Mudstones
Introduction
Mudstones, especially those deposited in a marine environment, contain important clues to depositional
processes and to conditions existing in the oceans at the time of their deposition. To interpret the origin of these
mudstones we need to examine sedimentary structures, bioturbation, and microfossil contents. We will also
examine nodules and the color of the mudstones.
Sedimentary Structures provide clues to the processes acting on the sediment during it's deposition. A problem
with mudrocks, unlike sandstones, is that they are fine-grained and typically the sedimentary structures
associated with these rocks are also very small scale. Consequently, these structures may be missed. Another
problem with mudrocks is that they are usually recessive and are not always well exposed like the sandstones.
Therefore, they are hard to study. Furthermore, difficulties may arise when attempting to determine depositional
processes of a marine mudstone because it is difficult to directly observe processes acting in this environment.
Our only access to these deep marine environments has been through small submersibles. However, even with
these limitations in mind with can still gather reams of data on mudstones to make logical interpretations of
depositional processes and environments.
Laminations--Are generally produced by a change in grain size or composition. Many times these laminae
result from a muddy layer passing to a silt or sandy layer. Clays found in mudstones are deposited in quiet
water environments. A change from a clay layer to a sandy/silty layer typically indicates that a change in current
energy occurred in the depositional environment. The quiet water environment in which the clays were being
deposited was briefly subjected to a higher energy current that transported the coarser and heavier sand grains.
The question is what led to this change in current energy. Fluctuations in strength of currents flowing parallel to
contours (contour currents) may cause some these alternating sandstone & mudstone laminae sea floor.
Alternatively, turbidity currents (density currents flowing downslope that consist of a mixture of water, mud,
and or silt, and or sand) may lead to the production of these laminae. Mudstones deposited in a shelf
environment may also show alternating mudstone and siltstone/fine sandstone layers that reflect storm waves
hitting on the seafloor, leading to the deposition of the silt layers. Storm waves will not reach the deep water
marine environments such as the continental slope and the abyssal plain
Massive/structureless mudstone and bioturbation--Massive structureless mudstone may result from very
viscous mudflows (a slurried flow of mud and water moving downslope) or from low density turbidity currents.
Conversely, massive mudstones may result from intense bioturbation (churning of sediment by organism such as
worms, clams, shrimp, etc.) and the destruction of laminae. Bioturbation indicates that there was enough
oxygen in the depositional environment for organisms to subsist. Laminae in deep marine environments are
best preserved where oxygen levels are sufficiently low to exclude burrowing organisms. This does not mean
that all laminae found in rocks were formed in an oxygen poor environment. Sandstones deposited in rivers,
alluvial fans, and marine environments contain laminae formed in these oxygenated environments. If sediment
in a particular environment is deposited at high rates, then these high rates may make the environment hostile to
organism.
Microfossils--Microfossils are important constituents of marine mudrocks. One generally needs a hand lens to
observe these fossils. Microfossils found in mudstone result from planktonic organisms that secret a hard shell
which die and fall to the sea floor. Benthic (organisms living on the sea floor) microfossils are also found in
mudrocks. Microfossils are important because they are not only used for dating the rocks but also because they
provide important clues about environmental conditions existing at the time the sediment was deposited. For
example, there is a particular foraminifera (calcareous planktonic organism)that coils in a certain direction when
the water temperature is cooler and it coils in the opposite direction when the water is warmer. Diatoms
(siliceous planktonic plants) require a high nutrient content in the waters and are therefore mostly found in
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abundance in areas of the oceans where ocean water flows upward from some depth bringing with it nutrients.
This process of upward moving ocean water is termed upwelling. Additionally, the presence or absence of
calcareous shells in a mudstone deposited in the deep ocean also give clues about whether the sediment was
deposited above or below the Calcium Carbonate Compensation Depth (CCD). At depth the ocean is
undersaturated with respect to calcium carbonate (we will discuss this in future lectures). As a result the
calcium carbonate will dissolve. That depth in the ocean where the rate of calcium carbonate dissolution equals
the rate of supply of calcium carbonate is known as the CCD.
Color--The color of mudstones (see text and supplementary article) is principally related to the organic content
and the oxidation state of the iron in a mudrock. A black to gray mudrock usually indicates a high organic
content. Organic matter requires special conditions for preservation. Organic matter is best preserved in
oxygen-deficient environments. In the presence of oxygen, the organic matter oxidizes and generates carbon
dioxide and water. What conditions led to these oxygen-poor environments? A gray color also indicates the
presence of organic matter, but not to the extent of amounts found in black mudrocks. A green color generally
reflects the presence of illite, chlorite and biotite and the absence of appreciable amounts of organic matter and
hematite. A red color indicates that hematite is present in the rock. Hematite forms in an oxidizing
environment. One must never use color as the lone criteria for determining the nature of the depositional
environment. The color of the mudrock can change with burial and when exposed to elements in outcrop.
Nodules--Nodules are spherical, elliptical to flattened which are mostly composed of calcite or dolomite.
