Geology 109L Lab 1A: Sedimentary Particles and Flow Velocity
Goal: The purpose of this lab is to introduce you to sedimentary particles (the most common
constituents that make up sedimentary rocks) and how they reflect depositional processes. Both the
particles and the rocks they compose contain abundant information about the source of the
sediment, transport processes, and the environment in which they were deposited. This information
can be used to understand everything from the tectonic setting to why a specific fossil is present or
absent.
Special Note: The boundaries between geological subfields are not sharp, and particularly in the
case of identifying the sources of sediment, think about using all of your knowledge from
mineralogy, petrology, and tectonics to help you interpret the rocks. One of the things you will find
in these labs is that information from other classes can be very useful and you can apply the
knowledge you gain in this lab to your other classes!
Sedimentary Particle Composition and Origin:
The large variety of sedimentary particles can tell you about the source of the sediment and
the depositional environment. The sediment source can, in turn, tell you about the tectonic
environment, transport processes, and resident or upstream flora and fauna. There are two basic
types of sedimentary particles.
1) Particles derived by the breaking down of previously existing rocks. These are often
called "terrigenous" because they originated on land and are transported to the site of deposition.
Particles are typically composed of siliciclastic minerals such as quartz, feldspar, heavy minerals
and rock fragments. Volcaniclastic particles, such as pumice fragments, volcanogenic feldspars,
volcanic rock fragments and glass shards, are both igneous and sedimentary. Carbonate clasts and
shells can also be terrigenous, if they are eroded from an older rock. However, they are more often
considered part of the next category. Transported particles contain abundant (or sometimes sparse)
information about uplift and erosion (tectonics), transport processes, and climate.
2) Minerals and organic matter that formed in place by chemical and biochemical
processes. These “autochthonous” sediments form within the depositional environment and
provide information mostly on conditions specific to where they were deposited. They include
organisms, such as corals, moluscs, some algae, diatoms, and radiolaria that precipitate CaCO3 or
SiO2 shells or skeletons. When they die, the remains become sedimentary particles that can be
lithified into a limestone or chert. In environments where these organisms are abundant, the
sediments are composed mostly of their shells. Evaporites and organic-rich rocks are also
autochthonous. The process of evaporation concentrates ions in water causing minerals such as
gypsum and halite (rock salt) to precipitate. In swamps and bogs, plant debris can build up and
form coal during burial, whereas in anoxic basins, the remains of pelagic organisms accumulate
resulting in deposition of organic-rich shales. All of these deposits contain important information
on the chemistry of the depositional environment and the organisms living there.
Texture
Sediment texture is defined as the size, sorting, shape and arrangement (fabric) of the
grains that make up a sedimentary rock.
1. Size: Particles can range in size from house-size boulders to microscopic clays. (There won't be
any boulders in lab, but there are some interesting ones in Puta Creek below Monticello Dam west
of Winters.) The grain size can tell us about the speed and turbulence of the flow that deposited the
particles. Mud, being composed of very small particles, is deposited in low energy environments or
in pockets of low energy between larger particles where it is protected from being washed away. To
transport large grains, a higher energy flow is required. It takes far more energy to move a boulder
or cobble down a river than it does to move silt and clay; this intuitively obvious statement is
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actually a very important key to interpreting sedimentary rocks. We will use semi-quantitative
versions of this concept extensively in class.
The grain size of clastic rocks is related to a number of factors in addition to flow speed:
1) The size of the minerals in the source area influences the final grain-size
distribution. For example, an argillite-rich (shale) source area will not yield coarse sand
grains unlike a granitic source area. Thus, no matter what the flow speed, sand will not be
transported or deposited if it does not exist in the source area. Similarly, if clays are not
present, shales can not accumulate.
2) The fracturing, dissolution, and abrasion of grains during transport act to
decrease their size. The longer grains are actively transported, the smaller they become.
The rate of decrease depends on the original size of the grain, the energy of the transporting
system, the length of time in the system, and the composition of the grain itself. As a
general rule, the greater the distance from the source area, the smaller and better rounded the
grain will be.
3) The flow conditions under which the sediment is deposited influence the grain
size. A high mountain stream deposits coarser material (boulders and cobbles with areas of
gravel and sand) than the lower Sacramento River (mostly sand and mud). This is because
turbulence and flow velocities are vastly different, and these two variables are the most
important controls on the maximum grain size that the streams can move.
