Download Lab 2: Sedimentary Environments, Rocks, and Structures

Lab 2: Sedimentary Environments, Rocks, and Structures
Sedimentary rocks account for a negligibly small fraction of Earth’s mass, yet they are
commonly encountered because the processes that form them are ubiquitous in the near-surface
environment. Thus, they preserve the history of that portion of the planet that is most familiar.
Sedimentary rocks indicate paleoenvironment, i.e. ancient climates and ecology. Sometimes
they provide the only remaining evidence of former mountain ranges or shallow seas.
Sedimentary rocks are also essentially the only type of rock that contains fossils, which not only
are indicative of previous environments, but also are crucial in dating and correlating rock units.
Sedimentary rocks also provide a record of previous geologic hazards such as seismic events,
volcanic eruptions (ash deposits), storms, and fluctuations in climate. Furthermore, key
economic natural resources involve sedimentary rocks. Resources such as coal, oil, natural gas,
gypsum, aggregate (sand and gravel), and salt are all found within sediments.
Sedimentary Structures
Sedimentary structures such as stratification (layering), ripple marks, cross-bedding, and
mudcracks can be preserved in sedimentary rocks. These structures provide important
information about depositional environments such as flow direction, climate (arid, semi-arid, or
humid) and setting (e.g. fluvial, lacustrine, or marine). These structures also may indicate which
direction was originally up within the rock. Tectonic forces can fold and overturn rocks, so
establishing the original orientation is not always easy, but is often useful.
Bedding
Sedimentary rocks will often be deposited in discrete layers, which leads to a particularly
important sedimentary structure called bedding. Bedding layers can range in thickness from
millimeters to tens of meters. Typically, though not always, bedding is originally horizontal in
orientation; tilted bedding indicates that the rock has been deformed in some way.
Lithification
Sedimentary rocks start out as loose sediment. To become a sedimentary rock, the
sediments must be lithified, which involves compaction and cementation. Compaction occurs
through pressure via deep burial. Water is removed and the grains are packed tightly together.
During cementation, minerals such as quartz, calcite, or hematite precipitate out of water and fill
the spaces between the clasts, locking them together. The term friable describes a poorly
cemented rock that falls apart easily. Note: rocks can become friable either because they were
never cemented thoroughly, or because the cement has been re-dissolved and removed.
Depositional Environments
A sedimentary environment is a geographic location characterized by a particular
combination of geologic processes and environmental conditions. Geologic processes include
the currents that transport and deposit sediments (water, wind, or ice) and the plate tectonic
settings that affect sedimentation. For example, the geologic processes of a beach environment
include the dynamics of waves crashing against the shore, shoreline currents, and the distribution
of sediments on the beach. Environmental conditions include the kind and amount of water
(ocean, lake, river, arid land), the landscape (lowland, mountain, coastal plain, shallow ocean,
deep ocean), and biological activity.
Location
Depositional
Environment
Lakes
(Lacustrine)
Glacial
Alluvial Fan
Continental
Environments
Rivers and Streams
(Fluvial)
Floodplains
Swamps, Marshes, or
Bogs
Evaporite Basins
Desert (Eolian)
Deltas
Shoreline
Environments
Beaches
Shallow marine or
continental shelf
Deep marine
Marine
Environments
Reefs
Characteristics
Lake deposits are generally low-energy
environments where fine-grained sediments are
deposited in thin layers. Lakes that freeze over
seasonally may develop varves: alternating layers
of light (coarser) and dark (finer) sediments.
Large lakes can also have higher-energy, sandy
beaches.
Glacial deposits form along the margins of and
beneath glacial ice. Because ice can transport any
size grain (unlike water or wind), deposits are
typically very poorly-sorted
Alluvial fans are fan-shaped wedges of sediment
deposited along the margin of a steep slope.
They often contain a lot of coarse-grained,
moderately to poorly-sorted sediment.
Rivers and streams typically deposit medium- to
coarse-grained (sand to cobble-sized) sediment in
their channels. River sediment is often
moderately- to well-sorted.
Floodplains are relatively low-energy
environments where finer (clay, silt, fine sand)
sediments are deposited in well-defined layers.
These plains are only periodically wet and when
they dry out mudcracks often develop.
Swamps are typically rich in organic material that
is buried and compressed to form coal
Shallow basins in arid regions and lagoons may
become supersaturated and precipitate evaporite
minerals.
Eolian environments are arid and typically have
winds that transport and sort medium- and finegrained sediment (sand to silt). Eolian sediments
are often well-sorted and show well-developed
cross-bedding.
Deltas form where rivers and streams enter larger
bodies of water. They often contain fluvial-type
deposits as well as swampy environments.
