Allochtoniczne skały paleozoiczne w osadach wizeńskich Gór Bardzkich (Sudety)

Bolesław Wajsprych


Allochthonous Paleozoic rocks in the Visean of the Bardzkie Mts. (Sudetes)

Bardzkie Mts. represent, themselves, one of the large geological units of the Middle Sudetes. Its regional position in relation to some other major units of the region is shown in Figure 1. The unit is composed of the Ordovician, Silurian, Devonian, and Lower Carboniferous rocks. For simplicity, however, a division into the pre-Carboniferous and the Lower Carboniferous rocks will also be used here. Litostratigraphy of the pre-Carboniferous rocks of the Bardzkie Mts. is as follows: the oldest are Ordovician sandstones and quartzites; overlying are the Silurian lydites and graptolithebearing shales; whole the above (younger) rock sequence, lithologically monotonous and lacking faiuna, was considered by previous authors as a separate lithostratigraphic unit of Lower to Middle Devonian age and informai termed „Zdanów Beds” (Oberc, 1957, 1973). More recently, however, Haydukiewicz (1974, 1977) has shown on the basis of conodont fauna that complete Devonian profile is represented within the Zdanów Beds. The Zdanów Beds are comprised of shaily mudstones, clayey shales, clayey-siliceous and siliceous shales. Within this rock sequence, radiolarites are also found to be quite common in various parts of the profile (Skandy, 1972; Haydukiewicz, 1974). Towards the top of the Zdanów Beds a considerable increase in the amount of siliceous sediment's is observed, this suggesting persistence of deep regime throughout the Devonian time. Lithostratigraphy of the Lower Carboniferous rocks of Bardzkie Mts. is discussed separately. In all of the previous studies, the Bardzkie Mts. were considered as a fragment of broad Caledono-Variscan geosyncline. The Bardzkie Mts. unit was thought therefore to be of complex origin and interpreted as formed during a considerably long time (from Ordovician to Lower Carboniferous inclusive), with the sedimentation being frequently interrupted by orogenic movements. Such a model of the Bardzkie Mts. evolution was discussed in detail by Oberc (op. cit.). In his opinion, the earliest folding took place in the region discussed immediatly after the deposition of Zdanów Beds, i.e., after the Lower Devonian. After the Middle Devonian, which was thought by this author to represent a stratigraphic gap, two successive marine transgression (in the Upper Devonian and Lower Carboniferous, respectively) took place, these also being separated by a sedimentary gap. It seems clear now that this picture of the Bardzkie Mts. evolution should be considerably changed when the results of Haydukiewicz (op. cit.) are taken into account. This is primarily because a continuity of sedimentation throughout the Devonian has been evidenced by the latter author. New field observations of the present author indicate that the pre-Carboniferous rocks form part of a large allochtonous rock-complex (melange) and the Bardzkie Mts. basin originated perhaps not till the Lower Visean. Source area for the Ordovician, Silurian, and Devonian rocks of this complex is recognized to be the eastern part Of the KaczaWsfkie Mts. (Wajsprych, 1978). The Kaczawskie Mts. are composed of folded and epizonally metamorphosed eugeosynclinal deposits. Sedimentary rocks represent main lithological component of this sequence and these are predominated by clayey shales, clayey-siliceous and siliceous shales accompanied by rocks of volcano-sedimentary complex (mainly greenstones). The problem of the age of folding and metamorphism in this region is closely related to the present author’s general concept and is therefore discussed in the end of this summary.

In the vertical profile of the Lower Carboniferous deposits of Bardzkie Mts., three lithostratigraphic units are formally distinguished by the present author. In ascending order these are: the Nowa Wieś Formation, the Srebrna Góra Formation, and the Zdanow complex (Fig. 3). These three units together form a higher order lithostratigraphic unit, namely the Bardzkie Mts. Group. In this paper, an informal term ,,olisthon of the Bardzkie Mts.” is also used and this (genetic) term is an equivalent of the formal (descriptive) term „Bardzkie Mts. Group”. The term „olisthon” has been adopted here, according to Hoedemaeker’s (1973) definition, to emphasize the resedimented nature of the entire rock-sequence comprising the Bardzkie Mts. Group. The stratigraphic position and the time interval of formation of the olisthon of Bardzkie Mts. are shown in Figure 3. The Nowa Wieś Formation is represented by sedimentary breccias, conglomerates and subordinate sandstones, the sequence being composed of gneissic material and limestone material in its lower and upper portions, respectively. Field evidence (Wajsprych, op. cit.) suggest that the deposits resulted from gravitational sliding of loose debris from the surface of the Sowie Mts. „gneissic floe”. The sandstones of Srebrna Góra Formation are typical flysch deposits developed in distal facies, their deposition being associated with the orogenic processes in the Kaczawskie Mts. eugeosyncline (Wajsprych, 1978). The melange of Zdanów complex (shortened term „Zdanów Melange” is also used here) is a thick layer of deposits which exhibit do implicated, chaotic structure and variable pétrographib composition. The term „melange” is used here according to Greenly (1919) as a purely descriptive term designating a deposit being a mixture of highly variable components.

