Coal petrology and anomalous coalification of Middle and late pleistocene peat and soft brown coal from the Tengchong Basin, Western Yunnan, People's Republic of China

Abstract

In the Tengchong Basin, Western Yunnan, People's Republic of China, the absolute age of Middle Pleistocene soft brown coal is about 600,000 B.P. and Late Pleistocene peat, part of which had been transformed into soft brown coal, is about 75,000 to 10,000 B.P.. The forma is the world's youngest soft brown coal. Thus, this basin affords an opportunity for studying the transformation of peat to soft brown coal. Our study of early coalification is based on comprehensive methods including the following: coal-petrological techniques, radiometric dating, botanical methods, sporopollen analysis, electron probe (EPMA), scannign electron microscopy (SEM), proximate and ultimate analyses, and pyrolysis-gas chromatography (PY-GC). In view of the fact that the most abundant information on premacerals and macerals is obtained from peat and soft brown coal, we propose a revised classification system of macerals of soft brown coal which is also used for peat. The premacerals and macerals of peat and soft brown coal from the Tengchong Basin are divided into four maceral groups: the huminite, liptinite, liptohuminite (newly proposed) and inertinite maceral groups. The liptohuminite group includes the macerals lying between liptinite and huminite maceral groups. Therefore, in optical properties, chemical composition, technological properties and genesis, these macerals show transitional characteristics. There are also some new interpretations of the genesis of several macerals. For example, sclerotinite-like phlobaphinite appears to have originated from the polyhedral cell tissue of teh endomycorriza of several kinds of plants. The authors subdivide different types of microstructures of some macerals with the scanning electron microscope (SEM) and use both sporo-pollen analysis and microfluorescence photometry to determine some fluorescence regularities in early coalification. The coalification of Tengchong peat and soft brown coal is quite different from normal coalification in two respects: (1) the coalification occurred during a brief period of time, and (2) there are anomalous factors of coalification such as high paleogeothermal gradient introduced by volcanic activity and shallow burial. Thus, we suggest the term “anomalous coalification”.