Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Geology of the Quaternary fluvial sediments at the confluence of the Labe and the Jizera rivers
AbstractContrary to the Upper Cretaceous monotonous bedrock the Upper Cenozoic continental deposits are more diverse. They are represented besides the aeolian loess and sand, and eventually by several lithological types of slope sediments, particularly by fluvial sandy gravel preserved at different altitudes above the rivers. Fluvial sediments in the environs of the rivers Labe (Elbe) and Jizera confluence differ from those known in the upstream and downstream parts of the valley. The whole group of Lower Pleistocene Labe high terraces is missing here. The existing high gravel relics belong to the right tributaries of the main stream. Proper Labe terraces are represented by the impoverished group of middle and low terraces (Fig. 1). Compared with the rather simple structure of older terraces, the geology and morphology of the lowest Labe terrace and the flood plain is rather complex. The Upper Pleistocene valley terrace gravel, the Holocene fill-in-fill aggradations as well as the erosional relics of the lowest Middle Pleistocene terrace are incorporated into it and appear in the same morphological level. Even the Cretaceous bedrock crops out within it in places (Fig. 2). The expressive fan-shaped body developed along the lowermost part of the Jizera River valley (Fig. 3) is composed fully of the fluvial sand with occasional pebble interlayers. The sediments show relatively thin but laterally constant sets of shallow trough cross - and low angle planar cross-bedding. Horizontal bedding is also relatively frequent. The pebbly interbeds are rather massive. All architectural elements were generated by dynamic stream flows in the mode of a braided river mostly in shallow wide scours and by surficial sheet flows. Some elements were deposited in shallow distributive channels. The architectural elements give evidence to the fluvial type af the fan (in sense of Czajka 1958) rather than that of the classical alluvial fan type. The fan stratigraphically belongs, contrary to the previous dating, to the Upper Pleistocene. The marked "20m terrace" (VII a), which follows the Labe right bank as far as the Mělník area, dated previously correspondingly also to the Middle Pleistocene, represents a direct downstream continuation of the Jizera fan and should be, therefore, of the same age. The lower level (VII b) is a lowered erosional surface of the same terrace body.