Sedimentologi

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Sedimentologi Kamal Roslan Mohamed
MARINE REALM MORPHOLOGY PROCESSES
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INTRODUCTION
The oceans and seas of the world cover almost
three-quarters of the surface of the planet and
are very important areas of sediment
accumulation. There is considerable variety in
the sedimentation that occurs in the marine
realm, but there are a number of physical,
chemical and biological processes that are common
to many of the marine environments. Physical
processes include the formation of currents
driven by winds, water density, temperate and
salinity variations and tidal forces these have
a strong effect on the transport and deposition
of sediment in the seas. Chemical reactions in
seawater lead to the formation of new minerals
and the modification of detrital sediment. The
seas also team with life long before there was
life on land organisms evolved in the marine
realm and continue to occupy many habitats within
the waters and on the sea floor.
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DIVISIONS OF THE MARINE REALM
The bathymetry, the shape and depth of the sea
floor, is fundamentally determined by the plate
tectonic processes that create ocean basins by
sea-floor spreading.
The spreading ridges are areas of young, hot
basaltic crust that is relatively buoyant and
typically around 2500m below sea level. Away from
the ridges the water depth increases as the older
crust cools and subsides, and most of the ocean
floor is between about 4000 and 5000m below sea
level.
A cross-section from the continental shelf
through the continental slope and rise down to
the abyssal plain.
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DIVISIONS OF THE MARINE REALM
At the ocean margins the transition from ocean
crust to continental crust underlies the
continental rise and the continental slope, which
are the lower and upper parts of the bathymetric
profile from the deep ocean to the shelf.
The angle of the continental slope is relatively
steep, usually between about 2 and 7, while the
continental rise is a lower angle slope down to
the edge of the abyssal plain. The continental
shelf itself is underlain by continental crust,
and the junction between the shelf and the slope
usually occurs at about 200m below sea level at
present-day margins
A cross-section from the continental shelf
through the continental slope and rise down to
the abyssal plain.
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INTRODUCTION
The shelf area, down to 200m water depth, is
called the neritic zone, the bathyal zone
corresponds to the continental slope and extends
from 200m to 2000m water depth, while the abyssal
zone is the ocean floor below 2000 m.

• Depth-related divisions of the marine realm
• broad divisions are defined by water depth
• the shelf is described in terms of the depth to
  which different processes interact with the sea
  floor, and the actual depths vary according to
  the characteristics of the shelf.

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INTRODUCTION
The shelf (neritic environment) can be usefully
further divided into depth-controlled zones
according to tidal processes, waves and storms
affect the shelf. The foreshore is the region
between mean high water and mean low water marks
of the tides

• Depth-related divisions of the marine realm
• broad divisions are defined by water depth
• the shelf is described in terms of the depth to
  which different processes interact with the sea
  floor, and the actual depths vary according to
  the characteristics of the shelf.

The shoreface is defined as the region of the
shelf between the low-tide mark and the depth to
which waves normally affect the sea bottom, and
this is the fair weather wave base.
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INTRODUCTION
This deeper shelf area between the fair weather
and storm wave bases is called the offshore-
transition zone. The offshore zone is the
region below storm wave base and extends out to
the shelfedge break at around 200m depth.

• Depth-related divisions of the marine realm
• broad divisions are defined by water depth
• the shelf is described in terms of the depth to
  which different processes interact with the sea
  floor, and the actual depths vary according to
  the characteristics of the shelf.

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TIDES
The Moon exerts a gravitational force on the
Earth and although ocean water is strongly
attracted gravitationally to the Earth, it also
experiences a small gravitational attraction from
the Moon. The water that is closest to the Moon
experiences the largest gravitational attraction
and this creates a bulge of water, a tidal bulge,
on that side of the Earth. The bulge on the
opposite side, facing away from the Moon, can be
thought of as being the result of the Earth being
pulled away from that water mass by the
gravitational force of the Moon.
The gravitational force of the Sun and Moon act
on the Earth and on anything on the surface,
including the water masses in oceans.
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TIDES
During the diurnal tidal cycle the direction of
flow reverses from ebb (offshore) to flood
(onshore). The current velocity also varies from
peaks at the mid points of ebb and flood flow,
reducing to zero at high and low tide slack water
before accelerating again.
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Sedimentary structures generated by tidal currents
An analysis of current directions recorded by
cross-bedding in sands deposited by tidal
currents may show a bimodal (two main directions
of flow) and bipolar (two opposite directions of
flow) pattern. bipolar cross-stratification may
be seen in a single ertical section produced by
alternating directions of migration of ripples or
dunes. This is known as herringbone
cross-stratification
Herringbone cross-stratification in sandstone
beds (width of view 1.5 m).
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Sedimentary structures generated by tidal currents
At the time of high or low tide when the current
is changing direction there is a short period
when there is no flow. When the water is
relatively still some of the suspended load may
be deposited as a thin layer of mud. When the
current becomes stronger during the next tide,
the mud layer is not necessarily removed because
the clay-rich sediment is cohesive and this makes
it resistant to erosion. Mud drapes formed in
this way can be seen in wave and current ripple
laminated sands deposited in shallow water in
places such as tidal mud flats.

