Sequence stratigraphy
• Scheme for considering stratigraphy in terms of sea level fluctuations and
depositional sequences bound by unconformities.
• Initially developed by geologists and geophysicists of Exxon in USA.
• A common sense way of looking into sedimentary succession.
Key concepts and definitions
Accommodation space
• Space locally available within sedimentary basins – potential at any point
for sediment to accumulate.
• In absence of accumulation space, an estuary, for example, would act
only
l as a zone off ttransfer
f off water
t and
d sediment
di t from
f continent
ti t to
t marine
i
realm.
• If accommodation space is available sands and mud can build up in the
estuary to form a deposit.
• The balance between accommodation space and sediment supply
determines the character of the stratigraphy developed.
Creation of accommodation space
• IIn shallow
h ll marine
i realm l accommodation
d ti space iis created
t db
by a rise
i iin
sea level relative to the sediment-water interface (the sea bed) at that point.
• Relative sea level rise can be achieved by
1. the sea level itself rise
or
2. the sea bed subsiding
• In the shallow marine setting the upper limit of accommodation space is
defined byy sea level and sediment would build up p to sea level if sediment
supply is sufficient.
• In abyssal depths accommodation space is not a factor because the amount
of water above the ocean floor does not directly influence sedimentation at
that point.
Base level
• Base level (of deposition and erosion) is generally regarded as a global
reference surface to which continental denudation and marine aggradation
gg
tend to proceed.
• This surface is dynamic, moving up and down through time relative to the
center of the earth in p
parallel with eustatic rises and falls in sea level. For
simplicity, base level is often approximated with the sea level.
Aggradation, progradation and retrogradation
Sedimentary succesion built up is a balance between rate of creation of
accommodation space and rate of sediment supply.
Aggradation: If the rate of space created and the rate of sediment supply to
fill it are exactly in balance the deposit will built up vertically.
Along a shore line the beach will stay in the same position with time and the
shoreline does not shift landwards or seawards.
All the facies belts remain fixed in position up through the stratigraphic
section.
Progradation
If rate of sediment supply higher than rate of creation of accommodation
space, the deposits build up and upon filling the space available also builds
outward.
t d
Along a coast the beach will build out into foreshore and shoreface will shift
basinwards.
basinwards
The vertical section will show upwards change to shallower facies.
This is also called regression of the depositional environments.
Forced regression
Regression due to relative sea level fall.
Different from that regression which is due to rate of sediment supply
exceeding rate of creation of accommodation space.
A sea level fall can be due to absolute fall in global sea level or uplift of the
sea bed
b dd due tto ttectonism
t i – rate
t off creation
ti off accommodation
d ti space is i
negative.
In this circumstances
circumstances, the shift in facies basinwards is accompanied by
erosion in the shallower environments.
A downstepping geometry of deposits is formed during forced regression.
regression
Retrogradation
Creation of accommodation space exceeding sediment supply – more
commonly known as transgression.
Characterized by landward shift of facies resulting in upward change to
deeper water facies in vertical profile.
Depositional sequences and systems tracts
The pattern
Th tt off stratigraphy
t ti h created
t d by
b balance
b l between
b t the
th rate
t off supply
l off
sediment and the rate of creation of accommodation space form the basis
for the division of a basin fill succession.
Depositional sequence – a package of sediment deposited during a particular
period in the basin history.
It is bounded at the at its top and base by unconformities or their correlative
conformities.
It represents a period of deposition between two episodes of significant sea
level fall.
In sequence stratigraphy depositional sequences are primary means of
dividing up a basin fill succession.
If relati
relative
e sea le
level
el changes are basin
basin-wide,
ide seq
sequence
ence bo
boundaries
ndaries ma
may be
recognized in different vertical sections through the basin stratigraphy and
used as a means of correlation.
Sequence boundary
Sequence boundary
Concept of Unconformity-bounded sequence
Note- Unconformities are restricted
to basin-margins than basin-interior
Concept of sequence in sequence
stratigraphy. Correlative conformities allow
tracing sequences across entire
sedimentary basin
Sequence boundaries
Due to significant (several tens of m) relative sea level fall, the exposed
beach, foreshore and shoreface will be subjected to erosion.
