Sediments and sedimentary rocks

1. EASC 229: STRATIGRAPHY AND SEDIMENTATION
SEDIMENTS AND SEDIMENTARY ROCKS
Jennifer E. Agbetsoamedo
Department of Earth Science
University of Ghana, Legon
April, 2021 1

2. WHAT IS SEDIMENT ?
• Sediment is solid material that is moved and deposited in a new location.
• Sediment can consist of rocks and minerals, as well as the remains of plants and
animals.
• It can be as small as a grain of sand or as large as a boulder.
• Sediment moves from one place to another through the process of erosion.
• Erosion is the removal and transportation of rock or soil. Erosion can move sediment
through water, ice, or wind.
• Sediment is usually characterized by a specific type of fabric (texture) and this defines
the sediment.
• A good wealth of information regarding the history of sedimentation can be deduced
from studying the texture of a sediment type.
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3. TEXTURE OF SEDIMENT
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Sedimentary texture encompasses three fundamental properties of sedimentary rocks:
grain size, grain shape (form, roundness, spericity and surface texture [microrelief] of
grains), and fabric (grain packing and orientation). Grain size and shape are properties of
individual grains. Fabric is a property of grain aggregates.
Textures can be clastic, where they are composed of grains from pre-existing rocks
(allochthonous grains) or crystalline, where the crystals (“grains” sensu lato) grew from a
fluid producing an interlocking mosaic of crystals.
Knowledge of sediment properties like texture and mineralogical compositions
can be interpreted to provide very vital information regarding the depositional
history of the sediment.
Vital information on depositional histories include:
• Energy levels in the environment
• Distance of transportation
• source area lithology,
• paleoclimate,
• tectonic activity,
• processes acting in the depositional basin, and
• time duration in the basin.

4. TEXTURE OF SAND - INTERPRETATION
• Texture is an indicator of energy levels in the environment of deposition (i.e., the
place where sediment accumulates):
(i) Beach
(ii) Riverbed
(iii) Lake
(iv) Delta
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• Moving water (such as waves or currents) is considered to be a high energy
environment.
• Quiet water or still water (water without waves or currents) is considered to be a
low energy environment.
• Deep water environments (e.g bottom of the oceans and lakes) are characterized by
quiet water, because wave motion is restricted to the upper part of the water column.
• SO, WHAT INFORMATION DO SEDIMENTS PROVIDE IN DETERMINING THE ENERGY
LEVELS IN THE DEPOSITIONAL ENVIRONMENT.

5. • Coarse-grained sediments such as sand & gravel indicate high energy environments.
• A large amount of energy is required to transport gravel-sized clasts, and moving
water is required to transport sand.
TEXTURE OF SAND – INTERPRETATION (GRAIN SIZE)
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• Fine-grained sediments like clay and silt indicate low energy environments.
• There is insufficient energy to bring larger clasts into the environment.
• If the water were moving, the clay would not be able to settle out and be deposited on
the bottom.

6. • The degree of roundness of sand grains helps in knowing the distance of transportation
Texture of Sand – Interpretation (Grain Shape)
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• Sphericity is a measure of how spherical an object is.
• Sphericity refers to "equal dimensions".
E.g., a ball would have high sphericity, and so would a cube (high sphericity, but low
roundness).

7. • In a sedimentary environment, a well rounded sand grain indicates that the sediment
has been transported for a long distance from the original source area, and that it has
been in the depositional environment for a long time.
TEXTURE OF SAND – INTERPRETATION (GRAIN SHAPE)
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• Angularity or roundness is a measure of the distance of transportation
• Angular sand grains have, most likely, only been transported for a short distance from
the source area, or they have been in the depositional environment for a short time.
• The environment of deposition is also a factor in sand grain roundness.
• Sands from desert environments tend to be more rounded than sands from beaches.

8. • Sorting is a measure of how similar grain sizes are within a sediment or rock and tells
us about the relative strength of the current before it dropped (deposited) its cargo.
• In a current of water or air, the larger and denser grains fall faster than the smaller
grains. That is, for the same material, size determines settling velocity.
TEXTURE OF SAND – INTERPRETATION (SORTING)
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DEGREE OF SORTING
• Well-sorted grains indicate that the sediment was probably transported for a long time
in a fairly high energy environment
• Poorly sorted grains signify that the sediment has not been transported very far from
the source area.

