Sedimentologi

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Sedimentologi Kamal Roslan Mohamed
Shallow Sandy Seas
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INTRODUCTION
Shallow marine environments are areas of
accumulation of substantial amounts of
terrigenous clastic material brought in by rivers
from the continental realm. Offshore from most
coastlines there is a region of shallow water,
the continental shelf, which may stretch tens to
hundreds of kilometres out to sea before the
water deepens down to the abyssal depths of ocean
basins. Terrigenous clastic material is
distributed on shelves by tides, waves, storms
and ocean currents these processes sort the
material by grain size and deposit areas of sand
and mud, which form thick, extensive sandstone
and mudstone bodies in the stratigraphic record.
Characteristic facies can be recognised as the
products of transport and deposition by tides and
storm/wave processes. Deposition in shallow
marine environments is sensitive to changes in
sea level.
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SHALLOW MARINE ENVIRONMENTS OF TERRIGENOUS
CLASTIC DEPOSITION
The continental shelves and epicontinental seas
are important sites of deposition of sand and mud
in the worlds oceans and account for over half
the volume of ocean sediments. These
successions can be very thick, over 10,000 m,
because deposition may be very long-lived and can
continue uninterrupted for tens of millions of
years. They occur as largely undeformed strata
around the edges of continents and also in
orogenic belts, where the collision of
continental plates has forced beds deposited in
shallow marine environments high up into mountain
ranges.
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Sediment supply to shallow seas
The supply of sediment to shelves is a
fundamental control on shallow marine
environments and depositional facies of shelves
and epicontinental seas. If the area lies
adjacent to an uplifted continental region and
there is a drainage pattern of rivers delivering
detritus to the coast, the shallow-marine
sedimentation will be dominated by terrigenous
clastic deposits. The highest concentrations of
clastic sediment will be near the mouths of major
rivers adjacent coastal regions will also be
supplied with sediment by longshore movement of
material by waves, storms and tides.
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Sediment supply to shallow seas
Shallow seas that are not supplied by much
terrigenous material may be areas of carbonate
sedimentation, especially if they are in lower
latitudes where the climate is relatively warm.
In cooler climates where carbonate production
is slower, shelves and shallow seas with low
terrigenous sediment supply are considered to be
starved. The rate of sediment accumulation is
slow and may be exceeded by the rate of
subsidence of the sea floor such that the
environment becomes gradually deeper with time.
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Characteristics of shallow marine sands
Texturally, the grains of sand will have suffered
a degree of abrasion and the processes of
turbulent flow during transport will separate the
material into different grain sizes. The
compositional maturity will probably be greater
than the equivalent continental deposits, because
the more labile minerals and grains (such as
feldspar and lithic fragments) are broken down
during transport shallow marine sands are
commonly dominated by quartz grains. Shallow
seas are rich in marine life, including many
organisms that have calcareous shells and
skeletons. The remains of these biogenic hard
parts are a major component of shelf carbonate
deposits, but can also be very abundant in sands
and muds deposited in these seas.
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Characteristics of shallow marine sands
Shallow seas are environments rich in animal
life, particularly benthic organisms that can
leave traces of their activity in the sediments.
Bioturbation may form features that are
recognisable of the activities of a particular
type of organism, but also results in a general
churning of the sediment, homogenising it into
apparently structureless masses. Primary
sedimentary structures (wave ripples, hummocky
cross-stratification, trough rossbedding, and so
on) are not always preserved in shelf sediments
because of the effects of bioturbation.
Bioturbation is most intense in shallower water
and is frequently more abundant in sandy sediment
than in muddy deposits.
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Facies distribution across shelf
Shoreface The shallower parts of the shelf are
within the depth zone for wave action and any
sediment will be extensively reworked by wave
processes. Sands deposited in these settings may
preserve wave-ripple cross-lamination and
horizontal stratification. Streaks of mud in
flaser beds deposited during intervals of lower
wave energy become more common in the deposits of
slightly deeper water further offshore
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Facies distribution across shelf
Offshore transition zone In the offshore
transition zone, between the fairweather and
storm wave bases on storm-dominated shelves,
sands are deposited and reworked by storms. A
storm creates conditions for the formation of
bedforms and sedimentary structures that seem to
be exclusive to storm-influenced environments.
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Facies distribution across shelf
Offshore The outer shelf area below storm wave
base, the offshore zone, is predominantly a
region of mud deposition. The sediments are
commonly grey because this part of the sea floor
is relatively poorly oxygenated allowing some
preservation of organic matter within the mud.
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Characteristics of a shallow-marine succession
The offshore facies mainly consists of mudstone
beds with some bioturbation. This is overlain
by offshore transition facies made up of sandy
tempestite beds interbedded with bioturbated
mudstone. The tempestite beds have erosional
bases, are normally graded and show some
hummockyswaley cross-stratification. The
thickness of the sandstone beds generally
increases up through the succession, and the
deposits of the shallower part of this zone show
more SCS than HCS.
A schematic graphic sedimentary log of
a storm-dominated succession.
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Characteristics of a shallow-marine succession
The shoreface is characterised by sandy beds with
symmetrical (wave) ripple lamination, horizontal
stratification and SCS, although sedimentary
structures may be obscured by intense
bioturbation. Sandstone beds in the shoreface
may show a broad lens shape if they were
deposited as localised ridges on the shallow sea
floor. The top of the succession may be capped
by foreshore facies.
A schematic graphic sedimentary log of
a storm-dominated succession.
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Deposition on tide-dominated shelves
Offshore sand ridges Near shorelines that
experience strong tidal currents large sand
ridges are found on modern shelves. The ridges
form parallel to the shoreline in water depths of
up to 50m and may be tens of metres high, in
places rising almost to sea level. The sands are
moderately well sorted, medium grained but the
deposits may include some mud occurring as clay
laminae deposited during slack phases of the
tidal flow.
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Deposition on tide-dominated shelves
Tidal sandwaves and sand ribbons Currents
generated by tides affect the sea bed tens of
metres below sea level and are strong enough to
move large quantities of sand in shallow marine
environments. The effects of waves and storms
are largely removed by tidal currents reworking
the material in macrotidal regimes and only the
tidal signature is left in the stratigraphic
record.
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Deposition on tide-dominated shelves
Tidal sandwaves and sand ribbons In seas with
moderate tidal effects the influence of tides is
seen in shallower water, but storm deposits are
preserved in the offshore transition zone in
these mixed storm/tidal shelf settings.
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A schematic sedimentary log through a
tidally influenced shelf succession.
A schematic graphic sedimentary log of
a storm-dominated succession.
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Characteristics of deposits of shallow sandy seas

• lithology mainly sand and mud, with some gravel
• mineralogy mature quartz sands, shelly sands
• texture generally moderately to well sorted
• bed geometry sheets of variable thickness,
  large lenses formed by ridges and bars
• sedimentary structures cross-bedding, cross-
  and horizontal lamination, hummocky and swaley
  crossstratification
• palaeocurrents flow directions very variable,
  reflecting tidal currents, longshore drift, etc.
• fossils often diverse and abundant, benthic
  forms are characteristic
• colour often pale yellow-brown sands or grey
  sands and muds
• facies associations may be overlain or
  underlain by coastal, deltaic, estuarine or
  deeper marine facies.

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SEKIAN