Petrografia osadów dewonu w strefie Gościno – Człuchów niecki pomorskiej

Abstract

PETROGRAPHY OF THE DEVONIAN SEDIMENTS IN THE GOŚCINO - CZŁUCHÓW ZONE OF THE POMERANIA TROUGH (NORTH-WESTERN POLAND)

Summary

Devonian sediments the thickness which exceeds 1 000 m are known from about 30 boreholes situated in the Pomerania (Fig. 1). There two principal facies zones have been recognized: the lithologically differentiated marginal zone where sandstones and limestones predominate and the deeper zone of the basin chiefly with fairly monotonous marly sediments (Figs. 2 and 3). Petrographic studies of the latter zone are reported in the present paper. The most common microfacies have been identified, the lithological division has been established, and the interpretation of the depositional environment has been attempted (Figs. 4-7). Three depositional phases have been distinguished within the Devonian sequence: the transgressive phase, the phase of the relatively deepest basin and the regressive phase.
During the transgressive phase (Fig. 4, complexes CA - CD) mainly clastic and subordinately marly or carbonate sediments were formed in the littoral zone and possibly also on tidal flats close to the land of an undiversified relief. Sandstones, frequently silty, with bioturbational structures (Pl. V, Fig. 24) belong to the sublitharenites and are well-sorted, fine-grained, and have subrounded grains (PI. I, Figs. 8-10). Claystones, marls, and marly limestones occur in subordinate amounts. The bulk of the Devonian sequence representing the relatively deepest zone of the basin (Fig. 4, complexes CE - CG and GA - GC) is composed of marls and locally of claystones (Pl. I, Fig. 11; PI. II, Figs. 12-14). The main mineral constituents are: illite, chlorite, carbonate micrite, and dispersed pyrite. The mineral composition, depositional structures, and faunal assemblage are indicative of undisturbed deposition in slightly agitated waters and under highly reducing conditions. The transition towards the overlying sediments of the regressive phase (Fig. 4, complexes CH - CI and GD - GF) is gradual and of oscillatory type. The principal sediments of this phase are marls and marly limestones (Pls. III and IV, Figs. 16-20) of a characteristic nodular structure (Pl. V, Figs. 25 and 26). As compared with the lower complexes, the bioclasts are much more significant until packed biomicrite is formed, although sparse biomicrite (according to R. L. Folk's classification, 1968) is the most common variety. The faunal assemblage is more diversified. These rocks are supposed to have been formed in a more mobile water better aired and with clearer water, possibly in the shallow-neritic zone. In the Człuchów area it could have been the fore-reef zone. The nodular structure is related to depositional boudinage. Allochemical limestones of the GF complex in the Gościno sequence occupy a particular position. Unsorted or poorly sorted biosparites, biopelsparites, and pelsparites (Pl. IV, Figs. 20-22) and less frequently biomicrites originated in mobile and shallow waters zone with wave action and bottom currents. The provenance area was moderately active and it could have been a shoals belt on the border of high energy waters and the landward slope of these shoals.