Download Sedimentary Rocks

Sedimentary Rocks
A Basic Introduction
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WtoW globe image by Tatiana Baeva, to whom I am grateful.
Sedimentary Rock – A rock formed of material weathered from a pre-existing rock (the “parent” or
“source” rock), eroded and transported to a different place by one or more transport mechanisms,
deposited, and then lithified. (Note: even sediment that has undergone all of these things except
lithification are usually treated as rocks. The loose sand of Providence Canyon, GA, for example, is
treated as a rock stratum even though it is by no means lithified. It is called the “Providence Sand”.)
Most of the “rocks” in the southeastern Coastal Plains are not lithified.
The Providence Sand in its “type section” at Providence Canyon, GA. The uppermost red layer is a weathered residuum of the younger Clayton Formation
From the definition you can see that there are a series of steps that must happen in order to form a
sedimentary rock. Those steps must, in fact, happen in a specific order. They are:
1) Weathering of the source rock. (We have covered this in some detail previously.)
2) Erosion of the weathered material. (Remember that until this happens we do not consider the
weathered material sediment. Instead we call it “regolith” or “soil” as appropriate. As soon as
it is moved we begin to consider it sediment.)
3) Transport of the sediment. (The system that originally moves the material also begins the
transport stage and so these are often considered together. Remember though that there may
be a complex history of repetitive erosion and transport by several systems before the
sediment arrives at its final resting place. The next few slides elaborate on the erosion and
transport steps in the process.)
4) Deposition of the sediment. (This happens, of course at some place where no process is able
to remove it to yet another location. Were it to move again it could not undergo the last step
in the temporary location. This will be covered in a separate slide show.)
5) Lithification. (A set of processes exist that take loose sediment (if it is loose to begin with) and
harden it to rock. These will be discussed individually later in this slideshow.)
EROSION and TRANSPORT
Consider Stone Mountain, GA. The igneous rock of this stock is weathering in the
hot Georgia Piedmont both chemically (hydrolysis and oxidation) and
mechanically (exfoliation, abrasion, impacts, Frost wedging, tree root wedging … )
Imagine what will happen when the loose, exfoliated slab shown by the arrow
slides off the steep side of the mountain.
TRANSPORT (cont)
At the first budge, the rock is a large gravel particle, moving under the influence of gravity alone. This type of movement – gravity-driven
movement down a slope – is called mass wasting and we will return to it later in the term.
Mass wasting will only be a viable transport mechanism until the rock gets to the bottom (red arrow). Beyond that the slope is not steep
enough. But this rock will begin its travels via mass wasting.
When the falling rock slams into the rocks
already at the bottom of the hill it and
they will be broken into smaller pieces
(including sand and finer material) by the
impact. Even though the large gravel will
not be moveable without further
weathering, the finer pieces will.
Suppose a strong wind is blowing
southeastward (blue arrow) when the
rock hits and breaks. The smaller broken
pieces may be lifted and blown for some
distance downwind by this, continuing the
transport step via another transport
system (red squiggle). Of course this can
also happen later, after the sand has sat in
place for a while.
N
~1000 feet
Or imagine that the wind never blows hard enough to lift some particles. The sediment is still above the level of the lakes that you see,
and as Mr. Bill always says, “mister gravity always wins.” Mud, sand, and even fine gravel can be washed down even fairly gentle hills by
sheetwash during and after a hard rain. We will come back to sheetwash later in the term. The point is that the movement of material
toward the lakes around the mountain is inevitable. The lakes, of course, are man made. As the story continues we have to back up in
time to a point before the dam was built that formed the lakes. Then Stone Mountain Creek and its tributaries flowed freely through
their valleys. Once sediment is in a stream its transport becomes much easier.
TRANSPORT (cont)
Stone Mountain Creek flows a
few miles eastward and joins the
Yellow River (location 1). The
Yellow flows southward and joins
the Alcovy River between
Jackson and Monticello to form
the Ocmulgee River (loc. 2), the
Ocmulgee flows southward for a
long way, then makes an
eastward bend. Soon it is joined
by the Oconee River near
Hazlehurst (loc. 3) to form the
Altamaha.
All of these are lowland streams
with generally low gradients, so
the sediment may spend
centuries on sand bars along the
streams. The ultimate goal of
the streams, however, is the
Altamaha Delta, just north of
Saint Simons Island (loc. 4) and
the river system has the
potential ultimately to deliver
the sediment there as well.
1
2
3
4
TRANSPORT (cont)
When our sediment from Stone
Mountain reaches the delta it is
not necessarily done. The finer
material is likely to remain
suspended in the sea water and
move out onto the continental
shelf or beyond.
In addition, waves create a
longshore transport system that
moves sediment away from the
mouths/deltas of streams and
parallel to the shore. The barrier
islands of the Mid-Atlantic and
Southeastern U.S. are all made of
sand transported this way.
Storms and tidal currents may
also move sediment offshore and
onto the continental shelf at any
point. Some of that sediment
may cross the shelf, go down the
slope and end up on the
continental rise.
