Download GG 101L SEDIMENTARY ROCKS SUPPLEMENTARY READING

GG 101L
SEDIMENTARY ROCKS SUPPLEMENTARY READING
Sedimentary rocks are common in continental parts of the Earth, but not very common here in
Hawai‘i. In this lab you will learn how to identify some of the more common sedimentary rocks, and
also what these rocks can tell you about the conditions under which they formed. Unlike igneous rocks,
which can form just about anywhere, sedimentary rocks form in particular environments and under
particular conditions. They are therefore very useful for unraveling Earth’s history.
I SEDIMENTARY ROCKS
There are two main types of sedimentary rocks – those that are made up of pieces of other rocks
(sometimes called detrital or clastic sedimentary rocks), and those that form by precipitating minerals
out of water by either biological or non-biological processes (sometimes called chemical or biochemical
sedimentary rocks). It can get a bit confusing because there are some sedimentary rocks that form from
a combination of both clastic and precipitating processes. Importantly, however, all of these
sedimentary rock types form under normal conditions of temperature and pressure with regard to the
Earth’s surface.
Detrital Sedimentary Rocks
These are probably the most well-known types of sedimentary rocks, and include sandstones,
shales, breccias, mudstones, conglomerates, and many others. Detrital sedimentary rocks are important
because they contain a record of the starting rock type out of which the particles were eroded, the
transport mechanism and distance that the particles moved under, and the conditions under which the
particles were deposited, and finally the conditions under which the particles were sedimented together
(lithified) to form the eventual rock. Detrital (or clastic) sedimentary rocks are classified by a number
of characteristics, including the size of the particles, their shape, whether or not they’re all the same size
and composition, etc. Your lab book discusses these characteristics in more detail.
Chemical and Biochemical Sedimentary Rocks
Chemical and biochemical sedimentary rocks either form by non-biological precipitation out of
chemical-laden water, or by biological processes, or by some combination of these. The convention is
to consider any rock that at some time had life involved in its formation to be a biochemical
sedimentary rock. Thus, even if it is made up of a whole bunch of broken seashell pieces (and thus the
animals were actually dead before the pieces became a rock), it is still considered to be a biochemical
sedimentary rock rather than a detrital sedimentary rock.
Characteristics of Sedimentary Rocks
The following sections describe different properties of sedimentary rocks that are used in
classifying them. Keep in mind that the ultimate goal in geology is not to come up with a rock name, it
is to understand the circumstances under which the rock formed. The rock name is just a shortcut way
to describe the rock so that other geologists will be able to readily understand the rock’s significance
and where it fits into a geological story.
Grain size
Detrital sedimentary rocks are classified most generally by the size of the particles (usually called
grains) making up the rock. Figure 4.1A in your lab book illustrates the size classifications of
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sediment, and Table 4.4 compares grain size to the eventual sedimentary rocks that develop from these
sediments. From finest to coarsest, the sediments (and corresponding sedimentary rock) are: clay
(claystone, shale), silt (siltstone, shale), sand (sandstone), granules + pebbles + cobbles + boulders
(conglomerate or breccia).
Thus, merely by measuring the dominant size of the grains in a sedimentary rock, you can go a long
way toward identifying the rock. Do geologists go around with straight-edges measuring individual
grains in rocks? Not usually. Instead, there are some general rules that they follow. For example, it is
pretty easy to tell if the grains are bigger than 2 mm across and therefore telling pebbles from sand is
easy. Additionally, if the grains are too small to be seen individually, they are finer than sand and must
therefore be silt or clay. How do you tell the difference between silt and clay? Your lab book suggests
grinding a bit between your teeth. If it feels gritty, it is silt. If it feels smooth it is clay. However, in
GG101 labs, it is pretty common for students to test every single rock with HCl, so perhaps you don’t
want to go around biting all the fine-grained rock samples. Instead, try scraping the rock with your
fingernail instead of your teeth.
