O niektórych skałach krzemionkowych w Polsce

Marian Kamieński, Zdzisław Sokalski


On some siliceous rocks in Poland

The authors draw attention to certain siliceous and silicocalcareous rocks which on account of their physical and chemical properties seem to be of particular interest. In the first place the authors took an interest in the region lying to the north of Sandomierz. At the village of Piotrowice, near Zawichost, exposed are calcareous rocks, of «opoka» type (i. e. siliceous marls), appertaining to the Lower Turonian. Occurring among them are varieties that are particularly light-weighing, very porous, highly hygroscopic, and composed almost exclusively of opal substance. On the basis of entirely cursory field-observations it may be assumed that these siliceous rocks occur in the form of narrow belts, not more than several metres wide, running WNW—ESE. In the morphology of the terrain they stand out as small ravines, running in the above-mentioned direction. Their length has not been determined. It will be possible to determine it on the basis of appropriate exploratory work. What we have at Piotrowice are probably several such belts. In the southernmost belt of siliceous rocks, situated at the southern boundary of the village of Piotrowice, seven artificial excavations were dug out along a line 125 metres long. In no case did the authors succeed in reaching the bottom of the above-mentioned siliceous rocks. In all cases their appearance was uniform (light creamcoloured when moist, and snow-white after drying), and only in the fop layers did they display distinct limonitic incrustations, the latter arranged ribbon-like. As far as the origin of the Piotrowice siliceous rocks is concerned, they must be considered as a product of décalcification of the surrounding «opoka» (siliceous marls). It appears that this bed-rock is also composed — apart from calcium carbonate — of opal substance. Quartz grains, argillaceous substance and iron hydroxides occur in but very small amounts. When the «opoka» is treated with dilute hydrochloric acid for several hours, it changes into a very light-weighing and very porous rock, macroscopically and microscopically corresponding to the above-mentioned siliceous rock. Under the action of hydrochloric acid the «opoka» does not disintegrate; it remains compact. The siliceous rocks were not produced as the result of a surface process of décalcification, although the latter process does manifest itself in places; these rocks were formed under certain particular conditions. The waters leaching the calcium carbonate had favourable conditions of circulating only in certain zones, arranged in a parallel manner to one another. The possibility occurs here of taking into consideration the existence of a number of small faults and fissures,, running from WNW to ESE, which facilitated the action of water. Perhaps they ought to be associated with the dislocation — accepted by Samsonowicz — that cuts off the Tarłów graben from the south and is included in the system of strong dislocations which according to the above-mentioned author originated in post-Cretaceous times perhaps in the Palaeogene. Another region to which was directed the authors’ attention was the Lublin upland where the best developed deposit exposed at th e earth surface — apart from the Quaternary — is the Upper Cretaceous, appertaining to the Senonian. The basis for the interest in the Lublin upland and its Senonian «opoka» was the work of Sta rzyński who, linking up his work with older geological ones, distinguishes in the Lublin upland within the Senonian bed-rock a calcareous variety (containing, on the average, 60 to 80% of CaCO(3) and, also on the average, 30,5% of an insoluble residue) and a calcareo-siliceous variety. The latter contains 30 to 60% of CaCO(3) and, on the average* 54% of an insoluble residue. In the calcareo-siliceous «opoka» Starzyński distinguishes yet certain varieties which he characterizes in a general manner as arenaceous. In their chemical composition they display a maximum of 30% of CaCO(3), and frequently they are even deprived of the latter constituent. Some of these varieties are yellowish, compact and with a rough fracture; they manifest a preponderance of grains of clastic quartz and glauconite, joined together with an opal cementing material. By their appearance they correspond to sandstones. Other pelitic varieties have the appearance of white-coloured rock, also compact, but one which can be whittled with a knife. In a general manner Starzyński defines these pelitic varieties as a siliceous (spongiolitic) «opoka» resembling gaizes. They are particularly interesting. Their characteristic properties are that they are light-weighing and that — in a similar manner as the siliceous rocks from Piotrowice — they are strongly hygroscopic and adhere to the tongue. These properties, together with their porosity, are more and more strongly marked, as the rocks contain less and less calcium carbonate. When treated with hydrochloric acid, the pelitic varieties leave a residue composed, above all, of an opal substance in which occasionally is observable the presence of sponge spicules, although the latter are hardly ever well preserved. In very small quantities are present: quartz grains, glauconite, and minute scales of mica. The chemical analyses of the pelitic varieties are listed in the Polish text. On the subject of the origin of the siliceous «opoka», the following remarks occur. Starzyński assigns to it a geographical range (a narrow belt adjacent to Roztocze) and treats it as a separate fades. On the other hand, on a number of examples the above-mentioned author presents proof of secondary surface processes of décalcification of the original «opoka». Therefore, on the background of Starzy ń ski’s data, we can consider the siliceous «opoka» as a rock a priori dependent upon the conditions of sedimentation; in certain cases this rock was subjected to still additional décalcification. Observations from the region of Józefów and Opole confirm, above all, the process of secondary décalcification ; this process has been proved to take place in a very irregular fashion. Frequently it is not evident at all, and where it does occur, it reaches down to a variable depth : from several centimetres to several score. In some cases, probably, the above-mentioned thickness is much greater, inasmuch as Starzyński mentions enormous blocks of the siliceous «opoka», without stating, however, their whereabouts. It has been also observed that the process of leaching out of the carbonates — which seems understandable — took place in consequence of the action of humic acids in a more intense manner in places where the rock was overlaid by forest undergrowth, although deviations from this rule are also observable. The chemical analyses quoted in the Polish text inform us as to the course taken by the process of décalcification of the primary rocks belonging to the «opoka» type and containing CaCO(3) in comparatively considerable quantities. These analyses pertain to the previously discussed rocks of the Lower Turonian from Piotrowice and the Senonian rocks from Wrzelowiec, a locality situated on the Lublin upland, between Józefów and Opole. The décalcification process is not a phenomenon of particularly rare occurrence. It has been observed, for instance, by Premik in the Upper Senonian of the vicinity of Sieradz. On a small scale it was discovered by the authors in the Cracow Senonian, and they had also observed in the vicinity of Niemirów and Magierów that the local Senonian «opoka» in its surface zone is also deprived of calcium carbonate and that in its chemical composition (analysis quoted in the Polish text) it resembles the decalcified rocks previously discussed. It turns out, consequently, that certain calcareous rocks containing in their composition opal of organic origin, forming as if a framework impregnated with calcium carbonate, may be subjected to a décalcification process, to a various depth, dependent upon local conditions. As a result of the calcium carbonate being leached out, the rocks frequently do not lose their original cohesion, becoming only light-weighing and porous. They adsorb water to a large degree, they are characterized by considerable refractoriness and, perforce, they are enriched in SiO(2). By these properties the above-mentioned siliceous rocks approximate to diatomaceous earth, and they can serve as a substitute for the latter in certain fields of its numerous applications. As demonstrated by investigations, especially the varieties deprived of argillaceous substance, or poor in the latter, are deserving of particular attention. If we consider the fact that crude diatomaceous earth sometimes requires far-reaching processes of refinement, in conjunction therewith there arises the question whether it would not be also possible to utilize as a substitute for diatomaceous earth — by decalcifying them artificially — the previously discussed primary rocks which occur at Piotrowice, within the Lublin upland, or else are known from other regions, and which apart from calcium carbonate contain opal substance. Laboratory work carried out on the original «opoka» from Piotrowice gives an interesting basis for further investigation in this direction.

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