Grain sizes are commonly reported using the Wentworth Scale (p. 12, textbook), which is a
scale based on particle diameter in mm. The phi (Ø ) scale quantifies the Wentworth Scale by
taking the negative logarithm base 2 of the grain size in millimeters. The phi scale is useful for
statistical analyses of grain size and grain sorting. In lab, you can use either the words or the phi
value for reporting grain size.
2. Sorting: Sorting refers to the size range of grains in the sediment or rock. A well sorted
sediment is one that consists of a single sized sediment (for example, most grains are fine sand). A
poorly sorted sediment is one that contains a broad range of grain sizes (for example, both boulders
and find sand, as well as cobbles, gravel, etc.). Sorting is a good qualitative measure of the
consistency of flow during deposition. Well sorted sediments are the result of consistent flows that
are barely strong enough to transport the largest particles and are plenty strong enough to transport
smaller particles. All the small particles get washed down flow. A well-sorted beach sand is a good
example: the waves are strong enough to transport the larger grains and they sort out smaller
particles and wash them off shore. Different beaches have different grain sizes due substantially to
wave size (which correlates to water velocity on the beach) and sediment source. A poorly sorted
sediment is often the result of very rapid deposition, i.e. it was unceremoniously and rapidly
dumped into place without time for flows to sort the sediment by grain size. Flood deposits and
land slides are two good examples. (Of course, if the source of sediment consists only of one grain
size, all deposits will be well sorted.) Poorly sorted sediments are also deposited in low energy,
fine-grained environments where large, shell secreting organisms live. Here, currents are too slow
to wash away the shells or to wash in any outside grains larger than mud. Thus, large grains remain
mixed in with the fine grains that the currents can just barely transport.
The determination of sorting in a sediment can be approached qualitatively (which is usually
good enough for most sedimentologists and stratigraphers) or statistically. Statistical studies of
sorting usually involve sieving unconsolidated sediments to determine the abundance of each grain
size. They are usually restricted to studies of process sedimentology where the researchers are
trying to understand the details of particular types of flow. In studies of lithified rocks, the
sediments can not be easily sieved and statistical grain size distributions require extensive
microscope work. In this class, as in many sedimentological studies, the qualitative approach is
sufficient and most practical.
To estimate sorting, identify the finest and coarsest abundant grains in the sample. If the
size range is all within one size class on the Wentworth scale, it is well sorted (or very well sorted);
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if the size range covers 4 size classes on the Wentworth scale, it is poorly sorted. Note that the size
range within one category is much larger for large particles. Thus, a conglomerate composed of
cobbles ranging in size from 50 to 60 mm in diameter covers only 1 size class and is well sorted,
whereas a sandstone with grains ranging from 0.1 to 1 mm covers 3 size classes and is only
moderately sorted. The following table give the number of classes versus sorting “name”. You can
also use the figures in the textbook, p. 20, to get an intuitive feeling for what well sorted versus
poorly sorted sediments look like.
Standard
Deviation of φ
<0.35
0.35-0.5
0.5-0.71
0.71-1.0
1.0-2.0
>2.0
Number of size
classes Present
1
1
2
3
4
5-8
Sorting
Very Well Sorted
Well Sorted
Moderately Well Sorted
Moderately Sorted
Poorly Sorted
Very Poorly Sorted
3. Shape (p. 21, textbook): The shape of sedimentary grains can be described in terms of sphericity
(spherical shape) or roundness (the smoothness of the grain edges). Sphericity refers to how close
the particle approaches the shape of a sphere versus a rod or a plate. Roundness (or angularity) is
how rounded (or jagged) the edges of the particle is. You can have a well-rounded particle with low
sphericity (think of a great skipping stone that is flat and very smooth), as well as have a very
angular particle with high sphericity.
Sphericity reflects the inherent physical properties of a mineral grain and is often
determined by the shape of the mineral prior to weathering, the shape of the organism in the case of
skeletal grains, or zones of weakness in the rock for lithic (rock) fragments. Minerals which
display a conchoidal fracture (quartz) will eventually show a high degree of sphericity, whereas a
mica grain with basal cleavage will never look like a ball bearing because it breaks much more easily
into flat plates. The sphericity of particles can be described as spheroidal, meaning it is close to
being equidimensional; platy, meaning disk-like; or prismatic, meaning rod-like. Use these three
terms to answer the questions in lab.
Roundness is determined by the abrasion of the particles during transport and varies with
the transportation distance and energy. The longer a grain is actively transported, the rounder the
edges become because small protrusions get broken off. Faster flows round grains more quickly
because collisions between grains are more forceful. Soft minerals round more quickly than hard
minerals. Often, sand sized soft minerals are broken into microscopic particles.