The shelf becomes very narrow at
Miami, FL and any sediment
brought down the longshore
system that far is funneled to the
continental rise anyway.
TRANSPORT (cont)
Part of the point of this story is to introduce to you the idea of an environment of deposition or a sedimentary
environment. We have talked about hypothetical sedimentary particles being eroded and transported by these
various systems: mass wasting, wind, streams, rivers, deltas, the longshore transport system, systems that
move sediment onto the shelf, or across it to the continental rise. (Only the very finest of detrital sediment
makes it far onto the abyssal plains). But, of course, the sediment doesn’t necessarily, or even usually, go
directly from source to rise in one step. It is deposited at various points along the way, potentially many, many
times, and if conditions are right it can stay in one of those places permanently, become lithified, and
eventually be studied as a sedimentary rock. The next slide will elaborate on deposition. For now we are still
interested in transport.
The main point of the story so far is to have you realize that the potential for transport of material is enormous.
When we are talking about these systems in detail later in the term we will introduce the notion of base level
and specifically for the present point, base level of erosion. For virtually every system that base level is sea
level. Anything above sea level is fair game for erosion. Not a square inch of land is exempt. Sedimentary
material can be moved by gravity, wind, water, glacial ice, or any combination of them from any point above
sea level. Transport begins then and can continue through a complex series of systems before the sediment
arrives at its final resting place.
That resting place may not even be at or just below sea level. Notice that all these systems operate because of
gravity, and that gravity will move things to the lowest possible point unless something interferes. The lowest
place to which sediment can move depends upon the type of continental margin that occurs near base level. In
the Atlantic that is the continental rise or abyssal plain. In the Pacific it is the bottom of a trench. Not all
sediment gets there – something usually interferes – but a lot of it does.
DEPOSITION
Well over 90% of all sedimentary rocks ever studied were deposited in the environments indicated by the light red here. At this point all the rocks we
see at the surface and most we see only in boreholes have been studied, so it’s safe to say that virtually every sedimentary rock that was ever formed
was deposited here.
We do find rocks made of sediment deposited by mass wasting, by wind, by glaciers, by streams (particularly lowland rivers like the Altamaha),
environments well above base level. Similarly, there are sediments and sedimentary rocks from the abyssal plains. Both groups, however, are pretty
rare. Almost all sedimentary rocks are from coastal (deltas, tidal areas, barriers), continental shelf, or continental rise environments.
The crux of the matter is
base level. Expect erosion
above it and deposition
below it. There are
exceptions to bot of those,
but they are unusual.
Everything in green
here can be
considered potential
source rock.
Decreasing
Probability of
Preservation
Decreasing
Probability of
Preservation
Everything in blue can be
considered a likely
environment of deposition
(including the Great Lakes).
DEPOSITION (Cont.)
You should get the idea from mention of the “environment of deposition” (or “depositional environment”) or
“sedimentary environment” that the notion of an environment is important in understanding deposition.
For a sedimentary particle to move, the system doing the moving must have enough oomph – enough energy – to
move it. If the system’s energy becomes lower it may not be able to carry the sediment it was carrying before,
and it will be forced to deposit some or all of it. Thus the energy of the transporting medium is an important part
of the depositional environment. However, that is never the only thing that influences how the sediment is
deposited, or other characteristics of the deposit, or the organisms that inhabit the environment.
Florida
This is the delta of the Suwannee River in Florida. Within a narrow
coastal zone the one-way flow energy of the river ceases (its slope
drops to zero), the banks that once constrained the flow disappear, the
purely fresh water of the river system changes to brackish as it mixes
with the sea water, and finally , not far offshore, the water is purely salt
water.
The delta accumulates because of the immediate lowering of energy at
the river’s mouth, but the other factors mean that the sediment
accumulates in different patterns in the river valley, on the delta, and
on the shelf. The change from current to wave and tide energies play a
role in how the sediment behaves. The different chemistries play a
role. And all these things also control what kinds of plants and animals
can survive. The plants and animals influence how the sediment
behaves. It is all very complex and so the sediment reacts in very
complex ways.
Gulf of
Mexico
~1 mile
The point is that we can eventually look at the sediment and make very
good interpretations of what kind of environment allowed it to settle.
We study the sediment, the fossils in it, the relationships to other
environments, and so on. We will come back a little later and see the
basics of how to do all this.
LITHIFICATION – 1) Compaction.
There are three principle ways in which loose sediment can become hardened or lithified. Or that sediment becomes hardened as
it forms. Each works best upon certain types of rocks and some don’t work at all, or not very well, for some kinds of rocks.
The first type to understand is lithification by compaction. This only works in sediment with a very high content of clay or
similarly fine material, and the more fine sediment, the better. It works because the arrangement of charges on crystal faces is
not homogenous, and this is particularly true of clay minerals. Some parts of a crystal face are slightly positive, some are slightly
negative, or in the case of clay minerals, an entire face may be positive or negative. In large particles this is true too, but in those
large sediments the grain size means that gravity and other forces for outweigh the electrical differences. In very small grains,
however, the cohesion provided by different charges can hold the rock together.
