SEDIMENTARY ROCKS:
Diagenisis-all chem bio and phys changes that happen to sediments after they are
deposited. This happens within a few KM of the crust at temps less than 200 degrees C.
Diagenesis includesRecrystalization-Development of more stable minerals from less stable ones.
Lithification-sediments  rock through compactation and cementation. Natural
rocks include calcite, silica, and iron oxide.
Types of Sedimentarty enviroments
Continental- Rivers and streams, Glacial (tills, moraines), Wind, (Aeolian)
Transitional (shoreline)- Tidal Flats, Beaches, Lagoons, Deltas.
Marine- Shallow, up to 200 meters
Deep-Deeper than 200m.
Sedimentary facies
-Each unit (called a facies) has characteristics reflecting a particular environment.
-Different sediments often accumulate adjacent to one another at the same time.
Detrital
-Conglomerate
-Sandstone
-Siltstone
-Mudstone
-Shale
Chemical
-Limestone
-Dolostone
-Chert (Si)
-Evaporite (Nacl, etc)
Sedimentary StructuresStrata or beds-layers
Bedding planesCross bedding-inclined layers
Mud cracks-shrinkage on exposure to air.
Fossils- Remains or traces of prehistoric life preserved by sediment or sedimentary rock.
2 types, body and trace fossils.
Sedimentary rocks are simply weathering products that have been turned into solid rock.
> they give us:
1. Insight into earth history and life on earth over time
2. Major energy and metal deposits (e.g., coal, oil, placer gold)
> sedimentary rocks constitute about 5% of the earth's crust by volume, but 75% of the
surface area of the crust
>> this is because sedimentary rocks form at the surface of the earth (they are like the
"icing" on the cake)
Sedimentary Cycle
> we can think of the processes that lead to the formation of a sedimentary rock as a cycle
>> first, a rock must be brought up to the earth's surface by tectonic uplift
>> then it is exposed to weathering and erosion
>>> this yields two kinds of products: (1.) mineral and rock fragments, and (2.) dissolved
ions
>> these materials can then be transported by a wide range of transporting agents, such
as rivers, wind, glaciers, ocean currents
>> deposition can then occur to form a sediment pile due to:
>>> (1) decrease in the energy of the transporting medium in the case of the mineral and
rock fragments
>>> (2) a change in the chemical conditions in the case of the dissolved ions
>>Lithification- finally, sediment can be turned to solid rock by the process called
lithification, which involves two processes:
1. Physical: the compaction of the sediment (due to pressure of overlying layers)
with subsequent release of water (this is particularly important for the finegrained clay sediments
2. Chemical: particles in contact, under the pressure of the overlying layers, can
recrystallize and anneal at the points of contact, and water solutions passing
through the sediment can precipitate cement on particle surface to cement
together the sediment (for water to flow through, the sediment must be porous and
permeable). The three most common cements are microcrystalline calcite, quartz
and hematite. Gypsum can be a cement, and in a rock with lots of clay the clay
itself can act like a cement since the clay particles stick together so well.
>> the solid sedimentary rock that results can re-enter the cycle by being uplifted and
eroded (or, of course, it could be further buried and metamorphosed, or even melted to
generate a magma)
Classification of Sedimentary Rocks:
There are two kinds of sedimentary rocks: (1) clastic, and (2) chemical. Clastic rocks
form from the cementing together of particles of rock and mineral fragments. Chemical
sedimentary rocks form from the precipitation of ions out of solution. We need to discuss
the classification scheme for each type separately.
(1) Clastic (also known as Detrital) Sedimentary Rocks
A. Particle Size:
>particle size is the main criterion in the classification of clastic sedimentary rocks
>> this criterion is chosen because it reflects the energy of the transporting medium (see
your classification scheme in the lab manual, or in the textbook)
>>> the coarser the particle size, the higher the energy that was needed to transport it
>>> the names for clastic sedimentary rocks range from claystone (or mudstone or shale)
for the lowest energy of transport, through siltstone and sandstone to conglomerate for
the highest energy
>>> think of a fast flowing (i.e., high energy) river flowing into an ocean: all particle
sizes can be carried by the river, but when the river reaches the ocean, it loses energy. In
doing so, it dumps the coarse gravel first, and progressively finer stuff as it loses more
and more energy, until, at the lowest energy, it can only carry fine clay farther out to sea.
