Acta Geologica Polonica

Lithology and paleotectonic development of the carboniferous in the Wierzchowo area (Western Pomerania)

Against the geological background of the Koszalin-Chojnice region, the paleotectonic-facial development is here discussed of the Dinantian deposits found in boreholes in the Wierzchowo area. The lithological complexes of the Tournaisian and Lower Visean are described showing i.a. the presence of thick greywacke tuffaceous sandstones. An analysis of the geological sections confirms the block nature of the Carboniferous structure in the Wierzchowo area, also showing that it is cut by a NNW-SSE fault which downthrows several hundred metres the eastern wing of the Carboniferous structure. The formation of this fault is referred by the writer to the Sudetic phase of the Variscan syn-diastrophism, i.e. prior to the dislocation of the whole Koszalin-Chojnice region and the en bloc upheaval of the Wierzchowo structure, probably during the Asturian phase.

Between 1960 and 1974 some scores -of deep boreholes have been drilled within the Koszalin-Chojnice region showing that the Zechstein there is underlaid by various members of the Carboniferous or older deposits. No complete Lower or Upper Carboniferous profile has, however, been reached in anyone of these boreholes owing to the complicated en bloc tectonics experienced by the region under consideration during the Carboniferous, probably also the Lower Permian.

The present paper comprises a description of the Carboniferous sediments based on 12 boreholes situated. in the SW part of the KoszaIin-Chojnice region within the Wierzchowo area (Western Pomerania - NW Poland) (Fig. 1). They are there overlaid directly by the Zechstein (except in boreholeprofile W.4·where it is discordantly overlaid by conglomerate deposits probably Saxonian in age), and in no borehole have they been pierced.

A detailed biostratigraphic analysis of the Carboniferous from the Wierzchowo area has not as yet been completed but even at the present stage it may be reasonably supposed that it is represented by various Tournaisian and Lower Visean members.

The thickness of the Carboniferous deposits, reached so far, ranges from 7.7 m (borehole W.1) to 228.5 m (borehole W.10) and it is 994.0 m only in borehole W.4. The coring differs strongly and, as a rule, it is not ,complete as shown in Table 1.

In what lithology is concerned the Carboniferous has here -developed as carbonate and clastic rocks occurring in various proportions. On the whole they are strongly diagenetized, occasionally dolomitized and . this effaces their original character often hindering the correct identification. Their description has been based on traditional macroscopic observations and an analysis of thin sections. Detailed descriptions of the profiles here considered were published by the present write in 1976b.

Because of the limited area under investigation no attempt has been made to differentiate formal lithostratigraphic units. This will not be possible before the Carboniferous throughout the Koszalin-Chojnice region has been thoroughly worked out. The data available so far show clearly that the Carboniferous deposits here are characterized by strong variability in lithology and thickness (Korejwo 1975, 1976a, Matyja 1976) resulting from a complicated geological structure and tectonic processes connected with the Variscan diastrophism (Dadlez 1974).

On the basis of macro- and microscopic observations of core fragments and the interpretation of electric logging it has been possible roughly to distinguish in the particular profiles more or less similarly developed and mutually comparable complexes (Figs 2-5). They have been marked by lettering while their stratigraphic position is shown by Arabic figures. The correlation is based mainly on borehole profiles W.4 and W.9. Dinantian rocks of Tournaisian and Strunian age, 1000 m thick, have been reached in the former profile, while somewhat younger Dinantian members have been encountered in profile W.9.

AlI the profiles, W.4 excepted, have been shown on the same scale. Four diagrammatic geological sections (Fig. 6a-d) along the lines shown in Fig. 1 have been presented to illustrate the spatial arrangement and the thickness of the particular complexes.

