Pliocene-Pleistocene drainage network in the Makó Trough: Mapping on 3D seismic data

András Horányi, Orsolya Sztanó (TXM & ELTE), Péter Szafián, Gábor Bada, Anita Horváth (TXM)

The Makó Trough is one of the deepest Neogene depressions of the Pannonian Basin. From 12 to 5 Ma, sediments of Lake Pannon accumulated, whereas later, starting from the Pliocene, rivers coming in from the Alps and Western Carpathians were feeding the trough. Across most of the basin, the Pliocene-Pleistocene fluvial-alluvial sedimentary beds, formed over a period of approximately 5 million years, reached a total thickness of over 2000 m. These beds carry exceptionally detailed information that can be used, through the mapping of former larger river beds, to draw conclusions with respect to paleo-hydrography, paleo-climatology, structural-sedimentary evolution, and the dimensions and connectivity of potential reservoirs.

This thick and extensive alluvial sequence and the meander network can be mapped, enabling us to track chronological changes in high resolution from the regression of Lake Pannon to the Quaternary on the 3D seismic data (two-way travel time between 1700 and 500 msec), and on well logs. The horizons and individual meanders on the alluvial plain can be studied in time slices in terms of various seismic attributes such as amplitude, lithology, and similarity. Also, the channel fill sequences from some of the greater rivers can be superbly identified in individual wells.

The most prominent features of the hydrography are the bends of the typically meandering rivers, whose dimensions, tortuosity, and flow direction vary in time and space. Initially, following the southward shift of the delta plain, when alluvial sedimentation is well under way across the basin, in the western part of the basin we find river bends with an amplitude of 2 to 3 km and highly tortuous rivers running NNW to SSW, parallel with the axis of the basin. The smaller streams running parallel to these follow the same structural trend. Later, both the degree of tortuosity and the bend dimensions seem to be decreasing (to an average amplitude of 0.8 to 1-3 km), due to a less wet climate and/or the shifting of the depocenter. About 700 m below the surface in the north-western corner of the trough, bends with amplitude of 4 to 5 km appear some time during the early Quaternary, followed by smaller rivers with 1-1.5 km bends to the south. All of this seems to suggest that, from this point on, the principal river of the region no longer travelled through the Makó Trough.