ZMIANY KSZTAŁTU ZIAREN KWARCOWYCH ORAZ SKŁADU MINERALNEGO PIASZCZYSTYCH OSADÓW MIOCENU, PLIOCENU I NIŻSZEGO CZWARTORZĘDU W ROWIE KLESZCZOWA

Abstract

CHANGES IN THE SHAPE OF QUARTZ GRAINS AND IN MINERAL COMPOSITION OF MIOCENE, PLIOCENE AND LOWER QUATERNARY SAND DEPOSITS IN THE KLESZCZÓW GRABEN

Abstract. Samples of Miocene, Pliocene and overlying Lower Pleistocene deposits collected from the Bełchatów brown coal mine were analysed for the composition of heavy and light minerals and the shape of quartz grains. The Miocene–Pliocene deposits represent lithostratigraphic units called the subcoal unit, coal unit, clayey-coal unit and the youngest clayey-sandy unit. The Lower Pleistocene deposits are represented by alluvial series. Mineral composition of the three lowest Miocene units is dominated by transparent resistant minerals. They are represented by tourmaline and disthene, which occasionally occur alternately, and staurolite. Rutile and zircon contents are much lower. The characteristic feature of this mineral assemblage is a very low content of garnets and lack of non-resistant minerals. However, a different
composition of heavy minerals is observed in deposits representing the clayey-sandy unit. An abrupt increase in the amount of garnets is observed here, again with the dominance of tourmaline and a greater content of staurolite. Opaque minerals are represented in the Miocene- Pliocene deposits by common ilmenite and hematite. The heavy minerals composition of the Pleistocene deposits is similar to that observed
in the underlying Pliocene series, however the content of garnets is higher. The light fraction of all the Miocene–Pliocene units is dominated by variable amounts of quartz grains, proportional to the contents of the remaining constituents represented by sulphate minerals, clayey-ferruginous aggregates and lignite fragments. The individual lithological units exhibit distinct, specific mineral characteristics. Sulphate minerals are abundant in the lower two units. Quartz grains from the Miocene-Pliocene deposits show low shape variability in the vertical section. Microrelief of quartz grains from these deposits is conspicuous by pronounced variability. In the lower and upper parts of the deposits, EL-type (blunt shining) grains are definitely prevalent. In the middle part, the amount of EL grains decreases with the increase in ET grains (transitional). It allows suggesting that the original surface of rounded polished grains was largely destroyed by chemical etching. The preglacial deposits, included in the lowermost Quaternary and immediately overlying the Pliocene series, contain a considerable admixture of RMgrains (round matt). Their amount constantly increases to become dominant at the top. This is a very important criterion facilitating placing the boundary between the Upper Neogene deposits (Pliocene) and the Pleistocene succession. Mineral composition of the preglacial alluvial deposits and the overlying alluvial series is similar to that observed in the Pliocene deposits. Feldspars appear only above and their content increases upwards in the alluvial deposits. They were derived from glacial deposits.