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The secret of sedimentary rocks
10/26/10
Historically much more embedded in Anglo-Saxon countries, the study of sedimentary rocks and the processes
which create them were only rarely the subject of publications in French. Frédéric Boulvain, a professor at the
University of Liège, has just filled this gap by publishing, through Ellipses, a complete updating of his course
on 'Sedimentary Petrology' (1). Primarily addressed, obviously, to students who are tackling Earth Sciences
in their curricula, but also, he writes, 'to the curious with some general scientific baggage and who wish to
understand what sedimentary rocks tell us about the past and the present of our planet.'
As, let us remind ourselves, sedimentary rocks
cover…90% of the planet's surface. And it is they which enclose the majority of the raw materials used by
humankind. Beginning with coal, petrol or…water. Moreover, sedimentary rocks teach us enormously about
the history of the planet. 'Petrology is the study of rocks,' explains Frédéric Boulvain. 'What is important is
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their genesis: we wonder how they have been formed, and in asking ourselves this question, we open onto
something essential: the evolution of the Earth. As rocks are the witnesses to the environment in which it
was formed. And that is a piece of the jigsaw puzzle of understanding the planet. Thanks to rocks we can
reconstruct the landscapes of the past, as well as climate conditions and their evolutions.'
In sedimentary petrology that can go very far indeed: the rocks retain visible traces of waves, currents or living
organisms of course. 'I do a lot of actualism with my students. In other words we go to see the ripples of waves
on the beaches and then show equivalents which have been in the rocks for 325 millions of years! Actualism is
the basis of sedimentology. But it doesn't always work! The physical processes at work on the planet change
little over the course of time, but when life intervenes, it becomes a lot more complicated. Analogies are more
difficult as the ancient organisms are no longer there.'
(1) Pétrologie sédimentaire, Des roches aux processus, Ellipses éditeur. Collection « Technosup », Paris, 2010. 259 p.
But let us not go too quickly: we have yet to say what a 'sedimentary' rock is! And to do that the easiest thing
is to start from the well-known 'geological cycle.' Let's start from a rock. At the beginning it is found buried
under the earth, either low down or higher up. Through the erosion of what is above it, it will gradually be
brought to the surface. There, due to atmospheric conditions, pressure and temperature, it erodes. Grains
and chemical elements are stripped from it, transported and deposited. These are the sediments. Other
sediments accumulate above the first and, progressively, the grains descend. The temperature and the
pressure increases and the elements fuse into rocks. That is diagenesis, which forms the sedimentary rock with
which we are concerned. If the pressure and temperature increase once again, we obtain a metamorphic rock,
cooked, in fact. Yet higher temperatures and the rock becomes magma, which will end up crystallising and in
its turn climb to the surface. And the whole process can start again. Thus, when you are looking at a grain of
sand at the North Sea, it might have undergone two complete geological cycles. It could be a billion years old!'
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That definitely makes one look at the sand in a different way. This cycle, quickly summarised here, is the subject
of the first part of the book, concerned with processes. In the second part, sedimentary rocks themselves are
studied, as well as their 'interpretation,' in other words their formation modes and conditions. In this second part
the 'temporal' aspect is also examined. 'When we study a rock, we can estimate in what type of environment
it was formed, but it must not be forgotten that the rocks are found in superimposed layers and that they thus
give access to the evolution of environments over the course of time. This factor also needs to be integrated.'
Erosion, transport, deposit
We will direct those who are interested to the second part of the book. But the first, and these processes of
the formation of the sedimentary rocks which surround us, doubtless merit a little more illustration.
The first phenomenon: erosion. 'When a rock finds itself on the surface of the planet, it is not necessarily in
balance with the conditions of its formation,' explains Frédéric Boulvain. 'It is no longer in its natural milieu. It
will thus little by little break up. Through physical processes such as wind, frost, or temperature differences.
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Through decompression, quite simply, which creates joints in the rocks. Or through biological and chemical
processes, such as the action of roots or the dissolution of limestone rock, which creates the well-known karstic
phenomena: caves, stalactites and stalagmites, etc. This breaking up reduces the rock to its most resistant
ions and 'grains,' the well known quartz, notably.
These grains will subsequently be transported, and this is the second phenomenon. Here as well, the diversity
of the modes of transport is great. Wind, water, and ice can all transport the grains. Simple gravity as well,
which creates landslides. 'That is the only mode of sediment transport available on the moon,' stresses Frédéric
Boulvain. Other less known 'transporters' are gravitational flows, movements also due to gravity but in which
a fluid intervenes. Mud flows, for example. Even less well known and yet responsible for a good part of the
process of the depositing of oceanic sediments are the turbidity currents. A process of undersea transport
(following en earthquake for example) in which the grains, in turbulent suspension, can travel hundreds of
kilometres, forming sediments known as 'turbidites.' They were discovered for the first time in 1929 after they
caused the rupture of undersea telegraph cables between the United States and Great Britain.
