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LITHOSTRATIGRAPHIC SUBDIVISION OF LONJA FORMATION (APPROXIMATE PLIOCENE, PLEISTOCENE AND HOLOCENE) BASED ON SUBSURFACE AND SURFACE DATA

Entire subsurface clastic (Neogene and Quaternary) infill volume of the Sava Depression has been divided into lithostratigraphic units in the rank of formations while only some were subdivided into the rank of members (PLETIKAPIĆ, 1969 ; ŠIMON, 1980). This kind of approach was influenced by the Petroleum geology importance of determined formations, hence member subdivision and detailed analysis were only performed in the hydrocarbon bearing ones (e.g. Ivanić-Grad, Kloštar Ivanić and Široko Polje formation). Lonja formation presents the youngest most part of the Pannonian Basin infill and roughly relates to sediments of Pliocene, Pleistocene and Holocene age. As a formation of lesser importance, it was not analysed in detail and subdivided. As in more modern hydrocarbon explorations, an importance for gas accumulations arose, it became more interesting. An integrated analysis comprised of available outcrop sampling and analysis regarding lithological composition, heavy mineral fraction, micro and macro-fossil determination along with palynological analysis. Surface to subsurface data correlation was performed based on macrofossil (primarily Viviparus molluscs), microfossil (ostracoda) assemblage along with their approximate position to the subsurface model. In the next step, surface data was correlated with the available subsurface data (Well logs, rock cutting description, scarce seismic) resulting in subdivision of the Lonja formation into six members (Figure), primarily on well log analysis. Paleo-reconstruction of the environments that followed the shallowing of the Pannonian Lake after Miocene (LUČIĆ et al., 2001 ; MANDIĆ et al., 2015) was performed based on well log and surface data. As the result of the analysis, a difference was noted in the clastic material assemblage in contrast to Uppermost Miocene (Rhomboidea beds). Heavy mineral fraction lacked less resistant minerals in the base part of the formation (Hrastilnica, Ravneš and base part of the Vrbak member) suggest either long sediment transport or resedimentation of older clastic strata. An increase in the content of the less resistant minerals in heavy fraction in the uppermost part of the Vrbak member, could possibly relate to the increased uplift of today’s Moslavačka gora Mt. More specifically, when the Upper Miocene sandstones and marls deposited on Moslavačka gora Mt. were eroded, material for the infill was mixed together with the weathered magmatic and metamorphic complex. Possibility of re- sedimentation of the Pliocene strata was also confirmed with the finding of re-sedimented Miocene brackish dynociste in Viviparus abundant beds.

Litostratigraphy, Lonja Formation, Sava Depression, well logs

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