Stratygrafia i rozwój facjalny górnego dewonu między Olkuszem a Zawierciem

Abstract

SUMMARYThe litological features are here described of the carbonate Upper Devonian deposits observed in numerous boreholes in the eastern margin of the Upper Silesian Coal Basin. Six lithostratigraphic units have been distinguished within the investigated profiles. The presence of an abundant conodont fauna made it possible to find an almost continuous sequence of standard Upper Devonian zones and to establish the occurrence of the Frasnian and Famennian stages. The facies and paleogeographical analyses have reliably assigned the area under consideration to the vast Upper Devonian carbonate shelf of southern Poland.In southern Poland, along the north-eastern margin of the Upper Silesian Coal Basin, there extends a narrow belt of Paleozoic deposits, mostly Devonian and Carboniferous in age, folded during the Variscan orogenic cycle (cf. Znosko 1974). At the present time this zone constitutes the substratum of the Mesozoic deposits of the Cracow-Silesian monocline and it crops out but at a few places (Fig. 1).Six lithostratigraphic units have been distinguished among the investigated Upper Devonian deposits of the above mentioned area. These are in ascending order:  Nodular limestones. Micritic marly limestones displaying nodularity, with a few thin calcarenitic intercalations (PI. 12, Figs 1, 3–5; Pl. 13, Fig. 1). Total thickness 51 m.Styliolina limestones and shales. Thin- and evenly-bedded limestones with marly shale intercalations. In both the lithological types tentaculitoids, mostly styliolinids occur abundantly (PI. 13, Figs 2–4; PIs 14–15). Total thickness ranging from 42 to 46 m.Detrital limestones. Sediments showing a predominance of calcirudites and coarse-grained calcarenites, commonly with an admixture of quartzose sand. In the lower part (= calcirudites member) the detrital limestones form a nearly homogeneous unit same tens metres thick (PIs 16–18, Text-fig. 2). The upper part (= laminated limestones and calcirudites member) contains only isolated calcirudite intercalations within the well-bedded marly micritic limestones (PIs 19–21). Thickness (without the uppermost part): 110 m.Brown shales. Dark brown shales with a few thin micritic intercalations, unfossiliferous (Pl. 22, Figs 4–5). Total thickness unknown.Nodular-detrital limestones. Marly limestones commonly nodlar, with numerous erosional surfaces overlaid by intraformational conglomerates and calcarenites (Pl. 22, Figs 1–3; Pls 23–28; Text-figs 3–4). Thickness (without the uppermost part): ca 170 m.Black shales and limestones. Dark grey, fine-grained calcarenites often showing parallel or cross lamination, intercalated with black fossiIiferous shales (Pls 29–30). Thickness of the investigated profile: 24 m.The distribution of the lithotratigraphic units and their thicknesses in the investigated profiles are shown in Fig. 5.The biostratigraphic analyses are based exclusively on conodonts. The ca 4500 collected specimens are assigned to 109 form-taxa of the rank of species and subspecies belonging to 14 genera. 63 species of stratigraphic importance or interesting from the taxonomic point of view are figured on PIs 1–11. The collected conodonts allow the correlation of the profiles studied with the Ziegler's standard zonation as well as the biostratigraphic dating of the lithostratigraphic boundaries in the particular boreholes (cf. Fig. 5 and Tables 2–11). The investigated profiles represent a nearly continuous sequence of the Upper Devonian conodont zones.Discontinuities in the litho- and biostratigraphic record exist only between the lower and upper occurrence of the brown shales unit, and between the upper and lower occurrence of the nodular-detrital limestones unit and the black shales and limestones unit, respectively. triangularis Zone through the Middle crepida Zone at most; the latter occurs at the styriacus/costatus Zone boundary.The correlation of the Belgian stratotypes with the studied profiles via conodont zonation reveals that the lowermost of the investigated units i.e. the nodular limestones unit is younger than the Givetian/Frasian boundary. In turn, the black shales and limestones unit is slightly older than the upper limit of the Devonian period. The Fransian/Famenian boundary corresponds approximately to the transition from the detrital limestones unit to the brown shales in the BK-89 