This document provides an overview of delta depositional environments. It begins with an introduction to depositional environments and their classification. It then discusses marginal marine environments and focuses on deltaic systems. The key factors that control delta environments are described. The document outlines the various sub-environments found in a delta, such as the delta plain, front, and prodelta. It also discusses the variation in delta morphology and the processes involved in delta formation. Finally, it covers the morphological units of deltas and methods of delta classification.

2. Contents
 Introduction
 Classification of depositional environments
 Depositional environments
 Marginal-marine environments
 Deltaic system
 Controls on Delta Environment
 Sub-environments of delta
 Variation in delta morphology
 Processes in model delta
 Morphological units of delta
 Classification of Deltas
 Typical sequence
 Importance of delta
 Sedimentary structure in deltas
 Structural features of deltaic reservoir

3. Introduction:
 To discuss deltas we should have great understanding
of depositional environment.
 Depositional Environments:
 Geomorphic setting in which a particular set of physical,
chemical, and biological processes operates to generate
a certain kind of sedimentary deposit
 Physical environment has “static” and dynamic
elements
􀂄 Static: basin geometry, sediment composition, water
depth, etc.
􀂄 Dynamic: currents (wind, water), precipitation, climate

4.  Chemical elements: pH, Eh, salinity, pCO2, etc.
 Biological aspects: activities of organisms
(burrowing, skeletal particles, etc.) and their remains
(e.g., peat)

5. Classification of Depositional
Environments:
 Nonmarine Environments
 Colluvial and Alluvial fans
 Fluvial environments
 Lacustrine environments
 Aeolian environments
 Costal(marginal marine) Environments
 River mouth environments
 Regressive river mouths:Deltas
 Transgressive river mouths:Estuaries
 Open shoreline(beach) environments
 Foreshore
 backshore
 Marine Environments
 Shallow marine environments
 Shoreface
 Inner and outer shelf
 Deep marine environments
 Continental slope, Abyssal plain(basin floor)

7. Depositional Environments:
􀂄 Objective: Use sedimentary deposits to
interpret depositional environments
􀂄 Physical, biological, chemical parameters
of an environment combine to produce a
body of sediment characterized by specific
textural, structural and compositional properties
􀂄 Distinctive bodies of sediments or
sedimentary rocks are facies

8. What happens… What we want…

9. Marginal marine environment:
 The marginal-marine(transitional) setting lies
along the boundary between the continental
and the marine depositional realms.
 It is a narrow zone dominated by
 Riverine
 Wave
 Tidal processes
 Salinities may range in different parts of the
system from fresh water through brackish
water to supersaline, depending upon the river
discharge and climatic conditions.

10. Deltaic system
 The term “delta” the Greek character
was used to describe the mouth of Nile
by Herodotus nearly 2500 years ago.
 A modern definition cites delta as “the
sub aerial and submerged contiguous
sediment mass deposited in a body of
water(ocean or lake) primarily by the
action of a river”(Moore and
Asquith,1971,p.2563).

11.  Deltas seldom form on active, subducting
continental margins because there is no stable
shallow shelf on which sediments can accumulate.
 Shape of Delta:
 The shape of a delta is not always the triangle that
suggested the name to Herodotus.
 Delta shape is influenced by
a) Sediment input
b) Wave energy
c) Tidal energy.

12. Controls on delta environment:
Magnitude of
fluvial discharge
Climate, tectonics, s Delta morphology
ubsidence, sedimen and sedimentary
Delta regime
t supply facies
topography
Magnitude of
wave and tidal
currents
Factors affecting delta regime, morphology and
facies (Elliot 1978a)

13. Subenvironments of Delta:
 Deltas are influenced by a complex combination
of fluvial and marine processes.
 Each delta has more than a dozen distinct
environments of deposition.
 These environments can be grouped into three
broad divisions:
1. The delta plain with the meandering flood plains
Swamps and beach complex
2. The steeper delta front
3. The broadly sloping prodelta, which grades into
the open shelf.

15. Variations in delta morphology:
 The combinations of factors that control delta
morphologies give rise to a wide spectrum of
possible delta characteristics.
 Two main factors are the most important in
determining the morphology of deltas:
1) Effect of grain size
2) Depth of water in delta are going to deposit

16. (a) a high proportion of suspended load results in a relatively
small mouth bar deposited from bedload and extensive delta-front
and prodelta deposits

17. (b) a higher proportion of bedload results in a delta with a higher
proportion of mouth bar gravels and sands.

18. (a) A delta prograding into shallow water will spread out as the
sediment is redistributed by shallow-water processes to form
extensive mouth-bar and delta-front facies.

