Geo Science 100 - Section 2
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SEDIMENTARY ROCKS
Sufficient for us is the testimony of things produced in the salt waters and
now found again in the high mountains, sometimes far from the sea.
Leonardo da Vinci
The term “sedimentary” is from Latin, meaning “to settle”. Sedimentary rocks are
formed on the surface of the earth by processes which include weathering, erosion,
transportation, deposition and lithification. Because of their position of formation
sedimentary rocks cover over 70% of the earth’s surface. (However on a volume basis
they represent less than 10% of the outer 10 km of the earth, the great majority of crustal
rocks being igneous and metamorphic.)
Sedimentary rocks are important and of interest to us for several reasons:
(1) They are abundant.
(2) They contain much evidence of processes which operated at the earth’s surface in
the past.
(3) They contain fossils of plants and animals and thus the available record of past
life on earth. (Because of temperature and pressure conditions very few fossils
are preserved in igneous or metamorphic rocks.)
(4) They are economically important because they contain the majority of fossil fuels
- petroleum, natural gas and coal; chemical sedimentary rocks are important
sources of halite, gypsum, potassium chloride, etc.; many important ore deposits
occur in sedimentary rocks, e.g. copper, lead, zinc, iron, manganese.
In the sedimentary rock labs we will consider: some of the processes responsible for the
formation of sedimentary rocks; a classification of sedimentary rocks; clastic rocks,
chemical rocks, sedimentary structures; sedimentary environments; how to identify
sedimentary rocks.
The Origin of Sedimentary Particles and Sedimentary Rock
We will investigate briefly the four steps in production of sedimentary rock: weathering,
transportation, deposition and lithification. The links between these steps are shown in
the diagram, for two major types of sedimentary rock.
(1) Weathering: Most minerals are stable only under the conditions in which they form.
Previously we looked at plutonic igneous rocks. These rocks looked hard and durable.
But because they formed deep beneath the earth’s surface the minerals in them are
unstable. When exposed at the earth’s surface they decompose due to chemical
weathering - the effects of contact with gases and water and naturally occurring acids.
New stable minerals form from the old minerals, and much material goes into solution.
Also physical weathering occurs whereby rocks are broken into smaller fragments called
lithics, and individual minerals.
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(2) Transportation: Materials generated by weathering may be transported by water,
wind, ice or gravity. Most particles move in suspension or as bed load by water or wind.
Products of chemical weathering by oxidation, hydrolysis and solution are carried in
solution. Weathering continues during transport. Also particles may be rounded, and
sorted by size en route to their environment of deposition.
(3) Deposition: Material which is transported as particles is deposited by settling.
Material which is dissolved is deposited by precipitation when the solution is evaporated
or a chemical reaction forms an insoluble material.
(4) Lithification: Lithification is the conversion of unconsolidated material to a
consolidated cohesive solid rock, by compaction, cementation and crystallization.
Precipitation of calcite or quartz as cement is a common process.
This four-step description of sedimentary rock formation is summarized in the chart
showing two major groups: (a) clastic (or detrital), and (b) chemical including biogenic
(or biochemical).
(a) Clastic (Detrital) rocks are derived from particles of minerals, rocks or
organic matter, products of physical weathering
(b) Chemical rocks are derived from materials produced by chemical weathering
and precipitated from solution by inorganic or by biogenic processes.
Next we will look at each rock group in terms of their textures and compositions.
DETRITAL (CLASTIC) SEDIMENTARY ROCKS
Textures of Clastic rocks
It is important to be able to differentiate between clastic rocks and crystalline rocks.
Clastic rocks are composed of clasts of various sizes, shapes (angular to rounded) which
are distinct and separate from one another. Porosity or cement may be visible among
grains.
Texture is important in that it provides evidence from which inferences may be made
concerning the mechanism and distance of transport, the nature of the source rocks, and
the geological environment in which deposition and lithification occurred. We will
consider the following aspects of texture: grain size, rounding, sorting, compaction and
cementation.
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1. Grain size
The size of particles in clastic rocks ranges from lithic fragments several meters in
diameter to extremely small (clay <1/256 mm). We use a simple 3 part classification
based on grain size.
