Unconformities Flashcards
What is an unconformity?
“a gap in time”
All unconformities represent a break in time or a period where no sediment was deposited.
What may cause an unconformity?
May be the result of a change in the environment or earth movements and erosion
What are the three types of unconformity?
Angular unconformity.
Disconformity.
Nonconformity
What is an angular unconformity?
old rocks tilted or dipped.
Then the unconformity
Younger rocks are horizontal
What is a disconformity?
old rocks are horizontal.
then the unconformity (erosion).
New rocks deposited horizontal.
Difficult to identify but still represent a gap in time
What is a nonconformity?
Older igneous rock is eroded and exposed at the surface.
Then the unconformity.
Then younger sedimentary rocks deposited
What is uniformitarianism?
The theory that the current earth processes have always been as they are now.
Slow incremental changes such as erosion
What is catastrophism?
Sudden, short lived, and violent eruptions.
e.g. earthquakes, floods, volcanic eruptions
What is gradualism?
Gradual (geo time) reform rather than sudden. Rocks found take a long time to form
Who produced the idea of uniformitarianism?
Charles Lyell
What are the 5 mass extinction events? (names)
Ordivian-silurian
Late devonian
Permo-triassic
Triassic-Jurassic
Cretaceous-tertiary
When was the ordivician-silurian extinction event?
443 Ma
When was the late devonian extinction event?
372 Ma
When was the permo-triassic extinction event?
252 Ma
When was the triassic-jurassic extinction event?
210 Ma
When was the cretaceous-tertiary extinction event?
65 Ma
What is the better known name for the permo-triassic extinction event?
The great dying
(also called End permian)
What is the better known name for the Cretaceous-tertiary extinction event?
KT
What caused the ordivician-silurian extinction event?
Ice age from plants absorbing CO2
What caused the late devonian extinction event?
Drops in oxygen in seas/oceans
What caused the permo-triassic extinction event?
Volcanic activity and climate change
What caused the triassic-jurassic extinction event?
Rising temps from volcanic activity
What caused the cretaceous-tertiary extinction event?
Massive volcanic eruptions, then an asteroid
How many species were affected by the ordivian-silurian extinction event?
86% marine species
How many species were affected by the permo-triassic extinction event?
96% of marine species
70% of terrestrial species
How many species were affected by the triassic-jurassic extinction event?
80%n marine eruption
How long did the ordivician-silurian extinction event last?
10 million years
How long did the late devonian extinction event last?
<3 million years
How long did the triassic-jurassic extinction event last?
3-4 million years
How long did the cretaceous-tertiary extinction event last?
<1 million years
What are the characteristics of sediments deposited by rivers?
Often sorted by size.
Some may end up in the ocean, then deposited quickly
Sediments called sub-mature
What are the characteristics of sediments transported by wind?
Mineralogically and texturally mature.
Softer material eroded by attrition and abrasion.
Sometimes calcite present.
High energy environment
What are the characteristics of sediments transported by glaciers?
Poorly sorted, very angular.
Texturally immature.
Seds called till (tillite if rock).
May also be transported by meltwater –> making them fluvial deposits
What are the characteristics of sediments transported in shallow marine environments?
Deposited in littoral zone (high energy) and on cont. shelf.
Sorted with distance
What is the littoral zone?
Area between very high and very low tide.
High energy environment
What is a conglomerate?
Sedimentary rock. Rounded clasts, poorly sorted, fluvial deposit
What are the three types of conglomerate?
Polymitic, oligomitic, monomitic
What is polymitic conglomerate?
A coarse grained rock containing clasts of many different rock types
What is oligomitic conglomerate?
A coarse grained sedimentary rock containing clasts of a few different rock types
What is monomitic conglomerate?
A coarse grained sedimentary rock containing clasts of a single rock type
What are the 6 sedimentary structures?
Ripple marks
crossbedding
graded bedding
desiccation cracks
salt pseudomorphs
imbricate structure
(technically bedding planes too)
What are symmetrical ripple marks?
Formed by oscillating currents.
- most common on beaches
- bidirectional flow
What are asymmetrical ripple marks?
Unidirectional flow
common in rivers and deserts
What is crossbedding?
Unidirectional flows form gentle slopes on upstream slope.
Dunes or bars migrate, slip face is preserved.
