| Abstrakt: |
More than ten years ago, Röhlich (2007, 2008) published interesting papers documenting the occurrences of slump structures in limestone strata of the Silurian and Devonian in the Barrandian area. These gravitational events are manifested here both by slump structures and deformations within the limestone beds (originally slides on the seabed or slope) as well as by mass wasting sediments (intraformational breccias - i.e. mudflows and debris flows).This study describes a new locality with well-developed slump structures not previously mentioned by Röhlich. The locality, called U topolů, is situated in the basal part of the Lochkov Formation, in the Radotín valley, near the village of Cikánka. The study is also adding a new information about two other known sites with slump structures - Lochkov (Lochkov Formation) and U kapličky in the basal Zlíchov Formation. Also discussed is the effect of slumps and gravitational currents on their underlying beds. All these localities are important stratigraphic sections in the Lower Devonian of the Barrandian (Chlupáč et al. 1998), and are located in the south-western part of Prague. The U topolů locality (Lochkov Formation) is a newly described slump structure (Fig. 1) that is exposed in a road cut, west of a thick crinoid bank (1.8 m) under which the Silurian/Devonian boundary runs. Due to the intense folding of the entire section into several synclinal and anticlinical structures, the slump structure occurs in the given road cut at least three times in (8 m, 14 m, 23 m west of the crinoid bank), so that its lateral variability can be observed. Further to the west, there are two other slump deformations in the sequence of strata which occur in stratigraphically upper parts of the Lochkov Formation. One of them is exposed straight in the road cut, but its tectonic origin cannot be excluded here (Fig. 2). The other one is located higher in the outcrop and exhibits a typical deformed and fragmentary internal structure (Fig. 3). The locality of Lochkov (Lochkov Formation) represents an outcrop with characteristic slump structures, and has more recently been described by Röhlich (2008). The major slump deformation can be observed in a sequence of tabular limestones and manifests itself as a thick folded anticline bank (Fig. 4). Other two thinner beds of intraformational breccias were found in the roof of this major deformation. One is about 75 cm thick, while the other one is 25-30 cm thick. Thin bedded, fine-grained bioclastic limestones with signs of turbidity sedimentation (lamination, current ripple marks, Figs 5, 6) are developed between all these slump structures. Slump structures identified at the U topolů and Lochkov localities are likely to be a record of the same gravitational event - on the sections recorded always not much high above the base of the Lochkovian Stage as slumps and intraformational breccias. The U kapličky locality (Zlíchov Formation) was originally described by Kettner (1917) as the so-called Chapel Coral Horizon which can be traced on the base of the Zlíchov Formation virtually within the whole SE flank of the Devonian basin (also referred to as the Zlíchovian Event - cf. Chlupáč - Kukal 1988). More recently has been described by Röhlich (2007, 2011). About 5 thick layers of intraformational breccias (i.e. mudflows and debris flows), deposited by gravitational currents, are developed in the lower part of the Zlíchov Formation. The first of them is more than 8 m thick. Slump structures lying under this very thick intraformational breccia (mudflow) are newly interpreted as deformations due to sudden load by overlying mudflow, and subsequent deformation of underlying unconsolidated sediment (Fig. 7). Another similar example of deformation in underlying bed of the intraformational breccia can be observed in the central part of the section (Fig. 8). The character of uneven base of the debris flows can be seen even in other parts of the section, e.g. eastwards, under the edge of the highway bridge (Fig. 9). The depth of unconsolidated carbonate sediment on then seabed may indicate the thicknesses of the deformed zones under the given intraformational breccias (i.e. mudflows, debris flows), reaching there about 0.5-1.1 m. [ABSTRACT FROM AUTHOR] |
