Brown coal or lignite, as it is also known, is a grade of coal intermediate between coal and peat. It is yellowish brown in color with a woody texture (fig1). Brown coal contains more moisture and less energy per kilogram than more mature coals. It also tends to dry and crumble when exposed to air. Brown coal is a second tier fuel that produces significant amounts of smoke. Large deposits are found in the United States, Canada, Germany, and elsewhere, chiefly in Tertiary formations.
German Brown coal localities are found from Koln in the west of Germany right across eastwards. The eastern material is not so profitable to mine, and many pits have closed. Cumulative brown coal production has been 187 million tons to 1996. Somewhere between 80-90% is used in electricity production, and power stations are a normal feature of the skyline in brown coal areas.
A large mine may have a life of 50 years, but it "moves" with time. There are problems of relocating and resettling people, land reclamation, management of river courses and water tables. For example, one pit required the relocation of 8,000 people. In this case, 70% of them wanted to retain their community and it was necessary to move the whole village.
Working pits target coal seams of 50-75 metres thickness. These are located at the central parts of large Tertiary basins. The map of the limits of 25 metre thick coal shows it to be very extensive. Normally, mining goes down to about 300metres depth, but it may occasionally reach 400 metres. The whole area has experienced deformation and NW/SE faulting. There are gentle dips. The faults post-date the coal, but may be contemporaneous with the overlying sands. In western Germany, the Tertiary is up to 800 metres thick. Terrestrial sediments are to the southeast, and marine to the north. There is evidence of marine transgression - causing interfingering.
The largest faults are about 600 metres throw. Most are much smaller than this, creating a stepped structure. The area is regarded as one experiencing ongoing extension of the order of a few mm per year. However, this is overridden by subsidence due to water extraction (about 5 metres of movement in total).
Lithostratigraphical work has been undertaken, utilising the clays for correlation purposes. In the coal layers, there are additional biostratigraphical markers. The coals are mainly Miocene, with some Pliocene.
Special machines have been built to extract the coal and the surrounding sediments. Transportation within the mine is by conveyor belts. The machines can be 220 metres long and 50 metres high. The cutting wheel is about 20 metres diameter.
THE MINE
The Tagebau Hambach pit, to the west of Essen (fig 2), was described by our guide as the "biggest hole in the world in soft rock". On the drive to the pit, we passed a large man-made hill on the western margin, 200m high and covered in trees. This hill is said to contain 1 billion cubic metres of material: the tip resulting from the early years of operating the mine.
Work commenced at the mine in 1978 but coal was not reached until 1984. The initial spend was 6 billion marks. The pit is currently about 6 km wide and 320 metres deep. 1600 people work today in the mine, although a few years ago, the figure was 6,000 people. The decline has come about because of the reduction in the world price of coal. At present rates of extraction, the mine will be workable until 2040. Before then, three villages will have to be relocated, plus a major railway line and a trunk road.
There are seven places of extraction at present - each with its giant machine. There are 95 km of conveyor belts . The central part conceals a Miocene coal that is 100 m thick. This is too deep to mine. Nevertheless, above this there are several exposed seams: each about 50 metres thick with sands between (fig. 3). Some clay horizons are thought to have been produced by rivers. In places, a Miocene fauna has been found in the clays: amphibians, reptiles, shark teeth, etc. This is the most northerly Miocene deposit known in Europe.
Each 50 metre thick coal was said, by our guide, to have been formed in an accumulating peat bog. Timescales have been estimated at 8 million years, but our guide thought this was too long and preferred a figure of 4 million years. In the deposits generally, over 300 plant species have been identified. Trunks of conifers and deciduous trees are present (not homogeneously) but in this pit they did not exceed 2 metres diameter. They were thus not mature trees. Our guide linked this to the peaty environment and its lack of potential for long term growth. A 3-to-1 compaction was thought to have taken place.
Examination of the coal showed a peaty matrix with fragmented bits of wood. The matrix was dominant. Wood was present, but quite a few surfaces lacked visible wood. The wood was in fragments that evidenced brittle fracture and little decay (fig. 4). In places, bedding was well developed, and nowhere was there signs of internal deformation. The mineral content of the coal is 2-3%, typical of the mineral content of plants. My own search failed to record any roots, and only one upright trunk - devoid of any root system (fig. 5).
Above this seam were sands (said to be marine) and then another coal. Every thing looked well bedded. The top of the upper coal was uncovered by our guide to reveal burrows. They are interpreted as the burrows of a non-marine mussel (Teredina) which were subsequently preserved as pyritised infills. Our guide first found these burrows in 1992. We were informed that burrows have also been found in wood (as well as the peat) in the top of this coal seam. The sand used to be thought of as marine - but are now reinterpreted as non-marine. We were not able to examine them closely, but it was apparent that cross-bedding was a feature of these 70 metre thick sand deposits. Some clay horizons within the sand are said to contain plant material.
We looked also at the overlying Pliocene sediments. In one locality, our guide drew our attention to fossil leaves incorporated into the sands. In addition, there were numerous examples of wooden branches and some trunks of trees. They were all in a highly fractured state, and none of the trunks had roots.
The uppermost Pliocene horizon is a thick clay, interpreted as a river deposit. A variety of fossil materials have been reported: from molluscs to mammals (including primates).
DISCUSSION
The most interesting questions concern whether this brown coal is in situ (autochthonous) or washed in (allochthonous). Relevant data, not all of which has been introduced above, with possible interpretations, are below. It is necessary to note that the depositional model for the allochthonous coal involves unusual and violent processes, whereas the depositional model for the autochthonous coal involves sustained and cyclical conditions over millions of years.
Criterion |
Allochthonous |
Autochthonous |
Bedded structures |
Water deposited |
?? |
Fragmented, not decayed, wood |
Catastrophism |
?? |
Lack of rooted trunks |
Not in situ |
?? |
Immature trees |
Destruction of young ecosystem |
Ecosystem not able to sustain large trees |
300 plant species |
Immature ecosystems? |
Changing flora with time |
High proportion of peaty matrix |
Degradation during transport |
?? |
Relative purity (2-3% mineral content) |
Water sorting of materials |
No insurges of sediment despite subsidence |
Great thickness |
Deep sedimentary basin and subsidence |
Subsidence and growth in situ |
Fossil soils |
Alleged soils are rare and may not be soils |
In situ growth |
Bark stripping |
Catastrophism |
Decay |
Absence of volcanics |
If volcanism caused the destruction, it was far away |
Mt St Helens is not a good analogy |
Interfingering of coal and marine sands |
Both are water lain and contemporaneous |
?? |
Siderite occurrences in certain areas |
Percolating groundwaters |
Percolating groundwaters |
Oriented wood in places |
Water laid |
Not a general pattern - a chance orientation? |
Fossil content of sands/muds |
Potential environmental relevance |
Potential environmental relevance |
From this comparison, the ONLY strong argument for in situ growth is that there are seat earths. Despite our strong request to our guide to show us an example, we did not see one. The reason was: shortage of time. A 50 metre thick coal should have numerous horizons within it where renewed growth would have occurred. However, there was no hint of such horizons. I looked carefully for root systems - but found nothing.
The arguments for the coal having been washed in, by violent processes, seems overwhelming. It remains an open question what the mechanisms were for this catastrophic model.
