Coal-Bearing Depositional Systems

Ever since the term “facies” was introduced into the geological literature by Gressly (1838/41), it has been subject to various interpretations. In this book the concept of Walther (1893/94), based on Gressly and recently reiterated by Murawski (1972), Middleton (1973) and Woodford (1973), is followed, in which facies encompasses all the physical, chemical and biological characteristics of an areally defined geological body in its present state. Facies is not synonymous with palaeo-environment but conclusions about the latter can usually be drawn after facies characteristics have been analysed. Facies characteristics are therefore indicators of the palaeo-environmental conditions under which a rock body has been formed.

Coal is an organic sediment which consists of coalifield vegetal matter. A broad distinction is made between humic and sapropelic coals of which the first type is far more frequent than the second type, which has been formed by subaquatic sedimentation of floating vegetation (algae) and allochthonous (= redeposited material, not formed in situ) organic matter. The phytogenic precursors of humic coals derived mainly from rooted autochthonous (= formed in situ) vegetation which grew in mires where they accumulated as peat. The latter is the first step in the coalification process by which the biomass is transformed into successive coal ranks which are expressed by such terms as (in order of increasing rank), brown coal, subbituminous, high, medium and low volatile bituminous coal, metabituminous coal, semi-anthracite and anthracite.

The transformation of vegetable matter into peat and coal is commonly regarded as proceeding in two steps, called the biochemical and physicochemical stage of coalification (Stach et al. 1982), respectively. Other terms, such as “first and second phase” (Mackowsky 1953), or “diagenetic and metamorphic stage” (Teichmüller 1962) have been used to describe the coalification process. During biochemical coalification organisms initiate and assist in the chemical decomposition of vegetal matter and its conversion into peat and brown coal. The results of this process, i.e. the type of peat and coal formed, depend on the phytogenic input and the environmental conditions under which it is transformed into peat. Different biological, chemical and physical constraints result in different peat types which during the subsequent physicochemical coalification are transformed into different coal types without losing their palaeo-environmental signature. Because of the causal links between coal types and depositional setting the following discussion will emphasise the conditions and results of biochemical coalification, whereas physicochemical coalification will be dealt with less rigorously.

The preceding discussion has shown that similar plant tissues can form a wide range of degradation products when subjected to varying degrees of humification and dehydration before being incorporated in the accumulating peat. On conversion into coal some of the differences between the peat components are lost whereas others are retained and may even become accentuated, such as differences between macerals in reflectance which develop at the beginning of physico-chemical coalification, before undergoing the kind of convergence illustrated in Fig. 3.29.

In the preceding chapters coal components have been classified on the basis of physical, chemical and genetic relationships. In the following discussion these will be employed in a filtering process that is designed to detect the signatures left behind by depositional environments in the form of a distinctive coal facies. The concept of depositional environment will be targeted at two levels: one is specific and refers to the type of mire and the conditions of peat accumulation within it, whereas the other takes a broader view and seeks to establish the relationship between coal facies and the sedimentary setting of the mire. The questions to be asked in the first case, which is the main subject of this chapter, will concern the hallmarks of rheotrophy and ombrotrophy, as well as the influence on coal type of the vegetal progenitors after the local variabilities have been filtered out. The aim is akin to what Walker (1980, Fig. 4) calls the “pure essence of environmental summary”, which constitutes a facies model obtained from the common denominator of a variety of local examples. The results of this enquiry are then linked to their sedimentary settings, such as delta plains, alluvial valleys and the like, which will be discussed in detail in Chap. 7.

Coal seams and their surrounding strata share several spatial and genetic relationships, some of which are common to all sediments, while others are specific to the transition from primarily inorganic to organic sedimentation and vice versa. Although coal seams and their enclosing inorganic sediments differ in many aspects, a mutual influence on each other’s composition and structural relationships is often observed in the vicinity of their contacts. Examples are the distribution of elements and minerals, which may be quite uniform in a vertical seam section, but undergo considerable changes in concentration near the sediment/coal interface (Nicholls 1968; Gluskoter et al. 1977; Pareek and Bardhan 1985). The observation of Dorsey and Kopp (1985) of a gradual upward decrease in elemental concentration in the Pewee Seam of the Wartburg Basin in Tennessee, U.S.A., followed by a sharp reversal of the trend (increase in Si, Al, Ti, K, Mg) below the seam roof is probably not an isolated occurrence. The authors regard the gradual upward element depletion as an indication of the decreasing influx into the Pewee swamp of terrigenous minerals, which was followed by renewed flooding and abundant sediment supply, thus terminating peat accumulation. While this interpretation is probably correct, an additional factor in the upward elemental depletion may be the recycling of essential elements, when plants cannot obtain sufficient nutrients from a deeply buried and water-logged soil, or because of the cessation of nutrient supply by flood waters.

Genetically related lithosomes constitute the integrated response of a sedimentary environment to the depositional process. Sedimentary environments are therefore integral parts of the hierarchy of sedimentation elements, since they consist of combinations of lower ranking sedimentation elements by which they can be identified. Palaeo-environmental analysis makes use of diagnostic combinations of these in many different forms, ranging from the superposition of characteristic deflections in geophysical logs to the regional changes observed in a fossil assemblage. In some cases palaeo-environmental conclusions can be reached only after the careful study of many different aspects of a vertical profile, while in others the genetically based depositional assessment becomes part of the logging process, whereby either outcrops, bore cores, geophysical signals or a combination of these are used (Fisher and McGowen 1967; Weber et al. 1984; Hamilton and Beckett 1984; Hamilton 1986).

The preceding chapter has shown that in the course of undisturbed sedimentation the various depositional environments produce specific orders of superposition of lithofacies which are recognisable in the stratigraphic column as either rhythmic or cyclic patterns. The strongest cyclicity results from coastal settings, where on either side of the shoreline several contrasting depositional environments coexist which can be shifted readily by relative sea level variations. Coal measures interbedded with marine sediments therefore provide particularly good examples of cyclic sedimentation and were used by Weller (1930) to formulate the concept of cyclothems, which for many decades strongly influenced research into coal measure sedimentation (Fiege 1937, 1952; van Leckwijck 1948; Wanless 1931, 1950; Wanless and Weller 1932; Wanless and Shephard 1936; Weller 1956, 1958; Moore 1950; Moore 1959; Jessen 1956a–c, 1961; Jessen et al. 1952, and others). A comprehensive summary of the early work is given by Duff et al. (1967).

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 are in a mopping up stage, in sensu 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 molasse basins, the development of which accompanies or follows terminal geosynclinal tecto-orogenesis (Aubouin 1965).

These final annotations serve two purposes, which can be expressed in the double questions: what has been achieved and where do we go from here? As mentioned in the Preface, one of the aims of this monograph was to argue the case for coal facies analysis as a useful tool in palaeo-environmental reconstruction. This was mainly addressed to the sedimentologists who work in coal-bearing strata but often make little use of the wealth of information the enclosed coal can provide. The other aim was to sell sedimentology to the coal petrologists who could benefit from widening their palaeo-environmental enquiry to include modern sedimentological and stratigraphic methods. This strategy required a close integration in the layout of the text of the relevant aspects of coal science and sedimentology. “Integration” and “relevance” are the operative terms in this context. There are several good sedimentology texts on the market, and Stach’s classic coal petrology text is currently being re-written, but both approaches to the subject matter offer at the same time too much and not enough. More importantly, sedimentology and organic petrology are treated in the available texts as separate entities with few or no palaeo-environmental cross-references, which is the key element in this monograph.