Chronologic occurrence and characteristics of some underclays in North America
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Date
1984
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The Ohio State University
Abstract
Underclays directly underlie Mississippian through middle Tertiary coal beds in North America but are particularly common in the Pennsylvanian System. These light-gray to yellow-brown and variably pigmented clays are characterized by root impressions, concretions, lack of bedding, and slickensides. The origin of these seat rocks is controversial. They have been attributed to a paleosol, a substrate for developing coal swamps, or a transported colloidal weathering product. The mineralogy of Phanerozoic underclays is highly variable with major amounts of kaolinite, quartz, and illite, lesser amounts of mixed-layer clays and chlorite, and trace amounts of pyrite,zircon, tourmaline and rutile. The thickness of underclays also varies, underclays may extend beyond the overlying coalbed and coals are commonly found without underclays. Knowledge of the genesis of underclays will aid in the exploration for refractory clays. In addition, if underclay development can be tied to paleosol formation, then underclays may provide evidence for climatic changes in the Phanerozoic.