VARKONYI, LASZLO, MOL Hungarian Oil Plc., Geophysical Exploration Company, Budapest, Hungary; PAUL WEIMER and PETER VARNAI, Department of Geological Sciences and Energy and Mineral-Applied Research Center, University of Colorado, Boulder, CO

ABSTRACT: High-Frequency Depositional Sequences in Lacustrine Strata, Pannonian Basin, Southern Hungary

Sequence stratigraphic concepts were developed initially for marginal to marine strata with an emphasis on passive margins. Few published examples have addressed the potential applications of sequence stratigraphic concepts to lacustrine systems. Conceptually, depositional systems will respond to relative changes in lake level, and produce stratal patterns similar to those observed in marine settings.

A sequence stratigraphic study of the Pannonian basin, one of the several Mediterranean back-arc basins, indicates a complex lacustrine fill history during the late Miocene-early Pliocene. Analysis of 2200 km of multifold seismic data and fifty exploration wells in southern Hungary indicates that the basin was filled from northwest by thick siliciclastic sediments, and the average water depth of the lake gradually decreased from 400 to 0 m.

The Pannonian basin became isolated from the Mediterranean Sea at 10.5 Ma; because of increasing fresh water input, the basin's water became brackish, and then fresh throughout the late Miocene and early Pliocene. In spite of this isolation, three third-order and several fourth-order depositional sequences are interpreted in these lacustrine strata. The ages of the third-order sequence boundaries (8.2, 6.3 and 5.5 Ma) are based on magnetostratigraphic data. The high sediment input greatly exceeded the basin subsidence, which led to the development of the higher-order sequences resolvable on multifold seismic data. All of these third- and fourth-ordersequences are complete depositional sequences, composed of lowstand (LST), thin transgressive (TST), and highstand systems tracts (HST), nd bounded by Type 1 sequence boundaries. The higher frequency (fourth-order) lake level changes are superposed onto the third-order cycles. Therefore, during the falling limb of the third-order cycle, the fourth-order LST are thick, and fully developed, the HST are thin. During the rising limb of the third-order cycle, the LST are thinner, and only partly developed, whereas the HST are thicker.

The sequences were correlated and mapped in three dimensions, and helped define new stratigraphic plays in a mature basin, primarily turbidite systems lapping out against the slope. Maturation and migration studies indicate that there is high potential for petroleum accumulation in these stratigraphic traps.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.