A red-bed formation in the South Bohemian Basins, Czechoslovakia

Abstract

Tectonic activity, climatic changes and associated weathering cooperated during the formation of Upper Cretaceous red beds of the South Bohemian Basins known as the Klikov Formation. The sediments are clastic and comprise: (1) light-grey or yellow conglomeratic, course-to-medium sandstone beds; (2) generally finer red beds; and (3) grey beds. They alternate in asymmetrical cycles, in succession fining upwards. Small- and large-scale cyclothems were recognized. The large-scale cyclothems are closely connected with paleogeographical changes and tectonic evolution of the South Bohemian Basins in Early Senonian time. In regard to rapid vertical and lateral lithological variations, the common presence of graded bedding, poor sorting and lack of fossils, the sediments are comparable to the fluviatile facies of molasse, known as “flysch in molasse”. They were deposited from highly fluctuating currents and periodically overloaded streams in an alluvial and/or lacustrine environment, suffering continuously from tectonic unrest emanating from the Alpine—Carpathian geosyncline. Differences between red and grey sediments are related mostly to the primary distribution and preservation of organic matter and to some extent to the redistribution of iron during early diagenetic changes (e.g. red stains). Ancient weathered gneiss and granite profiles preserved beneath the sediments show development of either two (on granite) or three (on gneiss) horizons differing in degree of weathering and distribution of red colour within the profiles. The trend of weathering documented by chemical analyses and presence of kaolinite as the end-product points to the red podzolic soil formation process.