Utwory tufogeniczne kredy górnej Niżu Polskiego w otworze wiertniczym Tłuszcz

Maria Harapińska-Depciuch




In the bore hole Tłuszcz IG-l tuffaceous formations were for the first time encountered in the Upper Cretaceous deposits within the Polish Lowland area. The formations occur as an intercalation, 20 cm in thickness, found at a depth from 506,1 m to 506,3 m in chalk deposits of Lower Campanian age. As a result of macroscopic observations five various laminae (Fig. 1) were ascertained to occur in this intercalation. The laminae are built up of bentonites and chalk containing a considerable admixture of montmorillonite.
Macroscopically, these are schistose rocks, grey in colour, swelling and decomposing in water. Under the microscope, the rock sample taken from the lowermost laminae C1 displays a crystalloclastic-aleuritic-psammitic texture, and a feebly directional structure. The main component constituting the rock body is represented by fine-scaly montmorillonite, in which single crystalloblasts of quartz and feldspars occur, from 0,02 - 0,1 mm in size. Among feldspar representatives plagioclases occur twinned according to albite law. Fragments of rock matrix, as well as muscovite and biotite are rarely found.
Under the microscope the rocks from the laminae C2 and C3 show an aleurite-pelitic texture and a directionless structure. Montmorillonite is here the main component; it contains a small admixture of carbonate substance, the amount of which increases in the samples from the laminae j1 and j2, at the cost of montmorillonite.
On the basis of the microscope examinations of powder pattern made of pyroclastic material, differences in grain size can be ascertained, since the largest grains occur in the lowermost bentonite, where grain diameter ranges from 0,06 mm to 0,2 mm, rarely amounting to 0,3 mm. Upwards, grain diameter diminishes: in the laminae C2 and C3 there are found grains 0,06­–0,1 mm in diameter; in the laminae j1 and j2 these are 0,08 mm in size.
Feldspars and quartz are predominating components of clastic fraction in the bentonites. Rock matrix, volcanic glass, muscovite and biotite are found in considerably lesser quantities. Zircon, garnets, rutile and hornblende are accessory minerals here. Feldspars are represented by anorthoclase with an angle 2Va being about 45°, oligoclase with 20–25%. An, and andesite with 38–45% An. Quartz occurs in the form of sharp-edged fragments having shell-like fracture, and, accidentally, as well-rounded grains shaped so probably due to resedimentation.
Curves of thermal analyses (Fig. 2) demonstrate that in the samples of the lowermost laminae C1 and C2, endothermal effects characteristic of montmorillonite occur. On the other hand, in the samples taken from the laminae C3, j1 and j2, endothermal effects gradually diminish at the cost of endothermal effects of calcite so that in the last laminae j2 only calcite endothermal effects may be observed.
In addition, on the basis of chemical analyses made on the samples from the laminae C1, C2 and C3, we may state that montmorillonite is here the main component. It contains a little admixture of calcite in samples C1 and C2 and a fairly high admixture of this mineral in sample C3. Thus, it is possible to ascertain that this is a bentonite intercalation of tuffaceous origin. In all probability, volcanic ash falling into the sedimentary basin was an original material there. Chemical analyses particularly, however, iron contents (4,52% by weight) and magnesium contents (3,79% by weight), which represent rather an original content of these components in rocks, as well as feldspars represented by oligoclase and andesite (beside alkaline feldspars), allow to draw a conclusion that rocks resembling dacites were mother rocks to volcanic ash.
Magmatic source of the volcanic ash can hardly be discussed here, since only one occurrence point of tuffaceous formations has so far been encountered in the Polish Lowland area. Maybe, the source is connected with the Cretaceous Carpathian magmatic phase, although other sources can also be taken  into account.


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