So, at a beach, there is a progression from coarse sediment high up on the beach, to the
finest clay out in the deeper water offshore.
B. Degree of Sorting:
> this is a subsidiary criterion for the classification of clastic sedimentary rocks
>> it reflects the grain-size distribution of the rock (are all the particles the same size, or
is there a range of sizes?), which tells you a lot about the conditions under which the rock
formed
a. Well-Sorted: the grains are all the same size
> this implies that the transporting agents acted for a long time on the material, sorting
out and removing particles smaller and larger, leaving the uniform grain-size material
(e.g., think of a beach, where waves push the coarser pebbles up high on the shore, and
wash the fine clay out to sea, leaving the sand in between in the shallow water of the
beach)
b. Poorly-Sorted: there is a wide range in grain size
> this implies, generally, rapid dumping of sediment into the site of deposition, so that
there was no opportunity to separate the particles by grain size
> consider the name sandstone for a rock: this just tells us that the particles are, for the
most part, sand sized
>> we need a further division to reflect the degree of sorting: a quartz sandstone is wellsorted, and a greywacke (with sand and clay intermixed) is poorly-sorted.
>Note: the degree of sorting is important for the permeability of the sediment or
sedimentary rock. In a quartz sandstone, to become so well sorted, all the quartz grains
will be nice, uniform-sized, rounded particles. This results in open spaces between the
grains even when they are packed together. Hence, it has high porosity and permeability.
In a greywacke, those spaces are filled up by smaller clay particles, and the permeability
will be much lower.
C. Mineralogy and Grain Shapes
> this is a subsidiary criterion for the classification of clastic sedimentary rocks
>> it reflects the intensity of weathering and/or the distance/time of transport of the
sediment
>leads to concept of maturity versus immaturity:
>> Mature: the particles have been weathered a long time and have traveled a long way;
this breaks down the low-resistance minerals and concentrates the high-resistance
minerals, and rounds the grains of quartz, and allows sorting to take place (e.g., quartz
sandstone is 100% quartz; conglomerate has clasts with rounded edges)
>> Immature: rapid deposition with little transport preserves the low-resistance minerals
(e.g., feldspar, olivine, etc.), and results in little sorting and grains that are not well
rounded (e.g., arkose, which is a sandstone with a high amount of feldspar in it; breccia,
which is a rock with large fragments that are very angular)
(2) Chemical Sedimentary Rocks
> the classification of chemical sedimentary rocks is based on their chemical
composition
>> following are some of the different kinds
a) Limestone: composed of microcrystalline calcite
>> formed in warm sub-tropical to tropical seas
>> organisms extract Ca and carbon dioxide from sea-water to build their shells; when
the organisms die, the shells and shell fragments can accumulate and get cemented
together to make limestone
>>> hence, we call limestone a BIOCHEMICAL sedimentary rock, because of the role of
biological life in its formation
>> an important type of limestone is REEF LIMESTONE, formed from the
transformation of a coral reef into a solid rock; the coral reef has a lot of empty spaces, so
that the REEF LIMESTONE, unlike most other limestone, can be highly porous and
permeable
b) Chert: composed of microcrystalline quartz
>>some marine micro-organisms extract Si and O from seawater to build their shells out
of quartz; when they die and sink to the sea bottom, they can form a layer that cements
into chert
>>> chert is thus, like limestone, a BIOCHEMICAL sedimentary rock
c). Banded Iron Formation (BIF): this is our main source of Fe
>> this rock is found as iron-oxide layers (magnetite or hematite) alternating with chert
(hence the banding); the layers can be as thin as a few millimeters
>> huge BIF deposits are restricted to rocks between 2.2 and 1.9 billion years old
> how does BIF form?