Particularly for water-borne sediments, grain size is often an indication of the distance that the
sediment traveled. This is because if you followed any stream or river from its headwaters to where it
enters an ocean or lake, you would notice that the flow velocity would decrease along the way. There is
a direct relationship between the velocity of flowing water and the size of particles that it can carry,
with faster water being able to carry larger particles. Thus a single stream may be characterized by
swiftly flowing water at its head that is pushing boulders and cobbles down a relatively steep stream
bed. Farther downstream, slower flow carries only pebbles. Even farther downstream there may be
some sand, and eventually, when the stream has merged with other streams into a slow-flowing lazy
river, only silt or clay will be transported. Of course the character of streams and rivers changes
depending on whether it is flooding or not, so the size of the particles that can be transported may also
change.
Grain size is not as diagnostic for chemical and biochemical sedimentary rocks. However, there are
some cases where grain size is part of the characteristic used to identify them. For example, coquina
consists of a coarse-grained aggregate of shell fragments whereas micritic limestone consists of claysized particles of calcite and clay. Some chemical and biochemical sedimentary rocks don’t really have
a sedimentary “grain size” at all because they precipitated directly from water. Instead of being
fragments of previous rocks they are a crystalline aggregate, sometimes having a texture that looks
more like an igneous rock.
Sorting
Sorting refers to the variety of grain sizes in a rock. A well-sorted sedimentary rock is one in
which all the grains have the same size. A poorly-sorted sedimentary rock contains grains of many
different sizes. Interestingly (some might say confusingly), engineers use the term “well sorted” to
refer to just the opposite - materials where there is a big range of grain sizes. This is because such
materials are much better for mixing with cement and other construction-oriented uses. But we’re
geologists… Sorting is important because it is closely tied to the environment under which the
sediment was deposited, to the mechanism that carried the sediment to its final resting place, and to the
distance that the sediment traveled before becoming lithified. Wind, for example, is very good at
moving sand-sized grains, tends to blow silt and clay particles away, and doesn’t often move pebbles.
Wind-deposited sandstones therefore tend to be very well sorted. Meanwhile, the silt and clay that is
blown away eventually lands somewhere to form a different (but also well sorted) rock called loess.
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Certain transport environments are not very good at sorting particles by size. Rapidly flowing
streams, mudflows, landslides, and glaciers are capable of transporting everything from boulders to clay
particles. If all the particles are deposited directly from one of these high-energy environments, the
resulting sedimentary rock will be very poorly-sorted.
Grain Shape
If the grains are big enough, their shapes hold important clues to the amount of energy in the
environment that the sediments were deposited in and also to the amount of time and/or distance over
which the sediments were transported. High-energy streams crash boulders and pebbles together so
they often have broken, angular shapes. On high-energy ocean and lake beaches, pebbles and boulders
are constantly ground and smashed against each other so they end up with well-rounded shapes. Sand
grains hit against each other while being blown by the wind, and whether or how they break depends on
what they consist of. Quartz sand grains, being very hard, rarely fracture all the way through. Instead,
tiny chips are broken off when they crash together and the eventual result is a frosted surface much like
beach glass. Feldspar sand grains are much more likely to break apart when they collide (usually along
cleavage planes), so they become smaller and smaller while blowing in the wind. Eventually, all the
feldspar will be broken into tiny pieces that get blown away completely, leaving behind a sandstone
consisting only of round, frosted quartz grains.
Composition
The composition of the particles in detrital or chemical/biochemical sedimentary rocks is an
important clue to their origin, and therefore important for classifying them. Calcite, for example, is
produced almost exclusively by biochemical processes, so any sedimentary rock consisting of calcite is
classified as chemical or biochemical (even if it may happen to look like a sandstone). As we noted
above, depending on the distance over which wind-blown sand is carried, the overall composition can
change from a mixture of quartz and feldspar eventually to pure quartz. Thus, looking at the
composition can tell you something about the distance that detrital sediments were carried, which is
important if your goal is to reconstruct ancient environments.
