Transcript Slide 1

Sedimentologi
Kamal Roslan Mohamed
CONTINENTS:
SOURCES OF
SEDIMENT
INTRODUCTION
The ultimate source of the clastic and
chemical deposits on land and in the
oceans is the continental realm, where
weathering and erosion generate the
sediment that is carried as bedload, in
suspension or as dissolved salts to
environments of deposition.
Thermal and tectonic processes in the
Earth’s mantle and crust generate
regions of uplift and subsidence, which
respectively act as sources and sinks
for sediment.
Weathering and erosion processes
acting on bedrock exposed in uplifted
regions are strongly controlled by
climate and topography.
FROM SOURCE OF SEDIMENT TO FORMATION OF STRATA
In the creation of sediments and
sedimentary rocks the ultimate
source of most sediment is
bedrock exposed on the
continents.
The starting point is the uplift of
pre-existing bedrock of igneous,
metamorphic or sedimentary
origin.
Once elevated this bedrock
undergoes weathering at the
land surface to create clastic
detritus and release ions into
solution in surface and
nearsurface waters.
The pathway of processes involved in the formation
of a succession of clastic sedimentary rocks, part of
the rock cycle.
FROM SOURCE OF SEDIMENT TO FORMATION OF STRATA
Erosion follows, the process of
removal of the weathered
material from the bedrock
surface, allowing the transport
of material as dissolved or
particulate matter by a variety of
mechanisms.
Eventually the sediment will be
deposited by physical, chemical
and biogenic processes in a
sedimentary environment on
land or in the sea.
The final stage is the lithification
of the sediment to form
sedimentary rocks, which may
then be exposed at the surface
by tectonic processes.
The pathway of processes involved in the formation
of a succession of clastic sedimentary rocks, part of
the rock cycle.
These processes are part of the
sequence of events referred to as the
rock cycle.
MOUNTAIN-BUILDING PROCESSES
Plate tectonic theory
provides a framework of
understanding the
processes that lead to the
formation of mountains,
aswell as providing an
explanation for how all the
main morphological
features of the crust have
formed throughout most of
Earth history.
The boundaries of the present-day principal tectonic plates.
Plate movements and associated igneous activity create the topographic
contours of the surface of the Earth that are then modified by erosion and
deposition. Areas of high ground on the surface of the globe today can be
related to plate boundaries.
GLOBAL CLIMATE
The climate belts around
the world are principally
controlled by latitude.
The amount of energy
from the Sun per unit
area is less in polar
regions than in the
equatorial zones so
there is a temperature
gradient from each pole
to the Equator.
The present-day world climate belts.
These temperature variations determine the atmospheric pressure belts:
high pressure regions occur at the poles where cold air sinks and low
pressure at the Equator where the air is heated up, expands and rises.
These differences in pressure give rise to winds, which move air masses
between areas of high pressure in the subtropical and polar zones to
regions of low pressure in between them.
WEATHERING PROCESSES
Rock that is close to the land
surface is subject to physical and
chemical modification by a number
of different weathering processes.
These processes generally start
with water percolating down into
joints formed by stress release as
the rock comes close to the surface,
and are most intense at the surface
and in the soil profile.
Weathering is the breakdown and
alteration of bedrock by mechanical
and chemical processes that create
a regolith (layer of loose material),
which is then available for transport
away from the site.
Physical weathering
Chemical weathering
These are processes that break
the solid rock into pieces and
may separate the different
minerals without involving any
chemical reactions.
These processes involve changes
to the minerals that make up a
rock.
- Freeze–thaw action
- Salt growth
- Temperature changes
- Solution
- Hydrolysis
- Oxidation
Physical weathering -Freeze–thaw action
Water entering cracks in rock
expands upon freezing, forcing the
cracks to widen; this process is also
known as frost shattering and it is
extremely effective in areas that
regularly fluctuate around 08C, such
as high mountains in temperate
climates and in polar regions
Physical weathering - Salt growth
Seawater or other water containing
dissolved salts may also penetrate
into cracks, especially in coastal
areas. Upon evaporation of the
water, salt crystals form and their
growth generates localised, but
significant, forces that can further
open cracks in the rock.
Physical weathering - Temperature changes
Changes in temperature probably
play a role in the physical
breakdown of rock.
Rapid changes in temperature
occur in some desert areas where
the temperature can fluctuate by
several tens of degrees Celsius
between day and night; if different
minerals expand and contract at
different rates, the internal forces
created could cause the rock to
split.
This process is referred to as
exfoliation, as thin layers break off
the surface of the rock.
Chemical weathering - Solution
Most rock-forming silicate minerals have very
low solubility in pure water at the temperatures
at the Earth’s surface and so most rock types
are not susceptible to rapid solution.
It is only under conditions of strongly alkaline
waters that silica becomes moderately soluble.
Carbonate minerals are moderately soluble,
especially if the groundwater (water passing
through bedrock close to the surface) is acidic.
Most soluble are evaporite minerals such as
halite (sodium chloride) and gypsum, which
locally can form an important component of
sedimentary bedrock.
Chemical weathering - Hydrolysis
Hydrolysis reactions depend upon
the dissociation of H2O into Hþ and
OH ions that occurs when there is
an acidifying agent present.
Natural acids that are important in
promoting hydrolysis include
carbonic acid (formed by the
solution of carbon dioxide in water)
and humic acids, a range of acids
formed by the bacterial breakdown
of organic matter in soils.
Many silicates undergo hydrolysis
reactions, for example the formation
of kaolinite (a clay mineral) from
orthoclase (a feldspar) by reaction
with water.
