Factors controlling Cenozoic anthracogenesis in the Polish Lowlands

The accumulation of large amounts of phytogenic matter, leading to the formation of lignite deposits of economic importance, has been determined by two groups of external factors: (a) climatic factors determining the indispensable production of organic matter and (b) geological factors allowing preservation of this matter in the sediment and its diagenetic transformation in the process of coalification. The overriding item among climatic factors was obviously the production of suitable amounts of phytogenic matter. It could be accumulated only under favourable vegetation conditions and therefore a warm and humid climate was a condition sine qua non for the intensive production of phytogenic matter. Problems of lignite origin do not generally relate to the lack of production of phytogenic material, but to preventing preservation in the sediment due to rapid oxidation of this material in very warm conditions. Therefore, the coal-forming process was constrained by two critical temperature values, which had to be neither too low nor too high, wherein sufficiently high humidity persisted throughout the process. Therefore, the range of mean annual temperature, which ensured favourable conditions for the growth and preservation of phytogenic matter, was from approx. 15.5 to 24°C. In the Cenozoic, such conditions commenced in the Early Oligocene and persisted up to the later Miocene – this was the interval of the most intense anthracogenesis in the Polish Lowlands. In the widespread lowland areas, lush swamp forests and peat fens developed, and thick lignite seams reflect the accumulation of phytogenic matter. This interval began with the cooling at the Eocene/Oligocene boundary and terminated at the beginning of the next cooling and drying phase known as that of the “C4 grassland” in the Late Miocene. Both the critical points are related to the surpassing of temperature limits: too high for the preservation of phytogenic deposits, and too low for the extensive development of lignite-forming vegetation. An important condition for the emergence of a large thickness of phytogenic sediments is primarily an existence of the accomodation space, where a considerable amount of plant matter might accumulate. This occurred only in conditions of dynamic equilibrium between the growth of plant matter and the lowering of the depositional surface, which ensured stabilization of the groundwater level. The rate of subsidence of the depositional surface must be balanced by the rate of vegetation growth. Therefore, no single lignite seam corresponding in age to the whole period of potential accumulation was formed at that time. Rather, a few lignite seams, separated by thick successions of mineral deposits, then formed. The vegetation of the wetlands which created the individual lignite seams was similar, this being mostly a facies element. Differences in the composition of vegetation are found mainly in plant communities outside peat-fens and it is the plants outside of the wetlands which allowed for subsequent dating of the lignite seams. The thermophilous vegetation was replaced by plants of lower thermal requirements during the progressive climate cooling towards the end of the Miocene. The ultimate cooling and completion of peat/lignite production was generated by the Middle Miocene uplift of the Carpathians arc in the Alpine orogeny. This natural barrier considerably limited the circulation of warm and humid air masses from the south to the Polish Lowlands area.