Deltaic systems usmaan

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Institute of Geology, Punjab university Lahore

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2Deltaic Systems

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Presented to:

Prof. Dr. Shahid Ghazi

Presented by:

UsMaan Sikandar

Roll No. 29

UsMaan

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4 Contents

Introduction Delta Classification Fan Delta Physiographic Characteristics of Deltas Ancient Deltaic Systems Indus River Delta Conclusion

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5 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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7 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 deltas1. 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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9 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 sedi ment removal by currents.

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10 Delta Classification The distribution and characteristics of deltas are controlled by a complex set of in terrelated 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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11 Delta Classification

Deltas can be classified in several ways, however, classifica tion 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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12 A classification of deltas based on dominant processes

Dominant process

Environments

Sand facies

Example

FLUVIAL Radiating bird-footdistributary/levee systems

Radiating mouthbar sands

Mississippi

MARINE WavesDistributariestruncated bybarrier sands

Arcuate delta-frontbarrier sands

Nile and Niger

TidesExtensive tidal flats and scoured braided estuaries

Delta-front sheetsand

Ganges-Brahmaputra

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13 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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17 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 ex tend from within the channel mouth out onto the subaqueous delta-front platform.

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18 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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19 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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20 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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22 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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23 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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25 Ancient Deltaic systemsAncient 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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26 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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28 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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