Statistical analysis of Neogene sediment thickness deposited during the first transtensional and first transpressional evolutionary stages in the Bjelovar Sub-basin, Northern Croatia

Abstract

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.