>> we believe that BIF indicates that the earth's atmosphere in its early history was
oxygen-poor; this allowed iron, when weathered from rocks, to be in the reduced Fe2+
state
>>> in that form, it is highly soluble, and abundant iron was carried to the seas and held
in solution there
>> the oxygen level of the atmosphere rose, until by 2.2 billion years ago the level was
sufficiently high to oxidize much of the iron in the seawater, changing it to Fe3+
>>> Fe3+ has very low solubility, and so it was precipitated out as iron-formation layers
>> the alternating chert/iron-oxide layers probably represented yearly cycles
>> once the oxygen level got high enough to precipitate much of the iron, subsequent
weathering of rocks at surface would lead to little iron being carried to the sea, as most
would remain in soils as solid iron oxides, since the Fe3+ is so insoluble
>> BIF is our main source of iron: it is highly concentrated in iron, and the iron is in the
form of an oxide, not a silicate
d) Evaporites: these are sedimentary rocks formed by direct precipitation during the
evaporation of saline sea water
>> some kind of enclosed sea is needed to allow sufficient evaporation to reach the point
of saturation (e.g., the Red Sea)
>> e.g., gypsum (CaSO4 . 2H2O)
>> e.g., potash (sylvite: Kcl)
>> e.g., rock salt (halite: NaCl)
> since NaCl has the highest concentration of all the salts dissolved in seawater, rock salt
is the most abundant evaporite when an enclosed sea evaporates
Abundance of Sedimentary Rocks:
> Shale is the most abundant sedimentary rock, representing > 50% of all sed rocks
>> this is because feldspar is the most abundant mineral in the earth's crust (almost
entirely in igneous and metamorphic rocks), and it weathers to clay, the main ingredient
of shale
> Sandstone constitutes about 20% of all sedimentary rocks
> Limestone constitutes about 10% of all sedimentary rocks
WHAT DO SEDIMENTARY ROCKS TELL US ABOUT THE GLOBAL-SCALE
CONDITIONS AT THE SURFACE OF THE EARTH?
(1) there has been free water at the earth's surface since at least 3.9 billion years ago
>> we can find sedimentary rocks with evidence of deposition in water back that far in
time
(2) the oxygen content of the atmosphere has changed over time
>> it was very low in the early earth history
>> banded iron formation reveals this to us
FEATURES OF SEDIMENTARY ROCKS
> features of sedimentary rocks can be used to interpret the geological environment under
which they formed
>> here are some examples of such features, and what they mean:
1. Bedding:
>> beds are normally deposited horizontally; the change from one bed to the next
overlying bed must represent a change in conditions (for example, a change from
sandstone to shale shows a decrease in the energy of the transporting medium)
>> within a single bed, we often see CROSS-BEDS, where there are internal "beds" at an
angle to the main horizontal bed
>>> it indicates that the sediment was deposited by the action of a unidirectional flow of
air or water, and you can determine the direction the flow came from by the nature of the
cross-beds (see the textbook)
2. Ripple Marks:
>> most of us have seen ripple marks at the beach
>> they represent the action of a flowing fluid (water or wind)
>>> in water, it represents shallow water, as wave action (which causes the ripples) can
only cause an effect a small depth into the water
>>> if the ripple marks are symmetric, then they formed from the back-and-forth action
that occurs in ocean or lake water near shore
>>> if the ripple marks are assymetric, then there must have been a unidirectional flow of
air or water (and we can tell the direction the flow came from)
3. Mud Cracks:
>> mud cracks form when wet mud is exposed to the air and the mud dries up; when it
dries up, it shrinks and forms cracks (usually crudely hexagonal in shape); if these cracks
are buried, they can be preserved as fossil mud cracks
>> they clearly must form in shallow water; an ideal spot is in a tidal flat, where the sea
is constantly going in and out regularly due to the tides
4. Fossils:
>> when life forms die, they can be buried and preserved in the resultant sedimentary
rocks
>> the soft organic parts rarely get preserved, but the hard shells often do
>> preserved evidence of life forms are called fossils
>> by studying the nature of the fossils, and comparing them to modern life forms, we
can say something about where (i.e., the conditions of formation of the sediment) the
sediment was deposited
5. Grain Size:
>> we talked earlier about how the grain size of clastic sedimentary rocks gives an
