Paleocene Coal Basins
Paleocene Coal Basins
The tectonic history of the Rocky Mountains and Northern Great Plains regions between 65 million and 55 million years ago provides both the backdrop and important controls for accumulation of high-quality coals. The Paleocene was a time of significant mountain building and basin develop-ment. The basins, which progressively developed from southwest to northeast during early, middle, and late Paleocene, became sites of coal accumulation.
The Wind River basin provides a good example of a typical tectonically controlled basin. Prior to 70 million years ago, sedimentation was relatively uniform across the future basin, and there was little if any uplifting or downwarping. In early Paleocene time, about 65 million years ago, the north side of the future basin began to accumulate sediments at a faster rate than the south side, because of uplifting of the Granite Mountains to the south. At about this time, Cretaceous rocks were being eroded and redeposited along the south side of the future basin. By middle Paleocene time, the Wind River basin began accumulating a thick succession of lake sediments on its north side as the basin was rapidly depressed due to crustal loading from thrust faulting along its northern margin. At this time the Owl Creek Mountains began to form. These tectonic events were similar to present-day earthquake-faulting events in and near the Los Angeles, California, area. Meanwhile, on the south side of the basin, rivers were accumulating sediments from the erosion of much older rocks.
Later, during the late Paleocene, the Wind River basin accumulated an even thicker wedge of lake sediments, but during this phase of development, the lake bordered the thrust-faulted margin of the growing Owl Creek Mountains. Mud, silt, and sand derived from the erosion of older rocks from these mountains accumulated on the north side, while on the south side of the basin, river deposits reflected increasingly deep erosion of older sedimentary and granitic rocks from the Granite Mountains. This southern succession of sediments indicates the "unroofing" of the crust as the region to the south was uplifted. On the north side of the basin, very rapid uplifting of the Owl Creek Mountains began about 10 million years later than the uplifting on the south side of the basin.
Similar tectonic settings in the Bighorn and Hanna basins provided environments of peat accumulation and resulting coal deposits, namely rapid subsidence caused by thrust faulting and associated crustal loading along basin margins.