PEAT FORMATION: INITIAL PHASE OF COAL SYSTEMS

Climate and the degree of weathering of the catchment area are first order controls on the quality of organic matter accumulation and resultant coal quality. Producing bituminous coal deposits average ~18 percent ash, are laterally extensive, and are typically >0.6 m thick. The original peat precursors to coal, therefore, were <18 percent ash, laterally extensive, and thick. Even though modern terrestrial organic-rich deposits can be found in a range of temperature regimes and a variety of landscapes (maritime hillsides, boreal glacial terrains, and cold temperate to tropical deltas and coastal plains), few of these deposits consist of low-ash peat with sufficient lateral extent and thickness to become economic coal.

Water quality controls the mineral matter content of organic-rich deposits. Water quality in modern organic-rich deposits is a function of (a) the climate (rainfall and temperature regimes), which determines the balance of precipitation/evapotranspiration and recharge/discharge, and (b) the solute load from the water source. Oligotrophic conditions (low nutrient availability as a result of low solutes concentration) and the absence of sediment transport into the organic deposits are both critical to the formation of low-ash peat. These conditions result when water sources to the peat are exclusively precipitation and/or water recharge from highly leached (weathered) terrains. Low nutrient concentrations in peat pore-water may restrict plant growth, but more importantly, microbial degradation of the organic matter is suppressed and low-ash peat is the result. In contrast, mesotrophic (moderate nutrient content) peat-forming environments occur where a higher proportion of the water is from surface water runoff and ground water discharge from moderately leached terrains. As a result, microbial organic matter degradation and sulfur fixation tend to be greater than in oligotrophic deposits. Mesotrophic peat deposits are precursors to high-ash, high-sulfur coal. Eutrophic (high nutrient content) organic-rich deposits receive high levels of dissolved ions from surface water, ground water, or sea water and often receive significant clastic influx. Eutrophic organic-rich deposits are likely to result in carbonaceous mudstones but rarely if ever result in economic coal deposits.