Inherited and syndepositional structural control on the evolution of the slope of Lake Pannon, Northern Somogy, Hungary

Abstract

The prograding shelf margin of Lake Pannon reached and ran across the Northern Somogy area between 8.8 and 8 Ma in an overall NNW–SSE direction. At about the same time, the slope crossed in the deep-water area of the Zala Basin.
However, as a result of the shallow water depth, in the area of the Transdanubian Range and near the zone of Lake Balaton the slope could not develop. South of this area, at Northern Somogy, the slope progradation became uniform again. The aim of this study was to understand the controlling effects of relative lake-level changes and structural movements on slope sedimentation, based on the interpretation of approximately 1800 km of 2D seismic reflection.
The retro-deformation of the seismic sections revealed that the characteristic morphological features (deep basins and elevated highs) of the present-day pre-Pannonian basement existed before and fundamentally influenced the Pannonian sedimentation. This dissected morphology is a result of the complex structural evolution of the area, which can be subdivided into four main phases. During the Late Palaeogene to Early Miocene a transpressional phase with strikeslip and reverse faults resulted in the juxtaposition of the distinct basement units. This phase was followed by syn-rift extension and the development of the deep sub-basins bounded by normal faults during the Karpatian to the Middle Miocene. Before and during the slope progradation the structural elements exhibit a complex transpressional–transtensional deformation. This phase is responsible for the uneven basement morphology before the slope progradation. This influenced the thickness variations of the basin-filling marls and turbiditic sandstones and, also the direction of slope progradation locally. Later, during the fourth phase, the older structures were reactivated and the Pannonian sedimentary succession uplifted and folded during the neotectonic inversion of the area.
As a consequence of the highly irregular basement morphology, the Pannonian strata vary significantly over short distances. The inherited sub-basins drew the prograding slope, while the elevated edges acted as a barriers and deflected it. The uneven basement relief also influenced the local water depth of the lake and this is reflected by the varying height of the slope: at the basinal areas, the thickness of the slope sediments is greater compared to that of the elevated ones.
Based on the regional seismic mapping, the area studied was filled by two slopes prograding from different directions. The north-western slope prograded from the area of the present-day Tapolca Basin towards the S–SE into the Mezőcsokonya Trough. The north-eastern slope prograded towards the SW and its progradation was influenced by the Ozora Trough and its elevated south-eastern margin, the Tamási Edge. These slopes were merged around the Igal High. In general, the slope prograded towards the S and the slope advanced 35 km within ca. 0.7 million years.
The relative lake-level changes during the progradation are marked by the shelf-edge trajectory of the advancing clinoforms: the shelf margin is constructed of alternating aggradational and progradational units, which indicates repeated rising and stagnant lake levels. These cycles can be explained by climatic changes with a periodicity of ca. 100 ky. At one location it was possible to identify two overlying slope sequences which indicates a major relative lake-level rise, however it was not traceable in the neighbouring sections. Features indicating relative lake-level fall were not apparent on the studied sections. However, unconformities could be identified and these are the results of the superposition of two slopes, prograding from different directions.