Compositional differences between Besko and Otryt sandstones of the Krosno Beds (Oligocene) in Eastern part of the Silesian Tectonic Unit

Abstrakt

The aim of this research was to quantitatively and qualitatively analyse under microscope sandstones representing two lithofacies called Otryt and Besko (or Lesko) facies of the Krosno Formation (Oligocene), Silesian Tectonic Unit in the E part of the Polish Outer Carpathians. Four samples were taken into consideration — two from each lithofacies. Special attention was paid to  structural and textural characteristics and  mineral composition that reflects mainly  provenance. The results have been compared and analysed between and within facies, in terms of source and transport of the material, mineralogical and textural maturity and sedimentary processes that affect the deposition. Ślączka & Unrug (1972) and Shideler at al. (1976) concluded that mineralogical variations exhibited by sandstones in Krosno Formation were attributed to both regional (provenance) and local (sorting) effects.

The samples have been taken from the northern (Otryt)  and southern (Besko) limbs of the Otryt Syncline. They represent different sources of the material -  Silesian Ridge in NW in case of Besko facies and Maramures Massif in the the SE in case of Otryt facies (Wendorff 1981). The result of the analysis confirm statement about two different sources.

Transport and deposition mechanism results in considerable difference in grain size and its distribution. Besko facies sandstones display classic Bouma sequence at structures (T_{b - c}) indicative of deposition from normally diluted, waning turbidity currents, with incremental deposition layer-by-layer (Talling 2012) and this results in very fine grained sandstones and in better sorting. Deposits of that lithofacies appear much relatively uniform and well organized (Wendorff 1987). By contrast, sedimentary features of the Otryt facies suggest deposition from rapidly decelerating high density turbidity currents — “sandy debris flows” (Talling 2012), conditions and mechanisms of transportation and deposition must have been very variable (Wendorff 1987). These sandstones are medium grained, but the bottom sample is characterized by coarse tail caused by rapid deposition. The detrital material of the top sample was deposited also of high density turbidity current, however slightly less dense than that of the bottom sample.

The diversity of mineral composition between Besko and Otryt lithofacies is very clear. There are probably three reasons for that — completely different source rocks, initially different maturity of the detritus involved in density flows and the depositional processes within the basin . Besko (Leslo) lithofacies is dominated by nonundulatory quartz. Other components are rather minor. The only difference between carbonate components is in the proportion of detrital calcite/limestone (or dolomite) to carbonate cement, which may have resulted from diagenetic processes. Carbonate cements are very rich in both samples. In Otryt facies mineral composition is more differentiated. Quartz predominates, but not as distinctly as in Besko facies. The most abundant and the coarsest are polycrystalline grains. Nonundulatory quartz is similarly abundant. Altered K-feldspars and metamorphic rock fragments are important however they occur less frequently. The bottom and top samples differ in quantity of coarse to very coarse metamorphic rock fragments and polycrystalline quartz — the bottom sample is richer in such grains. This is the result of grain-size sorting — coarser grains have been deposited first and this sorting process affects the mineral composition of the top-of-bed sample in which the coarse tail is absent and the proportion of metamorphic grains is less pronounced. There is very little cement in both samples. Next to high mineralogical maturity, Besko facies sandstones show also high textural and  mineralogical maturity (presence of well-rounded grains and high quartz content). On the other hand, Otryt facies sandstones are immature both texturally and mineralogically, which results from both source area composition and rapid deposition.

According to provenance diagrams (Dickinson 1983), source of the Besko facies is recycled orogen, but close to craton interior field. Whereas source of the Otryt facies should be recycled orogen too.

Despite the fact that all samples are the same age and occur in one formation,  they are totally different.

In summary, differences in mineral composition and textural features, observed under the microscope, between the analysed samples of the Besko/Lesko and Otryt lithofacies turbidite sandstones reveal different mineral composition of the source areas and are comparable to the macroscopically observed sedimentary features. The latter indicate pronounced differences in the transportation and deposition mechanisms – from normally diluted turbidity currents to rapidly decelerating high sandy debris flows.