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

Early Miocene transpressional wrenching yielded a series of NW–SE-elongated pull-apart basins 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 this system. Due to the karstic environment, the hard-water, alkaline, long-lived 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 detailed insights into 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. 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 uppermost littoral zone inhabited by starfruit (Damasonium) meadows. The flooding of the aerated, limy mud plain introduced H+ ions from organic-matter decay reactions into the shallow lake. This decreased its pH level, with catastrophic consequences for its biota. The ecosystem then stabilized during the orbitally-forced, dry climate phases. Based on the mollusk record, streams still influenced the marginal lake environment and rich organic-matter production created swamps and mires. The onset of mollusk radiation in the section correlates with stabilized lake alkalinity, as indicated by the disappearance of starfruits, ongoing authigenic carbonate production and by coal seams representing textbook examples for coal formation in alkaline environments. The inferred basinal setting fits well with the pull-apart basin model, pointing to the presence of an extended shallow ramp in front of a steep, fault-induced hillside of the hinterland.