assignment 200

151PTE321 Engineering Geology

Lecture 6

Learning Outcomes

After completing this chapter the student will:

• familiarize himself with common types of sedimentary basins and their

formation;

• learn the relationship between the formation of sedimentary basins and

tectonics;

• familiarize himself with the different kind of sedimentary rocks deposited in

different sedimentary basins;

• learn what are the most favorable sedimentary basins for O&G

accumulations.

Sedimentary Basins

• Sedimentary basins are regions where sediment accumulates into

successions of hundreds to thousands of metres in thickness over areas of

thousands to millions of square kilometres.

• The underlying control on the formation of sedimentary basins is plate

tectonics and hence basins are normally classified in terms of their position

in relation to plate tectonic settings and tectonic processes.

• Each basin type has distinctive features, and the characteristics of

sedimentation and the stratigraphic succession that develops in a rift valley

can be seen to be distinctly different from those of an ocean trench.

• The sedimentary rocks in a basin provide a record of the tectonic history

of the area.

• They also provide the record of the effects of other controls on deposition,

such as climate, base level and sediment supply.

Sedimentary Basins

Three main settings of basin formation can be recognized:

1. basins associated with regional extension within and between plates;

2. basins related to convergent plate boundaries;

3. basins associated with strike-slip plate boundaries.

Basins Related to Lithospheric Extension

• The motion of tectonic plates produces in some areas the extension of the

lithosphere and in other places compression.

• In the early stages of extension Rifts form and are typically sites of

continental sedimentation.

Sedimentary Basins

Rift Basins

• In regions of extension

continental crust fractures to

produce rifts, which are structural

valleys bound by extensional

(normal) faults.

• The down-faulted blocks are

referred to as graben and the up-

faulted areas as horsts.

• The structural weakness in the

crust and high heat flow

associated with rifting may result

in volcanic activity.

• Sediment is supplied from the

rift flanks or brought in by rivers

flowing along the axis of the rift.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Rift Basins

• In regions of extension

continental crust fractures to

produce rifts, which are

structural valleys bound by

extensional (normal) faults.

• The down-faulted blocks are

referred to as graben and the

up-faulted areas as horsts.

• The structural weakness in the

crust and high heat flow

associated with rifting may result

in volcanic activity.

• Sediment is supplied from the

rift flanks or brought in by rivers

flowing along the axis of the rift.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Intracratonic Basins

• These are areas of broad

subsidence within a

continental block (craton)

away from plate margins or

regions of orogeny.

• Rifts are therefore areas of

high heat flow, a high

geothermal gradient.

• When geothermal gradient is

reduced the crust cools,

contracts and sinks

resulting in thermal

subsidence.

• Fluvial and lacustrine

sediments are commonly

encountered in intra-cratonic

basins. from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Proto-oceanic troughs

• As the extension of the continental

crust continues this leads to

thinning and eventually to rupture.

• Basaltic magmas rise to the

surface in the axis of the rift and

start to form new oceanic crust.

• The basin will be wholly or partly

flooded by seawater.

• Rivers will be depositing

sediment to shelf areas and out into

deeper water in the axis of the

trough as turbidity currents.

• Connection to the open ocean may be intermittent during the early stage of

basin formation and in arid areas with high evaporation rates the basin may

periodically desiccate. Evaporites (gypsum or halite) may form in these

circumstances.

• This stage is known as a ‘proto-oceanic trough’ and is the first stage in the

initiation of an ocean basin: the remnant flanks of the rift become the passive

margins of the ocean basin as it develops.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Passive margins

• The regions of continental crust

and the transition to oceanic

crust along the edges of

spreading oceans basins are

known as passive margins.

• Passive because no subduction is

happening in this region.

• The continental crust is

commonly thinned in this region

and there may be a zone of

transitional crust before fully

oceanic crust of the ocean basin

is encountered. Transitional crust

forms by basaltic magmas

injecting into continental crust in a

diffuse zone as a proto-oceanic

trough develops.

• Subsidence of the passive margin is due

to continued cooling of the lithosphere as

the heat source of the spreading centre

becomes further away, augmented by

the load on the crust due to the pile of

sediment that accumulates.