Textural Maturity: The sorting and shape of grains, as well as their composition, can be
summarized into a description of the textural maturity of a sediment or rock. An immature
sediment/rock will contain poorly sorted, angular grains and will often include soft and
soluble minerals. It is called immature because its characteristics were not changed
substantially by transport processes. A texturally mature sediment/rock consists of well
rounded grains that are well sorted. Soft and soluble minerals are rare. High textural
maturity generally is a sign of a long transport history that changed the grain characteristics
and sorted them by size. Textural maturity is an important feature to note when interpreting
sedimentary sequences, because differences reflect variations in source rock, depositional
environment, transportation processes, climate, geography, and a host of other conditions.
4. Fabric: Fabric is the spatial arrangement and orientation of the grains in a sedimentary rock.
(Because this lab is focused on sediments before they are lithified, fabric will not be analyzed in this
lab. However, you will need to understand it for later labs.) Some fabric elements, such as the
orientation of non-spherical grains, can indicate paleocurrent directions. For example, elongated
Lab 1A, Page 3
pebbles, gravel or cobbles tend to stack up (imbricate) in a position that is the most
hydrodynamically stable. This would be with the flat faces gently dipping up-current.
Another useful fabric is grain packing which refers to the spacing or density of grains in the
rock. Well packed grains are in contact with each other and the rock is said to be “grain
supported”. In contrast, a “matrix supported” rock contains grains floating in a matrix of finer
grained sediment where the grains are not in contact with each other. Grain-supported rocks tend to
be deposited when sediments are either well sorted or the proportion of large grains is high relative
to small grains. Matrix-supported rocks form from sediments that are poorly sorted, were
deposited in quiet water environments with autochthonous shells supported by fine-grained
sediment, or from flows with very high concentrations of fine grained sediment like mud flows.
Thus, grain packing has complex origins, but when used with measures of sorting, analysis of
sedimentary structures, and characterization of depositional environments, it is very useful. Many
more fabric elements are also visible in thin sections, but we will not be looking at those in this lab.
Composition
The composition of sediments and sedimentary rocks provides both descriptive and genetic
information. The resistance of certain minerals to physical and chemical weathering and the
abrasive effects of transport makes their abundance within a sediment an important indicator of the
intensity and duration of the processes that led to deposition. For example, if soft, unstable
minerals are present, weathering and abrasion were not substantial during transport. In addition, the
composition can reveal much about the provenance (source terrain) and can aid in the interpretation
of regional tectonics. For example, abundant clasts of andesitic volcanic fragments within a sand
matrix may suggest that the sediment was deposited in proximity to a volcanic arc (depending on
other sedimentary characteristics), which implies deposition within a convergent margin tectonic
regime. Compositional information is commonly so important that in many sedimentary rock
classification schemes, compositional characteristics are used as modifiers to the rock name. For
example, a quartz dominated sandstone would be called a quartz arenite whereas a feldspar-rich
sandstone would be called a feldspathic arenite. As you read through the following review of some
common particle compositions, think about possible origins of the different minerals.
Quartz: Quartz is the most common sedimentary particle. Quartz is very hard (7 on Mohs scale),
has no cleavage, and is chemically stable. Thus it is only slightly abraded during transport, and it
does not dissolve significantly. A sediment that starts out with diverse sand compositions will
eventually end up being dominated by quartz with sufficient transport and weathering, because
quartz is both very abundant and very resistant to weathering and fracturing. Quartz can be well
rounded or angular, depending on how long it has been in the transportation cycle.
Feldspar: Feldspars are a group of important rock-forming minerals. Plagioclase (CaAl2 Si2 O8 )
weathers quickly, is gray or creamy to white colored, has two directions of cleavage and striations,
which are only visible on large crystals. Volcanogenic feldspar is usually plagioclase and tends to
be soft, euhedral (you can see the crystal shape), and chalky in texture and color. Potassium
feldspar (KAlSi2 O8 ) is pink to cream in color, has two directions of cleavage but has no striations.
Sodium feldspar is similar looking to K-spar. Na- and K-spar are chemically more stable than
plagioclase, but they tend to alter to clay minerals as they weather. Feldspars have a hardness of 6
on Mohs scale. Thus, they abrade more rapidly than quartz during transport.
Mica: Micas are soft, and most sedimentary micas are either biotite (dark) or muscovite (light).
They are platy and shiny, and weather quickly to clays. Micas abrade rapidly during transport.
