Source-to-sink system for peat accumulation in marginal basins of the South China Sea with the Qiongdongnan Basin as an example

Abstract

In the coal-bearing deposits of the Qiongdongnan Basin in the South China Sea, coal seams are for the most part extremely thin, typically less than 0.5 m. Coal streaks are commonly observed but are not concentrated. In addition, the ash content of the coal components is very high, indicating that the geological conditions of peat accumulation and coal formation were very unstable. For these reasons, it can be speculated that at the time of development, peat mires were relatively poor, in terms of inheritance and continuity, and were commonly not preserved. Coal measures, coal seams (in situ peat accumulation over long distances) and dispersed terrigenous sedimentary organic matter, which together constitute a type of complex coal source-to-sink system found in basin-marginal basins, include three aspects. (1) The material is homologous: whether autochthonous accumulation, allochthonous accumulation or dispersed organic matter is present, the total organic matter source is homologous, i.e. suitable for development and plant growth. (2) The process of organic matter transport (channel) is divided into three forms: vertical accumulation in situ, transport to relatively fixed areas by flowing water, and transport by other mechanisms with long-distance dispersion and drift. (3) There are three types of aggregation forms: vertical accumulation in situ, transport from flowing water to relatively fixed areas away from where plants grow and aggregate, and deposition in deep-water basins fixed with other substances; the latter type requires a period of time for long-distance dispersion and drift, including channel flow and surface flow. KEY POINTS From plant growth to peat formation, the ‘source–channel–sink system’ of coal formation is ‘sensitivity to sedimentary environments and complexity of distribution’. Convergence of coal-series, coal seam (in situ peat aggregation and allochthonous aggregation), and terrestrial dispersed organic matter deposition, constitute a complex coal-measure source–sink system. Source includes substance homology, channels include in situ vertical accumulation and dispersion drift in short and long distances, with convergence forms including in situ vertical accretion, remote sink and dispersion in deep-water basin.