9. Sorting of the sediments also suggest the mode of deposition and transportation.
Short distance transport = poorly sorted angular grains.
Long distance transport = well-rounded & well-sorted sediments
It also helps in knowing the energy conditions of the river or wind.
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• The degree of sorting also suggests fluctuating energy levels, and a fairly short time in
the depositional environment.

10. Textural maturity is a concept which proposes that as sediments experience the input of
mechanical energy, they pass through a series of four stages.
Texture of Sand – Interpretation (Textural Maturity)
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(i) Stage 1- Immature: sediment contains mud (clay and/or silt)
(ii) Stage 2 – Submature: poorly sorted sediment with no mud
(iii) Stage 3- Mature: well sorted sediment with no mud
(iv) Stage 4 – Supermature: well sorted and rounded sediment with no mud
Three steps are involved in textural maturity:
(i) Winnowing or washing out of fines - makes an immature sediment become
submature
(ii) Sorting of grain sizes - makes a submature sediment become mature
(iii) Rounding - makes a mature sediment become supermature

11. MINERALOGICAL COMPOSITION OF SAND
• Sandstones are classified based on the composition of their grains, and the minerals
in sand/sandstones can be identified using microscope and/or hand lens
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• Sandstones are named on the basis of mainly three components, namely;
1. Quartz grains
2. Feldspar grains
3. Fine-grained rock fragment grains.
(i) Quartz sandstone or quartz arenite - dominated by quartz, implies a long time in
depositional basin.
(ii) Arkose – feldspar-dominated. This implies a short time in the depositional basin,
because feldspar typically weathers quickly to clay. Arkose also implies rapid erosion and
arid climate.
MAJOR TYPES OF SANDSTONES
(iii) Litharenite or lithic sandstone (commonly but vaguely called graywacke);
dominated by rock fragment grains.
This implies rapid erosion and deposition, temperate or arid (not humid) climate

12. • “Sand" is a texture term, not a composition term.
• In some areas, sands may be composed almost entirely of minerals other than
quartz and feldspar.
• E.g., sands can be composed of gypsum or olivine grains.
MINERALOGICAL COMPOSITION OF SAND
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• Other minor minerals in sand are heavy minerals such as; staurolite kyanite and
garnet and, all of which are metamorphic minerals.
• Others like olivine, pyroxene and amphibole are constituents of mafic igneous rocks.
• Other heavy minerals are; tourmaline, zircon, apatite, magnetite, ilmenite, hematite,
pyrite, rutile.
• Heavy minerals are important indicators, from which we can know the type of rocks that
existed in the sediment source area.

13. • Each mineral has unique features that enable the mineral to be identified in sand.
READ ON DETAILED DISTINGUISHING FEATURES OF THE VARIOUS MINERALS
MINERALS IDENTIFICATION IN SANDS
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• Sands from continental sources of sediments may be composed almost entirely of
quartz,
• Sands from warm, shallow, tropical seas, may be entirely or almost entirely made up
of calcium carbonate (CaCO3) grains such as calcite and aragonite.
• Calcium carbonate sediment may be made up entirely or almost entirely of
microscopic shells such as the remains of planktonic organisms, along with broken
shells and coral.
• Other calcium carbonate grains may be spherical or highly rounded. These grains are
known as oolites.

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INTERPRETING GEOLOGIC RECORDS
OF SEDIMENTARY ROCKS
• Textures and compositions of sedimentary rocks (e.g., sandstone)may be used to interpret
the history of the sand, including;
(i) source area lithology,
(ii) paleoclimate,
(iii) tectonic activity,
(iv) processes acting in the depositional basin, and
(v) time duration in the basin.
• Composition gives the key information such as minerals or rock fragments present in the
source area.
SOURCE AREA LITHOLOGY
• Sand-sized quartz grains could come from the weathering of source area rocks such as
granite, gneiss, or other sandstones which contain quartz (i.e. recycled sandstones).
• Sand-sized feldspar grains could come from the weathering of source area rocks such as
granite or gneiss.
• Sand-sized rock fragment grains come from the weathering of fine-grained source rocks.
Possibilities include shale, slate, phyllite, basalt, rhyolite, andesite, chert, and possibly
schist.

