Paleoenvironmental reconstruction of Lake Pannon (Upper Miocene) successions in South Zselic, Hungary

Abstract

In the last decade, lots of new results were published on the evolution of Lake Pannon based on borehole and seismic data from the Drava Basin and the vicinity of the Mecsek Mts. However, we still know relatively little about the geological structure of the transitional region, the area of South Zselic, located west of the Mecsek Mountains in the southern part of Hungary. This is a key area where both subsurface and outcrop data can help to connect the sedimentary environments, especially the deltaic systems of Lake Pannon between the Mecsek Mountains and the surrounding basins.

In this study, a detailed analysis is provided for three roughly coeval Pannonian (late Neogene) localities, where shallow lacustrine deposits crop out. Facies analysis was performed based on field observations in abandoned sand pits around Szigetvár; eight facies units and four facies associations were distinguished. The latter represent channel-filling (CH), terminal distributary channel (TDC), mouth bar (MB) and offshore (FF) depositional settings. Three main sedimentation processes impacted the formation of the investigated strata. These include sedimentation in delta plain channels of different size, valley incision and flooding caused by lake level rise. Channels of two different scales were identified from the sandy units. The larger ones represent distributary channels, while the small ones are interpreted as terminal distributary channels with connected mouth bars. Two main transport directions (towards SSE and WSW) were determined. The clay-prone lithological unit was formed as a result of flooding and it has been deposited above a wave ravinement surface. The paleoenvironmental reconstruction suggests that the channels and mouth bars formed within multi-level, complex incised valley fills: a larger, main and two smaller tributary valleys were reconstructed. The incision was created by a lake-level drop of 15-20 m, and then it gradually filled up by deltaic deposits as the water level rose. The overlying clay beds appearing in all outcrops were deposited after flooding reached the shelf. By comparing the findings with other known valley fills from the sediments of Lake Pannon, we determined that the studied outcrops represent lake-level drops on the scale of 15-20 m, which might have been common in Lake Pannon. Our results contribute to understand the architecture of incised valleys, terminal distributary channels, mouth bars and deltas formed in lacustrine environments, as well as the effect of minor water level fall events of lakes.