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Wyodak, Big George, and Felix are multibillion-ton coal deposits in Paleocene and Eocene rocks along the eastern flank of the Powder River Basin; they contain beds of subbituminous coal as much as 61 m (200 ft) thick, and average thicknesses are 30.5, 34, and 7.6 m (100, 113, and 25 ft), respectively. These thick coal deposits are elongate north-south; they split to the east and to the west. Laramide structural movements (subsidence and uplift) provided the necessary tectonic control for establishing and prolonging optimal depositional environments and conditions in early Tertiary time.

During Paleocene and Eocene time, quiescent periods of thick coal deposition on the eastern flank of the subsiding basin were interspersed with episodes of clastic influx from uplifted eastern source areas. Pivotal effects of western (basin) subsidence and eastern (Black Hills) uplifts produced linear fulcrum areas, in dynamic equilibrium, on the eastern flank of the basin. The fulcrum areas were elongate north-south, parallel to the strike of west-tilted paleoslopes: they were centers for swamp development and coal deposition. Optimal conditions prevailed just west of the fulcrum, where subsidence compensated for upward-building peat accumulation. During prolonged periods of thick coal deposition, those ecosystems were shifted west by eastern uplifts and east by western subsidence. The geometries of the Wyodak, Big George, and Felix coal deposits reflect those processes and indicate how thick coal deposits evolved as products of basin evolution.

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