Coarse - grained
Medium - grained
>2 mm diameter (gravel size)
conglomerates have rounded clasts
breccias have angular clasts
1/16 - 2 mm diameter (sand size)
arenites include sandstones
 quartz arenite - quartz is the dominant framework
component
 arkose - feldspar is the dominant framework component
 litharenite - rock fragments are the dominant framework
component
<1/16 mm diameter
 silt size (1/256 to 1/16mm in diameter):
silt stones
 clay sized material (1/256 mm in diameter):
shale - rock is fissile - cleavages of the clay minerals are
parallel to each other - imparts a tendency to split into thin
layers to the rock
mudstone - non-fissile - cleavages are not parallel and the
rock has a blocky fracture

Fine – grained
2. Rounding
Rounding refers to the degree of smoothness of grains. Rounding depends on many
factors such as mineralogy and degree of weathering. For our purposes we will consider
that particles have been subject to abrasion during transport, thus the more rounded the
particle the greater the distance or time of transport by water or wind. Transport by ice
does not produce rounding of grains. A reduction in size is associated with increased
rounding as particles are transported.
The diagram illustrates relative angularity and roundness in particles. Use a handlens or
microscope to observe the characteristics of fine - and medium grained rocks.
3. Sorting
Sorting is the degree to which grains in a clastic rock are uniform in size. This property
is very useful in identifying the agent of transportation. Wind-transported particles are
very well sorted. Streams sort sediment moderately to well as particles are deposited.
Glaciers do not directly cause sorting.
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Well sorted material has grains of one dominant size (and often of one composition) and
usually have been transported by wind or streams. [note that to engineers well sorted
means to contain a wide range of particle sizes]
Poorly sorted material contains several sizes of particle, as found in glacial till, in which
particles may range in size from clay to very large boulders.
4. Compaction
Compaction is the process in which sediment grains are squeezed closer together due to
the weight of the overlying sediments. Water and gas are forced out of the sediment and
porosity is reduced.
5. Cementation
Cementation is the process by which clasts are bound together by minerals precipitated
from groundwater in pore spaces. The most common cements are calcite, quartz,
hematite and limonite. It is useful to be able to distinguish among cements. Calcite can
be identified by a drop of HCl. Quartz cement results in a very hard rock from which it is
difficult to dislodge grains. Hematite and limonite, which are iron oxide minerals, give a
reddish brown colour to rocks.
Composition of Clastic Rocks
The majority of clasts in clastic sedimentary rocks are composed of one or more common
constituents, most of which are familiar to you.
1. Minerals  Quartz
 Feldspars (plagioclase, K-feldspar)
 Micas (muscovite, biotite)
 Clay minerals such as kaolinite
 Iron oxides such as magnetite, hematite, limonite
2. Lithic clasts such as gneiss, gabbro
3. Biogenic clasts-organically produced fragments such as plant remains.
Maturity of Clastic Rocks
Maturity is a term which refers to both mechanical factors and chemical factors
1. Chemical maturity refers to the stability of the constituents in the temperature, pressure
and chemical environment. Minerals of igneous and metamorphic rocks crystallize at
elevated T and P, and are therefore unstable in sedimentary conditions at the Earth’s
surface. Ferromagnesian silicates would be unstable clasts whereas quartz would be a
more stable clast. Unstable silicates usually alter to clay minerals by chemical
weathering.
Mechanical maturity refers to the above-mentioned degree of rounding and sorting. An
aggregate of well-rounded and well-sorted particles is considered mechanically mature,
whereas a rock containing poorly-sorted angular clasts is considered mechanically immature.
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CHEMICAL SEDIMENTARY ROCKS
Textures of Chemical Rocks
1. Crystalline Texture
Crystalline Texture consists of interlocking crystals formed by minerals precipitated from
seawater, groundwater or lakes. The texture is similar to that of igneous rocks. Four
size categories are used:
Coarse crystalline >2 mm
Medium crystalline 1/16 - 2 mm
Fine or microcrystalline 1/256 - 1/16 mm
Cryptocrystalline <1/256 mm
2. Skeletal Texture
Skeletal Texture occurs when shells or parts of shells dominate over other material, such
as cement. Shell composition is usually calcium carbonate and the cement is usually
calcium carbonate. Such a texture originates in a beach or reef environment.
3. Oölitic Texture
Oölitic texture is formed of oöliths, which are 0.1mm to 1.5 mm well-rounded
carbonate particles. The nucleus is a small clast around which have accumulated
concentric layers of aragonite or high-Mg calcite. They are found forming today in
such environments as the Persian Gulf, and Great Salt Lake, U.S.A..
