-section, a change in sedimentation rate occurs, from approximately 10cm/kyr in Unit 1 to 20cm/kyr in Units 2 and 3. Unit 3 with its coal-limestone facies is interpreted as a small lake delta (GIERLOWSKI-KORDESCH et al., 1991), formed from an increase in water and sediment input along with an increase in the accommodation space.

Sinj basin; Miocene; Dinaride Lake System; lacustrine/palustrine carbonates

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