Studia petrograficzne nad arkozą kwaczalską

Maria Turnau-Morawska, K. Łydka


Petrographic study of the arkose of Kwaczała

The characteristic sediment of the Eastern part of the Polish Coal Basin, the arkose of Kwaczala, has been of interest for petrographers for a score of years because of important paleogeographic and sedimentologic problems which could be solved by an exact knowledge of these rocks. Recently St. Siedlecki (in 1951) defined its stratigraphical position as belonging to the Middle-Stephanian and his field observations gave some new elucidation as to the sedimentation conditions of the arkose. A systematic laboratory investigation of the arkose has not however been performed till now. The authors made a preliminary attempt to obtain a petrographic characteristic of the chief types of the sediment and some conclusions as to the provenance of rock fragments and their sedimentation conditions. T he main part of the sediment is composed of a coarse-grained and coarse-bedded, poorly cemented arkose, but it contains also subordinate intercalations of gravels and clays, the last often in form of rounded pebbles. All the rock types were submitted to a microscopic analysis. The investigation of gravels shows, that even pebbles composed of unstable minerals are preserved in a rather unaltered condition, so they must have been transported on a short distance. Besides the most abundant pebbles of vein quartz and chert the gravels contain feldspatic rocks as gneiss granite, mica schist, riolite, tuff and sandstone. The mineral composition of the magmatic and metamorphic rocks is rather uniform. The chief feldspar is the potash feldspar, namely microperthite and the only observed dark mineral is biotite of a characteristic red-brown colour, often with zircon inclusions and pleochroic halos. The rocks which afforded the detrital elements must have been rich in silica and potash. Some pebbles show a petrographic analogy with Precarpathian exotic rocks, investigated lately by T. Wieser (1948) in the Silesian Cretaceous of the Wadowice area. This fact confirms the supposition of St. Siedlecki that the detrital material of the investigated sediment derived from a Precarpathian mass, lying to the South of the Coal Basin and covered now with nappes of the Carpathian chain. The petrographic analysis of the proper arkose sediment shows that the rocks are poorly cemented and mostly they are loose arkosic sands. The cement is composed of sericite, iron compounds or calcite, the first type of cement is syngenetic, whereas the two others are epigenetic. The percentage of feldspars in the rocks seldom excceeds 25% which amount is the lowest limit to call a sandstone arkose, otherwise it should be named an ortoquartzite or a greywacke. Arkose are supposed to be formed during or immediately after the orogenic stage of the diastrophic cycle. The arkose of Kwaczala was deposited (according to St. Siedlecki) after the Asturic orogenic stage. As they present petrographically a type passing in some facies into feldspatic greywacke one may suppose that the closing geosyncline reached the stage of beginning subsidence. Rock fragments composing a part of the arkose are similar to several types of pebbles forming the gravel intercalations. One may therefore suppose that the provenance of gravel and sand was connected with the same mass and the variation of grain coarseness depended on climatic changes which conditioned the rapidity of sedimentation. The clay intercalations and clay pebbles were also submitted to a microscopic analysis. It appeared that the mineral composition of arkose and clay is qualitatively similar. The clays are composed of quartz,, feldspar, biotite, sericite but contain much more than the arkose opaque matter of iron oxides and clay minerals, in which only kaolinite can be recognized. The clay of the shaly intercalations demonstrated under the microscope a fine lamination with rhythmic variation of layers richer and poorer in argillaceous matter. — The clay pebbles show an analogy of mineral composition with clay intercalations but they seem to be more argillaceous and they contain fragments of formerly deposited and consolidated clays. This fact presents an illustration of sedimentary processes on the investigated area. Periodic playas filled up by clay sediments dried and indurated during the rainless season, fragments were eroded by running water in wet season, mixed up with clay again and finally, with increasing rainfall, were introduced as clay pebbles into the arenaceous sediment. In the end part of the paper the authors consider the kaolinisation process of the investigated arkose, the cause, environment and geological time of the process. St. Małkowski in 1925 presumed that arkose lying upon coal beds could be kaolinised by deeper water, passing from underneath through coal seams and saturated with carbon dioxide. The arkose of Kwaczała overlying the Carboniferous beds could be altered in such a manner but the authors do not see at present enough field evidence to suppose such a process and to exclude the action of surface water. As to the geological time of the kaolinisation the authors presume that it took place after the deposition of the arkose but before covering it with younger sediments. The colloidal silica formed as a result of the kaolinisation impregnated the trunks of araucarias which were deposited as allochtonic within the sediments. If the trunks were covered with sediments before their lithification they would be changed to carbonaceous matter.

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