Sedimentology of a Permian playa lake: the Boda Claystone Formation, Hungary

Abstract

The Upper Permian Boda Claystone Formation (BCF) in SW Hungary has been previously been identified as a saline lake deposit. A country-wide screening found this 800-1000 m thick succession the most suitable for the disposal of high-level radioactive waste in Hungary, and research into this formation has consequently been intensified since. The investigations included a detailed study of the sedimentological characteristics. Data obtained by mapping of the 25 km 2 outcrop area of the formation and from more than 40 boreholes were processed. The sedimentary structures were investigated on outcrop to microscopic scales, and cycles in the succession were interpreted. The main lithofacies, sedimentary structures and ichnofossils are presented. They indicate that the major part of the succession was deposited in a playa mudflat and is not of lacustrine origin in a strict sense. The lake sediments are represented by laminated and ripple-marked/flaser-type cross-laminated claystones and siltstones and by massive dolomites; trace fossils include crawling traces and burrows. Partial or complete drying out of the lake commonly occurred after the formation of carbonate mud by evaporation. Periodic fluvial influx is recorded by cross-bedded sandstones and unsorted gravelly sandstones of up to pebble-sized angular grains. Fenestral and stromatolitic structures reflect the repeated appearance of playa mudflat conditions. The silty claystones, which compose the major part of the succession, lost their primary structures due to pedogenic processes and indicate prolonged subaerial intervals with soil formation and only ephemeral inundations. The presence of pedogenic carbonate concretions supports the interpretation of an arid climate and a relatively shallow groundwater table. Drying-out events shown by desiccation cracks and authigenic breccias can be traced all over the succession. The various facies form small-scale sedimentary cycles showing a shallowing-upward trend and the growing influence of aridity and subaerial exposure.

Paleofluid Fingerprint as an Independent Paleogeographic Correlation Tool: An Example from Pennsylvanian Sandstones and Neighboring Crystalline Rocks (Tisia Composite Terrane, S Hungary)

“…In the Mecsek Mountains (Figure 3), the Permian sedimentary record contains playa lake and saline mudflat deposits (Boda Claystone Formation; Kungurian to Capitanian), including evaporites such as gypsum, anhydrite, and rare pseudomorphs after hopper halite [72][73][74][75]. Consequently, Na-rich playa fluids were available during the Late Permian in the study area.…”

The colour analysis method applied to homogeneous rocks

“…The BCF was deposited in a rifting basin on the southern margin of Europe during the Permian. An intramontane basin developed, with playa lakes and under arid climatic conditions (Varga et al, 2005;Konrád et al, 2010). Subsidence and basin formation were driven by crustal reorganisation and re-equilibration, following the compressional phases of Variscan orogeny (Vozárová et al, 2009).…”

“…Subsidence and basin formation were driven by crustal reorganisation and re-equilibration, following the compressional phases of Variscan orogeny (Vozárová et al, 2009). The surroundings of the playa basin had a moderate relief, a consequence of long-lived denudation of the crystalline base ment (Konrád et al, 2010). Correlation with areas forming part of the European platform is hindered by the fact that the controlling processes of the post-orogenic evolution of the Variscan fold-and-thrust belt were very different from those in the foreland basins (McCann et al, 2008).…”

Cycles and rhythms within the Boda Claystone Formation in WELL Ib-4

“…The continuously cored well reached the top of the BCF at Geologic map with depth contours of the top of the BCF (after Konrád et al 2010) 494.6 m and was terminated in the same formation at the depth of 708.6 meters. At present this is the best sequence of the formation available for study within the Gorica area (the northwestern part of the known occurrence) and shows typical differences compared to the rocks exposed and drilled within the Western Mecsek anticline.…”