ABSTRACT

The Warrior coal field of Alabama is stratigraphically in the upper part of the Lower Pennsylvanian Pottsville Formation and structurally in the eastern part of the Black Warrior foreland basin. In the Warrior coal field, the Pottsville Formation has been divided into a lower non-coal-productive sandstone-rich sequence and an upper coal-productive sequence that contains less sandstone. The productive coal beds extend southwestward from the mining area downdip into the deeper part of the Black Warrior structural basin. Because the deep part of the basin is beyond the limits of conventional coal exploration, study of the stratigraphy of coal beds must rely on data from petroleum wells. For example, the density log as run in petroleum wells is sensitive to the presence of coal beds more than a few inches thick, but evidently does not provide a reliable measure of the thickness of individual coal beds. Relative abundance of coal can be stated in terms of numbers of beds, but because of the limitations of the available data, thicknesses of coals presently are not accurately determined. Distribution of coal beds in the Black Warrior basin reflects controls by depositional environments from delta plain to barrier islands; the various depositional environments shifted across the Black Warrior basin through time.

The lower sandstone-rich coal-poor part of the Pottsville has been interpreted as barrier sediments in the mining area. To the southwest in the deeper Black Warrior basin, coal beds are more numerous within the sandstone-dominated sequence. The area of most numerous coals trends northwestward parallel with the trend of the barrier islands, and evidently is in a back-barrier location.

The coal-productive upper Pottsville is informally divided into "coal groups" (in ascending order, Black Creek, Mary Lee, Pratt, Cobb, Gwin, Utley, and Brookwood), each of which includes several coal beds. The Black Creek, Mary Lee, and Utley coal groups are associated with northeast-trending delta-distributary sandstones. The areas of most numerous coals also trend northeastward and are laterally adjacent to relatively thick distributary sandstones, suggesting coal accumulation in backswamp environments. The most numerous coals in the Pratt coal group are in an area that trends northwestward parallel with and southwest of a northwest-trending linear sandstone, suggesting coal accumulation in a back-barrier environment. Equivalents of the Cobb, Gwin, and Brookwood coal groups contain little coal in the deep part of the Black Warrior basin.