The erosion
Th i will
ill b
be preserved
d as an unconformity
f it surface
f iin stratigraphic
t ti hi
record.
Further basinwards,
basinwards the relative sea level fall may not have exposed the outer
shelf to create an erosion surface.
However, shallow water deposits of beach and shoreface will overlie outer
However
shelf deeper water facies.
If sea level fall is as g
great as the depth
p of water at the shelf edge,
g , the
unconformity may develop over the whole shelf.
The surface created by the sea level fall is called sequence boundary –
generally marked by an unconformity and its correlative conformity, the
equivalent surface in the outer shelf where there was no erosion.
Depositional sequences,
Sequence boundaries,
& system tracts related
To relative rise & fall of
Sea level on a margin with
g
Distinct shelf edge.
Systems tracts
Between the events of relative sea level fall which create the sequence
boundaries there are normally periods of sea level rise.
The sediments preserved between sequence boundaries are deposited during
this period and show patterns of transgression, aggradation and progradation.
A systems
t ttractt is
i a subdivision
bdi i i off a sequence which
hi h iincludes
l d allll strata
t t
accumulated across the basin during a particular stage of shoreline shift.
Systems tracts are related to stages in the rise and fall in sea level
level.
Transgressive systems tract
Transgressive systems tract forms during periods of most rapid rise in sea
level.
Characterized by general landward shift in facies belts – shallow water facies
are overlain
l i bby d
deep water
t ffacies.
i
Rate of creation of accommodation space is greater than rate of supply of
sediment.
sediment
Highstand systems tract
As rate of sea level rise decrease
decrease, rate of creation of accommodation space
becomes balanced by rate of sediment supply and aggradation occurs.
When sea level reaches its p
peak little accommodation space
p is created and
sediments starts to prograde.
The resultant vertical sequence shows shallowing upwards.
The succession of deposits showing aggradation and progradation formed
during a period of relative sea level high is called highstand systems tract.
Depositional sequences,
Sequence boundaries,
& system tracts related
To relative rise & fall of
Sea level on a margin with
g
Distinct shelf edge.
Systems tracts on shelves with
a ramp geometry
Lowstand systems tract
A fall in sea level exposes inner shelf and initiates erosion to form a sequence
boundary.
This erosion continues as the sea level falls to its lowest point.
Two different systems tract may form during the period of low sea level.
If sea level falls to or below the level of the edge of the shelf, the whole shelf
area is exposed to erosion.
Most of the sedimentation occurs on the slope and basin beyond the edge of
the shelf.
These deep water deposits make up the bulk of the sedimenation to form
a lowstand systems tract.
Falling stage systems tract or forced regressive wedge systems tract
If the margin is gently sloping down into deeper water, a basinward shift in
facies belts will occur during the period of low sea level.
Shallow water facies will overlie deep water facies – progradational
sequence.
The deposition
Th d iti off th
these stage
t off the
th cycle
l bbetween
t onsett and
d end
d off sea
level fall is called falling stage systems tract or forced regressive wedge
systems tract.
Maximum flooding surface
This is the horizon which represents furthest landward extent of marine
conditions.
This surface separates the transgressive and highstand systems tract.
May be recognized in a vertical succession by a change from retrogradation to
aggradation/progradation
Subdivisions of depositional sequences and systems tracts:
parasequences
During formation of a depositional sequence the changes in sea level may
not be following simple curves.
For example
F l a progradational
d ti l package
k iin a hi
highstand
h t d systems
t ttractt may
consist of a series of smaller packages of beds.
Each smaller packages may show an upward shallowing,
shallowing progradational trend
trend.
A group of beds showing such type shallowing upward trend is called
parasequence.
parasequence
A parasequence is formed during a change in sea level.
Bounding surfaces of parasequences are called marine flooding surfaces.
This is different from maximum flooding surfaces which mark top of
transgressive systems tract.
A depositional sequence consists of a number of parasequences.
