Tertiary coal beds in the Powder River basin are the nation's largest resource of low-sulfur subbituminous coal. In northeastern Wyoming the north-trending depositional axis of the asymmetric, 100-km-wide basin is within 20 km of the west margin. Interlocking lines of cross sections prepared from geophysical logs of oil and gas wells and coal exploratory holes and set to a sea level datum, outline patterns, locations, and trends of thick coal deposition. Coal beds on the east flank of the basin dip west about 1/2° and tend to merge eastward. Five Paleocene (Fort Union) coals merge along the eastern margin to form a north-trending, 35-m-thick deposit known as the Wyodak coal. Near the depositional axis, five or more Eocene (Wasatch) coals merge to form a north-trendin , 60-m-thick deposit known as the Lake deSmet coal. Across the intervening area two or more succeeding coals merge locally on broad depressed subsurfaces to form north-trending belts of thick coal, and succeeding belts of thick coal are offset westward toward the depositional axis. Lines of cross sections oriented north-south roughly parallel to the depositional strike provide stratigraphic control on the coal. Maximum structural relief (about 400 m) of coal beds on the east flank relates to basin subsidence during deposition of coal-bearing rocks when rates of subsidence progressively increased westward. North-trending patterns of thick coal deposition across paleoslopes are associated with connected areas for which optimum paleoenvironments were stable for extended periods of time and ptimum rates of subsidence were maintained.

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