Sedimentary BasinsA basin is simply a depression in the earth’s crust formed by prolonged subsidence. Sedimentary basins form and evolve in a framework of moving lithospheric plates, horizontal movements control:

Isostatic responses Thermal state Crustal thickness

These produce vertical changes such that: uplifted areas are the parent rocks, depressed areas form the basins.

We can use stratigraphic successions in basin fills to decipher the tectonic history of a region and solve large-scale geological problems, for example:

Uplift history of mountain belts Reconstruct past tectonic settings Reconstruct past climatic regimes

There are three main controls on basin setting: Type of lithosphere (e.g., continental / oceanic) Proximity of a plate margin (e.g., intracontinental / margin) Type of nearest plate margin (e.g., divergent / convergent / transform)

Controls on arrangement of strata in basins: Tectonics (subsidence and uplift) Climate (temperature, rainfall, oceanographic) Eustasy (sediment supply and space for sedimentation)

Basin Types:

Divergent settings: formed by extension and represent a continuum from continental (terrestrial) rifts to proto-oceanic rifts (where new ocean floor is formed) to passive continental margins (where new oceans are formed). Not all rifts become oceans, “failed rifts” are more accurately “failed oceans”

Terrestrial rift valleys (e.g., East African Rift) Proto-oceanic rifts (e.g., Red Sea) Intraplate settings (e.g., passive continental margins and intracratonic basins)

Convergent settings: Backarc basins Trench basins Forearc basins Foreland basins

Foreland basins (flexure by tectonic loading). Typically, the load is a fold-and-thrust belt, and the basin develops in front of the load. Essentially “moats” to the mountain belt. Two broad categories:

1) Collisional (peripheral) foreland basins Continent-continent collision: Ganges Arc-continent collision: Timor trough Arc-arc collision: Pacific around Japan

2) Retroarc foreland basins: formed on continental crust behind arcs during oceanic subduction

E.g., North American Cordillera

Transform settings: typically small, short-lived basins that develop in response to localised extension related to strike-slip faulting (not pure)

Transtensional basins (e.g., pull-apart basins) Transpressional basins (e.g., San Andreas Fault Zone basins)

Subsidence mechanisms

Only two/three major ones. Tectonic settings influence the mechanisms by which a basin subsides

1) Changes in crustal/lithospheric thickness: Mechanical stretching leads to large scale fractures with subsidence between faults

(rift basins) Locally, small basins form by small amounts of extension in strike-slip fault zones

(pull apart basins) Thinning also includes thermal mechanisms such as cooling and subsidence of

oceanic crust as it moves away from the spreading centres (ocean basins) and continental crust that stretches without faulting subsides when the heat source is removed (intracratonic sag basins)

2) Tectonic loading: flexure of the lithosphere in response to loading at convergent settings Flexure of continental crust in continental collision zones, where the load is created

by the mountain belt (foreland basins) Flexure of oceanic crust as it approaches subduction zones, created by the mass of

magmatic arc as well as slab pull (oceanic trench basins) Flexure of continental-oceanic crust between the subduction zone and magmatic arc

(forearc basin)3) Sediment and volcanic loading: subsidence due to the weight of added sediment and

volcanic rocks. This loading is not considered a primary mechanism but amplifies the tectonic and thermal mechanisms in all basins

4) Subcrustal loading: increases the density of the crust through magmatic underplating5) Dynamic effects: related to the flow of the asthenosphere, primarily through subduction of

cold lithosphere, but also to mantle convection and plumes (plumes can also be very important in the development of continental rifts and the breakup of supercontinents)

6) Crustal densification: related to changes in P/T conditions in the crust and/or intrusion of high-density melts into lower-density crust.

Subsidence and Stratigraphic Patterns:Generally, basins that undergo similar subsidence histories tend to show similar broad-scale stratigraphic patterns in their in-fills. This is useful in determining the tectonic setting of ancient basins and predict stratigraphic patterns for mineral exploration in basins.

Consider: overall shape of curve, rate of subsidence, role of heat flow – compare this to cross sections of actual basins (major structural features, stratigraphic features, thickness, geometry, lateral and vertical patterns.

Rift Basins:

Canning Basin (rifting example)

Describe the major features of the basin and basin fill on this diagram: Basin is precambrian, basin-fill is phanerozoic (erosive surface) Normal faults dominate and have produced a series of uneven depo-centres (grabens, half-

grabens and horsts)

Which of these features indicates the basin type?

High heat flow is related to early basin formation Rift margins are uplifted because grabens are

down-thrown Steepest part if earliest (negative exponential) Most accommodation created early on, gradually

declines Has two stages: rifting slows after heat flow peaks

(post-rift = sag phase) Faults form early on Rift margins begin to fall during sag phase

Eucla basin (rifting example)

Describe the major vertical stratigraphic pattern in this basin fill Rifting started in late-Jurassic Rifting and subsidence was not even Half-grabens fill as wedges Two phases of filling – half grabens filled syn-rift Rift margins likely the source of JK sediments Faulting stops when rifting stops

How does this pattern relate to your subsidence curve? Clastics in lower part, carbonates on top Syn and post-rift curves can easily be distinguished, lots of space during syn-rift (thick

sediment packages) and not as much post-rift

Foreland basins (flexure)

Foreland basin

Heat flow of no importance (background) Can contain fore-bulge Two stages: syn and post orogenic Syn-orogenic subsidence is steep Post-orogenic rebound is due to erosion of the

mountain

Accommodation and Fill is influenced by: Eustasy

Describe the vertical stratigraphic pattern that would develop in this basin based on the series of diagrams above.

Repeated stratigraphy Initial base unconformity Shallowing up facies (transition from turbidites to fluvial or conglomerates) Big unconformity when orogenic rebound starts as well (basin-fill itself starts to erode) Width of basin increased over time Bulge migrates back over time Early basin formation creates lots of space but not much erosion. Therefore, deposits can

appear very deep after this space is made (e.g., turbidites)

What happens to the basin fill at (d)? What is the resulting geological feature?

Foreland Basin B2

What are the major features of the basin and basin fill in this diagram?

The sediments deposited in the Quaternary were derived by erosion of what?

Strike-slip basins:

Describe any lateral and vertical stratigraphic patterns in this basin fill

What is/are the main subsidence mechanism/s operating? And what is the evidence?

Note the size of the basin and thickness of the basin fill. How does this give a clue to the type of basin?

Extra:

What large-scale features are similar in the basin types depicted in the diagrams examined in exercise 2?

Which features help separate the basins into the three broad types?

Reading exercise: aim to identify major features such as

Size (length vs width) Cross-sectional shape Important tectonic structures (at basin margins and within the basin) Fill thickness Vertical and lateral stratigraphic patterns Provencance (type and location)