indication of the energy of the transporting medium, and hence the environment of
formation
6. Degree of Sorting:
>> we talked earlier about how the degree of sorting (e.g., well- versus poorly-sorted)
gave information about the nature of transport and deposition of the sediment
7. Grain or Rock Composition and/or Grain Shapes
>> we talked earlier about how the "maturity" of the sedimentary rock gave information
on the nature of transport and deposition of the sediment
ECONOMIC RESOURCES IN SEDIMENTARY ROCKS
> we'll discuss only two examples: (1) placer gold, and (2) oil
(1) Placer Gold:
> gold that is weathered out of igneous rocks can get carried down streams as gold
nuggets
>> due to the high density of gold, to find big gold nuggets you have to look in the
stream where the river has high energy; this is where the lower density silicate rock and
mineral fragments are in the form of gravel
>>> so, when you pan for gold you look for the gravel bars
>> these deposits of gold are called "placer" deposits
>>> if such stream beds are subsequently buried and turned into solid rock, you can still
mine them by underground or open-pit mining (for example, the world's richest gold
deposits are sandstone "placer" deposits in South Africa (the Witswatersrand deposits)
formed some 3 billion years ago
(2) Oil:
> oil is one of the FOSSIL fuels
> fossil fuels are stored solar energy, as they form from the accumulation of dead
organisms and plants (we call them hydrocarbons: composed of C, H, and O)
>> this includes coal, tar, oil and gas (oil, tar, and gas are known as petroleum products
from the Latin petros: rock and oleum: oil)
> coal forms from the accumulation of dead plant matter (fronds and ferns) in continental
swampy regions
>> if buried beneath a thick pile of sediments, such accumulations can be squeezed and
heated, with a resultant increase in C content
>> they can turn into a solid rock called coal, which we can mine and burn
But we want to concentrate on PETROLEUM
Origin of Petroleum
> it begins in a marine environment
(NOTE: although life has existed on earth for almost its whole history, abundant life
forms have only existed for the last 600 million years (i.e., the last 10% of earth's
history). Thus, most oil is produced from sedimentary rocks less than 600 million years
old, with most coming from rocks less than 100 million years old, since a lot of older oil
has had a chance to be destroyed)
Step 1:
> microscopic organisms die and fall to the sea floor
> most get oxidized and are lost to us, but if they get buried by anoxic muds (i.e., a
reducing environment) they can be preserved
>> hence, the beginnings of petroleum is as solid organic molecules (kerogens) preserved
in muds that, when transformed into sedimentary rocks, become SHALES and
MUDSTONES (these are what we call the SOURCE rocks for petroleum)
>>> Note: when people talk about OIL SHALE, they are actually referring to shale with
high amounts of solid kerogens in them; this kerogen is NOT in the form of a fluid like
oil, so the name oil shale is not correct. To get the hydrocarbons out, the rock must be
mined and crushed and treated to liberate the hydrocarbons
Step 2:
> the source rock shale and mudstone then gets buried under more sediments, and the
increased temperature and pressure, after millions of years, breaks down the complex
solid kerogen molecules into simpler molecules that are fluid (tar, oil and gas)
>> the temperature range for this to happen is between 50 degrees Centigrade and 120
degrees Centigrade; as the temperature increases, the tar goes to oil, which, under higher
temperature, goes to gas. Above 120 degrees Centigrade, the hydrocarbons are essentially
destroyed
Step 3:
> compaction of the source rock then squeezes the fluid tar, oil and gas out, and these
fluids of very low density will try to migrate through the rocks towards the surface
Step 4:
> on the way up, some of the petroleum can get trapped
>> estimates suggest that only about 0.1% of the petroleum actually gets trapped, with
the other 99.9% reaching the surface and getting "burnt up" in the oxygen atmosphere
How does oil and gas get trapped?
> it needs a rock with high porosity and permeability (i.e., like a sponge)
>> we call such a rock a TRAP rock (or RESERVOIR rock)
> the most common trap rocks are:
a. Quartz sandstone (60% of all petroleum traps are qtz sandstone)
b. Reef limestone (39% of all petroleum traps are reef limestone)
BUT!! If the petroleum can flow into the trap rocks, surely it can flow out again!