Beaches occur on the margins of large bodies of
water. They generally contain deposits of wellsorted, medium-grained sediment with planar
bedding.
Shallow marine environments are formed on the
margins of continents, on the continental shelf.
Associated deposits are typically medium- to
fine-grained and well sorted. In warm (subtropical to tropical) environments these sediments
may be calcite-rich.
Deep marine areas receive relatively little clastic
sediment. Common deep marine sediments are
either very fine-grained or microcrystalline (from
recrystallization of microscopic silica-producing
organisms).
Organic structures composed of calcium
carbonate-secreting organisms (i.e. coral) built up
on continental shelves or oceanic volcanic islands
Rock Type(s) Formed
shale, siltstone
Conglomerate and sometimes
breccia
If glacial origin is known, the
rock is called a “tillite”
Conglomerate, sandstone, and
sometimes breccia
Sandstone, siltstone, some
conglomerate
Siltstone, shale
Coal
Rock gypsum, Rock salt,
crystalline limestone
Sandstone, siltstone
Sandstone, siltstone, shale
Sandstone
Sandstone, siltstone, shale,
diatomite, oolitic limestone,
chalk
(Depends on supply of clastic
and chemical sediments)
Chert, shale
Limestone (fossiliferous)
Figure 7.2 Depositional Environments
Clastic Sedimentary Rocks
Clastic sediments are made of particles of mineral or rock fragments, known as clasts,
that have been weathered from preexisting rock and transported by gravity, water, ice, or air.
Chemical weathering involves the dissolution or decomposition of these minerals, whereas
mechanical weathering consists of processes such as abrasion and cracking that do not change
the mineral content of the material. During transport, clasts are abraded and become increasingly
rounded (smooth surfaced) and equidimensional (spherical). Chemically and mechanically
stable minerals, such as quartz, survive this transportation better than the less stable minerals and
are therefore concentrated in sediments that have been transported for long times and distances.
Transportation also tends to segregate particles by size. High-energy environments (e.g. rivers,
coasts) can transport large clasts, while low energy environments (deep ocean) can transport only
small clasts. Different transport processes can be more or less selective about which grain sizes
are moved, therefore the degree of sorting can be indicative of the transport medium. For
example, glacial ice is not selective at all and can move the widest range of clasts (tiny clay-sized
particles to boulders the size of buildings), while wind typically moves grains sized within the
narrow range of silt to fine sand. A sediment with clasts of uniform size is known as well-sorted,
while one containing a wide range of clast sizes is poorly-sorted. Sediments that consist
primarily of well-sorted, rounded, and spherical quartz grains indicate that the material has been
subjected to long or repeated periods of transport and is designated as mature. On the other
hand, sediments that consist of various minerals and rock fragments that are angular, nonspherical, and poorly-sorted are indicative of sediments that have not been transported far and
are called immature. Factors relating to maturity are outlined in the following tables and figures:
Stability of common minerals under surficial weathering conditions. Note that there is a
relationship between this series and Bowen’s reaction series. The minerals formed at highest
temperatures and pressures are the least stable, while those formed at lower temperatures and
pressures (closer to surface conditions) are more stable.
Most Stable
Least Stable
Fe Oxides
Al Oxides
Quartz
Clay minerals
Muscovite
Potassium Feldspar
Biotite
Sodium-rich Plagioclase
Amphibole
Pyroxene
Calcium-rich Plagioclase
Olivine
Degrees of rounding and sphericity. The degree of rounding is often a function of transport
duration; the longer a clast is in transport, the more rounded it will become. The degree of
sphericity also depends somewhat on transport duration, though some mineral grains start out
more equidimensional than others. Quartz, for example, often is found in nearly
equidimensional grains in granite, while amphibole and feldspar crystals are elongated.
Guide to grain size. Note that (a) A conglomerate may instead be called a breccia if its clasts
are angular. (b) Sand can be further divided into fine sand (1/16 to about 1/8 mm), medium sand
(1/8 to 1 mm) and coarse sand (1 to 2 mm). (c) The term clay can refer either to a range of grain
size (< 1/256 mm) or to a family of sheet silicate minerals known as clay minerals.
Name of particle
Range limits of
diameter (mm)
Names of loose
sediment
Name of
consolidated rock
Boulder
Cobble
Pebble
Sand
Silt
Clay
> 256
64 to 256
2 to 64
1/16 to 2
1/256 to 1/16
< 1/256
boulder gravel
cobble gravel
pebble gravel
sand
silt
clay
boulder conglomerate
cobble conglomerate
pebble conglomerate
sandstone
siltstone
mudstone and shale
Guide to grain sorting. Note that the degree of sorting is independent of the absolute sizes of
the grains involved.