Chemical weathering - Oxidation
The most widespread evidence of
oxidation is the formation of iron
oxides and hydroxides from
minerals containing iron.
The distinctive red-orange rust
colour of ferric iron oxides may be
seen in many rocks exposed at the
surface, even though the amount of
iron present may be very small.
The products of weathering
Material produced by weathering and erosion of material exposed on
continental land masses is referred to as terrigenous (meaning derived
from land).
Terrigenous clastic detritus comprises minerals weathered out of bedrock,
lithic fragments and new minerals formed by weathering processes.
Stable minerals such as quartz are relatively unaffected by chemical
weathering processes and physical weathering simply separates the
quartz crystals from each other and from other minerals in the rock.
Micas and orthoclase feldspars are relatively resistant to these processes,
whereas plagioclase feldspars, amphiboles, pyroxenes and olivines all
react very readily under surface conditions and are only rarely carried
away from the site of weathering in an unaltered state.
The products of weathering
The most important products of the
chemical weathering of silicates are
clay minerals.
A wide range of clay minerals form
as a result of the breakdown of
different bedrock minerals under
different chemical conditions; the
most common are kaolinite, illite,
chlorite and montmorillonite.
Oxides of aluminium (bauxite) and
iron (mainly haematite) also form
under conditions of extreme
chemical weathering.
The relative stability of common silicate minerals
under chemical weathering.
Soil development
Soil formation is an important
stage in the transformation of
bedrock and regolith into detritus
available for transport and
deposition. In situ (in place)
physical and chemical weathering
of bedrock creates a soil that may
be further modified by biogenic
processes.
An in situ soil profile with a division into different
horizons according to presence of organic
matter and degree of breakdown of the regolith.
EROSION AND TRANSPORT
Weathering is the in situ breakdown of bedrock
and erosion is the removal of regolith material.
Loose material on the land surface may be
transported downslope under gravity, it may be
washed by water, blown away by wind, scoured
by ice or moved by a combination of these
processes.
Falls, slides and slumps are responsible for
moving vast quantities of material downslope in
mountain areas but they do not move detritus
very far, only down to the floor of the valleys.
The transport of detritus over greater distances
normally involves water, although ice and wind
also play an important role in some
environments
DENUDATION AND LANDSCAPE EVOLUTION
The lowering of the land surface by the combination of weathering and
erosion is termed denudation.
Weathering and erosion processes are to some extent interdependent: it
is the combination of these processes that are of most relevance to
sedimentary geology, namely the rates and magnitudes at which
denudation occurs and the implications that this has on the supply of
material to sedimentary environments.
Rates of denudation are determined by a combination of topographic and
climatic factors, which in turn influence soil development and vegetation,
both of which also affect weathering and erosion.
In addition, different bedrock lithologies respond in different ways to these
combinations of physical, chemical and biological processes.
Topography and relief
A distinction needs to be made between the altitude
of a terrain and its relief, which is the change in the
height of the ground over the area.
A plateau region may be thousands of metres above
sea level but if it is flat there may be little difference
in the rates of denudation across the plateau and a
lowland region with a comparable climate.
With increasing relief the mechanical denudation
rate increases as erosion processes are more
efficient.
Rock falls and landslides are clearly more frequent
on steep slopes than in areas of subdued
topography: stream flow and overland water flow are
faster across steeper slopes and hence have more
erosive power.
Climate controls on denudation processes
Chemical weathering processes are affected by factors
that control the rate and the pathway of the reactions.
Water is essential to all chemical weathering processes
and hence these reactions are suppressed where water
is scarce (e.g. in deserts).
Temperature is also important, because most chemical
reactions are more vigorous at higher temperatures; hot
climates therefore favour chemical weathering.
Water chemistry affects the reactions: the presence of
acids enhances hydrolysis and dissolved oxidising
agents facilitate oxidation reactions.
The rates and efficiency of the reactions vary with
different bedrock types.
Bedrock lithology and denudation
The type of bedrock is a fundamental control on
the rates and patterns of denudation.
The greatest variability is seen in humid
climates where chemical weathering processes
are dominant because different lithologies are
broken down, and hence eroded, at widely
different rates.
Quartz-rich rocks are least susceptible to
breakdown, whereas mafic rocks such as
basalts are rapidly weathered and eroded.
Limestone bedrock is primarily weathered by
dissolution, and the pattern of denudation is
therefore dominated by development of karst
scenery.
Soils and denudation
Soil development has an important role in
weathering processes.
Water is retained in soils and hence the
thickness of the soil profile influences
how much water is available.
Biochemical reactions in soils create
acids, collectively known as humic acids,
which increase rates of solution of
carbonate bedrock.
Soils are host to plants and animals,
which also play a role in breaking down
bedrock, especially roots that can
penetrate deep into the rock and widen
fractures.
Vegetation and denudation
The types of vegetation and the coverage
they have over the land surface are
determined by the climate regime, which
is in turn influenced by the latitude and
altitude.
A dense vegetation cover is very effective
at protecting the bedrock and its
overlying regolith from erosion by rain
impact and overland flow of water.
Even steep mountain slopes can be
effectively stabilised by plants.
TECTONICS AND DENUDATION
The creation of the topography of the continental land surface is
fundamentally controlled by plate tectonic processes and mantle behaviour
but surface processes, particularly erosion, play an important role in
modifying the landscape.
Denudation results in the
removal of material from
the uplifted bedrock and
this reduces the mass of
material in these areas.
This removal of mass
results in isostatic uplift.
Uplift due to thickening of the crust followed by erosion results in isostatic compensation as the
load of the rock mass eroded is removed. If the erosion is uneven then locally the removal of
mass from valleys can result in uplift of the mountain peaks between.
SEKIAN