>> true, but if the rocks are arranged the right way the oil can be held underground and
we can drill down and pump it out
>>> 80% of all petroleum traps underground are folded sedimentary rocks, with the
petroleum being trapped in an up-arch (i.e., an antiform), and the antiform must have an
impermeable CAP rock above a permeable TRAP rock for the petroleum to become held
there (see the textbook for diagram)
> Note importance of SALT DOMES in creating petroleum traps:
>> evaporitic salt layers are very interesting, as salt is a low-density, plastic material
>>> under the pressure of overlying rocks, the salt can be mobilized, and it will rise up to
form large dome structures (SALT DOMES) that push up the overlying layers to form
antiform structures
>>> this creates arches for trapping petroleum; as well, petroleum can migrate up
permeable layers and get trapped against the impermeable salt dome
Oil and the Plate Tectonics Framework
Consider the importance of Passive Continental Margins
> when a new ocean opens up (e.g., the Red Sea), two new continental margins are
formed (i.e., boundaries between ocean crust and continental crust)
>> rivers run down to the new sea from the two new continental halves, and sediment
starts to build up on these margins
>> as the ocean opens wider and wider, more and more sediment gets deposited on these
continental margins, reaching thicknesses of up to 15 kilometers
>>> these margins then are ideal sites for the formation of petroleum deposits, because
marine organisms thrive in the shallow continental shelves, and the rapid sedimentation
and subsequent burial to great depth is what is needed to preserve and then transform the
kerogens
>>> the eastern seaboard of the United States and Canada is a perfect example of such a
margin: we call such a margin a PASSIVE CONTINENTAL margin because not a lot is
happening at it. The continental lithosphere and the oceanic lithosphere are joined and are
moving together at the same rate in the same direction (contrast this with an ACTIVE
continental margin, like the west coast of South America, where the Pacific Ocean plate
is in collision with the South American plate, and subduction is going on and the Andes
mountains are being built.
> at the opening of an ocean, like the Red Sea, when passive continental margins are
born, there is a good chance that layers of rock salt will form on these margins, because
such a narrow sea can get closed off and evaporation can result in salt deposition
>> once the ocean is wider, salt no longer forms, but the early-formed salt layers get
deeply buried by other sediments, and the pressure can result in salt domes that can help
trap oil (e.g., in the Gulf of Mexico and Texas)
> as an ocean opens wider, what happens at a passive continental margin?
>> the margin sinks, partly because more sediment brought to the margin by rivers from
the continent add weight, but mainly because the oceanic lithosphere (i.e., lithosphere
with ocean crust on top) at the margin is getting farther away from the mid-ocean ridge as
time passes, and it gets cooler and denser and hence sinks
>>> this allows room for more and more sediment to pile up
>>> eventually, the oceanic lithosphere gets so dense that it snaps right off from the
continental lithosphere, and the passive margin turns into an active margin; this is what
happened in western North America, where the Rocky Mountains have been built
>>>> when that happens, the active mountain building there can produce folded rocks
that can trap petroleum; if you are to close to the real active part of the mountain
building, where there is lots of igneous activity, then the petroleum will be burnt up. But
if you are a little farther towards the continent, the milder folding and heating will allow
petroleum to be preserved. Alberta in western Canada is in that ideal situation with
respect to the Rocky Mountains
So, why are two-thirds of the oil reserves in the Middle East (with consequent political
ramifications)?
>> because there was geologically recently a large sea (the Tethys Sea, an arm of the
Pacific Ocean) between Africa and Eurasia
>>> lots of sediments piled up on the two passive margins of those continents on either
side of the sea (with the trapping of abundant hydrocarbons)
>>> then the sea started to close up, with the transformation of the margins into active
margins, producing nice fold structures to trap petroleum
>>>> since the process is geologically recent (last 100 million years), and there are nice
folds generated, and two continental margins pushed together, there are huge petroleum
deposits preserved there
Last revision: 15 November 2001
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