A multi-phase succession of karst sinkhole deposits in the tectonic zone of the Owadów-Brzezinki limestone quarry at the north-western margin of the Holy Cross Mts. was studied in detail, using petrographic, palynological and other palaeontological methods. Upper Jurassic limestones are covered by a residual reddish brown clay layer of terra rossa type and a yellow-brown, loose calcareous deposit of a silt fraction. The yellow-brown calcareous silt consists of rhombohedral calcite grains, which may have been formed by the disaggregation of speleothems. The colouration of both deposits and clay mineral admixtures point to intensive weathering under a subtropical climate. The successive deposits of the sinkholes include coaly clay and a layer of whitish, microcrystalline, siliceous sinter. The coaly clay yielded a rich Middle Miocene palynofloral assemblage, indicative of a warm temperate and humid climate, as well as freshwater algae, which inhabited surface-water pools. The origin of the siliceous sinter probably is related to the activity of a hot spring. The youngest, massive infillings of the sinkholes consist of grey silty clays, which yielded marine bivalves, gastropods and Dichotomites ammonites, diagnostic for the Upper Valanginian as well as white yellow quartz sands of Upper Hauterivian? – Middle Albian age. The massive siliciclastic deposits probably slumped into the sinkholes during their final collapse. The sinkhole infillings document the development and timing of karst processes, Middle Miocene climatic conditions and the original succession of Lower Cretaceous–Neogene strata of the study area, which were removed from the surface by erosion.
