| Themes > Science > Earth Sciences > Geology > Coal > Formation of Coal > The Late Carboniferous |
Coal bearing basins from the Late Carboniferous are important because their coal mining activities for more than 200 years have been a basis for world wide economic development. A huge amount of geological data has been collected on these basins and a lot of models developed that extend our understanding of coal-bearing deposystems (Murchinson & Westoll, 1968; Rahmani & Flores, 1984; Diessel, 1992). Two principal groups of processes must be understood to describe the origin of coal: peat formation and peat preservation. Local conditions for peat formation and preservation have been discussed by many authors. Like the formation of other biogenous sediments the formation of peat can occur under a broad range of physical conditions. Local groundwater play an important role. A constantly rising groundwater table favours the steady growth of mires, and buffers the siliciclastic input due to the rising base level in the hinterland. Therefore, in sea-level influenced sedimentary systems, "coal-prone" sequences are considered to be favourably induced by transgressions, when accommodation potential in the coastal plains is balanced by the rate of sediment flux vs. peat growth (Cross, 1988). The formation of peat-bearing deposystems cannot be assigned to a single sedimentary environment, with coal-bearing strata known from siliciclastic, carbonate, and mixed systems. Peat has been shown to form a stable substrate that is difficult to erode and behaves like a hardground in carbonatic systems (McCabe, 1988). Erosion of seams is rarely documented. As such, peat caps the underlying strata and forces regressive siliciclastic systems to widen and form braided plains and then to incise deeply and form valleys. This is the major reason why even under high energy conditions, siliciclastic, nonmarine settings like intraorogenic basins, the probability of peat-preservation remains relatively high. For the Late Palaeozoic, coal generation has been observed on a global scale. Pangean configuration forced widespread homogenous climatic conditions in the central parts of the supercontinent. Alleghanian and Variscan orogenesis produced similar tectonic settings along the strike of the orogenic front. Besides foreland-basins, subduction-related thermal basins and intraorogenic strike-slip basins at the convergent margins of the evolving supercontinent, large sag-basins were able to form on the consolidated continental crust of the interior of the supercontinent. High subsidence and high sedimentation rates in the evolving foreland basins under an optimal climate provided the conditions for formation and preservation of peat bearing sequences. Siliciclastic underfilling of internal sag-basins also contributed accomodation space for large amounts of biogenous sediments, including peat-bearing systems. The sedimentation of Late Carboniferous coal-bearing strata was originally attributed to cyclic processes. In North America, two contrasting exogenic models, based on tectonic activity or on eustatic sea-level changes were developed (Moore, 1929; Weller, 1930). Both models include schematic and repeating siliciclastic or carbonatic successions, called cyclothems. The concept of cyclothems did not take into consideration the close lateral associations of different environments within deposystems. The sequence stratigraphic concept offers the possibility to integrate knowledge on lateral change and the systematic stacking of environments (Aitken & Howell, 1996). In addition, sequence stratigraphic techniques can contribute to the global correlation of Late Carboniferous strata. New chronostratigraphic studies add knowledge on the duration of the Late Carboniferous (Suess, 1996). Some well documented case studies together with integrated numeric modeling scenarios demonstrate the global and local factors that triggered sedimentation in this favoured time of peat generation and preservation. |
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