Imbrication of Middle Triassic rocks near Öskü (Bakony Hills, Western Hungary)

Abstract

The Veszprém Plateau is situated on the south-eastern flank of the Transdanubian Range Unit. The most typical structural elements of the study area are the SE verging Litér, Veszprém and Hajmáskér Thrusts. These thrusts were formed during the Eoalpine stage in the mid- Cretaceous. During the work we carried out a detailed structural analysis of the study area. This area is built up by Middle Triassic basin and platform sediments. The stratigraphy of these features has been investigated in detail, but modern structural analysis has not yet been performed. The Middle Anisian Tagyon Formation is overlain by basinal sediments of the Late Anisian to Ladinian Buchenstein Group. The Group was thought to be subdivided into two parts by a dolomite rock mass. In previous studies concerning this subject, this dolomite was depicted as a prograding wedge of a Ladinian carbonate platform. The present work took on the task of clarifying the stratigraphical and structural position of these units. South of Öskü and in the Kikeri quarry, Tagyon Dolomite underlies the Buchenstein Group. The Upper Anisian – Ladinian cherty, crinoidal limestones and tuffitic clays are moderately folded and affected by reverse faults and detachments along bedding planes. The Tagyon Dolomite was thrust onto the Buchenstein Group along the newly-defined Öskü Thrust. This south-east-verging, NE–SW striking thrust could be traced alongstrike into the village of Öskü, and it resulted in a repetition of the narrow stripe of basinal formations.
Between these two belts the platform dolomite was gently folded as part of a ramp or fault-propagation anticline and its lower contact is tectonically truncated. This structural scenario suggests that the intervening platform dolomite between the two belts of the Buchenstein Group is the Middle Anisian Tagyon Formation. The structural model presented in this paper shows that within the study area the intervening platform dolomite unit does not stratigraphically subdivide the stripes of the basinal formations; in other words, it does not appear as a prograding wedge of a Ladinian carbonate platform. The Öskü Thrust is coeval with the major, SE verging thrusts of the Veszprém Plateau and it is Cretaceous in age. The thrust could be the same structure as the Hajmáskér Thrust because it has the same strike and is collinear with that thrust. However, the junction of the two thrusts is covered by Miocene rocks.
During the work which underpins the present study some Triassic structural elements were identified, such as neptunian dykes, and outcrop- and map-scale normal faults; furthermore, the slide direction of the already identified slump folds was measured. Based on these observations we suggest that the Middle Triassic platforms were disintegrated by a NE–SW extension. This direction is in good agreement with the previous works which envisaged NW–SE striking normal faults at the platform margins.