Sedimentology presentation and stratigraphy: processes, environments and deposits

Sedimentology presentation

1. GEOL 440
Sedimentology
and stratigraphy:
processes,
environments and
deposits
Lectures 16 & 17:
Deltaic Facies

2.  Basic processes
 Classification
 Facies
What controls
morphology and
facies of the
Nile Delta?

3. Early descriptions
of deltas
Gilbert, 1885
Pleistocene
Lake Bonneville,
Utah
Barrell, 1911: Devonian, Appalachians

4. Bhattacharya,2006
Progradation
and time

5. River-dominated
Wave-
dominated
Tide-
dominated
GANGES
MEKONG
ORINOC
O
Classification of deltas
Nile delta
A basic scheme we’ll use at first ... BUT modification by basinal processes
is not the only parameter controlling the morphology of deltas

6. River-dominated deltas
ESA, 6
th
Feb. 2007

7. Modes of interaction between sediment-laden
river water and basin water
1a. Homopycnal flow
r(inflow)=r(basin)

8. Modes of interaction between sediment-laden
river water and basin water
1b. Inertia-dominated deposition
lunate mouth bar
bar crest
steeper
bar front
(almost) flat
bar back
channel
foresets
mouth
Gilbert-type
delta
channel
bottom sets
downstream fining

9. Gilbert-type delta
top sets
foresets
bottom sets

10. Modes of interaction between sediment-laden
river water and basin water
2a. Hyperpycnal flow
Plane jet
bottom flow
Basin
Basin
Plan view
r(inflow)>r(basin)

11. Modes of interaction between sediment-laden
river water and basin water
2b. Friction-dominated deposition
Channel bifurcation
Curved levees
New bars & bifurcation
during progradation
Formation of triangular
mouth bar, rapidly
fining downstream
Rapid flow deceleration
& lateral expansion

12. Modes of interaction between sediment-laden
river water and basin water
3a. Hypopycnal flow
Basin
Basin
Plan view
r(inflow)<r(basin)
Plane jet
near water-
surface flow
Settling

13. Modes of interaction between sediment-laden
river water and basin water
3b. Buoyancy-dominated deposition
Do you expect this type of deposition to occur at low or high river
discharge?

14. Bhattacharya,2006
Summary:
Controls on
Sedimentation

15. N
DELTA PLAIN
10 km
River dominated: birds-foot delta (Mississippi-type)

16. Prograding mouth bar succession
Homogenous mud, finely laminated
or bioturbated, marine fauna
… representing shelf deposit
Silty/sandy slump sheet
… representing redeposited
(distal) mouth bar sediment
60
-
150 m

Finely laminated mud, bioturbation,
marine fauna, contorted bedding
… representing prodelta deposit
Interbedded mud/sand, wave & current ripple
x-lam., parallel/lenticular lam., bioturbation,
contorted bedding, possible cut & fill
… representing distal mouth bar deposit
Well-sorted sand, planar & ripple x-lam., tangential
x-bedding (bar crest); poorly sorted sand/silt, cut &
fill, small-scale trough x-bedding (bar back); large
channel fills near top (distributary channel)
Clean sand, low- & high-angle 3D strat.
… representing beach & dune deposits
Mississippi delta

17. -5
m
0
5
10
15
Distributary channel fill facies
Carboniferous, SW Wales
Basal coarse lag deposit above erosional surface
… representing channel avulsion & erosion
into delta front progradation succession
Massive, locally x-stratified sandstone with internal
erosion surfaces & some soft-sediment deformation
… representing main channel subject to flooding
Trough & planar x-bedded sandstone
with unidirectional palaeocurrents
… representing migrating dunes/bars in channel
Ripple-laminated, fine-grained sand- & siltstone,
plant-rich & with palaeosol-coal units
… representing filling of shallow channel after avulsion

18. Interdistributary Bay filling
Bhattacharya,2006

19. Interdistributary bay facies (sandy, proximal)
Overbank flooding
FU
s
: Alternating deposition
from sand-laden overbank
flows & fine suspended
load
CU: Levee progradation
2 to 10 m

