Seria żarnowiecka – najstarsze skały pokrywy osadowej w zachodniej części syneklizy pery bałtyckiej

Abstract

THE ŻARNOWIEC SERIES: THE OLDEST ROCKS OF THE SEDIMENTARY COVER FROM THE WESTERN PART OF THE PERIBALTIC SYNECLISE

Summary

A series of detrital rocks, called the Żarnowiec series, was found in the western part of the Peribaltic syneclise. The series directly overlies the Precambrian crystalline basement and is overlied by Lower Cambrian deposits with faunal record.

The petrographic studies were aimed at characterizing the rocks of that series on the basis of the quantitative data on mineral composition and granulation. The results obtained for core material from 6 boreholes (Fig. 1) are presented in the form of histogrammes (Figs. 2–7) and a graph of chemical composition (Fig. 8). Mean values obtained for selected parameters are given in Tables 1–2. They are also plotted on maps (Fig. 9) in order to show differentiation of the Żarnowiec series in this area.

The Żarnowiec series is represented by poorly sorted, coarse and medium-grained, dark, red-brown and, sometimes, gray or gray-green, poorly bedded detrital rocks. Accumulations of hematite and laminae enriched in heavy minerals are common. Conglomeratic parts of this series are characterized by high contribution of matrix and poor mechanical treatment of heavily weathered rock debris. A transition to typical Cambrian is accentuated by repeated occurrence of lighter-coloured and better – sorted sandstones.

The present mineral composition of these rocks comprises quartz, illite, secondary micas, chlorite, feldspars, alterated micaceous minerals, accessory minerals, hematite pigment and leucoxene. Fragments of coarse-grained basement rocks occurring in conglomeratic parts are completely decayed. Such composition is the result of epigenetic changes determined by thermodynamic conditions and the conditions connected with subsequent hydrothermal activity and leading to complete decay of impersistent components.

Mode of the development of the Żarnowiec series indicates local origin of the material and similarity of sedimentary conditions in various points throughout the sedimentary basin. The material was derived from partly weathered and lithologically differentiated rocks of the crystalline basement.

The Żarnowiec series begins sedimentary cycle of the sedimentary stage transgressing on the old stage of the crystalline basement and it is related to sedimentation of Palaeozoic deposits.