Podpermskie kompleksy skalne w strefie Koszalin – Chojnice

Abstract

W utworach dewonu i karbonu wyróżniono 10 głównych kompleksów skalnych.

Sedymentacja rozpoczęta zapewne w wyższym dolnym dewonie ma w strefie brzeżnej charakter głównie oldredowy do środkowego dewonu włącznie. W dewonie górnym dominują utwory wapienno-margliste, które w części środkowej obszaru przechodzą do dolnego karbonu, a w części północno-zachodniej zastąpione zostają przez piaskowce arkozowo-szarogłazowe, łupki i węglany. Sedymentacja przerwana jest dwoma krótkimi okresami erozji w najniższym dolnym karbonie i na pograniczu dolnego i górnego karbonu. Zasadnicza przebudowa tektoniczna i powszechne procesy denudacji nastąpiły na przełomie karbonu i permu.

SUB-PERMIAN ROCK COMPLEXES IN THE KOSZALIN – CHOJNICE ZONE

Since 1958 pre-Permian rocks have been encountered in more than 70 deep wells (Fig. 1) within a narrow Koszalin – Chojnice zone in north-western Poland.

The results of the biostratigraphic and petrographic investigations of these columns are, so far, scattered in numerous papers concerning either single wells or particular stratigraphic units, or else specific faunistic groups (see list of references).

The aim of this paper is· to outline a broad geological background for these data, presenting a pattern of main lithological bodies, and to draw some general palaeogeographic and palaeotectonic conclusions. Gross lithology, lithostratigraphic correlation, biostratigraphic determination of stages and other substantial data about the columns are given in Figs. 2–12. Correlations of the Ordovician and Silurian sequences are simplified because of uniform development of sediments and their comparatively clear biostratigraphic subdivision – the height of sections of individual stages has no reference here to their thickness since the sequences are fragmentary and strongly tectonically involved. Schematic pattern of the Devonian and Lower Carboniferous rock complexes, facies relations, thickness changes and the range of later erosion are illustrated in Figs. 13 and 14.

Strongly folded and deeply eroded Ordovician and Silurian strata are covered by the Devonian sediments. Two zones are discernible in the pre-Famennian sequences. The sedimentation in the marginal zone (Figs. 4, 6 and 13) began probably in the late Siegenian time with the alluvial clastic series of Old Red type; merely in the local isolated basins clays tones with anhydrite deposited.

This series is subdivided into a lower, Jamno complex (Upper Siegenian? - Eifelian?) and an upper, Wyszebórz complex (Givetian? - Frasnian?). They are separated by the intervening marine shales and carbonates of the Givetian (partly Eifelian?) Sianów complex and overlain by the limestones and marls of the Frasnian (Lower-Middle?) Koczała complex. The limestones of both marine complexes are often strongly dolomitized. Faunal assemblage consists of corals, brachiopods, bryozoans and algae. The succession as a whole is similar to those recorded from Peribaltic Syneclise (W. M. Kurszs, 1975) and mid-North Sea area (J. J. Pennington, 1975). All these occurrences together may constitute the posterosional remnants of an once continuous sedimentary cover, deposited along the southern margin of the Baltic shield.

In the basinal zone (Figs. 5 and 13) pre-Givetian deposits are not, so far, recognized. Stromatoporoid-coral limestones and marly shales of the oldest known Tuchola complex pass upwards into the shales and siltstones of the Silno complex and they in turn – into the fine-grained, grey sandstones and siltstones of the Chojnice complex. Fossils date the Silno and Tuchola complexes at the Givetian.

A very thick series of marls and nodular limestones containing brachiopods, cephalopods and echinoderms (Człuchów complex ­– Figs. 4–10 and 13) developed in the late Frasnian – Famennian times and covered the entire area. However, it was about three times thinner in the marginal zone than in the, basinal one and was later partly eroded there (Fig. 13). General facies and faunal change as well as the significant increase and differentiation of subsidence at the start of sedimentation of this complex were contemporaneous with major tectonic events in the neighbouring areas (Dnepr-Donetz aulacogen, W. A. Rasnitzyn, 1975) and coincided with one of turning points in the Phanerozoic history (M. A. White, 1977).

Marly sedimentation continued in the central part of the area into the Tournaisian (Figs. 11, 13 and 14). The same is perhaps valid for the western part, but an episode of the erosion (mid-Tournaisian?) played an important role there (Figs. 9 and 14). A considerable part of the Człuchów complex was removed at that time.

Three types of rocks predominate in the Lower Carboniferous succession of the last mentioned region: black, rough shales, arkosic-greywacke sandstones and carbonates (Figs. 7, 8, 10 and 14). First two of them are concentrated in the lower and middle parts of the sequence (Wierzchowo complex – Tournaisian, partly Visean), occurring there in variable proportions. The third type of rocks prevails in the upper part. These are marls, shales and carbonates with anhydrite nodules in the north-western segment of the region (Grzybowo complex) and their lateral equivalent – oolitic and detrital limestones, partly dolomitized in the south-eastern segment (Kurowo complex). Both contain scarce Visean fossils.

The facies pattern in the Devonian and Lower Carboniferous strata shows that the detrital material was transported mainly from the north and north-west.

Great changes in the development of the Lower Carboniferous sequences in Denmark (O. B. Christensen, 1971), on Rògen (N. Hoffmann, W. Lindert, D. Weyer, 1975) and in north-western Poland points to the considerable transverse differentiation in the Teisseyre – Tornquist zone.

Upper Carboniferous sedimentation was preceded by a rather short period of insignificant erosion. Upper Carboniferous strata – quartz sandstones, siltstones and shales with plant detritus, comprising probably the interval from the Namurian to the Westphalian (B?) – are represented by the incomplete sequences which survived the erosion. Most important tectonic and erosional activity took place

in the latest Carboniferous – earliest Permian times. First, it brought about the formation of block-fault structure of the area. The pattern of antithetic blocks was dominant and the throw of the greatest faults was as much as several thousand meters. The subsequent erosional processes removed huge masses of earlier deposits, having reached locally down to the Middle Devonian strata.

They have entirely destroyed the Devonian-Carboniferous cover north-east of the Koszalin – Chojnice zone. The present extent of these systems is clearly a tectonic-erosional one (Fig. 1). The Devonian and Carboniferous rocks supplied the material for the Lower Permian conglomerates which sedimented along the south-western side of the Koszalin – Chojnice zone.