• Features that indicate tidal influence of
  transport and deposition
• herringbone cross-stratification
• mud drapes on cross-bedding formed during the
  slack water stages of tidal cycles
• reactivation surfaces formed by erosion of part
  of a bedform when a current is reversed.

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Sedimentary structures generated by tidal currents
Cross-bedded sandstone in sets 35 cm thick with
the surfaces of individual cross-beds picked out
by thin layers of mud. Mud drapes on cross-beds
are interpreted as forming during slack water
stages in the tidal cycle.
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WAVE AND STORM PROCESSES
The depth to which surface waves affect a water
body is referred to as the wave base and on
continental shelves two levels can be
distinguished. The fair weather wave base is
the depth to which there is wave-influenced
motion under normal weather conditions. The
storm wave base is the depth waves reach when the
surface waves have a higher energy due to
stronger winds driving them. Below the storm
wave base the sea bed is not normally affected by
surface waves.
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MARINE FOSSILS
Shelves are areas of oxygenated waters
periodically swept by currents to bring in
nutrients. As such they are habitable
environments for many organisms that may live
swimming in water (planktonic) or on the sea
floor (benthic), either on the surface or within
the sediment. Plants and animals living in the
marine realm contribute detritus, modify other
sediments and create their own environments.
Modern shelf environments team with life and it
is rare to find an ancient shelf deposit that
does not contain some evidence for the organisms
that lived in the seas at the time.
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TRACE FOSSILS
Although body fossils provide physical evidence
of an organism having lived in the past, trace
fossils are evidence of the activity of an
organism. Traces include tracks of walking
animals, trails of worms, burrows of molluscs and
crustaceans, and are collectively called
ichnofauna.
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TRACE FOSSILS
Trace fossils are usually found on or within
sediment that was unconsolidated but with
sufficient strength to retain the shape of the
animals trace. Contrasts in sediment type
between a burrow and the host sediment are a
considerable aid to recognition. A distinction
is made between burrows formed in soft sediment
and borings made by organisms into hard substrate.
The characteristics of trace fossils are
influenced by the nature of the substrate. Boring
organisms cut sharp-sided traces into solid rock
or cemented sea floors (hardgrounds).
Semiconsolidated surfaces (firmgrounds) result in
well-defined burrows.
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TRACE FOSSILS
The different forms of trace fossils are given
names similar to those used in the classification
of animals and body fossils so, for example,
smaller vertical tubes in sands are called
Skolithos and a crawling trail produced by a
multilimbed organism is known as Cruziana.
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TRACE FOSSILS
Assemblages of trace fossil forms and their
relationship to the major divisions of the marine
realm. The assemblages are named after
characteristic ichnofauna and the type
ichnofossil does not need to be present in the
assemblage.
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TRACE FOSSILS
Examples of common trace fossils (a) bird
footprint (b) bivalve borings into rock (c)
vertical burrows in sandstone (Skolithos) (d)
large crustacean burrow (Ophiomorpha) (e)
complex burrows (Thalassanoides) (f) Zoophycos
(g) Palaeodictyon (h) Helmenthoides.
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MARINE ENVIRONMENTS SUMMARY
The physical processes of tides, waves and storms
in the marine realm define regions bounded by
water depth changes. The beach foreshore is the
highest energy depositional environment where
waves break and tides regularly expose and cover
the sea bed. At this interface between the land
and sea storms can periodically inundate
low-lying coastal plains with seawater. Across
the submerged shelf, waves, storms and tidal
currents affect the sea bed to different depths,
varying according to the range of the tides, the
fetch of the waves and the intensity of the
storms. Sedimentary structures can be used as
indicators of the effects of tidal currents,
waves in shallow water and storms in the offshore
transition zone. Further clues about the
environment of deposition are available from body
fossils and trace fossils found in shelf
sediments.
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SEKIAN