The sedimentation of slate

In respect to the type of slate, the tectonic position, the area from where the material steams as well as the physical and chemical conditions during the sedimentation are important.

The main area of the sedimentation of the original material is situated within the hemi -to pelagic areas of the passive continental margin in which, for example, the slates of Galicia (Spain), Thuringia (Germany) and Fair Haven (Vermont) deposited. Another area is the shallow open marine shelf in which the Left Rheinish slates and the slates of Anger and Cornwall deposited. Passive continental margins comprise a large continent which delivers a mature fine grained material, e.g. quartz and clay minerals.

A third and less common area of slate deposition is the back-arc region in which the material of the slates of Wales and the Lake District were deposited. Active continental margins which often comprise a back arc are characterized by volcanic mountains chains and a short transportation. Hence, the weathered material is coarser as those from passive margins and generally contains more feldspar. Because of this, the tectonic environment greatly influences the petrographic composition of slate.

Examples for modern passive continental margins are the east coast of North America or West Coast of Africa. A back-arc situation can be, for example, observed west of Japan.

The sediments of the hemi- to pelagic areas consist of organic matter and very fine grained siliclastic material and is considered to be a result of long time physical and/or chemical weathering processes. After transportation by large river systems and through the air they are deposited at the margin of the oceans. The continental margin (shore, shelf and slope) receives about 90 % of the land derived material and the rest is deposited in the deeper parts of the ocean.

Coming from deltas (e.g. the Mississippian delta) or other river systems the sediments are carried by currents to the continental shelf or further to the continental slope. Material not trapped on the shelves continues on beyond the shelf to the continental slope where very fine grained material accumulates.

In general, sediments on the slope are finer then sediments on the continental shelf. During the transportation and deposition 'gradded bedding' takes place. 'Gradded bedding' means a separation of grains according to their weight and density during deposition. This sedimentary process can occur several times and the sediments show a slight transition from bright siltic to dark pelitic areas.

The sedimentary composition comprises mainly quartz, feldspar and phyllosilicates (micas). In addition, organic matter in form of organic carbon and fossil bituma, carbonate as well as sulfide from iron, zinc and lead can occur.

The continental slope is often cut by submarine canyons in which turbidity currents run. Turbidity currents come from the margin of the shelf and are loaded with muddy or very small grains and thus have a higher density than clear water and will sink down. These currents can reach a high speed (up to 100 miles per hour) and at the end of those canyons the material comes to rest in form of a huge fan covering the ocean floors. This process can also takes place several times leading to a typical pattern: gradded beds can frequently show thinner psammitic intercalation's which can cover vast areas.

In contrast, the sedimentary pattern of the Thuringian slates shows a continuous pelitic sedimentation in a probably pelagic environment (Schubert & Steiner, 1972). Another pattern is exposed within the slates of Moravia (Czech Republic). Here the roofing slates are embedded in a rhythmic deposited graywacke-sandstone-slate sequence.

Slates of a shallow marine sedimentation occur within the Left Rheinish mountains and in Anger. For this environment fine grained sediments, separated by slate units, are common and storm events can lead to gradded or laminated bedding.

The development of back-arc basins is related with volcanic activity producing fine grained ashes. The American geologist Wentworth gave a good short statement about the genesis of this pyroclastic rocks: "They are igneous on the way up and sedimentary on the way down.". The last way is the important one for slate. The volcanic ashes will be sorted during the atmospheric transportation. Afterwards, they will be deposited in ocean basins known as 'pyroclastic rocks'. Under the microscope, for instance, one can see glassy fragments. Volcanic ashes may result in reddish or greenish coloured slates like the slates of the "Borrowdale Volcanic Group" in the Lake District.

Besides these physical processes, three processes control the chemical conditions through the oceanic water column: solution, production and masking (Reading, 1996).
Below some miles water depth the main solution of calcium carbonate occurs. This border is known as lysocline and can be located within the upper parts of aggressive deeper parts of the ocean (Sliter, Be & Berger (1975) in Reading, 1996). Several 100 feet deeper the carbonate compensation depth (CCD) follows. Below the CCD no or almost no carbonate will be deposited. In present oceans the CCD is at depth between 13,000-17,000 ft. (Reineck, 1984).

Under normal conditions planktonic organic material will be destroyed during its downward transportation by oxidation. This process occurs mainly in depth between 1,000-4,000 ft. Here the conditions show a minimum of oxygen and a maximum of CO₂.
Organic matter will only remain if the amount of oxygen is low and the production and supply of organic matter is high. If one of this components - organic matter, oxygen - shows a very high amount it can dominate the other ones. This relation is described as masking by Reading (1996).

In other words, the deeper the sediment is deposited, the less carbonate it will contain; conversely, the amount of organic matter and sulfides increases. This relation is important for the understanding of the nature of a slate in regard to its carbon and carbonate content. The knowledge about the conditions during the sedimentation as well as the regional geological situation can be useful fort the estimation of the general 'carbonate - carbon character' of a slate. Fig. shows this relation.

Slates with a high content of carbon occur in some parts of Thuringia. They are named 'sooty slate' and they are not resistant to weathering. Thus, they are not used as building material. According to the geological literature and own investigations, slates in Galicia were mainly deposited below the CCD.