An analysis of the geological sections has reliably confirmed the supposition of the existence of a dislocation, probably directed NNW-SSE, breaking up the Wierzchowo structure into two block (Fig. 1). The eastern wing with boreholes W.1, W.9 and W.13, has been downthrown as compared with the western wing. Borehole profiles W.9 and W.11 reasonably suggest the amplitude of the downthrow here to be a 350 m. On the basis of geological sections, structural maps have been compiled for the bottom Zechstein, the bottom of the limestone (As) directly underlying the Zechstein (borehole W.4 excepted), and the bottom of the greywacke sandstone (E) recognized in the western wing of the Wierzchowo structure (Fig. 7a-c).

The details of the various complexes reasonably suggest that a similar type of sedimentation prevailed in that part of the Lower Carboniferous basin now comprising the Wierzchowo area. The lowermost members of the Tournaisian (including the Strunian) are represented by a limestone complex (Al), unpierced at thickness of 334.0 m, marly at the base, organodetrital higher up·with a fairly abundant benthonic fauna. These limestones are overlaid by black calcareous mudstones intercalated by grey limestone (Bl ) and containing impressions of carbonized plant and faunal remains (117.0 m). These are in turn overlaid by grey compact limestones (12.0-15 in) slightly sandy and with fragments of fossil fauna (A2). Higher up there is another complex of black calcareous mudstones with limestone intercalations (B₂), up to 74 m in thickness, with fragrments of fossil· fauna. In borehole W.4 this complex is moreover overlaid by black calcareous mudstones with laminations of grey siltstone (C₁) and a detritus of carbonized flora (57.0 m). This complex has not been encountered in the remaining boreholes but the presence has been noted in them of fine-grained, greyish-brown, dolomitic limestones (A₃) (12.0-56.0 m), occasionally marly and with few fragments of carbonized plants and fragmentary brachiopods. They also contain an admixture of detrital material represented by quartz, feldspars and fragments of volcanic glass. Overlying this is a thick (54.0-163.0 m) complex of black and dark-grey mudstones intercalated by marly limestones with rare ooids (B₃), while in borehole W.4 it is overlaid by calcareous sandstones with intercalations of siltstones (D₁) 54.0 m thick.

Higher up there occurs a strongly characteristic complex of greywacke sandstones (E), often tuffaceous-like as is i.a. suggested by the presence of re-crystallized volcanic glass aggregates. In boreholes W.4, W.B and W.14 these sandstones are correspondiiJ.gly 104, 88 and 91 metres thick. The greywacke sandstones are here and there intercalated by thin laminae of oolithic sandy limestones, partly dolomitized (14.5-18 m) (A₄) or by mudstones and siitstones (C₃) occasionally with limestone (B₄) intercalations, also oolithic (5.0-10.6 in). These intercalations yield faunal fragments.

In several profiles the greywacks sandstones have been observed to be overlaid by a mudstone complex (B₅), 5.0-13.0 m thick, often variegated in colour, with thin siltstone intercalations with mica, plant detritus and faunal remains as well as with marly dolomitized limestone intercalations sometimes containing ooids.

The various types of carbonate and siltstone-mudstone sediments occurring in the profiles just above or within the greywacke complex are in general characterized by a strong admixture of quartz, feldspar and frągments of volcanic rocks. This fragmentary material shows features analogous with that occurring in the greywackes.

The greywackes or mudstones are overlaid ,by oolithic (As) dolomitized· grey limestone, slightly beige, cherry or brown coloured, with an admixture of detrital material and of anhydrite, occasionally of pyrite. Here and there laminae·of variegated calcareous mudstone are encountered, less often coaly streaks. Scarcity of faunal fragments. The thickness of the limestone ranges from 17.5 to 45.0 m but they have been pierced only in the W.4 borehole (37.0 m).