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16. PALEOCLIMATE
• Paleoclimate refers to the climate which existed in the source area (i.e. ancient climate).
• The climatic conditions existing at that time determines the weathering rates in the
source area.
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• In humid climates, feldspar weathers to clay by hydrolysis.
• The presence of feldspar in sand suggests that the climate was most likely arid.
• The dominance of quartz in the sand also indicates that the climate was humid.
• If the rock fragments in the sand are predominantly made up of rock types which would
weather rapidly (e.g., basalt), the existing climate was probably arid.
• However, if the rock fragments present in the sand are mainly the relatively stable type
(e.g., Shale, chert, slate); the climate that existed may have been temperate to humid.
• Finally, the presence of rock fragments and absence of rock types, suggests a good
compromise of temperate climate.

17. TECTONIC ACTIVITY
• Tectonic Activity can be classified as active & passive.
• Tectonically active environment refers to areas characterized by;
✓ steep slopes,
✓ lots of earthquake,
✓ mountains close to the sea,
✓ tectonic uplift and
✓ volcanic activity.
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• On the contrary, tectonically passive environment is typified by;
(i) broad, flat coastal plain,
(ii) few or no earthquakes
(iii) no uplift
(iv) no volcanic activity.

18. TECTONIC ACTIVITY
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• High content of feldspar & rock fragments in sand reflects high tectonic activity.
• High tectonic activity might result in rapid dumping of sediments into depositional basin
with little or no time for sorting.
• Sand composed predominantly of quartz suggests low tectonic activity (i.e. passive
setting).
• Low tectonic activity indicates little uplift, low rates of erosion, hence relatively little
sediment is supplied to the basin.
• Sediment in low tectonic environments is washed around for a long time and become
well sorted and rounded.
• Consequently, apart from quartz, other grains are likely to be destroyed by abrasion &
chemical reaction (weathering).

19. PROCESSES ACTING IN DEPOSITIONAL
ENVIRONMENT.
• The texture of sediment provides information regarding the energy levels (High or Low,
Consistency) in the depositional basin.
• Coarse sediments generally indicate high energy, whereas fine sediments reflect low
energy.
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• Well sorted sediments signify consistent, fairly high energy levels.
• Poorly sorted sediments are evidence for inconsistent energy levels - rapid dumping,
probably involving short episodes of high energy, followed by low energy conditions.

20. TIME DURATION IN DEPOSITIONAL ENVIRONMENT.
• The mineralogy and texture of sediments can be used to determine the time duration in
depositional environment.
• Sand with abundant quartz grains implies a long time in the depositional environment;
whereas sand dominated by feldspar or rock fragment grains suggests a short time in the
depositional environment.
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• Immature or submature sediments probably spent only a short time in the basin before
burial.
• On the other hand, Mature or supermature sediments were probably rolling around in
the basin for a long time prior to burial.
• Long time is required to produce rounded grains, most likely in a tectonically passive
environment.
• The more rounded the grain, the most likely situation that the sediment has been in the
depositional environment for long.
• Consequently, desert sands are often well rounded because of the "sandblasting"
process of wind transport.

21. SEDIMENT & SEDIMENTARY ROCKS
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• Sedimentary Processes
• Transportation - the movement of sediment from its point of origin; typically results in rounding
and sorting of sediments
• Deposition - occurs when sediment settles out or ceases being transported, or when a chemical
or organic sediment accumulates; the place where this occurs is the environment of deposition
• Lithification (consolidation) - the process of converting sediment into sedimentary rock by a
combination of compaction and cementation
• Sedimentary rocks are produced through lithification-transformation of loose
sediments into solid rock (sedimentary rock)
• Lithification processes involves: Compaction & Cementation
• Compaction-
• packing grains closely together resulting in the reduction in pore space and volume
• Cementation-
• the precipitation of a mineral on the surface of grains, and/or in pore spaces causing the
reduction in pore space, and increase in mechanical strength
• common cementing agents include: calcite, silica, iron oxide

22. • Lithification can take place at burial conditions that do not involve metamorphic
change.
• Lithification (for which diagenesis is a lofty synonym) can be fast or it can take place
over, or after, hundreds of millions of years.
• For the latter reasons, a sedimentary rock’s age is how long ago it accumulated as
sediments (and its age is not when it became a rock).
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LITHIFICATION: SEDIMENT /SEDIMENTARY ROCKS

