Štěrky na rozvodí Dyje a Pulkau jižně od Znojma (Česko–Rakousko): nové poznatky ve srovnání s dosavadním stratigrafickým zařazením

Abstract

A thin veneers of coarse gravel deposits (clast size up to > 15 cm) are preserved on many summits of the Dyje–Pulkau watershed upland on the Carpathian Foredeep surface near the Bohemian Massif in the area, where the Waitzendorf fault parallels the boundary between both the large geological units. Related rim of the latter unit is there topographically manifested by the current prominent marginal slope mostly developed in granitoids of the Dyje Massif. In the existing maps and corresponding explanatory notes the northwestern occurrences of the gravel were considered, without direct stratigraphic evidence, as marine deposits of the Lower Badenian or uppermost Karpatian age (Lower/Middle Miocene transition). This paper revises the assumed genesis and stratigraphic position of the gravel based on pebble grain size and petrography analysis, assessment of geomorphological setting, and comparison with other gravel deposits in the vicinity characterized by some similar properties. The study of pebble material on the watershed upland revealed an existence of two adjacent WNW–ESE trending gravel belts of different petrography: the southern with only dominant quartz-rich rocks and the quartz-rich northern but with a high portion of the Dyje Massif rocks. Gravel characteristics and its position above fine-grained Carpathian Foredeep sediments in the basement deposited in the Lower/Middle Miocene transition period (lower stratigraphic limit) indicate a later terrestrial origin of both the gravel belts by two adjacent streams flowing from the WNW towards the ESE and largely redeposi­ting coarse material eroded in older layers at former margin of the foredeep basin. In the southern and northern surroundings of the watershed upland a number of other gravel sites are preserved near or within the marginal slope whose coarse pebbles also overlay fine-grained basement clastic sequences of the Carpathian Foredeep. Their mutually different petrographic properties also indicate dissimilar source areas within marginal part of the basin presumably located in the WNW or NW, thus supporting quite uniform material transport direction in many subparallel traces. The general regional transport seems to be largely interrupted by a renewed activity of the Waitzendorf fault system predisposing the Bohemian Massif uplift against the current Carpathian Foredeep. A related strong stream incision into the uplifted surface included a newly created SW–NE reach of the Dyje river within the marginal slope between the Šobes meander and the town of Znojmo whose origin presumably started in the (Lower) Pliocene (higher stratigraphic limit) based on the supposed age of sedimentary remnants preserved within higher parts of the corresponding valley section. Thus the deposition of gravel at the Dyje–Pulkau watershed finished before or shortly after the uplift and the valley incision began. Genesis and stratigraphic position of the focused coarse sediments is likely different from those in the previous considerations.