During the early stages of the Middle Miocene transgression marine biota invaded the newly formed Paratethys Sea. Reefs and reef-like structures particularly flourished with life, supported by the favourable climate conditions. Miocene biogene buildups show a variety of fossil assemblages and lithologic features. The main reef-builders were bryozoans, coralline algae, corals, oysters, vermetids, sponges and serpulids. Water turbidity and depth were major ecological factors controlling the reef biota. Lithofacies types at the studied localities in Northern Croatia show significant similarity with the Upper Langhian (Middle Badenian) of the Leitha Mountains in Austria, but can also be well compared with Miocene to recent reef structures in the Mediterranean. The Miocene reef-like buildups exhibit different porosity rates and other lithological characteristics.

The Bjelovar Sub-basin is situated in Northern Croatia and constitutes the southwestern branch of the Drava Basin. The Neogene-Quaternary sedimentary section can be subdivided into three megacycles, which are separated by regional unconformities. The data analyzed in this study are related to the first megacycle, which is characterized by a tectonic-erosional unconformity above the Neogene basement (electric log, abbr. e-log marker Pt or Tg); its top is the regional e-log marker Rs7. It is generally assumed that the Rs7 marker separates Sarmatian and Pannonian sediments and indicates the period just after the first transtensional and first transpressional events. The youngest elog marker, Rs5, represents the Lower/Upper Pannonian boundary (in Croatian stratigraphic nomenclature) and more generally the early period of the second transtensional event. The lithology of the Lower and Middle Badenian is characterized by coarse and medium-grained sediments, while the Upper Badenian to Lower Pannonian section is dominantly pelitic as a result of flattening of the eroded paleorelief, as well as of reduced amounts of fans and energy. Thickness maps for the stratigraphic intervals Pt/Tg-Rs7 (4.9 Ma) and Rs7-Rs5 (2.2 Ma) were statistically analyzed using a regular grid with individual cells of 500 × 500 m. It is demonstrated that the first transpressional event in this sub-basin continued during the Lower Pannonian, causing a significant decrease of sediment thickness and a large unconformity in the east. Statistics based on histograms and averages confirmed that the first transtension and first transpression can be clearly distinguished by the presented methodology.