Rock fragments: As you can guess, pre-existing rock fragments can look like almost anything, from
black chunks of shale to pieces of granite, schist, volcanic rock, or limestone. During transport,
they commonly break up into component mineral grains.
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Oolites: These are little balls of CaCO3 that have been rolled around in a high energy carbonateprecipitating environment. They look kind of like fish eggs (or jawbreakers!) with a concentric
internal structure. They are common on beautiful tropical beaches, and you will see them in the
Bahamian lab.
Peloids: Peloids (or pellets) are either fecal pellets or some other sand-size spherical or ellipsoidal
particle that displays no internal structure. This is a catch-all term for "we are not sure what they
are" but they are composed of carbonate.
Skeletal Particles: Most of the skeletal particles you will see in sedimentary rocks in lab will be
from marine organisms, but they can also be derived from fresh water or terrestrial organisms.
Some examples of marine skeletal particles include: bivalve shells, bryozoans, gastropods,
echinoderm spines, and coral fragments. Identifying the organisms can be challenging, but
provides interesting information on depositional environments and paleobiology.
Lab 1A, Page 5
Geology 109L, lab 1: Sedimentary Particles
Name:
TA:
Lab Assignment
Part 1: Characterization of Sands
In this exercise, you will examine a variety of unconsolidated sediments collected on the Broome
Town Beach, Western Australia (BR7-00 samples), and from Putah Creek, California (numbers
only). Please take care not to spill or mix up the samples. For each sample, identify the three most
abundant grain compositions, and characterize each in terms of size, sphericity and rounding. Then
characterize the sorting of the entire sample. Use dilute HCl to test for calcite and aragonite.
Sample: BR7-00-1
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
3. Third Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
Overall Sample Sorting:
Sample: BR7-00-2
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
3. Third Most Abundant Grain Composition:
Grain size:
Sphericity:
Overall Sample Sorting:
Lab 1A, Page 6
Rounding:
Sample: BR7-00-3
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
3. Third Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
Overall Sample Sorting:
Sample: BR7-00-4
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
3. Third Most Abundant Grain Composition:
Grain size:
Sphericity:
Overall Sample Sorting:
Lab 1A, Page 7
Rounding:
Sample: BR7-00-5
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
3. Third Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
Overall Sample Sorting:
Sample: BR7-00-6
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
3. Third Most Abundant Grain Composition:
Grain size:
Sphericity:
Overall Sample Sorting:
Lab 1A, Page 8
Rounding:
Sample: BR7-00-7
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
3. Third Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
Overall Sample Sorting:
Sample: BR7-00-8
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
3. Third Most Abundant Grain Composition:
Grain size:
Sphericity:
Overall Sample Sorting:
Lab 1A, Page 9
Rounding:
Sample: 1
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
Overall Sample Sorting:
Sample: 2
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
Overall Sample Sorting:
Sample: 3
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Overall Sample Sorting:
Lab 1A, Page 10
Rounding:
Sample: 4
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
Overall Sample Sorting:
Sample: 5
1. Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
2. Second Most Abundant Grain Composition:
Grain size:
Sphericity:
Rounding:
Overall Sample Sorting:
Part 2: Interpretation of Your Results
Read the web page for the lab (http://www-geology.ucdavis.edu/~Gel109/Labs/Lab1.html) and use
the photographs to place the samples into context in your mind. Then answer the following
questions. (These questions might be hard for you to answer exactly the first day of class. I am
looking for qualitative answers with your best reasoning. Chapters 2-4 have more information.)
1. How does the grain size of the Broome samples change from off-shore to near the beach? What
does this trend suggest in terms of relative wave energy?
2. For Broome samples, how do the grain size changes correlate with the sedimentary structures
observed where each sample was collected? (Use the information on the web page.)
Lab 1A, Page 11
3. How does the sorting of the grains change from off-shore to near Broome beach? What does
this tell you about variations in flow speed?
4. For Broome samples, how are the shells different between off-shore and near shore samples?
Which shells are from organisms that lived in the environment where the sample was collected, and
which are more likely to have been transported into the environment where they were collected?
(Use the information on the web page.)
5. Based on sorting in Putah Creek samples 1-4, would you characterize flow in Putah Creek as
steady and constant or highly variable? For Putah Creek sample 5, the sorting is very different
from the other 4 samples. What does this suggest about the flow that deposited sample 5 relative to
the other samples?
6. For Putah Creek samples, would you characterize the samples as mature or immature? Why?
7. The sediment source for all Putah Creek samples is the same. Why does sample 5 have a
different dominant composition?
Lab 1A, Page 12