Charakterystyka geochemiczna utworów triasu z rejonu Polski północno-zachodniej

Abstract

GEOCHEMICAL CHARACTERISTICS OF TRIASSIC FORMATIONS FROM THE NORTH-WESTERN AREAS OF POLANDSummaryThe Triassic deposits have been studied from the north-western part of the Pomeranian swell (bore  holes Połczyn Zdrój, Kamień Pomorski and Kołobrzeg) and of the northern zone of the Fore Sudetic monocline (bore hole Gorzów Wielkopolski).In order to know the range of the chemical and mineralogical changes in properties of deposits, caused by a definite sedimentary environment, the following groups of geochemical coefficients have been determined: 1 – distribution of trace elements in various rock types and in their fractions, and a ratio of contents of these chemical elements to iron, 2 – mineral composition of clay fractions < 20 m separated from the rocks examined, 3 – pH, and oxidation-reduction potential of deposits.In general, 368 samples of the Triassic deposits were examined (Table 1). The results obtained allow to state that clay formations laid dawn within the internal zone of a sedimentary basin, or under conditions of quiet sedimentation, contain more trace elements than other deposits. On the other hand, tin shallow inland basins, or within their peripheral parts, an increase in these elements can be observed in sandstones.Assemblage of clay minerals that occur in the fractions separated from the claystones and sandstones, is uniform and consists of minerals that belong to the group of hydromicas, of kaolinite and of mixed-layer chlorite-montmorillonite minerals. Quantitative relations of these minerals are changing, particularly hydromica-kaolinite ratio, and define the different physic-chemical conditions of sedimentary environment. The value of the ratio in the rocks examined ranges from 0,2 to 12. It has also been demonstrated that the hydromica-kaolinite ratio below 3 is connected with the deposits laid down in shallow, peripheral, low-salinity water basins of lagoon or delta type. On the other hand, a higher ratio of these minerals (more than 3) is characteristic for the formations of strongly saline, internal zone of sedimentary basin, or for the series of deposits laid down under changing, near-shore conditions, periodically strongly influenced by marine environment.As a rule, mixed-layer minerals are rarely found in the fractions examined. An increased quantity of these minerals was reported to appear only in the deposits laid down in a basin of lagoonal type.On the basis of same defined geochemical rules, the Buntsandstein sea of the north-western part of the Pomeranian swell can be regarded as a shallow highly saline basin, periodically supplied with materials characterized by an increased amount of several trace elements. On the other hand, the basin of the northern zone of the Fore-Sudetic monocline is thought to represent a basin influenced by changing continental and marine environments. Keuper sea was a slightly saline and shallowing basin. Here predominated continental weathering material that originated from the north-western area of the Pomeranian swell. In the region of the bore hole Gorzów Wielkopolski, in turn, the Keuper sea revealed an increase in salinity and some changing influenced by continental and marine environments.