The most completely developed vertical profile of melange complex is between the vicinities of Zdanów and Mikołajów, along an ancient rack-railway line (see Fig. 2). The vertical profile of the Zdanow Melange shows a clear tripartition in the area. Its lowermost portion exhibits numerous features which are generally thought to be typical for olisthostrome (according to the definition of Flores, 1955, 1959; see also Abbate et al., 1970, and Hoedemaeker, 1973). The particular olistoliths are up to tens of metres in their diameter. An olistolith of Upper Devonian clayey-siliceous shales exposed on the Orzech Mts., is more than 100 m long, and 8 to 12 m thick. The outcrops in the vicinity of Zdanów, when combined, show that the Silurian and Devonian rodks cropping-out in area of about 4 km2 (Fig. 2) represent, themselves, an allochtonous rock body of slide-sheet nature (sensu Harrison and Falcon, 1934, and Harrison, 1936). This is termed, therefore, „Zdanów slide sheet” and seems to be included into the olisthostrome (Fig. 4). One of most striking features of the olisthostrome deposits is the considerable amount of pelitic- and silt-material in their matrix composition. Only at very top of the olisthostrome a rapid decrease in the amount of the latter materials is observed, this being accompanied by a respective sharp increase in the mean size of the olisthostrome’s components. These changes result in a continuous transition from the olisthostrome into a megabreccia, the latter being composed of large blocks which are several metres to tens of metres in diameter, although numerous rock-plates show diameter of hundred of metres. This deposit shows several analogies to the olistothrymmata of Richter (1973) and Richter and Mariolakos (1973). One of the largest components of the olistothrymmata unit abqye is the gneissic gigantolistolith of Mikołajów (sensu Gorier and Reutter, 1968). In formal lithostratigraphic division, the olistosthrome deposits are distinguished as the megabreccias of the Orzech Mt. Member and the olistothrymmata deposits as the megabreccias of the Ostróg Mt. Member. In the light of the evidence above, the allochtonous nature of the large fragment of pre-Carboniferous rocks seems to be clear. The problem remains, however, whether the entire of this rock sequence is of allochtonous origin. The problem, itself, appears difficult to solve particularly with respect to minor rock fragments (i.e., „minor” in 1:25 000 map scale). In the latter cases, however, an essential evidence is usually supported by: (1) the manner of occurrence of such rock fragments, (2) their relation to the Lower Carboniferous deposits, and (3) the style of geological structure and tectonics of the area. As a rule the chaotic structure of the deposits (manifested as spatial discontinuity of lithologies) and the disharmonie tectonics are observed there. Moreover, the rock series are frequently intensively overturned in such areas and the directional properties of various tectonic elements .exclude presence of any kind of regular fold structures. All these features together are typical, if not diagnostic, for allochtonous rook complexes (Maxwell, 1959; Hsü, 1968). As an example, the geological map of the northeastern Bardzkie Mts. (Fig. 5) is presented here after Finckh,(1926). The problem of allochtonous nature of the Bardzkie Mts. rock sequence seems to be more complex in fact and it needs therefore to be considered, in future, in some additional details. Namely, at the top of the Zdanów Melange the present author found some giant tectoino-sedimentary units to be present, the order of size and the structure of which both give reasons for the use of terms „sedimentary nappes” or „sliding nappes” (terms after Schwan, 1974, p. 253) for describing their character. It is proposed here to term thém „olistonappes” (Wajsprych, 1979). In the Bardzkie Mts., three such olistonappes are probably present. Their common feature is a considerable amount of Lower Carboniferous rocks, the latter being entirely developed in flysch facies. The allochtonous Lower Carboniferous sandstones (i.e., those present within the olistosthrome, olistothrymmata, and olistonappes) occur on a far larger area (than that occupied by autochtonous flysch deposits of the Srebrna Góra Formation. The latter occur only as a narrow belt along the margin of Sowie Mts. block and in the vicinity of Dzikowiec (Fig. 2). Source areas for the former deposits are