PERIMETER SEDIMENTATION IN A DEPRESSION SWAMP AS AN ALTERNATIVE TO RAISED MIRES AS A MEANS OF ACCUMULATING LOW-ASH PEAT

Ombrotropic (rain-fed), raised mires have frequently been interpreted as the origin of thick, anomalous, laterally contiguous, low-ash coal deposits. Numerous studies of potential modern analogues have focused on coastal raised mires found in tropical regions. While these mires do produce low-ash peat deposits, not all low-ash peat deposits must necessarily result from raised mires. The Raised Mire model has difficulty explaining extremely thick coal seams, some of which exceed 30 meters.

It is commonly accepted that thick coals were autochthonous peat deposits that accumulated in stagnant, acidic, fresh water, wooded swamps. Many researchers have used the Raised Mire model to explain conditions preventing the influx of sediment that would have increased the ash content of the resulting coals. Several problems arise with this analogue when associating it with thick coal deposits. The stunted growth character of raised mires would require several mires to be stacked atop of one another to create a thick coal deposit. Shrubs are the dominant vegetation of a raised mire. The thickest tertiary coals result from woody tissue (lignin), primarily from trees instead of shrubs.

An alternative model that provides for low-ash, low-sulfur coal is a depression swamp. Depression swamps occupy areas of low elevation surrounded by highlands. Drainage in depression swamps is poor and water movement is slow to nonexistent. Dissolved oxygen in these waters is very low, so complete breakdown of organic matter does not occur as readily as it does in the more oxidized environment of a raised mire. The climax vegetation in a depression swamp is dominated by trees, and plant diversity is limited due to reduced, low-oxygen, low-nutrient conditions and the constant presence of water. Sedimentation in depression swamps is limited to the periphery by simple gravity settling of clastic sediments and the immediate flocculation of the clay platelets when introduced to the acidic swamp water. Depression swamps, when combined with a subsiding regime, would not only create low-ash, low-sulfur coal, but would also provide for the accommodation space needed for thick coal deposits.