Deltaic systems usmaan

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Institute of Geology, Punjab university Lahore

Deltaic
Systems
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Presented to:
Prof. Dr. Shahid Ghazi
Presented by:
UsMaan Sikandar
Roll No. 29
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Contents
 Introduction
 Delta Classification
 Fan Delta
 Physiographic Characteristics of Deltas
 Ancient Deltaic Systems
 Indus River Delta
 Conclusion
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Introduction
The term “delta”, the Greek
character ∆, was used to
describe the mouth of Nile
by Herodotus nearly 2500
years ago.
A modern definition of
delta cites as “The sub-
aerial and sub-aqueous
contiguous sediment mass
deposited in a body of
water such as ocean or
lake primarily by the action
of a river”.
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Introduction
Deltas can form in lakes and inland seas as well as in
the ocean, but they are most important in the open
ocean. Much of the siliciclastic sediment transported
to coastal zones throughout geologic time has been
deposited in deltas.
Conditions required forming deltas
1. The river must carry a large amount of sediments.
2. The waves or current along the coast must not be
strong enough to remove the sediments
deposited by the river, at least not faster than the river
can deposit them.
3. The slope of the sea floor or lake at the river mouth
must be gentle or flat.
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Introduction
Ancient deltaic deposits have been identified in
stratigraphic successions of many ages, and deltaic
sediments are known to be important hosts for
petroleum and natural gas, coal, and some
minerals such as uranium.
Deltas are particularly common in the modern
ocean owing to post-Pleistocene sea-level rise
coupled with high sediment loads carried by many
rivers. High sea level increases sedimentation rates
on deltas because sediments are trapped by the
rising water, inhibiting sediment removal by
currents.
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Delta Classification
The distribution and characteristics of deltas are controlled
by a complex set of interrelated fluvial and
marine/lacustrine processes and environmental conditions
such as:
 climate
 water and sediment discharge
 river-mouth processes
 near shore wave power
 tides
 near shore currents
 winds
 slope and inclination
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Delta Classification
Deltas can be classified in several ways,
however, classification on the basis of delta-
front regime appears to be favored by most
geologists. Deltas are classified thus as
(1) Fluvial-dominated,
(2) Tide-dominated,
(3) Wave-dominated,
(4) Mixed process-dominated
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A classification of deltas based on
dominant processes
Dominant
process Environments Sand facies Example
FLUVIAL
Radiating bird-foot
distributary/
levee systems
Radiating
mouth
bar sands
Mississippi
MARINE Waves
Distributaries
truncated by
barrier sands
Arcuate
delta-front
barrier sands
Nile and
Niger
Tides
Extensive tidal flats
and scoured
braided estuaries
Delta-front
sheet
sand
Ganges-
Brahmaputr
a
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1-Fluvial-Dominated Deltas
The discharge of river water and sediment into a lake
or ocean is referred to as a jet. This discharge may
take place in following ways:
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Homopycnal flow :occurs when the density of the river
water is equal to the density of the standing water in
the basin. This type of flow is associated with rapid
mixing throughout the general flow. There is also abrupt
deposition of the sediments carried to the basin by the
river as the two water sources meet.
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hyperpycnal flow: River water that has higher
density than basin water flows beneath the basin
water, commonly during floods, generating a
vertically oriented, plane-jet flow called
hyperpycnal flow.
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hypopycnal flow: If river outflow is less dense than basin
water, as in rivers that flow into denser seawater or a
saline lake, it flows outward on top of the basin water as
a horizontally oriented plane jet called hypopycnal
flow.
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2-Tide-Dominated Deltas
If tidal currents are stronger
than river outflow, these
bidirectional currents can
redistribute river-mouth sedi-
ments, producing sand-filled,
funnel-shaped distributaries.
The distributary mouth bar may
be reworked into a series of
linear tidal ridges that replace
the bar and extend from within
the channel mouth out onto
the subaqueous delta-front
platform.
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3-Wave-Dominated Deltas
Strong waves cause rapid diffusion
and deceleration of river outflow
and produce constricted or
deflected river mouths. Distributary-
mouth deposits are reworked by
waves and are redistributed along
the delta front by long shore currents
to form wave-built shoreline features
such as beaches, barrier bars, and
spits. A smooth delta front, consisting
of well-developed, coalescent
beach ridges, may eventually be
generated.
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4-Mixed-Process Deltas
Mixed-process
deltas illustrate some
differences in
characteristics of
modern deltas that
are shaped by
processes that are
predominantly
fluvial, tidal, or
wave-related.
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Fan delta
A fan delta is a
coastal prism of
sediments delivered
by an alluvial fan
system and
deposited, mainly or
entirely sub-
aqueously, at the
interface between
the active fan and a
standing body of
water.
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Classification of deltas taking grain size, and hence
sediment supply mechanisms, into account. (Modified
from Orton & Reading 1993).
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Physiographic Characteristics
of Deltas
All deltas can be divided into sub aerial and sub
aqueous components, each of which can be
further subdivided.
Deltaic plain
The sub-aerial component of deltas, generally
larger than the subaqueous component. It is
divided into
 upper delta plain, which lies largely above high-
tide level
 lower delta plain, lying between low-tide mark
and the upper limit of tidal influence.
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Physiographic Characteristics
of Deltas
The subaqueous delta plain lies seaward of the
lower deltaic plain below low-tide water level.
Delta Front
The uppermost part of the subaqueous delta, lying
at water depths down to 10m or so.
Pro-delta
The remaining sea ward part of the subaqueous
delta.
The subaqueous delta may extend outward for
distances of a few kilometers to tens of kilometers,
and the pro-delta may extend to water depths as
much as 200-300 m.
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Ancient Deltaic systems
Ancient deltaic sediments have been reported in
stratigraphic successions of most ages, but they
appear to be particularly common in rocks of
Carboniferous and Tertiary age. These deltaic
sediments are known to be important hosts for
petroleum and natural gas, coal, and some minerals.
These deltas have same sedimentary processes which
are present in modern deltaic systems.
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Indus River Delta
 The present delta of the Indus is typically
triangular in shape and extends from Thatta up
to the sea. It covers an area of approximately
2600km2.
 Indus River Delta receives the highest average
wave energy of any major delta in the world at a
water depth of 10m. At the shoreline the Indus
Delta receives more wave energy in one day
than the Mississippi Delta gets in one year.
 The sediment discharged by the Indus settle on
the continental shelf but some are also
transported to the southeast by long-shore
currents. The lower margin of deltaic flood plain
is also rimmed by the tidal delta.
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Conclusions
 Deltas are important indicators of paleo-
climates, they provide us with drinking water,
and they can be good reservoirs for oil and gas.
 Deltas are commonly divided into constructive
deltas and destructive deltas.
 Modern delta characteristics can tell us about
ancient formations.
 In Pakistan, Indus River Delta is one of the largest
deltas of
the world which is destructive delta in its type.
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Deltaic systems usmaan

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