profile (see Fig. 5).A facies analysis shows that most of the lithostratigraphic units here described and generally characterized by the co-occurrence of marly micrite beds, as a rule bearing a poor benthic and abudant planctonic fauna, with detrital intercalations abounding in skeletons of diverse benthic organisms.Basing on analogies with similar facies from the Rhenisch Schiefergebirge (Babien 1956, Krebs 1971), Canada (Murray 1965) and the Urals (Chuvasov 1963), the former deposits may be interpreted as autochthonous sediments of the deeper shelf anaerobic abiotic conditions prevalent near the sediment/water interlace. On the other hand, the detrital intercalations, generally abounding in intraclasts together with abraded and sorted remains of echinoderms, diverse calcareous algae, foraminifers, brachiopods and bryozoans, are interpreted as allochthonous sediments redeposited from shallower areas within the carbonate shelf.Though the detrital intercalations seemingly resemble the so-called allodapic limestones or lime turbidites described by Meischner (1964) from the Rhenisch Schiefergebirge (also cf. Szulczewski 1968), the following important differences do not allow such an interpretation of the considered sediments:1. Frequent evidences of a strong intraformational reworking;2. Occurrence of detrital beds lacking allochthonous material;3. Absence of horizontal grading within the individual detrital beds.Basing on the sedimentary features of the detrital beds as well as on comparisons with similar Recent (e.g. Ball & al. 1967) and fossil (e.g. Jones & Dixon 1976) sediments, it is assumed that they have been formed under heavy storm conditions.Two "exceptional" lithostratigraphic units deviate from the above given pattern of the Upper Devonian facies. One of them (the lower part of the detrital limestones unit) is interpreted as resulting from the local abrasion of spatially restricted parts of the basin that have been uplifted owing to the synsedimentary block tectonics, hence similarly as e.g. "Scheck" in the Alpine Liassic (Hudson & Jenkyns 1969). In turn, the depositional environment of the brown shales unit has been probably influenced by some non-actualistic phenomena, planetary in rank, possibly by a climatic Frasnian/Famennian downbreak (see e.g. McLaren 1970; Copper 1977).As shown by the facies analysis, the area under consideration belonged to the vast Upper Devonian carbonate shelf resembling those described i.a. by Meischner (1971) from the Rhenisch Schiefergebirge, Chuvashov (1963) from the eastern part of the East-European Platform, or by Murray (1965) from Alberta in Canada. In southern Poland this shelf stretched to the east, at least to the western region of the Holy Cross Mts, as is reliably suggested by the close analogies of the facies under investigation to the sediments of the same age occurring farther east (cf. i.a. SzuIczewski1971; Kicuła & Żakowa 1972; Jurkiewicz 1975; see also Text-fig. 6). The western slope of the shelf here described, at the same time corresponding to its transition into the geosynclinal trough of the Eastern Sudetes, most probably coincided with the western margin of the so-called Cieszyn Block – a rigid structural element in the substratum of the Upper Silesian Coal Basin (cf. Znosko 1974). This seems to be indicated by the facies relations in the vicinity of Ostrava, described by Dvorak (1959), the facies development of the Devonian deposits in the Solarnia IG-1 borehole (Bukowy & Siewniak-Witruk 1975) as well as by the course of the axis of the East-Sudetic geosynclines (after Dvorak 19711) that parallels the above mentioned margin of the Cieszyn Block (Text-fig. 6). The development of the carbonate Devonian and Lower Carboniferous sediments within the region of the Cieszyn Block (e.g. Dembowski & Różkowski 1967; Konior 1988, 1969, 1971) suggests that they belong to the same carbonate shelf as those here considered. In turn, the southern closure of this shelf area may have been formed by a continental massif, the so-called Pre-Carpathian land (cf. Konior 1969; Ślączka 1976; Text-fig. 6). The presence of that paleogeographic element would account for the occurrence of relatively large amounts of terrigenous material in the sediments under consideration, the southwards shallowing of some of the shelf facies, and the general southwards thinning of the Devonian deposits in the substratum of the Western Carpathians.