Paleocene Coal Basins
Paleocene Coal Basins
The Clean Air Actwith its 1990 amendments placed a renewed emphasis on locating coals that are low in hazardous air pollutants (HAPs). The list of HAPs includes antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel, selenium, and uranium. Paleocene coals of the Rocky Mountains and Northern Great Plains regions are well known for their relatively low sulfur and ash contents, but detailed studies of the distribution of HAPs have only recently been undertaken.
Trends in contents of ash, sulfur, and HAPs and coal rank appear to be influenced by tectonic activity adjacent to and within peat-accumulating basins, the types of sediments in the peat, and drainage patterns of the basins. Depending on the tectonic setting at the time of peat accumulation and coalification, the basins are classified as active and passive.
Active basins, which include the Hanna, North Park, and Wind River basins, experienced rapid subsidence and changing drainage patterns during their evolution. Drainage changed from through-flowing rivers in open basins in early Paleocene to short-headed rivers in closed or partially closed basins later in the Paleocene. Lakes and primarily low-lying mires formed in these active basins. Intense tectonic activity associated with these basins increased the amount of reworked sediments from older rocks which entered the low-lying mires, therefore increasing concentrations of ash, sulfur, and some HAPs in the peats. The subsequent increase in depth of burial of the peats due to basin subsidence, with attendant increase in the temperature, elevated the rank of the resulting coals.
Passive basins, which include the Williston, Powder River, and Bull Mountain basins remained open throughout the Paleocene; rivers flowed through the basins during times of peat accumulation. Rivers established long-term drainage patterns confined by long-standing domed mires between river channels. This domed nature of these mires discouraged debris-laden water from intermixing with adjacent peat, resulting in lower concentrations of ash, sulfur, and HAPs than in the low-lying mires in the active basins. Less intense tectonic activity in the passive basins resulted in the shallower depth of burial of the peat, lower temperature, and therefore lower coal rank than resulting coals in the active basins.