Sedimentology and Palaeontology Lecture 17: Stratification and Sedimentary Structures Flashcards
Describe Stratification
Sedimentary rocks are stratified
- Layers in sedimentary rocks = strata
- Bedding planes = breaks in deposition
- Bed thickness ≠ time
Describe bedding and lamination
Bedding is the layering of sedimentary rock, while lamination is the thin layers within that rock.
Lamination is often regarded as planar structures one centimetre or less in thickness, whereas bedding layers are greater than one centimetre.
Bedding is typically > 1 cm thick
Lamination is typically < 1 cm thick
What is lamination defined by/ relevant variables
Defined by:
* Grain size
* Colour
* Composition
Represent:
* Depositional events
* Seasonal
* Chemical
* Biological
Define and describe Grading
Grading = change of grain size within a bed
Normal grading has larger grains at the bottom of bed and finer grains at the top of the bed.
Reverse grading has smaller grains at the bottom of the bed and larger grains at the top of the bed.
Define coarsening up/ fining up
Fining/coarsening-up = change of grain size across beds
Fining meaning grain size decreasing and coarsening meaning grain size increasing
Fining up meaning Finer sediments at the top bed
Coarsening up meaning coarser sediments at the top bed
Describe the movements of grains within a flow
Flow velocity decreases towards the base of a flow due to friction
- = boundary layer
Define a bedform
Sedimentary structures/surface patterns created by moving fluid over sediment.
Can be used to interpret ancient conditions
Describe the initiation of bedforms in a flow
Flow contracts over a step and velocity increases → transport
Flow expands over a pit and velocity decreases → deposition
Step - Increase in height of seabed/sediment bed
Pit - Decrease in height of seabed/sediment bed
Describe current ripple cross lamination
Current ripple cross lamination is a sedimentary structure that forms when ripples migrate and are deposited. It’s made up of subhorizontal layers with dipping laminae.
Stoss side is the step side, Lee side is the Pit side
Lee side avalanche laminae covered and preserved Stoss side eroded
Describe Cross-stratification terminology
Foreset = individual grain avalanche surfaces
* Set = single unit of cross-stratification (migration of bedform)
Co-set = stack of sets (depositional regime)
Describe characteristics of current ripples
Moderate flow velocity
- Grain size < 0.7 mm
(smooth bed) - Ripple index = λ/h = 8-20
Describe different kinds of cross ripples and what causes them
Climbing ripples = high sand flux
a large volume of sand is being transported by wind over a given area in a specific period of time
Starved ripples = low sand supply
Describe subaqueous dunes
Order of magnitude larger than ripples
- Where boundary layer ≈ flow thickness
- Fine sand to coarse gravel
- Wavelengths 60-100s cm
- Amplitudes 5-10s cm
Describe the sizes of current ripples and subaqueous dunes
Ripples - 1cm to 1m wavelength, 0.1mm to 10cm bedform height
Subaqueous dunes - 1m to 1000m wavelength, 1cm to 10m bedform height
Describe the various forms of cross bedding in subaqueous dunes
Tabular cross bedding
Foreset laminae dip down flow
Trough cross bedding
Describe Planar beds
Sedimentary structures that are nearly horizontal and made of sand or gravel.
Low flow velocity
* Lower stage flat beds in coarse sands and gravels
High flow velocity
* Upper stage flat beds
* Produce thin planar laminae and primary current lineation (PCL) (Lineation determined by current orientation, however not necessarily its direction)
Describe Standing waves and antidunes
Supercritical flow
* Very high velocity
* Surface waves from wind cannot propagate upstream
(Thin and fast flow)
Subcritical flow
* Moderate-high velocity
* Surface waves from wind propagate upstream
(Thick and slow flow)
