Sedimentologi
Kamal Roslan Mohamed
Shallow Sandy Seas
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.
SHALLOW MARINE ENVIRONMENTS OF
TERRIGENOUS CLASTIC DEPOSITION
The continental shelves and epicontinental seas
are important sites of deposition of sand and mud
in the world’s 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 longlived 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.
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 shallowmarine 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.
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.
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.
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.
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
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.
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.
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
hummocky–swaley 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.
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.
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.
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.
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.
A schematic
sedimentary log
through a tidally
influenced shelf
succession.
A schematic graphic
sedimentary log of a
storm-dominated
succession.
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, crossand 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.
SEKIAN