Lecture Thirty Three - Lithostratigraphy

Question 1

What is the basic idea of stratigraphy?

Answer 1

The study of rock layers (‘strata).
Vertical and horizontal (lateral) variations.
–> Igneous intrusions and extrusions and metamorphic rocks (follow the same principles).
Divided into stratigraphic ‘units,’ where strata distinct from this adjacent.

The law of superposition: 
An old idea - Avecinna (~1027 CE). 
Formalised by Nicolaus Steno. 
A few accept ions to the rule: 
- Thrust faults. 
- Overturned strata (recumbent folds). 
- Intrusive igneous rocks (dykes/sills). 
Salt diapires. 
Clastic dykes and liquefaction structures.

Question 2

What are the stratigraphic categories?

Answer 2

Stratigraphic units are not always distinguished based on rock (facies) characteristics.

Stratigraphic units can be defined by:

• Rock types = Lithostratigraphy.
• Fossil content = Biostratigraphy.
• Ages = Chronostratigraphy.

Question 3

What are the contact types of sedimentary rock layers?

Answer 3

Stratigraphic units separated by contacts.
Lithostrat units defined by distance lithological features.
- Lithostrat contacts = a change in rock formation processes.

Stratigraphic contact types:
Conformity = uninterrupted deposition.
- Gradational - continuous change in depositional style between units.
E.g. Sea level rise –> gradual change.
–> Arbitrary defining line but distinct end members.
- Abrupt = discontinuity of depositional style, but no detectable gap in deposition.
- Easily distinguished change in lithology.
- Negligile time gap Aka ‘diastems.’
E.g. Volcanic eruption - lava settles, then volcanic ash settles on top, distinct layers but settling at essentially same point in time.

Note - small discontinuities occur in all bedding planes.

Diastem = bedding microstructure -> grain size variation = minor disconuities.

Question 4

What are the names of the different unconformity contact types?

Answer 4

Unconformity = a boundary surface contact which represents a time gap between units.

Primary types:
Angular unconformity - Bedding discrepancy.
–> Angular sedimentary beds underneath horizontal overlying rock.
–> Lithofaces represent very different environments.
–> Hutton’s unconformity at Siccar Point in the UK is an example of this.

Disconformity = No bedding discrepancy, irregular (erosional) surface.
–> Sedimentary layers which have non-flat top layer, which have then had horizontal layers deposited onto.

Paraconformity = No bedding discrepancy or evident erosion.

• -> Horizontal layers of sediments.
• -> Morgan’s Hollow Unconformity = paraconformity: lithofacies (sub-envrionmental) change, but same formation… age discrepancy determined by different fossil assemblages (biostratigraphy).

Nonconformity = Sedimentary strata overlying metamorphic/igneous basement rock.

Question 5

Explain depositional rates of sedimentary layers.

Answer 5

Historically: Sedimentary stratigraphy was considered driven by shrote lived high energy events - ‘catatrophism.’
As opposed to gradual accumulation of strata over long periods of time - ‘gradualism.’

Now it is known that sedimentary depositional and erosion rates are strongly influence by grain size.
Finer = lower energy environment, more time required to accumulate and easier to erode.
Coarser = more likely to be deposited en masse and harder to erode.
hereford courser grained sediments are preserved better.

Short lived high energy events are over represented in the stratigraphic record.
Most of Earths history has been destroyed by metamorphism or eroded away and represented by unconformities.

Question 6

What is accommodation?

Answer 6

Space available for sediment accumulation (i.e. depositional).
Erosion is much more likely in subaerial environments.
–> Closely related to relative sea level in shallow marine environment.

In order to build and preserve a thick stratigraphic succession.

• -> We need to increase the accommodation space.
• -> Global (eustatic) sea level rise and or basin subsidence –> Relative sea level rise.

Question 7

Explain lateral stratigraphic variation and Walther’s Law.

Answer 7

Vertical changes in stratigraphy - well represented by start logs.
Many different environments and sedimentation styles occurring side by side, simultaneously.
The surface of the Earth represents the modern ‘time surface.’
- A stratographic surface who’s faces formed in a given geological time.

In a continuous stratographic succession:

1) Lateral facies variations on a given time surface represent environments that were taking place side by side.
2) Vertical facies variations represents environments that were also present laterally along any given time surface.

Question 8

Explain transgression and regression.

Answer 8

The most widely used application of Walther’s law.
Relative Sea Level (RSL) change and shoreline migration.
There are three primary contributors:
1) Sediment influx (-RSL).
2) Eustatic (global) seal level (+/- RSL).
3) Basin subsidence (+RSL).

If RSL is positive - Marine transgression (+/- delta retrogradation).
If RSL is zero - Results in aggradation.
If RSL is negative - Marine regression (+/-) delta progradation).

Question 9

Explain lithostratigraphic correlation.

Answer 9

Vertical succession (1-D) +/- reflect environmental changes through time (despite discontinuous depositions).
Can be represented by stratigraphic logs.
But facies always laterally.
Multiple strat logs can be used to correlate lithostrat units across entire basins.
Correlations are supported by a host of there data (the more data the better).

Question 10

What are well cores?

Answer 10

Continuous narrow cylinders of rock (~1-D).
Collected by drilling a cylindrical hole in a target stratigraphic succession and extracting the core.
Can be extremely long.
Most are several km deep.
The key to extracting stratigraphic data from the subsurface rocks - especially multiple cores in the same basin.