O halogenicznych zjawiskach krasowych w permie górnym na wyniesieniu Łeby

Józef Poborski



In normal stratigraphic column the Zechstein saline formation is divided into four stages: Z1, Z2, Z3 and Z4, corresponding to particular evaporitic (saline) cyclothems. However, the first geological cross-sections constructed for the area of Łeba elevation have shown abnormal profile of the Zechstein. This phenomenon is explained in the following way. In the area of Łeba elevation as well as in adjoining area of Baltic syneclize the Zechstein sedimentation started with deposition of typical stratigraphic members of the Z1 stage: (1) Weissliegendes, (2) copper-bearing shale, (3) Zechstein limestone, (4) lower anhydrite and (5) the oldest rock salt. However, close to the end of Z1 times the substratum of Zechstein sedimentary basin- was uplifted resulting in emergence of the oldest rock salt layer and a break in normal sedimentation of the evaporites. Figure 2 shows chronostratigraphic situation during the Late Permian (Zechstein) along the NW –SE section from the vicinities of Łeba to Elbląg area. Vertical strokes mark the part of the section corresponding to a gap in normal Zechstein sedimentation. It may be noted that this sedimentary gap also comprises the Z2 stage in the area of Łeba elevation. Close to the end of Z2 epoch, when climate changed from arid to humid, there wail an intense development of karst phenomena in Łeba elevation area. The oldest rock salt layer was subjected to leaching and there originated hydrographic network comprising dolines and karst depressions and a complex system of underground caves. Negative karst phenomena of such type were more intensely developing in the area of stronger uplift of the substratum (horsts) as it may be noted in the map (Fig. 1). At the end of the Z2 epoch all negative karst forms underwent complete infilling in result of development of positive halogenic karst phenomena. Residuum after solution of the oldest rock salt, i.e. gypsum, formed a typical gypsum cap. The gypsum or anhydrite was locally subjected to further secondary alteration under the influence of magnesium-potassium lye, which resulted in polihalitization of calcium sulphate, a common metasomatic process. The negative karst forms were finally filled with brines. “Descendant” salt precipitated from these saturated saline solutions as the main product of positive karst phenomena of halogenic nature. This resulted in complete infilling of crystal caves and karst depressions. At the beginning of the Z3 epoch the sea of gray saline clay transgressed the Łeba elevation area and sedimentation of calcium-sulphate-facies evaporites with a marked admixture of terrigenous clay suspension started.

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