Fossils
Fossils, evidence of organisms, are almost exclusive to sedimentary rock (as opposed to igneous
and metamorphic rocks), and indeed some sedimentary rocks consist wholly of fossils. Sometimes a
detrital sedimentary rock will contain fossils of some sort, and it will therefore be considered
“fossiliferous.” Many chemical and biochemical sedimentary rocks are also fossiliferous, but some are
not. There are chemical and biochemical sedimentary rocks that owe their existence to fossils that once
existed but which have been completely dissolved away. Limestone is a common biochemical
sedimentary rock that is derived mostly from ancient reefs. Some limestones look basically like a live
reef today, with shells, worm casings, coral fragments, sand grains, etc. Other limestones, however,
have been completely recrystallized, meaning that all the calcite that once made up the shells, worm
casings, coral fragments, and sand grains has been dissolved and then re-precipitated; no fossils remain.
And of course there is every gradation in between these extremes.
Fissile/Layered
Some sedimentary rocks are layered. This can be layering inherited from the initial deposition
process, and includes bedding in sandstones and siltstones. Fine layering, particularly if it is uniform in
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thickness, is called lamination. Other layering, particular to the finest-grained sedimentary rocks,
derives from the parallel orientation of mica minerals. This parallel orientation is partly due to the
deposition processes that took place, but mostly due to the development of clay minerals that occurs
after deposition takes place. This type of layering imparts a characteristic weakness in the rock that is
manifested by thin, platy layers that may flake off. Such rocks are said to be “fissile,” and the best
example is shale. Shales can be mostly silt-sized or clay-sized so there are silty shales and clay shales.
Sedimentary rocks that are neither layered nor fissile are “massive.”
Identifying the Characteristics of Sedimentary Rocks
1. Table 1 lists a number of characteristics of common sedimentary rocks. Figures 4 and 5 of your lab
manual give more details. Use Table 1, the choices listed below, the grainsize charts, and the lab
manual figures to identify the characteristics of the sedimentary samples supplied by your TA, and fill
in the blanks in Table 2.
Grain size choices
>2 mm (pebbles, cobbles, or boulders)
visible to 2 mm (sand - note that there are
subdivisions of sand)
gritty (silt)
really fine (clay)
looks crystalline (interlocking grains of various
or very small sizes)
Sorting choices
well-sorted
moderately sorted
poorly sorted
Grain shape choices
rounded
sub-rounded
sub-angular
angular
Composition choices (can be more than one)
quartz
feldspar
calcite
can’t tell
mix
Fossil choices
no fossils
few fossils
lots of fossils
Fissile/layered choices
fissile
layered (or laminated)
massive
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Table 1: Characteristics of Common Sedimentary Rocks
Rock
Grain size
Sorting
>2 mm
Poorly sorted
Breccia
>2 mm
Poorly sorted
Conglomerate
Grain Shape
angular
rounded
Composition
Can be lots of things
Can be lots of things
Fossils?
Usually no
sometimes
Fissile/Layered?
massive
massive
Quartz
Sandstone
Arkose
Sandstone
Greywacke/
Wackystone
Siltstone
Vis-2 mm
Well sorted
rounded
quartz
no
Massive or layered
Vis-2 mm
Rounded to subrounded
Sub-rounded to
sub-angular
Can’t tell
Massive or layered
Usually no
massive
sometimes
massive
Silty shale
Fine (gritty)
Well sorted
Can’t tell
sometimes
fissile
Claystone/
Mudstone
Very fine
Well sorted
Can’t tell
Quartz +
K-feldspar
Quartz, feldspar,
lithics, clay
Can’t really tell, but
mostly clay minerals
Can’t really tell, but
mostly clay minerals
Can’t really tell, but
mostly clay minerals
no
Fine (gritty)
Well sorted to
moderately sorted
Moderately sorted
to poorly sorted
Well sorted
sometimes
massive
Clay shale
Very fine
Well sorted
Can’t tell
Coquina
>2 mm
Moderately sorted
variable
Crystalline
Limestone
Micritic
Limestone
Chalk
Crystalline, can see n.a.
crystals
Crystalline, can’t
n.a.
see crystals
Very fine
Well sorted
Fossiliferous
Limestone
Oolitic
Limestone
Chert
Variable
crystalline
Coal
crystalline
Vis-2 mm, ± fines
sometimes
fissile
n.a.
