INTEGRATED SEQUENCE STRATIGRAPHICAL ANALYSIS OF THE NEOGENE LACUSTRINE PANNONIAN BASIN

Gábor Vakarcs¹ and Imre Magyar^2
1 MOL Pakistan Oil & Gas, Co., Islamabad, Pakastan; [email protected]
² MOL Hungarian Oil & Gas Co., Budapest, Hungary; [email protected]

During the last decade significant scientific progress has been made in the understanding of the Cenozoic sedimentary fill of the Pannonian Basin. Even though the stratigraphy of these sediments is well known, its sequence stratigraphic framework is much less understood. Stratigraphical problems debated for almost a century can be resolved by sequence stratigraphy, using reflection seismic and well log data in conjunction with biostratigraphic, magnetostratigraphic, radiometric, sedimentologic information and outcrop studies and the method can be applied to the lacustrine sedimentary strata of the Pannonian Basin.

Detailed sequence stratigraphic analysis allowed the interpretation of fourthy one depositional sequences within the middle Miocene - Pliocene lacustrine sediments. Depositional sequences were identified based on the analysis of published geological descriptions of outcrops and study of 12,000 km of 2D reflection seismic profiles and 225 hydrocarbon exploration wells. The sequences were stratigraphically positioned on the basis of radiometric and biostratigraphic data from 28 wells, and magnetostratigraphic data from 6 wells.

The Pannonian Basin is an integral part of the Alpine mountain belts of eastern central Europe. It overlies the Mesozoic thrust sheets of the eastern Alps, Carpathians, and Dinarides. The middle Miocene to present section of the Pannonian Basin is superimposed on a middle Eocene to early Miocene retroarc flexural basin complex, as a result of back-arc extension. The Paleogene basin evolution primarily was driven by compressional tectonics; the Neogene evolution was driven by extensional tectonics. Prior to the Sarmatian/Pannonian (middle Miocene) regional stage boundary, the Pannonian Basin became isolated from the world ocean. The Middle Miocene-Pliocene infill of the basin involved the advance of lacustrine deltas from the basin margins.

All of the lacustrine depositional sequences are complete sequences, containing lowstand or self margin, transgressive and highstand systems tracts and show the same stratal patterns as marine sequences. The duration of these sequences seems to be on the order of 0.1-0.5 Ma. and are believed to reflect Milankovitch-type climatic fluctuations in the drainage areas of rivers flowing into the Pannonian Lake. Most of the interpreted sequences are Type-1 sequences, related to erosional truncation. However, depending upon the sediment supply and the initial depositional profile, “apparent Type-2” sequences developed as well. There are several sequences where Type-1 and Type-2 behavior can be seen in the same sequence over a short lateral distance (10 km). These "apparent Type-2 sequences" reflect the initial depositional profile, and/or variations in difference of the sediment supply. The identified parasequences are mostly developed in the transgressive systems tracts and in the early highstand systems tracts.

The lacustrine depositional sequences show stratal patterns that are coeval and analogous to marine depositional sequences. Relative lake level changes appear to be responsible for the sequence development within the lacustrine setting. Within the areally limited extent of the Pannonian Lake, the sediment supply and the topography of the initial depositional surface were the major controlling factors. With high sediment supply, only Type-1 sequence boundaries developed. During the lowstand, lacustrine slope fans occurred on the deep slope (more than 400 m), with slope fans and/or prograding complexes in the shallow slope (100-400 m) environment. The ramp geometry is characterized only by a prograding complex. On the other hand, with low sediment supply Type-2 sequence boundaries are typically more common. Lacustrine slope fans characterize the deep slope, with mostly prograding complexes in the shallow slope and only prograding complexes in the ramp environment.

Tectonic activity was minimal during the post-rift phase of the back-arc Pannonian Basin and the rate of the thermal subsidence was relatively low. However, ongoing Neogene compression between the European and African plates resulted in a post-rift compression in the Pannonian Basin. Differential subsidence and broad uplifts, frequently seen on seismic sections in the Pannonian Basin, may represent the effect of changing intra-plate stress that strongly relates to the behavior of the different sequences.

The chronostratigraphy derived from detailed sequence-stratigraphy of seismic sections and well-logs was compared with all available age data. The results dramatically increased the stratigraphic resolution from millions of years to hundreds of thousands of years. The method used in this study developed a more precise chronostratigraphy and also increased the stratigraphic resolution of the lacustrine strata of the Neogene Pannonian Basin (upper Miocene-Pliocene), where traditional methods were difficult to apply.