2. Facies and facies associations Flashcards
define facies
A body of sediment or rock characterised by a particular combination of lithological, physical and biological characteristics that distinguishes it from the units surrounding it
once a facies has been defined it can be used to analyse vertical and/or lateral trends in the distribution of facies
broad armwavy disccussion
What can be used to describe facies?
- grain type and composition
- Texture (grainsize, sorting, roundness)
- sedimentary structures
- colour
- biological structures and content (trace fossils, body fossils)
Problems encountered with facies
- facies are defined based on observations
- it is commonly suggested that a single facies type is characteristic of a depositional environment but facies are often non unique
- it is more logical to tentatively assign processes to individual facies
Define facies association
a collection of multiple, genetically related facies formed within a single depositional system
- adding interpretation to the facies observations
How do we make the leap to an environmental interpretation?
Using analogues:
* Modern
* Ancient
* Theoretical
* Experimental
Where do turbidity currents occurr?
Anywhere in water where there are two different densities interacting
- NOT ONLY MARINE
What is the Bouma sequence?
A bed scale facies association commonly seen associated with a turbidite
Base -> Top
1) Massive, granules to sand
2) Parallel laminated sand
3) Cross laminated sand
4) Laminated silt
5) Mud
Describe a ‘point-bar’ facies association
Point bar is the inner part of a river bend
- eroding to the cut bank and so deeper point of stream and fastest energy
Base -> top
1) Coarse sand
2) fining upwards with dunes forming
3) Small straight dunes
4) planar beds with ripples
5) straight beds with likely vegetation
Define Architectural elements
‘building blocks of stratigraphy’
facies associations -> architectural
