MAPPING AND DATING QUATERNARY TERRACES OF THE VLTAVA RIVER FOR ASSESSING THE ACTIVITY OF THE HLUBOKÁ FAULT IN THE NEAR-REGION OF THE NPP TEMELIN (BUDEJOVICE BASIN, SOUTHERN BOHEMIAN MASSIF)

Dating and correlating fluvial terraces on both sides of the Hluboká Fault in Southern Bohemia intends to provide data on the long-term vertical displacement at the fault. The Budejovice Basin is a sedimentary basin within the Bohemian Massif filled with Cretaceous and Miocene sediments covered by Quaternary fluvial and colluvial deposits. Forming its NE margin, the NW-striking Hluboká Fault is separating the Budejovice Basin (hanging wall) from the Variscan crystalline basement (foot wall).

The Vltava River enters the Budejovice Basin in the south and leaves it after crossing the Hluboká Fault in the north. During the Pleistocene, the river accumulated clastic sediments forming terraces of different horizontal and vertical extent in the basin as well as in adjacent areas south and north of it. Since there is no data concerning the numerical age of the Quaternary sediments, luminescence dating is used as the key method for building a late Pleistocene stratigraphy of the fluvial deposits in this area.

Currently available data is derived from more than 60 outcrops, hand drillings and 30 shallow boreholes. Stratigraphic correlations rely on more than 20 luminescence ages. Additionally, archive data from more than 1000 drilling reports from the Czech Geological Survey (Geofond) in combination with a high-resolution DEM was used to create a model of the terrace bodies in the basin.

Results from field mapping in combination with borehole data show 5 terrace levels in the crystalline basement northeast of the Hluboká Fault (foot wall) and at least 4 levels within the Budejovice Basin (hanging wall). For the lowermost terrace level in the hanging wall it was possible to create a consistent stratigraphy with ages ranging from about 80 ka to the Holocene. The top of this terrace forms the present flood plain of the river. In the foot wall of the fault, time-correlated terrace sediments (~20 and 70 ka) are found at positions several metres above the current flood plain. The upper terrace levels are out of the dating range of the method.

In sum, the occurrence of fluvial sediments of similar age at different elevations as well as differences in the number and relative elevation of terraces on both sides of the Hluboká Fault point to vertical movements along this fault during the Pleistocene.