Kilka uwag o sedymentologicznych podstawach stratygrafii cechsztynu

Maciej Podemski




The scheme of Zechstein stratigraphy elaborated by G. Richter-Bernburg (1955) is characterized, apart from its numerous advantages, also by some inconsequences. A fact is striking, first of all, that clastic formations are of the same stratigraphical importance as chemical ones. They differ, however, considerably in their genesis, and are independent from each other. Moreover, clay horizons from the portions of the basin, remote far from the shore, for example, have not to be of the same age as the clastic formations of the near-shore fades, correlated with them. The fact that the so-called recessive formations are placed within the cyclothems that rest beneath them, makes another problem here. Upper anhydrites or covering anhydrites, corresponding to this name, for a certainly did not precipitate in the same solutions as the underlying rock or potassium salts. They come rather from the first portions of oceanic waters of the new transgression, which evaporated under conditions of shallow shelf. Under such conditions carbonates precipitated still during the water flow through the basin, thus sulphates were their first evaporation product in the farther portions of the basin. The first phase of chemical sedimentation in waters caming from the new transgression was broken up during the maximum inflow to these waters. After the break in connection between the Zechstein basin and the open ocean, the second phase of sedimentation began, in which chemical deposits characterized by normal development and classical sequence were laid down. According to this conception, the genesis of the recessive formations, mainly of anhydrites, is related with the evaporation of waters in which chemical formations were laid down above the annydrites under consideration. Thus, the so-called upper anhydrites or covering anhydrites should be referred to the overlying cyclothems, and not, as it has so far been in use, to the underlying ones. The author proposes to change their name into bottom anhydrites of a given cyclothem to stress better their new position.


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