Clastic sedimentary rock classification key.
Texture
Grain Size
Gravel (particles
larger than 2 mm)
Sand (particles
visible, but less
than 2 mm)
Clastic
Silt (particles not
visible, feels gritty
and cannot be
scratched by
fingernail)
Clay (particles not
visible, feels
smooth and is
easily scratched by
fingernail)
Composition
Gravel-sized clasts
mostly rock
fragments
Gravel-sized clasts
mostly rock
fragments
Commonly quartz,
feldspar, rock
fragments
Predominately
Quartz
Predominately
feldspar
Predominately
Lithic
Other
Texture
Rock Name
Rounded
grains
Conglomerate
Angular
grains
Breccia
Sandstone
Quartz
sandstone
Arkosic
sandstone
Lithic
sandstone
Most often quartz,
some feldspar
Siltstone
Clay minerals and
quartz
Mudstone or
shale
Chemical Sedimentary Rocks
Chemical sedimentary rocks are formed by the precipitation of compounds from aqueous
solutions. For example, limestone forms from the precipitation of calcium carbonate (calcite)
from seawater. Often, biology plays a key role in the formation of limestones as the calcite
comes from the shells of sea creatures. Another example of a chemical sedimentary rock is an
evaporite, a rock that forms when water is evaporated from closed basins in arid climates. As
evaporation progresses, the remaining water can become highly saline and eventually will
become supersaturated with respect to a variety of dissolved constituents, leading to their
precipitation from solution. Common evaporite minerals include gypsum and halite.
Silica is undersaturated in sea water so one would not expect to find it as a direct
precipitate from sea water. However, small siliceous organisms such as diatoms, radiolarians,
and some sponges are highly efficient in removing silica from sea water to form their skeletons.
After these organisms die they sink and accumulate on the sea floor. Many cherts are formed by
lithification and recrystallization of such deposits.
Organic Sedimentary Rocks
An unusual sedimentary rock is coal, a carbon-rich rock that forms when organic matter
(trees and other plant matter) is buried and compressed in an oxygen-poor environment so that
decomposition does not proceed. This is common in swampy settings.
Chemical and organic sedimentary rock classification key.
Texture
Composition
Microcrystalline quartz
Halite
Gypsum
Chemical
(crystalline)
Calcite
Dolomite
Organic
Biochemical
Plant material (carbon)
Other Properties
Scratches glass
Three perfect cleavages at
90°, tastes salty
Softer than fingernail,
cleavages not at 90°
Readily reacts with dilute
hydrochloric acid
Powdered rock reacts with
dilute hydrochloric acid
(much less reactive than
calcite)
Brown to black, low
specific gravity
Rock Name
Chert
Rock Salt
Rock
Gypsum
Limestone
Dolostone
Coal
Calcite
Fossils and fine grains
Fossiferous
Limestone
Calcite
Entirely composed of shell
fragments
Coquina
Calcite
Ooids (layered spheres)
Oolitic
Limestone
Quartz
Diatoms, very white color,
often low density and
friable
Diatomite
Station 1
a. Describe samples 1a and 1b.
Structure
Grain size
Sorting
Rounding
Flow
direction (e.
g. R to L)
Depositional
Environment
1a
1b
b. Examine samples 1a and 1b.
What sedimentary structure is common to both samples?
What is the flow direction in 1a? (R to L, L to R, or indeterminate?)
Which side is the top of sample 1b (A or B)?
Station 2
a. What sedimentary structure is present in samples 2a and 2b?
b. Which side is the top (A or B) and how is this shown?
2a___________________________________________
2b___________________________________________
c. Specimen 2b contains another prominent sedimentary structure.
What is this sedimentary structure and how did it form?
Station 3
Describe samples 3a and 3b.
Grain size
Sorting
Rounding
Structures
3a
3b
Sample 3a formed in a different type of environment than 3b. Was the environment for 3a
much higher energy, much lower energy, or did the two environments have a similar
energy? What indicates this?
Station 4
Describe sample 4a.
Grain size
Sorting
Rounding
Structure
Depositional
Environment
4a
Find the burrow in 4a. Would one expect to find other sedimentary structures? Why or
why not?
Station 5
Examine samples 5a and 5b.
What sedimentary structure is shared by these rocks?
How did it form?
Identify samples 5a and 5b.
5a_______________
5b_______________
Sample
6
7
8
9
10
11
12
Clastic or
Chemical
Representative
Grain Size
Sorting
Roundness and
Sphericity
Composition
Other (e.g.
fossil content)
Rock Name