The mineralogical characteristics of Maardu dictyonema shale

The composition of Maardu dictyonema shale as well as its structural and textural peculiarities were studied by means of the latest thermographic, optical and electroiimicroscopical methods. By polarisational microscopy it was stated that this dictyonema layer has an aleuropelitic structure and shistose texture. Typical of it is the pelitic predominance (the size of the grain being below 10 g). In addition to this fraction there occur some grains (their size varying between 10—25 p,) of mineral composition. In the samples there proved tobe quartz, kalifeldspar, plagioclase and different micas, e.g. chlorite biotite, muscovite, carboniferous detrite (1 —2%), and some pigmentic organic matteras well as gelose remnants of plants. Of authigenous minerals, there occur opal. chlorite, hydromica, pyrite, albite, and of accessory minerals zircon, tourmaline, apatite and garnet. Granatite, epidote, disthen, anatase, oxide of iron and hydroxides of iron, amphiboles and pyroxenes occur as singie grains. At the electron-microscopy of the fine-dispersed fraction it was stated that it consists mainly of hydromicas and inconsiderable quantities of quartz. By means of thermographic methods it was possible to define the raineralogical composition of the fine-dispersed component with greater precision, and thus the presence of kaolinite and organic matter was detected. The structural peculiarities, e.g. the finely dispersed structure, the presence of oligomicaceous, well-rounded precipitated breccias of argillaceous shist and the relatively high content of organic matter and iron dioxide allow the author to assume that Maardu dictyonema shale is a littoral formation. The lack of oxygen and thereafter a retreating environment could have originated only in lagoons peculiar of zones that were from time to time connected with the open sea, and it was here that the oxygen compounds were intensely formed and the organic matter accumulated and was conserved. The shallow-water condition of basins with dictyonema fauna is proved by the occurrence of precipitation breccias and sand interstratifications in the shist.