• The clastic sediment supply is largely

from the adjacent continental land area.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Stages in the Development of a Passive Margin

Sedimentary Basins Passive margins

• The supply of clastic sediments will be low in areas adjacent to desert

areas, and the margin will be a starved margin, experiencing a low clastic

sedimentation rate.

• In contrast, a large river system may carry large amounts of detritus and

build out a large deltaic wedge of sediment onto the margin.

• In the absence of terrigenous detrital supply, the shelf may be the site of

accumulation of large amounts of biogenic carbonate sediment, although

the volume and character of the material will be determined by the local

climate.

• Passive margins are important areas of accumulation of both carbonate

and clastic sediment: they may extend over tens to hundreds of

thousands of square kilometres and develop thicknesses of many

thousands of metres.

• They are also areas that are sensitive to the effects of eustatic changes

in sea level because most of the deposition occurs in water depths of up

to 100 m.

Detailed Cross-section of a Passive Margin

Atlantic Margin

Triassic rift valley sediments

Jurassic salt

Cretaceous &

Cenozoic sediments

What is the relative

age of the basalt?

Sedimentary Basins Ocean basins

• Basaltic crust formed at mid-oceanic ridges is hot and relatively buoyant.

• As the basin grows in size by new magmas created along the spreading

ridges, older crust moves away from the hot mid-ocean ridge.

• Cooling of the crust increases its density and decreases relative

buoyancy, so as crust moves away from the ridges, it sinks.

• Mid-ocean ridges are typically at depths of around 2500 m.

• The depth of the ocean basin increases away from the ridges to between

4,000 and 5,000 m where the basaltic crust is old and cool.

• The ocean floor is not a flat surface. Spreading ridges tend to be irregular,

offset by transform faults that create some areas of local topography.

• Isolated volcanoes and linear chains of volcanic activity related to

hotspots (mantle plumes) such as the Hawaiian Islands form submerged

seamounts or exposed islands.

Sedimentary Basins Ocean basins

• The shallow water environment may be a site of carbonate production

and the formation of reefs.

• In the deeper parts of the ocean basins sedimentation is mainly pelagic,

consisting of fine-grained biogenic detritus and clays. Nearer to the

edges of the basins terrigenous clastic material may be deposited as

turbidites.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Basins Related to Subduction

• A trough is created at the contact between the two plates as the downgoing plate

bends to enter the subduction zone : this is the ocean trench.

• The magmas generated by the melting of the subducted plate rise to the surface

through the overriding plate to create a line of volcanoes, or volcanic arc.

• Magma is created when the down going slab reaches 90 to 150 km depth.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Basins Related to Subduction

• Arc–trench systems are regions of plate convergence, however, the upper plate of

an active arc must be in extension in order for magmas to reach the surface and

generate volcanic activity.

• If the angle of subduction is steep then convergence is slower than subduction at

the trench, the upper plate is in net extension and an extensional backarc basin

forms (Dickinson 1980).

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Trenches

Ocean trenches are elongated, gently curving troughs that form where an oceanic plate

bends as it enters a subduction zone.

The bottoms of modern trenches are up to 10,000m below sea level, twice as deep as

the average bathymetry of the ocean floors.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Accretionary Complexes

A subducting plate can be thought of as a conveyor belt bringing ocean basin

deposits, mainly pelagic sediments and turbidites, to the edge of the

overriding plate.

In some places this sediment is carried down the subduction zone, but in

others it is sliced off as a package of strata that is then accreted on to the

overriding plate.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Forearc Basin

The main source of sediment to the basin is the volcanic arc and, if the arc

lies in continental crust, the hinterland of continental rocks.

Intraoceanic arcs are commonly starved of sediment because the island-arc

volcanic chain is the only source of detritus apart from pelagic sediment.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Backarc Basin

• Extensional backarc basins form where the angle of subduction of the

downgoing slab is steep and the rate of subduction is greater than the rate

of plate convergence.

• Rifting occurs in the region of the volcanic arc where the crust is hotter

and weaker.

• The principal source of sediment in a backarc basin formed in an oceanic

plate will be the active volcanic arc.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Basins related to Crustal Loading

Collision of plates involves a thickening of the lithosphere and the creation

of an orogenic belt, a mountain belt formed by collision of plates.

