Deltaic density currents and turbidity deposits related to maar crater rims and their importance for palaeogeographic reconstruction of the Bakony-Balaton Highland Volcanic Field, Hungary

Abstract

The Bakony-Balaton Highland Volcanic Field (BBHVF), active in the late Miocene, is located in the Central Pannonian Basin and consists of around 100 mostly alkaline basaltic eruptive centers. After volcanism, lake deposition took place inside the maar craters. Above the primary volcaniclastic deposits, thick maar-lake volcaniclastic sediments occur. The steeply dipping (25-35o), 25-30 cm thick, coarse-grained, inverse-to-normal graded beds of reworked tuff represent the foresets of large Gilbert-delta fronts built into the maar crater lakes of the BBHVF. The coarse-grained beds were deposited by low-density granule debris flows and grain flows. 10-15 cm thick beds of fine-grained, cross-bedded reworked volcaniclastic sandstone and mudstone beds are interbedded, probably deposited by turbulent sediment gravity flows. The delta fronts usually indicate transportation from north to south, suggesting a strong north-south trending fluvial system, active during or, shortly after volcanism in the BBHVF. The juvenile fragments of the deltaic sediments are often highly vesiculated, rounded/semirounded glassy lapilli. These suggest that the maar volcanism was related to widespread Strombolian-type explosive volcanism after the maar-forming phreatomagmatic events. Deposits derived from scoria cones were easily washed into the steep walled maar basins and deposited by debris flows into the maar lakes.