not completely recognized so far. It is thought that a considerable part of them was derived from the area of the Sowie Mts. block which formed, at that time, the bottom of the northern portion ^of primary Bardzkie Mts. basin (Fig. 6). At a final stage of the basin evolution the Sowie Mts. gneissic floe was uplifted and inclined to the south, and sliding of its (Lower Carboniferous) sedimentary cover into deeper parts of the basin and taken place (Fig. 6). This interpretation is primarily based on the petrographic composition and facies analysis of the sandstones. Their composition appears identical to that of the autochtonous deposits of the Srebrna Góra Formation. As regards the sedimentary facies, both the sand stone sequences above represent flyseh facies; however the allochtonous sandstones are clearly more proximal, whilst the autochtonous sandstones represent typical distal flysch-facies (Wajsprych, 1978). When transport from the north is considered (as evidenced by paleocurrent data; Wajsprych, 1978), localization of the proximal facies to the north from the distal one seems apparent. Thus, its location on the surface of the Sowie Mts. gneissic floe must be concluded. Certain part of the Lower Carboniferous deposits (e.g., sandlstones in the vicinities of Brzeźnica and Młynów, see Fig. 2) differs, however, in the petrographic composition from the sandstones of Srebrna Góra Formation. Their origin cannot be therefore adequately explained by the model shown in Figure 6. Looking for analogies to the model, these sandstones may be considered as coming from various parts of the Bardzkie Mts. basin and being derived in tectono-sedimentary settings similar to those explained by the model. It seems very probable that the evolution of the Bardzkie Mts. basin was associated, in its final stage, with destroying of its shelf areas and contemporaneous „downflow” of shelf sediments into central, deeper parts of the basin. This problem is actually investigated by the present author. In the northeastern Bardzkie Mts., in the vicinity of Mikołajów deposits being entirely composed of serpentinite material are found. These are mainly represented by turbidites („serpentinite flysch”) and occur at the top of the melange complex of Zdanów. This fact, together with regional geological relations, suggests that the ophiolitic massif of Grochowa- Braszowice (Figs. 1, 2 and 4) represents an allochtonous element. It is defined as the ophiolitic sedimentary nappe (Wajsprych, 1979). In summary, the components of the Zdanów melange can be groupped into two categories. One is represented by those which display no connection with the sedimentation in the Bardzkie Mts. basin. Second is represented by those melange components which come from the basin itself, their origin being due to the process which may be treated as an „autocannibalism of flyseh basin”. These two groups above are named „egzotic components” and „native components”, respectively (terms according to Hsü, 1968). The egzotic components of the Bardo melange are composed of the Ordovician,' Silurian and Devonian sedimentary rocks the crystalline rocks and the ophiolites, whilst the native ones are composed of the Lower Carboniferous sedimentary rocks.

According to the general model of the Lower Carboniferous sedimentation in Bardzkie Mts. basin (Wajsprych, 1978), the development of Bardzkie Mts. olisthon was caused by the orogenic processes in the Kaczawskie Mts. eugeosyncline. As a consequence of these relations, a conclusion comes out that the orogenesis of the Kaczawskie Mts. eugeosyncline took place in the time interval of the Bardzkie Mts. olisthon formation, i.e., from the end of early Visean to the Middle Late Visean (see Fig. 3). One would not find, however, direct confirmation of this conclusion in the existing stratigraphie data. The most recent paleontological studies evidenced the presence of epdmetamorphic Upper Devonian (Urbanek, 1975) and Lower Carboniferous (Chorowska, 1975) rocks in the Kaczawskie Mts. The problem itself should be considered, perhaps, from the standpoint of synorogenic sedimentation. It is thought here that during the whole time-interval of the Kaczawskie Mts. orogenesis the (synorogenic) sedimentation have taken place. From the other hand, the stratigraphy of rook-sequence of this type appears usually difficult to establish and should be based on a large number of data.

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