The compaction of mudstones can occur in a couple of ways. Initially the sediments may be up to 90% water. Removal of the
water, perhaps only as the grains settle more closely, perhaps by evaporation (below) or some other process brings the grains
close enough to begin to cohere and harden. You’ve all presumably seen this in drying mud. The resulting “mudchips” are
surprisingly hard. I’ve used the one in the left photo for over three decades to illustrate the process.
“Mudcracks” are the cracks between the mudchips in the right-hand photo. Once the water content gets low enough the mud
can no longer contract homogenously and so the cracks allow smaller parts of the layer to contract. At this point the edges begin
to curl upward. We’ll come back to this because there are not many environments where mud can get wet and then dry out, over
and over again. Mudcracks and mudchips are therefore good indicators of those environments.
LITHIFICATION – 2) Cementation.
Most kinds of sediments, and most particularly gravels, sands, and limestones, are lithified by having their grains (pebbles, sand grains,
fossils) cemented together by material precipitated in the spaces between them.
Geologists make thin sections of rocks by polishing a face flat and gluing it to a microscope slide, then cutting off most of the chip and
grinding it very thin – 3 microns, usually. This allows light to pass through most minerals and makes them easy to identify.
In this thin section photomicrograph (x55) the dominant mineral is quartz. You can see the outlines of what were originally quartz sand
grains (at A, for example) because each one has a thin rind of hematite (B) on its surface. (In color you’d see the reddish color of it.)
Beyond that rind, the mineral is again quartz, but that quartz
was not part of the original sand grains (C). Had it been, the
hematite rind would have been on the outside of that.
That quartz was precipitated between the loose grains by
groundwater with dissolved SiO4-4 in it, perhaps from
weathering of feldspar or other aluminosilicates.
But the story gets even more convoluted. Those grains, and
the cement around them, were weathered and eroded from
the sandstone and moved somewhere else and deposited.
Those grains were cemented by yet another round of quartz
cement with a different, finer texture, which you see as the
speckled material between the grains.
B
A
C
D
As the cement crystals grow within the intergranular pores
they lock the grains together, just as artificial cement locks
the gravel together in concrete. This lithifies the rock.
From Pettijohn, Potter and Siever, 1972
LITHIFICATION – 3) Crystallization From Water and Recrystallization.
This is a photograph of rock salt. If you look carefully you should be able to see the boundaries between numerous crystals.
The mineral, of course, is halite. It forms in only one way – precipitation from an evaporating water rich in Na and Cl ions.
Seawater works, of course (if it can be evaporated), and so does desert lake water. As the crystals grow larger and touch each
other they become locked together in much the same way that crystals become locked in an igneous rock. In fact, this should
remind you of phaneritic texture in, say, a granite.
The crystals do not appear to be cubic for the same reason that quartz crystals do not appear to be hexagonal in granite. The
proper arrangement of atoms is there, but the crystals grew into whatever space was availably, not freely into whatever form
they “wished”. This lithifies the rock as it is forming, just like in igneous rocks.
Something similar happens in other
rocks as well, and limestones in
particular. The modestly high
solubility of the calcite (or
aragonite) means that when fresh
water with Ca and CO3 in it passes
through the pores the individual
grains can increase in size, taking on
a texture very much like this.
That process, which also lithifies the
rock, occurs some time after
deposition of the loose sediment.
The rock does not lithify as it forms
(though the appearance is the same
as if it had), but later. This is called
recrystallization.
One Final Point
We have talked about sedimentary rocks as if deposition and lithification are the end of the story, but, of course, on a dynamic
Earth it never is the end of the story. One of three things may eventually happen to a sedimentary rock.
1) We saw an example of one possibility two slides back. In the photograph there we saw evidence that a sand from some source
rock had accumulated and become lithified by coarse cement. The resulting sandstone was then itself weathered and eroded to
provide sand grains for a second sandstone. In other words, any sedimentary rock encountered above base level is subject to
being the source rock for younger sediments.
2) We alluded to another
possibility when we explored
the origin of magma. At the
Peru-Chile Trench a slab of
oceanic crust is being forced
beneath the edge of South
America. But the basalt of the
plate itself is not the only
thing going down the chute.
It is carrying with it sediments
and sedimentary rocks from
the seafloor, the trench itself
(which gets a lot of sediment
from the mountains), and
even the edge of the
continent beneath the trench.
It also carries a lot of water
and the ions dissolved in it.
Thus sediments and
sedimentary rocks can serve
as the parent material for
igneous rocks.
3) We have not yet gotten to the third possibility, but we soon
will. The Himalaya has formed, and is still forming, as a result of
the force of India pushing against the southern edge of Asia. The
two landmasses are on separate tectonic plates and there is a
convergent boundary between them. Before they came into
contact there was a trench here, and a volcanic arc, and there
was a lot of sediment in that trench. There was also abyssal plain
with sediment on it, and a continental shelf on India with a lot on
it. You can bet that the heat and pressure that caused these
mountains to be uplifted also altered those sedimentary rocks
and their low temperature/pressure minerals.
Thus sediments and sedimentary rocks can serve as the parent
material (protoliths) for metamorphic rocks.