So parasequences represent sea level fluctuations of different orders
superimposed on longer episodes of sea level change which generate
the depositional sequences.
Causes of sea level fluctuations
1. Local changes in sea level
Tectonic forces and related thermal effects acting on margin of continents
result in land mass being raised or lowered with respect to sea level.
For example, in rifted basins blocks move down in the rift, but along the
flanks blocks may be uplifted,
Alonggppassive margins
g at the edges
g of ocean basins the continental crust
Is cooling and contracting, resulting in a relative rise in sea level.
In all these cases, the sea level changes are localized only to the region
affected by the thermal or tectonic event.
Effect is not global.
2. Glacio-eustasy
Melting and freezing of polar ice caps cause global sea level change.
More ice leads to sea level fall.
Melting of ice leads to sea level rise.
Ice sheets of the poles expanded and contracted during Quaternary, resulting
in world-wide change in sea level of 10’s of m.
Fluctuations between glacial and interglacial periods attributed to periodic
cooling and warming of world climate.
3. Thermotectonic causes
During supercontinent break up new oceanic spreading centers develop.
Mid oceanic ridges are at 2000 m water depth.
They take up more space in the ocean basins than older cooler oceanic
crust which sinks to a lower level of 4000-5000m.
More spreading ridges will cause oceanic crust occupy more space in the
ocean basins.
This will cause higher sea level.
During supercontinent formation - length of spreading centre less – fall of
sea level.
4. Other causes of global sea level change
Expansion of water during increase of global air temperature.
Results in sea level rise.
Have very limited effect – results in only a few m change in sea level.
A small change level of sea water is also possible by changing the
proportions of world’s water which is resident on the continents in rivers,
lakes and groundwater.
Cyclicity in changes in sea level
There are several orders of cyclicity in Phanerozoic superimposed upon each
other.
1. First order cycles
Long term fluctuations of sea level (100-300 m) spreading over hundreds of
millions of years.
Related to break up and formation of continents.
Sea level curve show rise during Cambrian to a peak in the early Ordovician
- due to formation of new spreading centres during dispersal of continents
prior to the amalgamation of Pangaea during Permian
Permian.
Fall of sea level during late Paleozoic due formation of Pangaea.
Rise of sea level at the end of Mesozoic due to break up of Pangaea.
2. Second order cycles
Superimposed on the first order cycles is a pattern of rises and falls with a
duration of 10’s of m.y.
Cause much speculated.
C
Changes
g in the rate of spreading
p g of mid oceanic ridges
g – most p
popular
p
explanation.
3. Third order cycles
Rise and fall of sea level of several 10’s of m and periodicity of 1-10 m.y.
No generall agreementt on causes – glacio-eustacy
N l i t or changes
h in
i th
the regional
i l
tectonic stresses acting on a basin.
4. Short term changes in sea level
periodicity 10,000 to 5,00,000 years – represent parasequences.
Magnitude of sea level change few m to 10-20m.
Cause - changes in global climate caused periodic wasting and accretion
of ice masses with a cyclicity of hundreds of thousands of years.
These global climatic variations related to behaviour of the Earth on its orbit
around the Sun and changes in the axis of rotation.
The orbital rhythms are recognized and their periodicity calculated by
Milankovitch.
So these cycles are now commonly known as Milankovitch cycles.
These cycles working, independently and in combination, exert fundamental
controls on global climate, leading to cycles of global warming and
cooling.
1. Changes in eccentricity of earth’s orbit
Longest period rhythm - about 1,00,000yrs
Due to changes of the eccentricity of the Earth’s orbit around the Sun.
The orbit is elliptical and changes its shape with time.
2 Changes in tilt of the Earth
2. Earth’s
s axis of rotation
The axis of rotation of Earth is oblique with respect to the plane of Earth’s
rotation around the Sun.
The angle of tilt changes over period of 40,000 yrs between 21.5 ˚ to 24.5˚.
3. recession of axis of rotation
Shortest periodicity – 21,000yrs.
Caused by wobble in axis of rotation
PRODUCTS IN DIFFERENT SYSTEMS TRACTS