Can’t really tell, but
mostly clay minerals
Calcite (shell
fragments)
calcite
Essentially all
fossils
sometimes
Massive, maybe
layered
massive
n.a.
Calcite
sometimes
Usually massive
Can’t tell
Soft calcite
Usually massive
variable
Fossil fragments
Essentially all
fossils
lots
rounded
Calcite (oolites)
sometimes
n.a.
n.a.
n.a.
n.a.
Silica (microcrystalline sometimes
quartz)
Carbon (black)
sometimes
variable
1-2 mm
Massive, maybe
layered
Massive, maybe
layered
Usually massive
Usually massive,
conchoidal fracture,
low density
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II Determining Sediment Size
In this portion of the lab, you will see how sedimentologists quantify the parameters of grain size
and sorting. The way this is done is to pour a dry sediment through a stack of sieves. The sieves with
the coarse spaces are on the top and those with ever-finer spaces are on the bottom. That way the fine
grains can settle through until they reach a level that they cannot get through. The set of sieves we will
use have mesh spacings of 2, 0.5, 0.25, 0.125, and 0.063 mm (it’s written on the sides of the sieves).
These sieves are kind of fragile and definitely are expensive to replace, so please treat them with care.
After you determine what percentage of the overall mass of the sample got caught in each sieve,
plot these, and you have a graphical illustration of both the sediment size and its sorting. Figure 1
shows how this is done. Note that the graph is kind of strange and the axes are kind of strange. The xaxis is in units of φ size (φ is the lower-case Greek letter f, by the way). Notice from Table 4 that each
physical mesh size (in mm) has a corresponding φ size. For extra credit, explain how you get φ size
from mesh size.
Table 4: φ size relationship
Mesh size
φ size
4 mm
-2
2 mm
-1
0.5 mm
1
0.25 mm
2
0.125 mm
3
0.063 mm
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< 0.63 mm 5*
*not really, but we’ll plot it this
way
The two numbers that you really want
to get are called Mdφ (the median
diameter) and σφ (the graphic standard
deviation). How do you do this?
First, plot your cumulative wt% and φ
size data on the probability paper.
Next, fit a smooth, curved line to your
data.
Mdφ is easy - just drop a vertical line
down from the intersection of your
curve and the 50% cumulative wt%
line, and read off the φ value. That will
be Mdφ.
Figure 1: Method for calculating Mdφ and σφ from sieve data plotted
on probability paper
To get σφ you drop vertical lines down
from the intersections of your line and
the 84% and 16% cumulative weight %
lines.
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These values are φ84 and φ16, respectively. σφ = (φ
φ84 - φ16)/2.
What are these Mdφ and σφ numbers and why would anyone care? The median grain size
diameter, Mdφ, is the grain size for which half the sample is finer and half the sample is coarser. If
Mdφ is small or negative, then the sample overall is coarse. If Mdφ is large and positive, then the
sample overall is fine. You hear the term “median” used a lot with house prices: “The median house
price on Kaua‘i in October was $680,000” means that of the houses sold on Kaua‘i in October, half of
them were cheaper than $680,000 and half of them were more expensive than $680,000 - a sad state of
affairs if you’re looking for a house there!
The other number, σφ is a measure of how much variability there is in the grain size. If σφ is big, it
means that the sample has a wide range of grain sizes, and a geologist would say that it is poorly sorted.
If σφ is small, then there is only a narrow range of grain sizes, and a geologist would say that the
sample is well sorted. Figure 2 illustrates how these two parameters vary.
Figure 2: diagram showing how Mdφ and σφ vary for different sieve samples.
In the left-hand diagram of Figure 2, there are three samples that have the same value of σφ, but
you see that each one intersects the 50 wt% line at a different place. This indicates that they have
different median grain sizes. The Mdφ values of samples A, B, and C would be about -2.4, -0.5, and
0.8 φ, respectively. In the diagram to the right, samples A, B, and C all have the same value of Mdφ
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(about 0.4 φ), but you can see that the ranges of φ16 and φ84 are quite different, so σφ would be
different. Specifically, σφ would increase from A to B to C, meaning that A, B, and C would be best-,
medium-, and poorest-sorted, respectively.