中英文资料对照外文翻译文献综述
Coal-Producing Tectonic Environments
This final chapter in the investigation of coal sedimentation is concerned with depositional aspects of the highest order of magnitude, namely, the influence of the crustal setting on peat accumulation. This is a broad and complex field which draws on information, gathered from many different disciplines of the earth sciences. Some of these are currently evolving quite rapidly, while others re in a “mopping up” stage, insensu Kuhn (1970) and Walker (1973), following recent scientific revolutions. An example of the latter is the replacement of the geosynclinal hypothesis in the early 1970s by the concept of plate tectonics. Even after a life span of 20 years, this new paradigm is still in the process of being refined and fitted out with conceptual subsets, as shown by the current emphasis on terrane analysis. It is therefore not possible at this stage to make a definitive statement on the chosen subject, but merely to outline the principle on which a modern geotectonic classification of coalfields can be established. Even this modest goal is fraught with difficulty, because the change from the predominantly static geosynclinal view of global tectonics to its modern, largely mobilistic interpretation has complicated the tectonic classification of some coalfields. While the tectonic status of many coalfields, e.g. those in foredeeps or foreland basins has changed relatively little, the setting of coals found in inter- and intramontane troughs, i.e. within orogenic cordilleras, cannot be
properly assessed without very careful study. According to the geosynclinal concept, practically all of these intradeeps, together with fore- and backdeeps, their extra-orogenic counterparts, were regarded as part of a group of molasses basins, the development of which accompanies or follows “terminal geosynclinal tecto-orrgenic” (Aubouin 1965). This fixist and strictly sequential interpretation (highlighted by the term “epieugeosyncline” of Kay 1951) has no place in modern geotectonic analysis, which views most orogenic belts as collages
of
autochthonous
and
allochthonous
terranes,
i.e.
as
tectono-stratigraphic assemblages with possibly coeval but heterogeneous stratigraphic records reflecting their origin in different geological and geographical domains (Monger and Price 1979, Monger et al. 1982). The tectonic setting, which influenced the formation of an allochthonous terrane assemblage before accretion, may have been very different in style and physically far removed from its resting place after docking. It follows that a multi-terrane orogen may contain a variety of coals formed at different times before and after terrane accretion. Moreover, contemporaneous pre-accretionary coal deposits formed in different terranes are likely to vary in coal types, coalification histories and tectonic styles, and all of these will in turn differ from the post-accretionary molasses coals, which alone reflect the conditiona prevailing in the orogen itself. Indeed, the situation may even be more complex, as will be discussed in Chap.9.3.2.2.
Plate tectonics has created its own nomenclature, of which only the essential terms will be used here. They will be supplemented by terms which are either descriptive, and therefore independent of geotectonic theory, or which have stood the test of time because they are useful in spite of their generic
association with now obsolete concepts. For example, the expressions “mio-” and “eugeosynclinal assemblage” have been kept here as reference term for shallow water marine (mainly shelf), and ocean floor pelagite, turbidite and ophiolite associationa, respectively. Moreover, reduced to a “miogeocline”, the miogeosyncline has in the North-American literature become a standard term for autochthonous, sedimentary terrace wedges onlapping continental margins. Also tectonic attributes of sediments, such as “synorogenic” flysch and “late syn- (folded) to postorogenic (non-folded)” molasses, respectively, can still be used in a plate tectonic context without unduly corrupting their relatively loose definitions. Particularly in the discussion of coalfields situated near convergent plate edges, the concept of molasses as the product of the destruction of the uplifted orogen is very useful. As in the previous discussion, it is not the purpose of this chapter to give detailed descriptions of a large number of cases but to select a few typical examples of coalfields and relate the essence of their architecture to their respective plate tectonic settings. 1 Early Examples of a Tectonic Classification of Coalfields
Large-scale coal formation can take place only in actively subsiding regions, for example in sedimentary basins. It is possible therefore to characterise the geotectonic environment occupied by a coal measure sequence in a manner similar to that which is applied to other sedimentary environments. Stutzer (1920) and Stille (1926) were among the first to recognise the genetic links between tectonism and the formation of coal. Stille, in particular, referred to the striking difference in terms of basin fill, number of coal seams present, their average thickness and proportion in relation to total coal measure thickness, which exist between the Carboniferous and Tertiary coal measures of Europe.
He attributed such dissimilarities to contrasting degrees of crustal mobility in the areas affected by the two main European coal-forming periods. His results are summarized in Table 9. 1. Even if differences in compaction ratios between the Tertiary brown and Carboniferous bituminous coals are taken into account (to a lesser degree the compaction applies to inter-seam sediments) the contrast is quite remarkable.
Later it was shown by von Bubnoff (1937) that the distribution of the world reserves of coal is also related to the geotectonic setting of coalfields. His conclusions are summarized in Table 9.2, which indicates that of all coal deposits known up to 1937, some 71% developed in former tectonically very active environments, particularly in the molasses foredeeps which develop adjacent to orogenic belts and receive much of the weathered debris washed down from the uplands.
Carboniferous coal measures in mobile Varican basins Average coal measure thickness Average number of coal seams Average seam thickness 3000m Tertiary coal measures in cratonic basins of Central Europe 150m 200 2 1m 15m