Pyrite, chert, and siderite nodules also exist. Nodules form in the sediment after the deposition, either
immediately after deposition or some time after deposition. Nodules result from the precipitation of a cement in
a local area of the rock. The formation of the nodules depends on geochemical conditions.
Laboratory
Microfossils
Foraminifera. Foraminifera are both planktonic and benthic, and they are predominantly marine. Plankton
consists of plants and animals that float in the ocean and that drift with the currents. Phytoplankton are plants
and zooplankton are animals. Foraminifera are animals that construct shells of calcium carbonate which can be
preserved. Foraminifera are crucial to reconstructing depositional environments and for dating.. If you examine
the California stages on a geological time scale you'll see such names as Wheelerian, Mohnian, Saucesian, etc.
These stages are constructed using foraminfera. Benthic foraminfera are important for environmental
interpretation because they are commonly preserved in the same environment in which they lived. Some of
these are temperature and pressure sensitive so that benthic foraminifera can be used as depth indicators.
Oxygen isotope ratios change with temperature, and these changes are preserved in the shells of planktonic
foraminifera. Consequently, foraminifera can be used to determine the whether waters were warm or cold when
they lived or when glacial and interglacial periods occurred. Foraminifera have been around since the
Cambrian.
Examine the following samples: (Look at page 119 of your text for microphotographs of foraminifera in thin
section).
Recent foraminifera in slides. Sketch a representative foram and put a scale on it.
Sample 121a. Miocene Monterey Formation, California. Find the benthic foraminifera in these small samples
using a handlens. Train your eyes to look for the microfossils.
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Examine the fusilinids in the matchbox and compare with those in sample 2027. 2027 is a limestone and not a
mudstone, but we will use it to examine the fusilinids. Make sure that you can see the wall structure of the
foraminifera in the fresh surface of the sample. Fusilinids are an important guide fossil for the Permian.
Sample 3817. Miocene Monterey Formation, California. This is probably a calcareous mudstone (mudstone
containing calcium carbonate). Find the foraminifera. Note on the weathered surface how the foraminifera
preferentially weather out. This is typical of calcium carbonate because precipitation combines with carbon
dioxide to form carbonic acid. H20 + CO2 > < H+ + HCO3- . This equation shows that carbon dioxide
dissolved in water generates hydrogen ions and increases the acidity (lowers the pH). This causes calcium
carbonate to dissolve. Note the laminations in this sample and the concentration of the foraminifera. The
Monterey Formation is both the major source and oil-bearing rock in California. The offshore oil platforms
present off of the Santa Barbara Formation are pumping mostly from the Monterey Formation.
1. What in this sample might indicate that the foraminifera have been transported and redeposited?
2. Was this deep marine sample deposited above or below the CCD? Explain
Sample from Rincon Formation. Late Oligocene to early Miocene, Santa Barbara area, California. Find the
foraminifera in this mudrock. Look for the white specs. The Rincon Formation underlies the Monterey
Formation in the Santa Barbara area and it is principally a slope deposit.
Sample from the Juncal Formation, Paleocene, Central California Area. The Juncal Formation is a submarine
fan deposit. Find the foraminifera. You are getting good if you can find these.
Examine sample 4021 (place a bit of sample on a small tray and examine with handlens). This sample is from
about 250-300m water depth, offshore Santa Barbara Island. As you can see, there is very little clastic sediment
in the sample. If one examines a topographic map of Santa Barbara Island you'll find that there are no major
drainage systems on the Island. Therefore, there is little clastic input to the offshore. Consequently, little
dilution of the accumulating carbonate sediment. Find the foraminifera in the sample.
Diatoms. Diatoms are phytoplankton composed of opal-A or amorphous silica (SiO2). Diatoms, because they
are plants, need to be in the area of maximum light penetration in the oceans. Generally this zone extends to a
depth of about 200m but varies depending on turbidity, waves, location of sun in the sky, etc. Diatoms are also
important environmental indicators. They are typically found in areas of high biologic productivity in the
oceans where, as mentioned above, upwelling occurs. They are also important for dating rocks. Ages for late
Miocene and early Pliocene rocks can be resolved to an amazing 200,000 years.
Look at the glass slides of the diatoms and
1. sketch a representative diatom. Do not use the slides marked station 1, station 2, etc. Most of these
diatoms are Recent and are from the Japan Sea. There are suggestions that diatoms have been around
since the Jurassic, but they are definitely present in the Cretaceous. With burial and temperature
increase diatoms are destroyed. So they might have been present in the Jurassic.
2. Also find and sketch a sponge spicule. Sponge spicules look like an elongate, narrow thorn. Sponge
spicules fall off the sponge when it dies and are composed of silica.
3. Also see if you can find the radiolarians in the slide marked radiolarians (only one exists, but you can
find them in the diatom slides also. Radiolarians are composed of amorphous opal A and they are
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zooplankton. Radiolarians occur from the early Paleozoic (Cambrian/Ordovician) to the present. Thick
accumulation of radiolarians led to the production of the famous Mesozoic Franciscan red cherts.
771A & 771B. Monterey Formation, late Miocene, Southern California.