In the western part of the Wierzchowo structure, the oolithic limestone is transgressively overlaid by the Zechstein. A complex, 31.0 thick, of variegated mudstones and siltstones (C₄) overlaid by a series of calcareous conglomerates (F), 78 m thick, occurs only in profile W.4 situated in the southernmost and lowermost 'part of the region here ·considered. The age of the calcareous conglomerates is controversial. In the eastern part of the Wierzchowo structure (east of the dislocation) where slightly younger Dinantian members (Fig. 5) have persisted overlying the oolithic limestone (As), there occurs in borehole profile W.9 a complex, 153 m thick, of calcareous mudstones intercalated by marly limestones and rare thin siltstone (B₆) laminae with fragmentary faunal remains. This complex is overlaid by black mudstones (C₅) intercalated . by dark-grey siltstones containing plant detritus and scanty traces of fauna. In the top the sediments are variegated in colour showing coaly streaks. The thickness of the complex (rached in boreholes W.9, W.1 and W.13) overlaid by the Zechstein is locally up to 42.0 metres.

The Wierzchowo structure lies within the uplifted Gościno-Wierzchowo region, as yet but poorly investigated. In the Gościno region the Zechstein is directly underlaid by Devonian sediments, while in the Wierzchowo region lying farther SE there occur Lower Carboniferous or Rotliegendes deposits (W.4).

The Carboniferous of the Wierzchowo area should be studied against the structural background of the whole Koszalin-Chojnice region where the Post-Caledonian cover, i.e. the Devonian and Carboniferous sediments have been affected by the Variscan syndiastrophism. Its earlier - Bretonian and Sudetic - phases were subjected to stronger epeirogenic movements. These are reflected in the character of deposits (a change of the carbonate lithofacies into a terrigenous - often coarse clastic - one) and are responsible for sedimentary erosional lacunae, occasionally also for local dislocations. The greywacke sandstones probably represent the synorogenic sedimentation of the off-shore zone of the basin. The presence in the deposits of pyroclastic material is also undoubtedly connected with diastrophism. The later phases in the Upper Carboniferous, i.a. the Asturian phase, have caused tectonic deformations throughout the region, mostly block-like in character. The particular blocks were, to a various degree, shifted in relation to each other, while the Pre-Zechstein erosion affected some Carboniferous members, occasionally even older deposits.

The vertical movements accompanied by increased supply of clastic material (after the Middle Tournaisian) were followed by gradual shall owing and final emersion of the basin. This may have taken place even before or during the Upper Visean. The area thus uplifted was probably dislocated owing to the Variscan syndiastrophism, chiefly during the Sudetic phase. The eastern wing was some hundred of metres downthrown in relation to the western wing, and it is here that have persisted the slightly younger Dinantian members (lower parts of the Visean). Only various Tournaisian members are present in the western wing, including very thick Strunian ones observed in the southernmost and lowermost part with borehole W.4. In both wings the Carboniferous deposits lie horizontally or sub-horizontally.

These movements, responsible for the dislocations, were followed by a rather long period of comparatively calm tectonic conditions, accompanied by strong erosion attacking the Carboniferous deposits. The Zechstein transgression invades the eroded sediments of the various Dinantian members (of the Lower Visean and Tournaisian) and only in the NW part of the Koszalin-Chojnice region, occasionally the Silesian or Upper Visean members. The Wierzchowo area was probably not uplifted until after the Lower Westphalian,. in result of the Asturian phase which caused the dislocation of the whole Koszalin-Chojnice region.

Another interesting problem suggested in this paper is the origin of the volcanic rock fragments encountered in the Lower Carboniferous of the Koszalin-Chojnice region. In view of its strong resemblance to the lower Permian volcanites observed in Rugia and Mecklenburg it has been supposed (Łącka & Turnau-Morawska 1968-1973) that the bulk of the aggregates in Pomerania belongs to the tuffogenic products antecedent to the Permian lava outflow. The presence is quite often observed of rather thick tuffogenic deposits followed by intrusions and extrusions of volcanites similar in petrographic character.

The Intrasudetic basin (Nowakowski 1968) is one of the examples illustrating such early phenomena of surface volcanism prior to the cardinal phase of pyrogenic activity of the eruptive Permian complex supplying pyroclastic material.