The Alps have formed by the closure of the Tethys Ocean as Africa has

moved northwards relative to Europe, and the Himalayas are the result of a

series of collisions related to the northward movement of India.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Basins related to Crustal Loading

Thickening of the crust will result in an additional load being placed on the

crust either side and causes a downward flexure of the crust to form

peripheral foreland basins.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Retroarc Foreland Basin

The thickness of the crust increases due to emplacement of magma in a

volcanic arc at a continental margin, resulting in flexure of the crust behind

the arc to form a retroarc foreland basin.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Strike-slip basins

• Most basins in strike-slip belts are

generally termed trans-tensional

basins.

• The overlap of two separate faults

can create regions of extension

between them known as pull-apart

basins.

• Such basins are typically rectangular

or rhombic in plan with widths and

lengths of only a few kilometres or

tens of kilometres.

• They are unusually deep, especially

compared with rift basins.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Strike-slip basins

Where there is a branching of faults a zone of extension exists between the

two branches forming a basin.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins The Wilson Cycle

Rift basins form

and evolve into proto-

oceanic troughs and

eventually into ocean

basins bordered by

passive margins. After

a period of tens to

hundreds of millions of

years the ocean basin

starts to close with

subduction zones

around the margins

consuming oceanic

crust. Final closure of

the ocean results in

continental collision

and the formation of

an orogenic belt.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins The Wilson Cycle

Rift basins form

and evolve into proto-oceanic

troughs and eventually into

ocean basins bordered by

passive margins. After

a period of tens to hundreds of

millions of years the ocean

basin starts to close with

subduction zones

around the margins consuming

oceanic crust. Final closure of

the ocean results in

continental collision

and the formation of an

orogenic belt.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins The Wilson Cycle

Within the Wilson Cycle,

• Rift basin: may be recognized by river and lake deposits overlying the

basement,

• Proto-oceanic trough stage: recognized by evaporites,

• Passive margin deposition: will be recorded by thick succession of shallow-

marine carbonate and clastic deposits.

• Forearc region: If this passive margin becomes a site of subduction, arc-

related volcanics will occur as the margin is transformed into a forearc

region of shallow-marine, arc-derived sedimentation.

• Upon complete closure of the ocean basin, loading by the orogenic belt may

then result in foreland flexure of this same area of the crust, and the

environment of deposition will become one of deeper water facies.

• As the mountain belt rises, more sediment will be shed into the foreland

basin and the stratigraphy will show a shallowing-up pattern.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

The Major Types of Sedimentary Basins

The major types of sedimentary basins are shown in their plate-tectonic settings.

The major physical cause or causes of subsidence for each case are shown below

the diagram. Some examples are indicated in top.

Michigan Basin

E. AfricaNevada

Offshore Calif.

Indonesia

E. Coast NA

Sedimentary Basins Strike-slip basins

The curvature of a single fault strand results in bends that are either

restraining bends (locally compressive) or releasing bends (locally

extensional): releasing bends form elliptical zones of subsidence.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Forearc Basin

The inner margin of a forearc basin is the edge of the volcanic arc and the

outer limit the accretionary complex formed on the leading edge of the upper

plate.

The basin may be underlain by either oceanic crust or a continental margin.

The thickness of sediments that can accumulate in a forearc setting is partly

controlled by the height of the accretionary complex: if this is close to sea

level the forearc basin may also fill to that level.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Trenches

They are also narrow, sometimes as little as 5 km across, although they may

be thousands of kilometres long.

Trenches formed along margins flanked by continental crust tend to be filled

with sediment derived from the adjacent land areas. Intra-oceanic trenches

are often starved of sediment because the only sources of material apart from

pelagic deposits are the islands of the volcanic arc.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009

Sedimentary Basins Basins related to Crustal Loading

When an ocean basin completely closes with the total elimination of

oceanic crust by subduction the two continental margins eventually

converge.

Where two continental plates converge subduction does not occur

because the thick, low-density continental lithosphere is too buoyant to be

subducted.

from G. Nichols, “Sedimentology and Stratigraphy”, 2009