1. Find the diatoms in both of these samples of diatomaceous mudstones. Also note the fish scales and fish
debris in these samples. 771A is comprised of mostly diatoms and clay. Note how light the sample is.
This is because the diatoms contain numerous pores in their tests and are relatively low density.
2. Place a drop of water on 771A and note how easily it is absorbed. Again, this is the result of the porous
diatoms.
Dinoflagellates. Dinoflagellates are phytoplankton and secrete a shell composed of an organic compound
termed sporopollenin which is preservable. Dinoflagellates are found in marine rocks and sediments from the
Permian to Recent . Dinoflagellates are interesting because they are responsible for the red tides present in the
oceans. The red tides form when plentiful nutrients are found in the oceans. The dinoflagellates, because of
this rich nutrient supply, multiply rapidly. A red pigment in the dinoflagellates result in the red color of the
ocean. Red tides can be dangerous because they secrete a toxin know as paralytic shellfish poison. One should
not eat shell fish during these red tides. Dinoflagellates cause the bioluminescence in the sea.
Sketch a representative dinoflagellate. The slides are of dinoflagellates that I collected during a red tide while
on a oceanography lab cruise. Imagine the tremendous amounts of dinoflagellates that were in the water. The
red tide occurred immediately following a period of heavy rains. Presumably, runoff waters collected nutrients
(nitrates and phosphates) from the land. These runoff waters flowed into the oceans via rivers, and the
dinoflagellates used these nutrients to proliferate.
SEDIMENTARY STRUCTURES/BIOTURBATION/NODULES/COLOR
SQC 8 Laminated mudstone. Sisquoc Formation, late Miocene to early Pliocene, Santa Maria basin, California.
Note the alternating light and dark laminae. These result from compositional changes rather than changes in the
size of the sediment. The dark layers are mostly clay with some silt.
1. What are the white layers composed of? These sediments were likely deposited in a slope environment
in an area of low oxygen levels where organisms were excluded from the substrate. The arrow indicates
the up direction.
2. How can you tell which way was up when the sediment was being deposited. Look at the white layer at
the base of the number M8 and follow it around to the smooth surface. Note that in places it is cut into
by the overlying gray layer. This probably resulted from the gray layer eroding or scouring into the top
of the white layer as it was being deposited. Deposition may have been by a turbidity current.
3. How would you classify these laminae (see book)
Sample 16--structureless mudstone, Sisquoc Formation, late Miocene to early Pliocene, Santa Maria basin,
California. Note that primary sedimentary structures are absent in this rock. The sediment was thoroughly
bioturbated.
1. What does bioturbation imply about oxygen levels in the depositional environment?
Sample 7381 LE5& 6076--Fissile shale. Shale is a fissile mudrock. Fissility is the property of splitting into thin
sheets. Fissility probably results from the alignment of clay particles or from the presence of laminae.
1. Would you expect to see fissility in a well bioturbated mudstone?
2. Sample 6076 borders on slate. Opposite the side of the sample which is marked by the number are what
appear to be infilled mudcracks. Mudcracks form in clay-rich sediments after they have been wet and
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then left to dry. This is an important environmental indicator. One would expect mudcracks to form in
a terrestrial environment but not in a marine environment.
Sample 5022--Moenkopi Shale, Arizona, Lower Triassic.
1. Find the casts of salt crystal on the surface of the sample. These crystals indicate that evaporation
exceeded precipitation and that the area in which the sediment deposited had water which subsequently
evaporated.
2. Also note the red color of the shale. This is typical of the Moenkopi Formation. The red color is due to
hematite and also helps to support a marginal marine environment. The central portion of this rock is a
siltstone.
Sample 3685--Argillite. Note the horizontal burrows. These were probably made by worms which fed on
organic matter. Horizontal burrows are more typical of deeper marine environments while vertical burrows are
more common in shallow marine environments.
Sample 3561--Formation? Age? Location: Colorado. Note that it is laminated and its very dark color. The dark
color is from the presence of organic matter.
Sample 3982--Chatsworth Formation, Cretaceous, Southern California--Note the green color of the mudstone.
The green color results from the presence of illite and chlorite and from an absence of organic matterand
hematite. Also note the alternating green and light layers. These layers consist of alternating mudstone and
siltstone/fine sandstone. The preservation of these layers does not reflect deposition in an oxygen-poor
environment but from changes in current strength. The green clay layers accumulated in a low energy
environment while the light coarser layers support and increase in current strength and probably reflect
deposition by a turbidity current.
1. Find the burrows in this sample. These burrows support deposition in an oxygenated marine
environment.
Sample 6313--Residual Mudstone, Silverado Formation, Paleocene, Orange County. This mudstone used to be
a very coarse sandstone that was deposited by a river. Intense weathering led to the alteration of the feldspars
and the biotite into clay. Quartz was dissolved by organic acids produced by vegetation when it decays. The
dark spherical objects are iron pisolites which are formed in soils developed in a tropical environment. They
were not deposited with the rock but formed in the sediment after deposition and alteration. These pisolites
form through the migration of and nucleation of iron. Not all soils will show this. Also note the red color of the
mudstone.
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