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The third phenomenon is depositing: 'logically, it occurs when the transporting agent disappears, as in the
case of ice, which melts, or when it slows down. In the latter case, the deposit is carried out depending on
sedimentary structure: in ripples, in dunes, etc. Certain of these structures are characteristic of a very particular
environment. Granulometry of the sediments, their size, etc., also instructs us about the nature and the speed
of the transporting agent.'
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The fourth and final phenomenon: the post-sedimentary evolution which will transform the sediment into
rock. In mixes (to summarise briefly) compaction phenomena under the weight of the following sediments
which consists of a reduction, through physical or chemical methods, of their original thickness and
physico-biochemical modifications (evolution of organic matter, precipitation of materials between the grains,
cementation, dissolution, etc.) which form the diagenesis. 'It might be an extraordinarily long phenomenon,
which takes millions of years, as is the case for sand and clay,' stresses Frédéric Boulvain.'But, and this is
a recent observation, that can also be very quick. On beaches in tropical zones, we have thus found rock
formations which contained…Coca-Cola bottles! The sandstones of tropical beaches are thus as hard as
concrete even if they have only been formed in several dozens of years.'
And diagenesis is also the crucial moment of the appearance (and the development) of porosity. Important,
as it is this porosity which allows rocks to become reservoirs (water, gas, petrol).
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A climate marker
Given the extraordinary diversity of the agents at work in each of the aforementioned processes, there is
reason to be fascinated in noting that the study of sedimentary rocks sometimes permits geologists, genuine
investigators, to know everything about the background history of a sediment: or, when, from the moment
the erosion has occurred, how the sediment was transported, where it was deposited, in what temperature
conditions and in what environment, and how it was turned into rock. And here it is nothing to do with a purely
scientific exercise: this understanding is crucial for prospecting! 'When we are looking for petrol, for example,
we are in effect looking for a very particular sedimentary body, and looking to be able to locate it in an ancient
landscape. Thanks to sedimentary petrology, we know what we are looking for, and where to look for it. The
same is true for other materials. We have in Wallonia two of the largest limestone prospecting firms in the
world. Here as well, they are looking for a very particular limestone, formed in very particular conditions.'
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Furthermore, in these times when the climate and its ancient evolution are being much studied, sedimentary
petrology also serves as a precious climate marker. 'It is almost the only direct experiment we can carry out
with the old climate,' points out Frédéric Boulvain. 'Studying rocks allows us to record an enormous amount
of information about the physico-chemical conditions in which they were formed. Recently the discipline has
been subject to a great conceptual revolution, brought about by oil fielders. From a very static vision of things
we have shifted to a reasoning based on the variation of parameters which sometimes have a global range,
such as the evolution of sea levels. That has enabled very precise and large scale correlations. Now we can
practically reconstitute the entire stages of the history of the planet, in identifying with certainty phenomena
which have occurred exactly at the same time at various points across the globe. We have a better global
understanding.'
If the book is a course aimed at specialised students, it has in addition obviously been designed not to
discourage any potential reader. Richly illustrated, crammed with concrete examples and diagrams, it presents
things clearly and broadly before plunging into detail. One can sense behind the clear writing and the fluid
organisation the hand of a powerful explainer, who must be a teacher. Frédéric Boulvain proclaims from the
rooftops his mission to spread knowledge and the genesis of this work is edifying in this respect: 'For a few
years,' he explains, 'I made my courses available to students on the internet, and regularly updated them.
Given the thinness of writing in French on the subject, this resource was progressively much used in diverse
francophone counties, in Quebec or in Africa for example. I receive between 700 to 800 hits a day! Last year,
Ellipses wrote to me offering to turn it into a book. And I only accepted on condition that the course remained
available on the web. As we have a mission to spread knowledge and the internet allows me to reach people
who cannot buy the book. Ellipses agreed, and it is an attitude to be emphasised.'
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Printed in 2,000 copies (but further editions are already planned) the book has already been used in the majority
of French universities, in Quebec, in Africa. 'And I am told that it has been seen in Bilbao and Brazil,' smiles
Frédéric Boulvain, who is currently working on the writing of a second work, which will be published in 2011
by the same publisher: Geology through fieldwork, the outcrop of a concept. 'A lot of people have problems
finding geologists capable of working in the field,' he explains. 'Fieldwork, in other words recognising rocks,
classifying them, producing maps, is nonetheless the basis of geology. In the francophone world, we have
sometimes tended to forget this. But not at Liège, where we have a recognised tradition of teaching on the
terrain. It has to be said that we live in a privileged place: even in remaining in the Liège region, one can see
just about everything there is to see from a geological point of view.'
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