23. • The Cementing material is usually secondary silica (siliceous cement), calcium
carbonate (carbonate cement), iron rich (ferruginous cement)
• Cement itself to some extent is the source of weakness in the sedimentary rocks
• Because cementing material and the clastic sediments are usually of different
composition, leading to heterogeneity in their physical characteristics.
CEMENTATION
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• Hence such rock will not behave homogeneously under stress, resulting into
development of cracks or fissures which develops in the cementing material.
• If the cement is Calcium Carbonate - it is undesirable, because calcite is also
susceptible to dissolution.
• However, if cementation process continues for longer span of time, cementation
will become more complete, which reduce the porosity and permeability in the
rock mass and increase competence.
• Shape of grains: i.e. if coarser grains are rounded or subrounded, cement material
will not have firm grip, and such rocks behave as incompetent rocks

24. Cementation
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26. IDENTIFICATION & CLASSIFICATION OF
SEDIMENTARY ROCKS
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• Sedimentary Rocks are categorized according to their origin as and how they form, as follows;
(1) Terrigenous sedimentary rocks (also called detrital or clastic sedimentary rocks)
(2) Chemical and biochemical sedimentary rocks, which form as chemical precipitates from
solution, or from the shells of organisms,
(3) Organic sedimentary rocks, formed by accumulation and consolidation of organic material .
• Sedimentary textures
• Clastic texture - sediment grains bound together in a rigid framework
• Crystalline texture - an arrangement of interlocking crystals that grow and interfere with
each other

27. CLASTIC SEDIMENTARY ROCKS
• Formed from broken rock fragments weathered and eroded by river, glacier, wind and
sea waves. These clastic sediments are found deposited in environments such as on
floodplains, beaches, in desert and on the sea floors.
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Clastic rocks
solidify
Matrix:
is the finer grains or
material that
surrounds the
larger clasts. It
consist of either
clay, silt and sand.
Any rock fragment
(size is > 4 mm=Pebble)
Cement: dissolved
substance
that binds
the
sediments.
1. Calcareous
2. siliceous
Fine-gravel/
Granule
(size <4mm)

28. CLAST SIZE OF CLASTIC
SEDIMENTARY ROCK
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Clastic rocks are classified on the basis of the grain size: e.g., conglomerate, sandstone, shale
etc.The size of clasts in clastic rocks is used to classify the rocks; Sizes range from gravel, sand,
silt to clay.
• Clastic sedimentary rocks with gravel-
sized clasts are sometimes referred to
as rudites (gravel) or rudaceous rocks.
E.g. conglomerate/breccia
Iron
oxide
• Clastic Sedimentary rocks with sand-sized grains
are called arenites (sand) or arenaceous rocks. E.g.
Sandstone. These rocks are either accumulated by
wind action or deposited under water action or
marine or lake environment

29. CLAST SIZE OF CLASTIC SEDIMENTARY ROCK
Quartz Sandstone also called quartz arenite
is composed mainly of quartz sand grains.
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• Arkose sandstone composed mainly of
pink or white feldspar grains, with quartz,
and generally some muscovite mica or
sand-sized rock fragments.
• This type of rock is also called Litharenite or
lithic sandstone is mainly composed of dark
sand-sized rock fragments, with some mica,
quartz, and feldspar grains in a clay-rich matrix.

30. CLAST SIZE OF CLASTIC SEDIMENTARY ROCK
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• Clastic sedimentary rocks with clay- and silt-
sized grains (mud) are known as argillites or
argillaceous rocks.
• e.g. shale, mudstone
• Finer-grained sedimentary rocks include shale,
siltstone, claystone, and/or mudstone
• Shale - contains both silt- and clay-sized
particles with a notable presence of thin
layers or laminations along which rock easily
splits (fissility)
• Siltstone - primarily silt-sized particles,
slightly coarser than shale, and lacks the
laminations and fissility of shale
• Claystone - primarily clay-sized particles but
lacks the fissility of shale
• Mudstone – similar to shale but more
massive & blocky, and lacks shale laminations
and fissility
• Claystone - primarily clay-sized particles
but lacks the fissility of shale

31. CLAST SHAPE OF CLASTIC SEDIMENTARY ROCK
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• The shape of clasts is important in the naming of coarse-grained clastic sedimentary
rock (e.g. rounded or angular).
• E.g.Conglomerate and breccia contain gravel-sized clasts surrounded by finer-grained
matrix.
Conglomerates have rounded clasts. Breccias contain angular clasts.