Crevasse splay
Erosional, FU sand

20. Delta lobe progradation-abandonment-reworking
Bhattacharya,2006

21. Porebski and Steel,2006
Deltas and
sea-level change

22. Wave-dominated deltas

23. Continuous
cuspate/ arcuate
beach shoreline
Rhône delta, France

24. Wave-dominated delta front succession
U. Cret. Dunvegan Fm., Alberta
0
m
5
10
CU mud to sandy mud to sand with pervasive
bioturbation, storm beds & hummocky x-strat.
... representing regressive shelf deposit
Fine-grained, bioturbated sand with
HCS & plane-parallel lamination
... representing wave-swept lower shoreface deposit
Fine- to medium-grained, cross-bedded sand
... representing wave-modified mouth
bar deposit on upper shoreface
Medium-grained sand, plane-parallel lam. & low-
angle x-strat., passing into sand/mud with plant
roots
... representing beach to non-marine deposit
Transgression
Similar to prograding beach-ridge
strand-plain succession due to weak
fluvial imprint

25. Tidal flood currents may
reach far into channel &
influence deposition
Funnel-shaped distributary
channel with bi-directional
sediment transport & mud
flocculation during slack water
Redistribution of mouth bar sand
into elongate tidal current ridges &
channels with preferred ebb- &
flood-dominated pathways
Tide-dominated deltas

26. Ganges-Brahmaputra
delta, Bangladesh
Himalayans
Brahmaputra
Jamuna

DELTA
Extensive tidal flats and
marshes dissected by
numerous tidal channels

27. Ganges-Brahmaputra delta, Bangladesh 100 km
Channel
Delta plain
(non-tidal)
Delta plain-
tidal flat
Tidal sand bar
Tidal channel
Tidal channel
deeps

28. 20
-
60 m

Tide-dominated delta facies succession
Ord River delta, Australia
Bioturbated mud passing into interbedded
mud-silt-sand, marine fauna
... representing open shelf to prodelta succession
Bidirectional x-bedded sand, mud drapes, tidal
bundles, reactivation surfaces, scours/channel fills
... representing migrating tidal current ridges
& inter-ridge channel fills on delta front
Alternating sand-silt-mud, x-lam., “flaser-linsen”
bedding, small channel fills, brackish-water fauna
... representing tidal flat facies on delta plain
Erosive, FU sand with herringbone x-strat., mud
drapes, tidal bundles, ripple x-lamination
... representing tidal channel on delta plain

29. Tidal flats and channels in Ord River delta

30. Influence
of grain
size
Bhattacharya,2006

31. Deltaic facies and architecture depend on:

 Density difference between inflow water and basin water
 River processes and modification by waves and tides
… but also on:

 Type of alluvial feeder or distributary system
 Grain size of transported sediment
 Gradient of inflow system
 Basinal water depth
 Type of sediment diffusion

32. Types of subaerial feeder system
A Very steep gradient (< 20º-30º), gravel-dominated alluvial systems:
 ephemeral, unconfined streams with mass flows
 small radius
 along mountain front and fjord margins
B Steep gradient (±0.4º), gravelly alluvial systems:
 multiple, highly mobile, bedload streams
 line source along delta front
 proglacial outwash plains, braided streams at head of
fjords/lakes
C Moderate gradient, gravelly-sandy alluvial systems:
 multiple, stable streams acting as line source
 mouth bar development
 at head of fjords and lakes
D Low gradient, sandy to muddy alluvial systems:
 widely spaced, highly stable, suspension load streams
 inflows act as point source and prograde in isolation
 birdfoot-type, mouth-bar deltas at margins of coastal lowlands

33. Classification of deltas ..coarse grained deltas

Postma, 1990

34. Source facies
 mainly marine mud (and lacustrine mud)
Reservoir facies
 mainly delta front sand (mouth bar & bar-finger sand)

relatively clean sand
adjacent to marine source rock
capped by interdistributary mud

sand body trend ^ coastline in fluvial- & tide-dominated
deltas
sand body trend // coastline in wave-dominated deltas

 other: barrier/beach sand
shelf sand
channel sand
Deltas and hydrocarbons

35. Hydrocarbon traps
 Main stratigraphic traps:

sand pinchouts in areas of maximum interfingering
between delta front sand and marine mud

isolated sand bodies in mud

 Main structural traps:

synsedimentary growth faults, induced by progradation
of delta front sand over prodelta mud, resulting in
oversteepening then failure
Th (downthrown side) = 5-10 x Th (upthrown side)

salt domes

36. Summary
Reading:
B&D: Chapter 15
Boggs; Chapter 9
Bhattacharya, New Facies Models Revisited
Reading, Chapter 6
Leeder, Chapter 22
• Rivers, waves, tides and influence on sediment dispersal
• Homo, hyper and hypo pycnal flows
• Patterns of sediment dispersal
• Vertical sequences
• Grain size/gradient/depth influences?
• Soft-sediment deformation?
• Think about the allo cyclic controls on deltaic sedimentation