STRATIGRAPHIC ARCHITECTURE AND SEQUENCE STRATRIGRAPHY OF LONG-TERM FLUVIAL RECORDS IN A CONTINENTAL INTERIOR BASIN, E HUNGARY

The study area is located in the eastern part of the Pannonian Basin system which is Europe’s largest continental interior basin. The thickness of the alluvial sedimentary complex exceeds 400 m. The present study is based mainly on subsurface facies analysis and the interpretation of well-logs of some 60 water-prospecting wells. In the course of the investigations traditional sedimentological interpretation was combined with sequence stratigraphic considerations. Different alluvial facies types as well as facies associations showing characteristic stacking pattern on the logs were recognized in the Quaternary succession. They were formed in channel belts, proximal and distal floodplain, floodbasin or wetland and on the distal part of an alluvial fan, respectively. Basin-scale facies mapping of the overall Quaternary sedimentary succession led to the recognition of the main channel belts.

The recognized facies associations show a characteristic vertical pattern in a successive order forming individual 40-100 m thick large-scale fluvial cycles. The basal member is made up of multistorey channel fill beds cut into the underlying floodplain deposits. This is overlain by an alternating sandy – muddy succession of channel fill and floodplain deposits forming the intermediate member. The upper member is comprised of silty-clayay floodplain deposits locally with very thin isolated silty-sandy bodies. The above mentioned 3 members represent a fining upward sedimentary cycle and they were interpreted as low, increasing and high accumulation space deposits, respectively, (i.e. LST, TST and HST in sequence stratigraphic terms). As the basal member generally lies above a regional erosional surface allocyclic controls can be assumed. In parts of the basin the cycle is not complete.

In a regional scale six cycles were differentiated above each other. Although these cycles were allocyclic it is an open question if they were tectonically or climatically driven. The fact, however, that six of them have been identified suggests that they represent the large-scale 400 ka Milankovic cycles during the Quaternary.