1.4 Coal-forming environments in the geological record
Figure 5 simplifies a typical vertical succession of sedimentary rocks found in many coalfields. The sequence from the base of the section upwards reveals the following:
When a mire starts to form, the first plants take root in underlying clays or sands that form the soil. Their rootlets are preserved beneath the coal seam as black carbonaceous markings, and the fossil soil in which they are found is called a seatearth (Figure 6). The presence of rootlets shows that the peat formed in situ, rather than being transported into the area by water currents.
The coal seam itself consists largely of plant material with small but variable amounts of mud. The seam itself can vary in thickness from a few millimetres to tens of metres.
Immediately above the coal seam there may occasionally be a mudstone containing rare but distinctive marine fossils (for example, brachiopods and cephalopods). This is unusual because most of the other fossil remains associated with coal-bearing sequences are normally of freshwater or land-based species. Where present, marine beds suggest that peat formation ceased as the sea flooded the area.
The muddy sediments overlying the coal seam pass upwards first into siltstones and then into sandstones. This sequence of rocks usually gets steadily coarser upwards, but variations are common. Fossils within this part of the sequence invariably indicate non-marine conditions. The sandstones and siltstones may show sedimentary features that indicate action of waves and currents in relatively shallow water. Some sandstones show evidence of river channels.
The sandstones often pass upwards into seatearths and another coal layer.
These vertical successions are thought to typify sediment deposition in certain areas of deltaic and coastal barrier environments similar to those shown in Figure 2. They are interpreted as representing initially flat low-lying sandy (or muddy) areas covered by vast freshwater lakes containing a variety of land plants growing in mires. Mire formation is then terminated by flooding of these areas, either by adjacent rivers bursting their banks, or by the sea flooding into the area. In both cases, these submerged areas would have filled up with mud, silt, and sand or, as depicted in Figure 5, mud grading into sand. Exactly what type of sediment overlies the mire depends on the source of the sediment (i.e. river or sea) and the processes involved in depositing it. Individual sedimentary successions can vary considerably, both laterally and vertically from one succession to the next. Eventually, this sediment pile will fill the submerged area enabling recolonization by plants and the establishment of a new mire.
This cycle of mire-flooding-sediment infill-mire, is repeated time and time again, which explains why there may be many seams stacked vertically in a coalfield. The sedimentary succession in coalfields can reach hundreds or even thousands of metres in thickness, even though all the sediments were deposited in shallow water.
What does this suggest about the stability of the land surface on which such sediments accumulated?
Great thicknesses of coal-bearing rocks are clear evidence that the sedimentary basins in which they formed were subsiding. The compaction of peat and mud (sand is relatively less compactable) would have been contributory factors, but cannot alone account for such sediment thicknesses.
Estimates of the current rate of subsidence for the Ganges and Nile deltas vary between 1 and 5 mm yr−1. Using a rate of 1 mm yr−1, calculate the time needed to deposit sufficient peat to form a 3 m coal seam. What assumptions does this calculation make?
As 10 m of peat are required to produce 1 m of coal (Section 1.2), a 30 m thickness of peat would eventually produce a 3 m coal seam.
A subsidence rate of 1 mm yr−1 is equivalent to 10−3 m yr−1.
So, 30 m of peat would be deposited in
This calculation assumes that neither sea-level nor the delta subsidence rate change during this time. It also assumes that all 30 m of peat are deposited before any further compaction occurs. In reality, the earliest formed peat will compact, making the calculated time span an overestimate.
Subsidence would not always have been uniform, so whilst mires existed in one part of the delta, sands, silts or muds were burying mires elsewhere. This variability results in seams that split laterally into two or more beds separated by bands of sandstone or carbonaceous shale, or that converge with an adjacent seam to become a single, thick one.
How do you think the vertical succession observed within raised mires might differ from the succession in Figure 5?
Most importantly, as raised mires can form far inland, flooding is less likely and so the cyclicity of mire-flooding-sediment infill-mire will not be seen. Instead, unimpeded development of the mire will mean that individual seams of greater thickness will develop. With flooding less likely, these coal seams will be less contaminated by sediment in comparison with deltaic and coastal barrier-formed coals.
The downside to coals formed in inland raised mires is that subsidence rates are not likely to be as high, and so total coalfield thicknesses are in general unlikely to be as great.