Composition of Chemical Rocks
Chemical sedimentary rocks are listed in the table showing five groups, their
composition, texture, and names of the most important rocks. The five compositional
groups are discussed separately below.
1. CARBONATES comprise the most abundant group. They consist of 50% or more
carbonate minerals, and give rise to a variety of limestones. Inorganic limestones
form by the precipitation of calcium carbonate in water. Travertine has been
precipitated from fresh water in springs or caves. Dolomite, or dolostone, may form
by direct precipitation, but it is thought that most originates by replacement of some
calcium in limestone by magnesium.
2. SILICEOUS rocks are composed mainly of polymorphs of microcrystalline quartz
(SiO2) formed by precipitation of silica in seawater. Examples are chert (light
colour), flint (dark colour from organic matter), jasper (red colour from iron oxide),
and agate (banded). Chert commonly occurs as nodules in limestone. Extensive chert
beds are thought to have originated as biochemical sediment from tests (skeletons) of
radiolaria and diatoms. It is hard to distinguish the origin of chert in hand specimen.
3. EVAPORITES are rocks composed of minerals which precipitated out of solution (in
lake or seawater) because of solar evaporation. Examples are halite, gypsum,
anhydrite and sylvite. Evaporites are indicative of environments of restricted water
circulation, e.g. a shallow arm of the sea, or of an excess of evaporation over
precipitation.
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4. “ORGANIC ROCKS” are not rocks in the strict sense of the definition because they
are not composed of minerals. However, such materials as peat and coal are
significant in that they indicate environmental conditions of formation. For example
a high rate of organic debris accumulation in stagnant lagoons may be inferred from
the presence of peat or coal deposits. The successive stages of coal formation are
related to increasing temperature and pressure during diagenesis and metamorphism.
burial
further burial
Metamorphism
PEAT
LIGNITE
BITUMINOUS
ANTHRACITE.
soft brown coal
soft black coal
hard, black coal
5. IRON FORMATIONS are a group of rocks which formed mainly over 2 billion years
ago and do not form today. Banded iron formations are a laminated sequence of iron
minerals and chert formed in conditions of low O2/CO2 ratios in the atmosphere and
ocean. The original precipitate was magnetite some of which later changed to
hematite and limonite.
SEDIMENTARY STRUCTURES
Silver blades of surf
falling crisp on rustling grit
shaping the shore as a mason
fondles and shapes his stone.
from Briggflatts (Bunting)
Sedimentary rocks display a variety of structures formed as sediments are transported and
deposited.
1. Stratification or Bedding
A bed or stratum is a horizontal layer of sediment that represents deposition under similar
conditions over a period of time. A bed differs from adjacent beds in terms of grain
size, shape, degree of sorting or colour. Surfaces between beds are bedding planes.
Many beds are homogeneous throughout. Others may exhibit systematic changes; for
example in graded bedding grain size increases from coarse at the bottom to fine at
the top.
2. Cross bedding (cross-stratification)
Cross bedding is inclined stratification, formed for example in sediment transported
unidirectionally by wind or water.
3. Ripple Marks
Sand grains deposited in moving air or water develop undulating surfaces of several
types. Two important examples are symmetrical and asymmetrical ripple marks.
4. Tool Marks
Tool marks may form on bedding planes when shells, sticks, pebbles or bones slide, roll
or bounce along a surface. Tool marks are holes or linear drag marks. Flutes are V6
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shaped depressions scoured into a surface by erosional action of currents, the V
pointing upstream.
5. Mud Cracks and Raindrop Impressions
Mud cracks, or desiccation cracks form when moist deposits of mud dry and contract.
When the cracks become in-filled with a different sediment. Mud cracks indicate the
original way up of a sedimentary sequence - the cracks are wider at the top. This is
helpful in determining whether rock layers have been overturned. Raindrop
impressions or hailstone imprints may form on exposed terrestrial surfaces as small
depressions which may be preserved by burial by sediment.
6. Trace fossils
Animals leave evidence of their existence, in the form of tracks, trails, burrows,
borings, nests and fossilized feces (coprolites).
SEDIMENTARY ENVIRONMENTS
The sedimentary environment is where sediment is deposited and the physical, chemical
and biological conditions at that place at the time of deposition.
