Lecture Twenty Nine - Sedimentary facies analysis Flashcards
What is a sedimentary facies?
Specific sedimentation events, processes and conditions produce distinctive deposits.
E.g. mudstone vs conglomerate.
E.g. crossbedded sandstone vs massive structureless sandstone.
The term facies describes an interval of sediment or rock that has unique definable characteristics that distinguishes it from ajacent facies.
What is a helpful about facies?
Faces is products of unique processes and conditions in the host envrionment.
Environment + process –> facies.
A powerful, empirical tool.
- By analysing the facies (geometry, lithology, sedimentary structures and fossils).
- Reliable and testable interpretations can be made about environments.
Environments include: tectonic settings, climate, topography and ecology.
What are subenvironments?
Environments consist of subenvironments.
Each subenvironment consists of distinctive facies.
Therefore, environments consist of faceis.
Recognising ancient environments involves analysing associations of multiple faceis.
Faces can be defined/described by a combination of:
- Texture.
- Depositional structures.
- Palaeocurrents.
- Fossils.
- Composition.
What are some descriptive properties of facies?
Distinctive properties –> distinguishes one facies from another and lend themselves to process interpretation.
Physical processes can effect geometry, lithology, sed structures and palaeocurrents of facies.
Chemical processes can effect lithology and sedimentary structures of facies.
Biological processes can effect lithology, sed structures and fossils in/of facies.
Describe facies geometry.
Faceis; restricted lateralls (spatially 2D) and vertically (~temporally 1D).
Preservation of 3D form or shape of facies is controlled by:
–> Interstitial medium and mechanism of sediment distribution.
E.g. Fans: talus aggregates (gravity driven sedimentation).
E.g. Planar laminations: pelagic zones (suspension sedimentation).
–> Pre-existing topography.
E.g. Sinuous channels (shoestrings) rivers or tides.
E.g. Wedges.onlaps: alluvial fans, submarine fans and volcanic cones.
–> Syn-depositional erosion (by current actions e.g. channel scours).
–> Post depositional uplift and erosion.
–> Tectonic deformation effects.
Describe lithology of sedimentary facies.
1) Texture:
- Physical properties of grain and aggrefates of grains.
E.g.
- Grain size variations.
- rounding of grains.
- Fabric.
- Porosity.
- Permeability.
- Reflects duration and energy levels of transporting agent and depositional environment.
2) Physical constituents: - Crystal fragments, rock (=lithic) fragments, skeletal fragments etc.
3) Composition: - Mineralogy, chemistry.
- Reflect type of source rock and cheistry of environment.
- Composition of source rock is more indicative of the sedimentary source rather than the depositional environment.
Very few lithological features are diagnostic of specific environments.
Specific depositional environments can be more easily decided if there is a number of different facies all found in the one outcrop. That is, a speicific combination of facies can only exist together in the same outcrop if there is a particular depositional environment.
Describe sedmentary structures.
Physical, chemical or biological in origin.
Reflect conditions and processes in sedimentary environments.
Few sedimentary structures uniquesly identify environments, except:
- Symmetrcal ripples (=wave shorelines).
- Low angle planar beach x lamination.
- Hummocky cross stratification (~storme shelf environments).
Assemblages of structures most important.
The sedimentary structures of some facies preserve the direction of current when the rock formed:
- Most commonly used:
- Cross stratification.
- Flute casts.
- Peddble imbrication.
What are some palaeocurrent indicators?
Particular environments produce specific current vector patterns.
- Braided rivers.
- Meandering rivers.
- Shoelines.
Rose diagrams can be used to describe these environments.
How can fossils be used to interpret environmental conditions?
Uses for geologist:
1) Evolutionary biology (palaeontology).
2) Dating sedimentary rocks (biostratigraphy) = changes in fossils caused by evolution…
3) Interpreting environmental conditions (palaeoecology).
– Logic: organisms present in a rock have evolved specific adaptations to that particular environment
- Natural selection: if they couldn’t survive there, they wouldn’t reproduce and provide us with fossils…
Body fossils (actial skeleton remains, casts, molds):
- Are they in place or reworked (been transported and broken up).
- Clues = preservation state (intact), not abraded.
Trace fossils:
- Burrows and tacks of grazing organisms.
- Organisms at different water depths produce different burrows.
Changes in fossil assembrages caused by changes in environmental conditions.
E.g. terrestrial to marine environments.
What is the check list that can be used for sedimentary facies?
1) Texture.
2) Depositionals structures.
3) Fossils.
4) Compositon.
How does stratigraphic logging work?
A stratigraphic log is graphical representation of stratigraphy, or how geology changes vertically.
A stratographic log of a sedimentary log of a sedimentary succession tells up how sedimentation envrionments changes through time.
The oldest is found at the base and the youngest at the top (unless there are also volvanic intrusions).
The graphical log represents a vertical successions perpendicular to bedding.
- If beds are tilted, they are returned to horisontal and upright.
- If folded, they are straightened to their underformed state.
Y-axis = height of strata (beds of interest).
X-axis = facies grain size - e.g. sandstone grains ‘stick out’ more than mudstone.
Guidlines to recognise facies from their outcrop presentation:
- Erosional profiles –> Grain sizes/cementation.
- Colour –> degree of weathering.
- Disruptive bedding structures –> Sedimentary processes.
Few environments are recognisable by jut one faceis characteristic.
Combinations of features (and adjacent features) are diagnostic together.
