December 18, 2022
Geology of Europe - words
It has been said that geology is a science of words.
Certainly every science (and probably every field of human endeavor) has its jargon.
In geology however, the name of the game is lots of words.
To be honest, geological science has been cluttered with an excess of jargon.
Jargon has changed over time, and jargon is different from region to region.
For a North American geologist (like myself) to begin studying the geology of Europe
requires mastering a good deal of new technology. Some of it I have encountered
in passing, or by accidental reading in the past of books written by older
european geologists.
This section is a sort of glossary with no intention of being complete.
Since I am a geologist, I won't spend time on terms I am familiar with,
and won't hesitate to define things using terms I am familiar with.
As I encounter words that I don't often see dealing with geology of
southwest north American, I will add them here.
Massif
This is one of those words with a vague meaning that varies from writer to write.
It can refer to a clump of mountains or to a section of the earths crusts that
tends to behave as a unit. It is probably elevated topographically at the present
time as well as being a structural unit of the crust.
Flysch
These are sedimentary rocks that form on the continental side of a mountain belt.
They are typically cycles of sandstone and shale, interpreted to have formed in
deep water by turbidite flows. The sandstones are graywacke like with micas and
other such minerals.
The term was coined by a Swiss geologist Bernhard Studer in 1827 and applied to formations
in the foreland of the Alps. He viewed them as river deposits whereas they are not
seen to be deep marine sediments.
Molasse
This and the word "Flysch" refer as much to a model for mountain building as they do to
actual rock types. In contrast to Flysch, which are deep marine sediments,
Molasse are terrestrial and shallow marine deposits. Both of these terms were coined
by European geologists developing models for the geology of the Alps.
Both flysch and molasse were deposited north of the Alps.
Nappe
A nappe is the upper sheet of a large thrust fault.
Displacement is on the order of several kilometers.
Large recumbent folds may be part of a nappe, but are not essential.
The actual fault plane is called a decollement.
Allochton
A large body of rock that has moved a long ways from its original location.
The key thought is "out of place".
Often an isolated erosional remnant of a thrust sheet, although that might
more properly be called a "klippe".
Listric normal fault
Every English schoolboy knows what a normal fault is, but when does one become "listric".
When it has a curved fault plane that is steep near the surface, and becomes shallow with depth.
How is such a thing recognized? By geophysical work no doubt, we can thank the folks out
hunting for oil with seismic gear for providing evidence for these.
It is important to bear in mind
that rock is more brittle near the surface, but becomes plastic at depth.
Hinterland - Foreland
Now we are definitely talking about orogenic models. In the context of an orogenic belt,
specifically the Alps in Europe, it goes something like this. The "foreland" is the
basin or platform north of the alps, accumulating sediments on the continent.
The "hinterland" is the ocean side of the alps (to the south, i.e. Africa).
All of this is now 100 year old obsolete (but nonetheless interesting) geologic thought
that needs to be reinterpreted in the current context of plate tectonics.
The idea of a forland (or foredeep) seems to still be a term in vogue and refers
to a basic accumulating sediment between an island arc and the contient.
Think of the Japan sea perhaps.
As a side note -- GSA special paper 555 published in 2022 is entitled
"Tectonic Evolution of the Sevier-Laramide Hinterland, Thrust Best, and Foreland,
and Postorogenic Slab Rollback (180-20 Ma). Apparently these terms have not
died out quite yet.
Aulacogen
Now here is something interesting. These are deep graben like basins with thick accumulations
of sedimentary rock. I am even familiar with one -- the Amargosa aulacogen near Death Valley.
I have taken note myself of amazingly thick accumulations of paleozoic limestones in this
area, and now I learn that it is a famous aulacogen! A map I have shows that it trends
more ore less east-west, which is not what you might expect given the north-south trend
of basin and range faulting in the area. On the other hand, the faults involved in the
aulacogen are old, whereas basin and range faulting is very young and related to more or
less "current" tectonic events by comparison.
Terrane
In particular "exotic terrane". If you are gonna have a terrane, let it be exotic.
Think of the coastal ranges of California. A terrane is crustal material that has been
transported by a plate and "accreted" onto the side of a continent.
The material can be pretty exotic for sure and may well include wedges and slices of
material that represent deep oceanic crust and who knows what else.
Many references in the above link look enticing.
Feedback? Questions?
Drop me a line!
Tom's Geology Info / tom@mmto.org