The Shale of Peel
Shale
Shale
(German: Tonschiefer) is a very fine-grained, clastic sedimentary rock
formed from mud that is a mix of flakes of clay minerals and tiny,
silt-sized particles of other minerals like quartz and calcite. Shale
is the most common sedimentary rock, it is characterized by its
tendency to split into thin layers, sometimes less than one centimeter
in thickness. This property is called "fissility". Shale is often used
as a synonym for mudrock.
Shale is typically gray to dark grey in color. The fine particles that
compose shale can remain suspended in water long after the larger
particles of sand have been deposited. As a result, shales are
typically deposited in very slow moving water and are often found in
lakes and lagoonal deposits, in river deltas, on floodplains and
offshore below the wave base. Most shales are marine in origin.
Shales and other mudrocks contain roughly 95 % of the organic matter in
all sedimentary rocks. Therefore shale is the most common source rock
for hydrocarbons (petroleum and natural gas). As already described, the
lack of coarse sediments in most shale beds reflect the absence of
strong currents in the waters of the depositional basin. These might
have oxygenated the waters and destroyed organic matter before it could
accumulate. The absence of carbonate rock in shale beds reflects the
absence of organisms that might have secreted carbonate skeletons, also
likely due to an anoxic environment. As a result, about 95 % of organic
matter in sedimentary rocks is found in shales and other mudrocks. The
organic matter in shale is converted over time from the original
proteins, polysaccharides, lipids, and other organic molecules to
kerogen, which at the higher temperatures found at greater dephts of
burial is further converted to graphite and petroleum.
The Shale of Peel
The location near Peel shows an extraordinary outcrop
of shale from the Niarbyl Formation, the only formation of the
so-called Dalby Group, a Silurian lithostratigraphic group on the west
coast of the Isle of Man in the Irish Sea. Therefore this place was
chosen for an Earthcache.
The name "Dalby Group" is derived from the village of Dalby near the
west coast of the island. Together with those of the adjoining Manx
Group, the rocks of the group have also previously been referred to as
the Manx Slate Series. The group comprises wacke sandstones with
siltstones and mudstones in the west of the island. The rocks here come
from a northern continent known as Laurentia of which Scotland and
North America are parts.
The age of the shale here dates back to the so-called Wenlock Epoch. It
is, sometimes referred as the Wenlockian, the second epoch of the
Silurian. The rocks from the Wenlock Epoch are about 433 - 427 millions
of years old.
Your task to log
the
Earthcache:
Answer the following questions via message in English or
German via my geocaching profile:
1.) Take a closer look at the shale you can find at the location. Give
me a description of the rock (color, surface, texture...) in your own
words. How does it feel to touch the rock?
2.) How thick are the thinnest and thickest rock layers you can see
here?
3.) At what approximate angle are the layers to the surface?
4.) In your opinion, were the layers originally deposited this way or
another way? Justify your opinion and explain, what might has happened
here over the Earth's history!
5.) Optional: Post a photo with your log, showing you and/or your GPS
near the location!
After you've sent me the message
with your answers, feel free to log! If there's something wrong,
I'll contact you via message!
Sources:
Blatt, H. & R. J. Tracy (1996): Petrology: Igneous, Sedimentary
and Metamorphic. New York, Basingstoke; Freeman.
en.wikipedia.org
Gradstein, F. M., Ogg, J. G. & A. G. Smith (2004): A Geologic
Time Scale 2004. Cambridge University Press.
manxgeology.com
Potter, P. E., Maynard, J. B. & W. A. Pryor (1980):
Sedimentology of shale: study guide and reference source. New York;
Springer.
webapps.bgs.ac.uk/lexicon
The pictures were taken by the author.
Enjoy the location!
The
most exciting way to learn about the Earth and its processes is to get
into the outdoors and experience it first-hand. Visiting an Earthcache
is a great outdoor activity the whole family can enjoy. An Earthcache
is a special place that people can visit to learn about a unique
geoscience feature or aspect of our Earth. Earthcaches include a set of
educational notes and the details about where to find the location
(latitude and longitude). Visitors to Earthcaches can see how our
planet has been shaped by geological processes, how we manage the
resources and how scientists gather evidence to learn about the Earth.
To find out more click HERE.