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 km2 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.