Charakterystyka mineralogiczno-petrograficzna serii pstrych łupków fliszu karpackiego oraz niektóre problemy ich sedymentacji i diagenezy

Abstract

Badania dotyczą serii pstrych łupków górnokredowych (godulskich) i eoceńskich jednostki śląskiej i skolskiej z rejonu Sanoka. Przedstawiono skład mineralny łupków i przewarstwiających ich mułowców, piaskowców oraz skał krzemionkowych. W nawiązaniu do dotychczasowych poglądów na genezę pstrych łupków, wysunięto hipotezę odnośnie do warunków ich sedymentacji. Podniesiono zagadnienie zmian diagenetycznych minerałów ilastych wchodzących w skład łupków, wiążąc z tymi przemianami proces sylifikacji w utworach fliszowych.

MINERALOGICAL-PETROGRAPHfC CHARACTERISTICS OF MOTTLED SHALE SERIES OF THE CARPATHIAN FLYSCH AND SOME PROBLEMS OF THEIR SEDIMENTATION AND DIAGENESIS

Upper Cretaceous and Eocene mottled shales of the Silesian and Skole tectonic-fades units from the Sanok area (eastern part of the Polish) were studies.

The Upper Cretaceous shales are usually in various shades of red in coulour, with green spots or streaks and with intercalations and irregular intergrowths of green shales. The Eocene clay shales are mainly (in 60-70%) represented by green and green-grey shales with red shale intercalations a few centimeters to several meters in thickness.

Granulation is similar and fairly stable in Upper Cretaceous and Eocene mottled shales. Two fractions predominate: below 2 mm (about 30%) and from 10 to 60 mm (about 60–70%). The content of clay minerals is greater than it would follow from their share in the fraction below 2 mm. This is because of the fact that during sedimentary analysis some clay minerals pass into coarser fractions including the psammite fraction in the from of not soaking aggregates.

Mineral composition of Upper Cretaceous and Eocene mottled shales is similar and fairly uniform. The shales consist of clay minerals (60-70%) and quartz (20-30%) as well as subordinate amounts of muscovite, glauconite, feldspars, calcite, iron hydroxides and sulphides, manganese oxides, debris of siltstones and organic matter. There are also found biotite, phosphates and heavy minerals (zircon, garnets, ruilte and tourmaline). The mottled shales are more or less calcareous in some places (as e.g. in the Monasterzec area) and then calcite belongs to rock-forming minerals.

Clay minerals of these shales primarily include beidelite and illite with mixed-layered phase of the illite-beidelite type. The share of kaolinite is small (of the order of a few percents). Exchangeable beidelite cations mainly include calcium, magnesium and, sometimes (e.g. at Monasterzec), sodium.

Attention should be paid to the presence of pyrogenic biotite and quartz, accumulations and thin veinlets of microcyrystalline silica as well as spherulitic aggregates of fibrous silica in the shales.

The mottled shales of the Eocene age are intercalated with silts tones and, sometimes, fine-grained sandstones. There are also found single thin (about 2 cm thick) layers of siliceous rocks with radiolarians, nodular carbonate deposits (lenses and layers thin and rapidly wedging out) and manganese and iron oxide nodules.

Green to green-grey, massive and hard siltstones are mainly found in green shales forming layers 1 to, 12 cm thick (3 cm thick on the average). They are aleurite to aleurite-psammite or, sometimes, aleurite-pelite or psammite in structure.

The texture is most often random and only sometimes stratified (alternating light- and dark-grey layers). Detrital material of these rocks is mainly represented by quartz, only a small part of which is pyrogenic. Quartz is accompanied by muscovite, glauconite, feldspars (orthoclase, microcline and plagioclases), calcite, biotite, phosphates, iron sulphides and hydroxides, manganese oxides, heavy minerals and debris of crystalline rocks and carbonized plants. Siltstone matrix is siliceous-clay or, sometimes, clay-calcareous or calcareous, usually of the groundmass type.

In the light of results obtained (and according to A. Slączka, 1963) it may be stated that Upper Cretaceous and Eocene mottled shales are autochtoneous whereas siltstones and sandstones intercalating Eocene shales represent deposits of ceasing turbidite currents. Colour of these rocks represents net result of sedimentary and diagenetic processes as well as hypergenetic factors. The essential differentiation in colour originated during sedimentation, under the conditions of repeated changes in hydrodynamic regime and morphology of floor of sedimentary basin, determining Eh and pH values.

Clay material of the mottled shales is either allochtoneous or autochtoneous. Illite and beidelite (originating in result of degradation of illite in the weathering zone) and some admixture of kaoline were supplied to the sedimentary basin from alimentary areas. The resulting deposit was enriched in beidelite from halmyrolysis of volcanic and presumably mainly subaerial material during sedimentation.

Beidelite became agraded into illite (throught mixed-layered phase) during diagenesis.

This was connected with discharge of free silica which subsequently formed cement in siltstones and sandstones and microcrystalline aggregates and veinlets in the shales.