Sedimentary environments range from organic reefs to glacial and deep sea. Also within
a particular type of environment there can be wide variations. For example there are
many coastal sedimentary environments on the coast of Vancouver Island, ranging from
low energy estuarine to high energy gravel beaches.
By studying modern sedimentary environments information on grain size and shape,
sedimentary structures and life forms may be used to infer the nature of ancient
environments of deposition from sedimentary rocks.
The following notes summarize important characteristics of sedimentary rocks formed in
each of several environments.
1. Alluvial fans are stream deposits that accumulate near a mountain front in a dry
basin. They typically contain poorly sorted coarse gravel and boulders. Fine grained
sand and silt may be deposited near the margins of the fan.
2. Floodplains are nearly flat expanses across which the great rivers of the world
typically meander before reaching the sea. A considerable amount of sediment is
deposited on these plains. Rocks formed in a floodplain environment are commonly
channels of cross-bedded sandstone deposited on the point bar of a meander and
enclosed in a shale which represents the finer horizontally-lain flood deposits.
3. Aeolian environments are those in which material is transported and deposited by
wind. Wind is an effective sorting agent and will selectively transport sand and dust,
leaving gravel behind. Dust-sized particles are lifted high in the atmosphere and may
be transported thousands of kilometers to where they accumulate as a blanket of
loess. Windblown sand commonly accumulates in dunes that are characterized by
well-sorted, fine grains. The dominant sedimentary structure in aeolian environments
is large-scale cross-bedding.
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4. Glaciers transport but do not effectively sort material. Thus glacial environments are
characterized by unsorted, unstratified accumulations of angular boulders, gravel,
sand, and fine silt. Glaciofluvial and glaciolacustrine deposits may be well sorted.
5. Deltas are large accumulations of sediment that are deposited at the mouths of rivers.
A delta is one of the most significant sedimentary environments and includes a
number of sub-environments such as stream channels, floodplains, beaches, bars, and
tidal flats. The deltaic deposit as a whole consists of a thick accumulation of sand,
silt and mud which fines outward and downwards (contrast this fining-downward
appearances with graded bedding, found in deep ocean environments).
6. Shoreline environments are characterized by beaches, bars and spits which
commonly develop along low coasts and partly enclose quiet water lagoons and tidal
flats. The sediment in these features is well washed by wave action and is typically
clean, well-sorted quartz sand. Behind the bars and adjacent to the beaches, fine silt
and mud are often deposited as tidal flats.
7. An organic reef is a solid structure composed of the shells and secretions of marine
organisms. The reef framework is typically built by coral and algae, but many other
types of organism contribute to the reef community. Together, these organisms
produce a highly fossiliferous limestone.
8. Shallow marine environments are found along the continental margins, and in the
past were even more extensive than today. Sediments deposited in shallow marine
waters form extensive layers of well-sorted sand, shale, and limestone, which
typically occur in a cyclical sequence as a result of shifting environments from
changes in sea level.
9. Deep ocean environments adjacent to the continents receive a considerable amount
of sediment transported from the continental margins by turbidity currents. Deep sea
deposits off of submarine fans or continental slopes are thus characterized by a
sequence of graded beds. Further offshore or in areas distant from sediment sources
fine grained deposits predominate. Shales can form anywhere, including far offshore.
Oozes and pelagic muds have fine grain sizes and slow deposition rates.
Diatomaceous earth and chert forms where siliceous microfossils like diatoms and
radiolaria collect (cool and high latitude waters) in the absence of other sediment or
at high productivity (blooms caused by upwelling) also (off desert coasts). Chalks are
formed from foraminifera in warmer and shallower quiet waters.
Walther’s Law & the Facies Concept: Armans Gressley noted the lateral change in
lithologies for particular bed of Triassic sediment, yet the underlying and overlying
lithologies remained constant. He reasoned that the environment of that middle bed was
different across its lateral extent. Johannes Walther realized that vertical sequences of
beds are due to the horizontal shift of neighboring environments with the passage of time.
Simply stated: Vertical successions of strata represent the lateral shift of adjacent
facies with time. Sandy beaches can laterally or vertically become muddy bays or
subaerial dunes by this reasoning, but not glacial moraines or deep sea turbidites, without
erosion or missing time.
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