engleski

Depositional history of the Miocene Lake Sinj (Dinaride Lake System, Croatia): the long-lived hard-water lake in a pull-apart tectonic setting

As a consequence of Early Miocene transpressional wrenching, a series of NW-SE elongated pull-apart basins developed in the Dinarides of Croatia and Bosnia and Herzegovina. They accommodated a huge lake system that gave rise to spectacular endemic mollusk radiation. Lake Sinj – moderately sized at 342 km2 – flooded the south-westernmost basin of the system. Due to its location in a thoroughly karstic environment, the hard-water, alkaline, ancient lake developed a sediment infill with an average thickness of 370 m, dominated by authigenic limestone. The studied section represents the upper third of the basinal infill and provides a detailed insight to the critical period of the lake and of the basinal evolution during the final stages of its filling. It comprises two large-scale shallowing-upward cycles, both starting with fossil-poor limestones gradually passing into coal-bearing carbonate rocks and coal seams, respectively. The fossil-poor intervals are interpreted as phases of repetitive acidification events due to changing lake level, which induced periodic drying and flooding of the infralittoral inhabited by starfruit (Damasonium) meadows. The flooding of the aerated, limy mud plane introduced H+ ions from organic matter decay reactions into the shallow lake. This decreased its pH level, with catastrophic consequences for its biota. The stabilization of the ecosystem followed during the orbitally forced, humid climatic phases where, judging from the mollusk record, streams influenced the marginal lake environment and rich organic matter production induced the formation of swamps and mires. Evidence that the stabilized lake alkalinity triggered a spectacular mollusk radiation is provided not only by ongoing authigenic carbonate production but also from coal seams that represent text-book examples for coal formation in alkaline environments. The inferred basinal setting fits well with the pull-apart basin model, pinpointing the presence of an extended shallow ramp in front of a steep fault-induced hillside of the hinterland.

Ancient Lakes; Hard-Water Lakes; Pull-Apart Basins; Authigenic Carbonates; Coals; Orbitally Forced Cycles

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