Peat/coal type and depositional environment—are they related?

Abstract

Four bogs in New Zealand were investigated in order to understand the relationship between peat type and depositional environment. This relationship is important because peat type translates into coal type, and coal types can ultimately be used to infer how and under what conditions the original peat bog formed. In our study, no correlation was found between peat type and depositional environment in the four bogs examined. Moreover, no correlation was found between peat type and either tectonic setting or climate. Water table level and degree of fluctuation are the only parameters which seem to have a good causative relationship on peat type. The bogs, Whangamarino, Moanatuatua and Kopouatai in the North Island and Sponge Swamp in the South Island, all have different depositional settings ranging from coastal plain, to fluvial-meandering and fluvial-braided river floodplain. We found no diagnostic peat types that would allow those different environments to be distinguished from studies of the peat. Data from other tropical and temperate climate peat bogs also support our contention that no diagnostic peat types can distinguish particular depositional settings. However, the level and variability of water table does have a correlation, one that is also seen in bogs elsewhere. From our observations, we infer that the validity of using maceral ratios (directly related to coal type) to indicate depositional environment should be questioned. At best, coal type only represents to what degree the original plant components were degraded, but not how they were degraded. To infer other parameters such as depositional environment, tectonic setting or climate, other data (e.g. distribution of surrounding sediment types, palynology, etc.) must be collected and assessed.