Petrografia osadów dewonu w strefie Jamno-Miastko niecki pomorskiej

Abstract

PETROGRAPHY OF THE DEVONIAN SEDIMENTS IN THE JAMNO – MIASTKO ZONE OF THE POMERANIA TROUGH (NORTH-WESTERNPOLAND)

Summary

The results of the second stage of petrographic studies of the Devonian deposits inWestern Pomeraniaare presented. While the first stage concerned mainly the neritic zone (J. Dadlez, 1975), the second one comprised the investigations of the marginal zone of the Devonian basin recognized in two boreholes, almost fully cored: Jamno IG 1 and Miastko 1 (Fig. 1). Simplified lithological columns of these boreholes and an attempt at their environmental interpretation are illustrated in Fig. 2. The more detailed petrographic results acquired in the particular complexes are given in Figs. 2–5, and the continental and marine sediments are separately discussed in the text.

Continental deposits occur mainly in the lower parts of both sequences (complexes JA, JC and MA, probably Lower and Middle Devonian in age). Immature, texturally variable sandstones and siltstones (PI. I, Figs. 6–8; PI. Il, Fig. 11; PI. III, Fig. 13) with frequent clayey lithoclasts predominate in these complexes and are thought to have been deposited in alluvial plains. Some segments of these columns might have been formed in river channels (member JC-2), some others – in the local isolated hypersaline basins (member MA-3 with the anhydrite nodules), still others – in the on-shore zone of the alluvial plains, periodically inundated by the sea (member JA-3 and considerable parts of the complex MB). Conglomerates (PI. I, Fig. 9; PI. II, Fig. 10), composed mostly of fragments of local claystones and carbonates, frequently also of quartz pebbles, generally play a subordinate role.

Marine sediments prevail in the upper part of the sequence, occurring either as thin members (complex MB, presumably Eifelian) or as thicker complexes (JB – Eifelian and JD – Frasnian). Clayey dolomites of members JD-l and JD-6 (PI. VI, Figs. 25–27), with parallel bedding and intercalations of red siltstones, are thought to be primary and deposited in supratidal zone. Clayey-silty sediments

(Pl. IV, Figs. 17, 18) as well as marls and limestones (PI. IV, Figs. 19, 20; Pl. V, Figs. 21–24) might have been formed in the tidal and subtidal zones. One of their varieties, rich in silty and sandy quartz grains, was probably connected with the near-shore belt, where the clastic material supplied by the rivers was distributed by long-shore currents. The carbonate sediments, which contain pellets and less abundant quartz grains, and which are partly dolomitized, might have been deposited farther from the shoreline, on the proximal slope of the near-shore barrier. The faunal assemblage, found in these sediments, seems to indicate that patches of reefs might have occurred along this barrier.

Marine and predominantly marine sequences are arranged in cyclic succession, the typical cyclothems being usually symmetrical (for example in complex MB – see Fig. 2).

The Devonian sequences discussed were deposited in the border zone between flat alluvial plains and the shallow marine basin. A correlation of the particular features of both columns leads to the conclusion that the relief of the alluvial plain and the contour of the shoreline were more differentiated in the area of Jamno than in the area of Miastko.