THE TRANSITION FROM A CLOSED TO AN OPEN LAKE IN THE PANNONIAN BASIN SYSTEM (CROATIA) DURING THE MIOCENE CLIMATIC OPTIMUM: SEDIMENTOLOGICAL EVIDENCE OF EARLY MIOCENE REGIONAL ARIDITY IN CENTRAL AND SOUTH-EASTERN EUROPE

In the area of the rift-type North Croatian Basin, the south-western part of the Pannonian Basin System, a lake characterized by mainly dolomite deposition evolved in the late Early Miocene. The well bedded and frequently horizontally laminated dolomites precipitated directly from the water body are associated with tuffites and marls, together with minerals such as analcime, hydrous Ca-bearing magnesium carbonate, and natrolite. Small fragments of carbonized terrestrial flora are dispersed within the facies. That indicates deposition in a shallow, up to a few metres deep hydrologically closed lake of highly alkaline waters controlled by an arid climate in the first stage of its evolution. The second evolutionary stage is represented by the intercalation of coarse-grained, poorly sorted and structureless horizontally bedded sandstones, gravelly in places, with dolomites. The sandstones are dominated by detrital quartz grains together with fragments of metamorphic and granitoid basement rocks. The content of carbonized terrestrial vegetation in sandstones increases significantly upwards. These deposits are the result of lacustrine gravity flows. The association of sandstones and dolomites reflects changes of hydrologically open and closed lacustrine environments controlled by the frequent alternation of humid and arid climates. The third stage, that probably commenced in the Middle Miocene, indicates the formation of a long-lived, hydrologically mostly open lake, characterized by deposition of siliciclastics by gravity flows. The whole lacustrine depositional sequence coincides with the Miocene Climatic Optimum generally characterized by hot and warm, and humid climates. However, the evolution of the closed lake that is correlative with similar lakes in northern Bosnia and central Serbia, together with the contemporaneous alluvial red beds and desert loess indicates the existence of an arid zone in the region that was confined by areas characterized by a more humid climate in the late Early Miocene.