19. (b) In deeper water the mouth bar is restricted to an area close to
the river mouth and much of the sediment is deposited by mass-
flow processes in deeper water.

20. Processes in a model delta:
 Reduced to its simplest elements, a delta forms
due to “unique hydrodynamic interaction” between
river water and seawater.
 There is sharp contrast in water density due to
salinity. As a result river water forms a plane jet
that spreads out and forms a layer over the
seawater.
 Current velocity diminishes radially from the jet
mouth, depositing sediments whose settling
velocities allow grain size to diminish radially from
the jet mouth.

21. Development
of the delta
through time by
progradation (A,
B, D) and
distributary
switching (C)
(From Davis,
1983)

22. Levees:
 Ridges on either side of the distributary channels
are termed “levees”.
 The sand carried in the stream is deposited along
the sides of the jet in the subaqueous
levees, where friction and mixing slow the flow.
 Distributary channels:
 It is channel that branches off and flow away from
a main channel or stream. Common feature of
delta.
 Distributary channel sands are abundantly:

23. I. Cross-bedded, with plenty of ripple cross-
lamination.
II. Scour-and-fill structures
III. Discontinuous clay lenses.

24. Distributary mouth bars:
 Further offshore, where friction and spreading
begins to slow the jet, sediments is dropped in the
distributary mouth bars.
 The distributary mouth bar sands are even more
complexly cross stratified because of the complex
current system that pass over them.
 Wood, debris and other organic matter carried
down the river during floods end up in the
distributary mouth bars.
 Between the distributaries on the delta plain are
wide, shallow “inter-distributary bays” and
“marshes” like the flood plains of the meandering
river. Much of inter-distributary sequence is built of
sand sheets from “crevasse splays” deposited.

25. Morphological units of deltas:
 Three main morphological units appear.
 Delta platform/plain:
 The delta platform is the sub-horizontal surface
nearest the jet mouth. It is basically composed of
sand and traversed by the distributary channel
and its flanking levees.
 Delta slope/front:
 The delta platform grades away from the source
into delta slope on which finer sands and silts
come to rest. Commonly burrowed, sand coarsing
upward, and have good porosity and permeability.

26. Pro-delta:
 Delta slope in turn passes down into the delta slope
on which finer silts and clays settle out of
suspension.pro-delta deposits rest unconformable on
marine shelf deposits, which may include nonclastic
components such as algal reefs.
Classically these three elements termed as 1-
bottomset, 2-foreset, 3-topset respectively.

27. Classification of delta:
 Deltas can be classified in several ways
(Nemec, 1990), however classification
on the basis of delta-front regime
(Galloway, 1975) appears to be favored
by most geologists.
 Deltas are classified thus as:
1) Fluvial-dominated delta
2) Tide-dominated delta
3) Wave-dominated delta

28. The forms of modern deltas: (a) the Nile delta, the „original‟ delta, (b) the
Mississippi delta, a river-dominated delta, (c) the Rhone delta, a wave-
dominated delta, (d) the Ganges delta, a tide-dominated delta.

29. Fluvial-dominated delta:
 A fluvial or river dominated delta has a large
volume of sediment and tends to be “lobate” when
there is a moderate sediment supply and
“elongate” when the sediment supply is large.
 If the sediment supply cannot keep up with the
erosive powers of tides, than the delta tends to be
very small.
 It occur where the tidal range is very low and the
tidal current action is very weak.

30. Example:
Mississippi delta:
 It is created when very large amounts of sediment
are carried into relatively quiet water.
 Partly because dredging has kept the major
distributary channels (locally called “passes”) fixed
in position for many decades, the Mississippi‟s
distributaries have built long fingers of sediment
out into the sea.
 The resulting shape has been termed a “birdfoot”
delta. Because of the dominance of stream
sedimentation that forms the fingerlike
distributaries, birdfoot deltas like the Mississippi‟s
are also referred to as stream-dominated deltas.

31. Stream-
dominated
delta
Aster satellite photo of the Mississippi River delta taken in 2001.

32. Tide-dominated delta:
 A tide dominated delta has many linear channels
parallel to the tidal flow and perpendicular to the
shore.
 It occur in regions where wave action is limited and
tidal ranges are generally in excess of 4
m, generating strong tidal currents -- have a major
effect on mixing of river water and seawater and on
sediment redistribution.
 These deltas form along a coast that is dominated by
strong tides, and the sediment is reshaped into tidal
bars that are aligned parallel to a tidal current.

