A chronostratigraphy for the Dinaride Lake System deposits of the Livno-Tomislavgrad Basin: the rise and fall of a long-lived lacustrine environment

Abstract

The Dinarides are an integral part of the Alpine orogenic belt and stretch over large parts of Slovenia, Croatia, Bosnia-Herzegovina, Monte Negro and Serbia. A great number of intra-montane basins formed in the interior of this Late Eocene to EarlyOligocene orogen during the Miocene. These basins harbored a suite of long-lived lakes, collectively called the Dinaride Lake System (DLS). Lake Livno, with its 600km2 of preserved surface, was the second largest of these Dinaride Lakes. At present, its deposits are divided between the Livno and Tomislavgrad basins, which were part of a single basin when Lake Livno first formed. High resolution age constraints for the over 2km basin infill have been lacking up to now, partly due to the endemic nature of its lacustrine fauna. This severely hampered geodynamic as well as paleoenvironmental reconstructions. Here, we present a chronostratigraphy based on radioisotopic and magnetostratigraphic data. 40Ar/39Ar measurements reveal that the Tušnica volcanic ash, found in between the Gomphoteriumbearing coal seams at the base of the basin infill, is 17.00±0.17 Ma old. 40Ar/39Ar dating of the Mandek ash, correlative to the uppermost sedimentary unit, provides an age of 14.68±0.16 Ma. Correlation of the composite magnetostratigraphy for the main lacustrine depositional phase to the Astronomically Tuned Neogene Time Scale is straightforward and reveals that the majority of the deposits of Lake Livno accumulated between 17 Ma and approximately 13 Ma. The disappearance of Lake Livno is most likely attributable to a change in tectonic regime. Calcarenites and breccias, derived from the basin margins, first entered the lake around 14.8 Ma and subsequently coarsened and thickened upwards. The basinmargins were apparently gradually uplifted before subsidence stalled. Comparison with chronostratigraphic data for other constituents of the DLS leads to the conclusion that their lifetimes largely coincide. Finally, we calibrate the most important marker fossils of the various Dinaride basins to the geological time scale and we present a new biochronological scheme for the DLS.