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33. CHEMICAL, BIOCHEMICAL SEDIMENTARY ROCKS
SEDIMENTARY ROCKS
• Derived from material that was once in solution and precipitates to form sediment.
• Two groups can be identified based on the source of the material;
(i) Directly precipitated as a result of physical processes (chemical origin), or
(ii) Through life processes (biochemical origin)
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• These types of rocks are often referred to as intrabasinal because they are formed within
the basin of deposition, rather than being transported into it.
• Chemical and biochemical sedimentary rocks can be divided into four sub-groups, namely;
(i) Evaporites
(ii) Siliceous Rocks
(iii) Ironstones
(iv) Carbonate Rocks
Evaporites - rocks or minerals with a crystalline texture that are formed from precipitation
from seawater or a saline lake (examples include gypsum, halite, borates, and other salts).
•Some types of evaporites are; Travertine, Rock Gypsum, Rock Salt (Halite)

34. CHEMICAL (EVAPORITES) SEDIMENTARY ROCKS
• Travertine (calcite) is formed by the evaporation in caves, river water or springs, consisting
of intergrown calcite crystals, and fizzes in acid
• Travertine includes;
(i) Stalactites and Stalagmites (cave-generated)
(ii) Tufa (spring-generated travertine)
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35. CHEMICAL (EVAPORITES) SEDIMENTARY ROCKS
• Rock Gypsum (Gypsum - CaSO4.2H2O) is formed by precipitation from sea
water after about 80% of the water has evaporated.
• Gypsum alters to anhydrite (CaSO4), by removal of water, normally caused by
burial to greater depths.
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• Rock Salt (Halite - NaCl) is formed by precipitation from sea water after about 90% of the
water has evaporated.

36. CHEMICAL (SILICEOUS) SEDIMENTARY ROCKS
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• Siliceous rocks are dominated by silica which precipitates from solution within
the basin of deposition. Siliceous rock includes;
(i) Chert
(ii) Opal
(iii) Diatomite.
• Siliceous sedimentary rock does not include quartz sandstone.
• Chert is a very fine grained silica sediment of chemical or biochemical origin. It is hard and
compact
• Opal is related to chert, but contains varying amounts of water.
• Diatomite is a rock composed of the siliceous (silica) skeletons of microscopic algae called
diatoms.
• Diatomite can be distinguished from chalk because it does not react with hydrochloric
acid.

37. CHEMICAL (CARBONATE) SEDIMENTARY ROCKS
• Carbonate rocks most commonly form in warm shallow seas.
• Carbonate rocks are made up of carbonate minerals, containing a carbonate (CO3) group.
Examples are;
(i) Calcite (CaCO3)
(ii) Aragonite (CaCO3)
(iii) Dolomite (CaMg(CO3)2)
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• Calcite and aragonite are polymorphs of calcium carbonate.
• Rocks which contain abundant calcium carbonate are often referred to as calcareous rocks.
• Carbonate minerals are easily identified because they effervesce readily in hydrochloric
acid.
• Dolomite frequently forms by chemical replacement of calcium and contains 50% or more.
• Rocks containing carbonate minerals are:
(i) Limestone (primarily composed of the minerals calcite and aragonite). It is one
of the most abundant chemical rock It is precipitated by organisms usually to
form a shell or other skeletal structure.
(ii) Dolostone (mainly composed of the mineral dolomite).

38. CHEMICAL (CARBONATE)SEDIMENTARY ROCKS
• Biochemical limestone - precipitated by the actions of organisms, and often are
comprised of the fossil remains of organisms
• Inorganic limestone – precipitated directly from water, without the aid of
organisms
• Limestone can be recrystallized, destroying original texture of the rock
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Fossiliferous limestone

39. CHEMICAL (IRONSTONES) SEDIMENTARY ROCKS
• Sedimentary Ironstones are rocks dominated by iron-bearing minerals such as hematite.
Examples include;
• Banded iron formations – formed in the ocean during early Precambrian time from iron
oxides in solution.
• Oolitic hematite/ oolitic ironstone
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• Organic sedimentary rocks are mainly composed of organic matter or carbon.
• Generally, organic sedimentary rocks do not contain minerals because, by definition,
minerals are inorganic substances.
ORGANIC SEDIMENTARY ROCKS

40. ORGANIC SEDIMENTARY ROCKS
• Peat is a sediment composed of plant fragments. Peat is transformed by burial pressure
and temperature to lignite
• Lignite changes to sooty bituminous coal with greater depth and duration of burial, and
higher temperatures
• Increasing metamorphism changes the bituminous coal to anthracite coal.
• The plant fossils in coal generally indicate deposition in fresh-water swamps.
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• Coal is therefore an organic rock that forms from the compaction of plant material that
has not completely decayed.