33. Example:
The Ganges-
Brahmaputra
Delta in
Bangladesh is a
good example of
a tide-dominated
delta.

34. Wave-dominated delta:
 A wave dominated delta is smoothly arcuate; the
wave action reworks the sediments and make such
deltas much sandier than other types of deltas.
 It occur where wave energy is high; out-flowing
freshwater behaves as a countercurrent, slowing
down oncoming wave crests and causing waves to
break in deeper water than normal.
 this leads to vigorous mixing, rapid deceleration of the
freshwater flow, and sediment deposition; wave action
reworks the deposited sediments to form sand bars
and beaches, creating a straight shoreline with only a
small protuberance at the distributary mouth.

35. Example:
The Nile Delta
It is a wave-dominated
delta that contains
barrier islands along its
ocean-ward side

36. mixed-process deltas:
 The examples discussed above illustrate some
differences in characteristics of modern deltas that
are shaped by processes that are predominantly
fluvial, tidal or wave related.
 Many deltas have characteristics that are
transitional between these end members types.
 Example:
 The Copper River delta in the Gulf of Alaska
provide an example of a delta that is strongly
influenced by tides but also experiences high
wave power(Galloway, 1976).

37. The Copper
River
Delta, Gulf
of Alaska

38. General delta patterns:
 A well-developed delta provides the whole gamut
of clastic sediment types from carbonaceous
mudstones to conglomerates.
 The proportions of the sediments types are
controlled chiefly by the interaction of fluvial
agency supplying the material and the marine
agency receiving it. This interaction leads to four
general deltas patterns:
I. High-destructive deltas
II. High-constructive deltas of birdfoot type
III. High-constructive deltas of lobate type
IV. Fan deltas

40. Typical sequence:
 The classical stratigraphic profile of deltaic
deposits shows a coarsening upward sequence
from the delta slope muds and silts to the
distributary mouth bar sands.
 This is opposite to the fining upward sequence
found in most meandering fluvial system.

41. Why deltas are so important???
 Ancient deltaic deposits are extremely important
economically.
 Due to variety of environments in the deltas, it
makes more important for a reservoir geologists.
 Many oil and gas-fields are in sedimentary
deposits associated with deltas.
 Located near the boundaries between marine
deposits, which include source sediments, and
non-marine deposits which represent the supply
zone for reservoir rocks, deltas are ideally situated
for the maximum interplay between source and
reservoir facies.

42.  They hosts most of the world‟s coal, and many
major petroleum provinces.
 The deltaic process is a way of deposition lobes of
sand (potential reservoir) into envelopes of
organic-rich marine muds (potential source beds).
 Deltaic environments deposit many potential
stratigraphic traps, including mouth bars, barrier
bars, and channels.
 Rapid deposition often leads to over-pressuring.
This may generate diapiric traps and roll-over
anticlines.

43. Sedimentary structures and
fossils:
 Numerous types of sedimentary structures such
as:
1. Cross bedding
2. Ripple marks
3. Bioturbation structures
4. Slump structure and
5. Mud diapirs occur in deltaic deposits.
A “Mud diapir” is a dome or fold in sediments that is
formed by the plastic deformation of mud
underlying sand or other sediments. Diapirs
called mudlumps, frequently emerge in
distributary mouth bar deposits,

44. Structural features in deltaic
reservoir:
 Structural features result from deformation of
sediments and rocks includes
faults, folds, tilting(dip), and fractures.
 Structural features can broadly divided into two
classes based on the timing of the deformation:
 Syndepositional deformation features:
 Syndepositional deformational processes, which
includes:
a. Slumping
b. Mud diapirism
c. Growth faulting are common in lower delta plain
environments and operate during delta formation.

45. Post-depositional deformation:
 Post-depositional deformational features includes:
a. Folding
b. Tilting, faulting, fracturing.
 This is due to tectonic forces and consequent
movement of earth‟s crust.
 Significances:
 Structural features can modify the sandstone body
geometry.
 These features are important in the
migration, accumulation and trapping of
petroleum.

46.  Migration of hydrocarbons from the source rock
enhanced by:
 Faulting and Fracturing.
 These accumulation or trapping of oil is caused by
permeability barriers which prevent further
migration of the petroleum.
 When the permeability barriers is a structural
feature, the reservoir is considered structural traps
 Fracture and faults are important on an inter-well
scale where they control the movement of both
injected and naturally occurring reservoir fluids
and may significantly affect the production of
hydrocarbons.

47. Examples in Pakistan:
 Samber Formation is source rock of
deltaic environment.
 Goru Formation is a reservoir rock of
deltaic environment.
 Its upper part acts as a seal rock.

48. Thank You.