Facies studies of coal and coal-bearing strata

Abstract

Abstract Prior to 1960 most discussions on the depositional environment of coal-bearing strata were related to theories of the origin of cyclothems. Work during the 1960s on modern deltas, particularly the Mississippi, led to the interpretation of the classic cyclothem as deltaic in origin, with coals originating in delta-top swamps. During the 1970s and early 1980s, detailed models of depositional environments were developed for coal-bearing strata. Coals have now been described in association with virtually all types of terrestrial depositional environments. There are surprisingly few published examples of facies analyses of coal-bearing strata. Detailed measured sections and meticulous descriptions of sedimentary structures are necessary for geologists to make independent assessments of published environmental interpretations and to make comparisons between various coal-bearing formations. The coal industry could benefit from more predictive facies models, which will require greater integration of coal and clastic facies studies. In most mires close to areas of active clastic deposition, ash content of peat is too high for them to be precursors of coal. Most coal must have originated as peats in raised mires or in low-lying mires well removed from active clastic environments. In the latter case, a significant hiatus in deposition may be represented by the contact of coal with underlying sediments. Subsurface studies, which allow an understanding of the three-dimensional character of coal-bearing strata, may provide clues as to the type of swamp in which a coal was deposited. Recent studies in the Upper Cretaceous of Alberta, for example, provide clear evidence that coals originated in mires well removed from contemporaneous clastic depositional systems.