A FIELD-BASED CLASSIFICATION FOR COALS AND THEIR UNDERLYING DEPOSITS

This study assessed the transition between coals and underlying siliciclastic sedimentary rocks that contain pedogenic features and are often referred to as underclays. This study considered coal from a pedogenic viewpoint, and found that the underclay-coal sequence is better described as a cumulate, polygenetic soil profile. The combination of the terms cumulate and polygenic is an attempt to emphasize the continuity of pedogenic processes while at the same time acknowledging the soil transition from a mineral soil (typically a protosol or gleysol) to an organic soil (histosol) with intermediate phases and with partial overprinting of the underlying mineral soil. This transition is interpreted as hydrologic succession in the three examples studied: the Cretaceous Menefee Formation (Mesa Verde Group) and Cretaceous Fruitland Formation, both in the San Juan Basin of Colorado, and the Carboniferous Pittsburgh Formation of the Appalachian Basin. A 4-m section in the Menefee Formation contains two cumulate, polygenetic paleosol profiles as well as several intervening protosols and gleysols. Each cumulate polygenetic paleosol consists of 10-25 cm of vitrinite-rich coal overlying massive siliciclastic mudrocks with in situ roots, burrows, wood and leaf fragments, ped structures, slickensides, mottling, and secondary sulfides such as pyrite. A coastal setting is indicated by Teredolites and an overlying transgressive sequence and condensed section. A 6-m section in the Fruitland Formation was interpreted as showing five cumulate polygenetic paleosol profiles. Each consist of coals up to 70 cm thick overlying dark organic-rich mudrocks containing wood and leaf fragments, ped structures, mottling, and secondary sulfides. The 4-m section studied within the Pittsburgh Formation consists of an 80 cm coal bed overlying a transitional zone of interbedded coals and underclays (rooted, mottled, massive mudstone with secondary sulfides), overlying fluvial deposits. The coal is truncated by a transgressive sequence. The presence of transition zones in all of these examples emphasize the continuous nature of pedogenic processes at locations with an evolutionary sequence from a mineral soil to an organic soil.