Dienstag, 18. September 2012

A geological overview of the Ruhr Carboniferous (Westphalium)

This time, my Blog will become a little bit off-topic. I recently took part in a field trip to the northern shores of the Baldeneysee, a lake in the South of Essen, Western Germany. This wasn't a botanical field trip but a geological one, because the rocky slopes and steep faces of the lake give a good overview about the geological history of North-Rhine Westphalia and the “Ruhrgebiet”. Of course, this includes also the history of coal, which was the main mineral resource of this region for a long time.

1) Historical overview

"Geologische Wand" in Essen, NRW

Our story begins about 300 Million years ago in an age, which is called Silesium; The a sub-part of the Carboniferous, which lasted from 359 Million Years to 299 Million years B.C.. The Silesium was again divided into several sections, which depends on the part of the world which was emerged from it. Important for my Article is the Westphalium, which conforms the Pennsylvanium in North America. The Westphalium lasted from 314 to 304 an is the era in which we find ourselves in this article

old headframe of coal mine "Zeche Carl Funke"

The Silesium was also the age of a major event in Earth's history: the collision of the two continents Gondwanaland and Laurasia. On Laurassia, this collision caused the formation of a large mountain range: the Variscan mountains. In Middle Europe, the Variscan mountains were located in the South of Germany, France and the Caucasian. In the north of the mountain there was the sea, and in between them a large marshland and river Delta (similar to the Mississippi estuary).

Flöz "Angelika" - black area (a)

This marshland was populated with many different plants (most of them prehistoric Horsetails) and was flooded again by the ocean. The plants, the rivers and cycle of flooding were responsible for the formation of strata, which we can see today.

2) Geomorphology 

  As I said it before, the marshlands was under the influence of different factors like flooding. Each factor was responsible for the formation of a different geological layer: a stratum. The main factors were:

  • Rivers: The Rivers in the marshlands carry sand as sediment, which was deposited with time. This sand became the Sandstone of today.
  • Roots: The Roots of the plants crossed the sandy soil of the River. Because of this, the sandstone in this area is more porous than rest. In German, this stratum is called Wurzelboden

    Flöz "Dünnebank" - a coal stratum (black areal)
  • Plants: Dead parts of the Plants fell into the river and become peat, which is a large reservoir for carbon. Over time, the peat has developed into coal and formed a coal stratum, which is called Flöz in German. This process is not uniform, why the proportion of carbon within the coal (and thus its value as energy source) can vary. The different types of coal are:

German name
US name
Carbon (in %)
Braunkohle (lignite)
65 - 75

Steinkohle (bituminous coal)
Sub-bituminous coal
75 - 81
Medium volatile bituminous coal
81 - 85
Medium volatile bituminous coal
85 – 87,5
Medium volatile bituminous coal
87,5 – 89,5
Low volatile bitumiuous coal
89,5 - 90
90,5 – 91,5
> 91,5

  • Ocean: Sea levels fluctuated during the time, which was mainly due to the melting of the Ice Age glaciers in the north. In warm periods, sea levels roses and flooded the prehistoric forest. The main sediment, which brought the sea, was clay. The Clay overlaid the peak, which was pressed to coal by this pressure later. Because of this, there is often a stratum of clay above coal layer

    .a clay Stratum above the Flöz
This sequence of Sand, “Wurzelboden”, coal and clay is typical for the soil in the “Ruhrgebiet”. But it should be said that this is not always the case. Sometimes, some strata are missing completely or are weak, A weak layer of coal for example, was formed during a period, where was not so many peat in the prehistoric forest.

Sandstone dosen't have to be yellowish.
He's more black, the yellow color is due to weathering

Moreover, the sequence of the strata is rarely as uniform as in the diagram. Responsible for this are geological processes, dragging or reorder layers. On the Baldeneysee, we are able to take a closer look at two of this processes: saddles and thrust fault.

3) Saddles

"Nöckersberger Saddle" - right side

Saddles are made where the rock is folded by tectonic processes, similar to folding paper. At one point, the rock rises, while it sinks on a different position (forming a hollow). This happens in different dimensions; some saddles are only small folds, while others are large ridges. It isn't also unusual, that minor saddles are existing in major hollows. In our case for example, the “Nöckersberger Saddle” is a minor saddle within the large “Bochumer hollow”.

"Nöckersberger Saddle" - left side

It's very easy to recognize such a saddle; especially if it is a smaller one. The strata being arranged obliquely. They fall on one side from the top left to bottom right and left from the upper right to bottom left.

4) Thrust fault

On the former area of the coal mine “Zeche Carl Funke” at the north shore of the Baldeney See you can find a Thrust fault. A Thrust fault (German: “Überschiebung”) is a break in Earth's crust; caused by tectonic movements, which have lifted one part of the rock upwards. By the associated forces, the rock finally broke and has been moved slightly upward. The unaffected rock slid under the shifted upwards rock.

Thrust-fault "Sutan" - right (older) stratum (c in my drawing)

The breaking point may be located quite easily. The rock is highly compressed in this area and the different strata cannot be distinguished with the eye.

Thrust-fault "Sutan" - left (younger) stratum (b in my drawing)

For this reason, in a thrust fault are older layers are overlying younger layer, what can be a little bit confusing for geological researches, because one part of a Flöz (coal strata) can be found several meters over the other part.

Thrust-fault "Sutan" - breaking point.

5) other phenomena

Saddles, hollows and Thrusts faults are large phenomenons, but they are no the only one in our areal. There are also some smaller, local features.

  • Fossils: in the different strata you can find many fossils of plants of the prehistoric marshland. This plants had a fluted stem, which leaves an impression in the rock. Most of them were prehistoric horsetails (Equisetum) or trees from the extinct genus Sigillaria and other genera.
petrified driftwood

fossilized tree bark of a Siggillaria
fossilized tree bark
  • Limonites: Limonites are solid chunks with a high content of oxidized iron. Thereby, they are much heavier than other rocks. The reason for their high iron content can vary greatly. For example, a dead animal can be surrounded by the peat, which leaded to an iron deposits. For this reason, Limonites can be very interesting for paleontologist, because they can contain some remains of prehistoric organisms. It's similar to a pinata. However, they can be dangerous for miners, because they can burst out during the digging and kill the workers.
  • Liesegang Rings: Liesegang rings also consist of oxidized iron, which was leached from higher regions, and then failed in an annular pattern. The exact cause is unknown but it is a typical phenomenon of iron.
Liesegang Rings
  • Tuff: Tuff is a volcanic rock, which was spitted out by volcanoes during their eruption. It has a reddish color and is very porous. In our areal, there was one deposit in a Flöz called “Angelika”. Tuff can be very useful for the historical classification of shifted strata. Its deposit acts as a red guideline. Strata with Tuff must been formed at the same time, because only at this time, a volcano erupted.

     Tuff (a) - the thin line, witch looks like a root

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