Coal bed thickness, lateral continuity, maceral content, ash content, and sulfur content are largely determined by the conditions that controlled the mire where the peat originally formed. Major factors are the type of mire, type and rate of vegetation growth, rate and degree of humification, rate of base-level change, and rate of clastic sediment input. These factors are influenced more by allogenic controls (tectonism and climate) than by autogenic controls (environment of deposition).
When plotted on paleogeographic maps, the global distribution of Cretaceous coals shows the importance of tectonism and climate in determining the location of coals. Cretaceous plate tectonics was dominated by the breakup of Pangea, but remarkably few coals accumulated either in associated rift basins or passive continental margins. By far the largest volumes of Cretaceous coal resources are located in the foreland basins that stretched along the western margin of the Western Interior seaway of North America. These basins were created by thrusting and crustal loading within the Western Cordillera, which began in the Late Jurassic in the Western Canada Sedimentary Basin, in the Aptian(?)/Albian on the North Slope of Alaska, and in the latest Albian in the western US.
Cretaceous coals of the world are distributed in a similar fashion to modern peats, and formed in mires in coastal regions, particularly near the equator where rainfall was presumably higher, or in high midlatitudes, where precipitation may have been relatively high and evaporation low. The major exception are the coals of the Western Interior of North America. Conditions that may have favored peat accumulation in that region are (1) the maintenance of high groundwater tables due to basin subsidence; (2) convection over a warm seaway that may have resulted in sufficient rainfall to permit peat accumulation along the coast; or (3) the development, if the ancestral Rockies were a major topographic feature, of a high-altitude low-pressure cell over the high terrain that created enough rainfall in the summer to maintain high groundwater levels.