What is graded bedding?
Largest/heaviest particles on bottom, finer on top.
What are desiccation cracks?
Loss of water due to evaporation. Each crack has v-shaped cross section with wide top and narrow bottom (surface gets more solar energy).
Only preserved if sediment fills cracks
What are salt pseudomorphs?
Cubic halite crystals form at surface of a bed due to evaporation of salty water.
When water re-enters, salt dissolves, leaving cubic moulds filled in by sediments
What is imbricate structure?
Pebbles rolled along stream bed and pile against each other.
Long axis are roughly parallel.
Pebbles are inclined (dip) upstream.
What is the paleoenvironment like for ripple marks?
High energy environment, by saltation.
Uni or bi directional.
Small = rivers, shallow marine
Large = dunes
What is the paleoenvironment like for cross bedding?
Moved by wind, river, or marine currents.
Unidirectional
small = rivers or wind
large = dunes
What is the paleoenvironment like for graded bedding?
Energy levels of water drop and sediment settles out of solution
Usually off continental shelf
What is the paleoenvironment like for desiccation cracks?
Only form in clay rich environments.
Arid areas with high evaporation rates, such as playa lakes
What is the paleoenvironment like for salt pseudomorphs?
Arid environments.
Sediment dries out in the heat
What is the paleoenvironment like for imbricate structure?
unidirectional rivers
Do ripple marks indicate way up?
Yes.
The edges of a ripple mark is concave (inwards)
Does cross bedding indicate way up?
Yes.
Concave (inwards).
Does graded bedding indicate way up?
Yes.
Large pebbles at bottom, finer at top
Do desiccation cracks indicate way up?
Yes. Wider at the top, narrower at bottom
Do salt pseudomorphs indicate way up?
No, not if completely filled.
If grown partially from the bottom then technically yes
Does imbricate structure indicate way up?
No
Do ripple marks indicate paleocurrent direction?
Symmetrical doesn’t as they are bidirectional.
Asymmetrical yes. Longer side is upstream
Does cross bedding indicate paleocurrent direction?
Yes.
Slope down is the direction
Does graded bedding indicate paleocurrent direction?
no
Do desiccation cracks indicate paleocurrent direction?
no
Does imbricate structure indicate paleocurrent direction?
Yes.
Dip is towards upstream.
How might velocity change along a river course?
Higher near the source and outside meanders.
Lower inside meanders, near mouth, and when tributary joins river
What types of deposits are breccia and conglomerate?
Fluvial
What is the energy like at braided streams?
Semi-arid areas.
May be low or high energy transport
What is deposition like at braided streams?
Sand and gravel deposited as energy drops.
Deposits are coarse.
As sediment deposits, river braids.
varied energy
What are wadi conglomerates?
Dry riverbed with water in rainy seasons.
Desert streams (wadis) = high energy.
Energy lost quickly when rain stops. Deposition rapid.
Poorly sorted conglomerates
What are aeolian sandstones?
Shaped by wind.
Red desert sandstone.
High energy wind + attrition.
Well sorted, rounded, sphericity.
Frosted due to frequent collisions.
Entirely quartz.
Large cross bedding possible
What are alluvial fan arkose?
Triangular deposits (<gravel).
Rushes down, reaches flat, then spreads and deposits.
Coarse grained at top, breccias in middle.
>25% feldspars with little weathering
What are playa lake evaporites?
When flooded, playa lake forms where fine grained seds and salts concentrate.
Least soluble salt come out of solution (deposit) first.
Tend to not flow to the sea due to desert environments.
Ions of Ca, K, Na in solution from upstream.
What are the two siliciclastic environments?
Beach environments and shallow seas
What is deposition like in beach environments?
Includes littoral zone.
Mostly deposited by rivers.
High energy bidirectional.
May find fossils.
Back of the beach = large clasts, boulders, etc
Further down = gravel, coarse sands
further = medium, fine sands
in water = silts, clays (muds)
Possibly limestones
What is deposition like in shallow seas?
Lowest tide to the cont. shelf.
Shallow = <130 m
Lower unidirectional energy.
Pebbles
Coarse-fine sands
Mud
What is the other name for sea level rise?
Transgression
What is the other name for sea level drop?
regression
What are facies?
Includes all the characteristics of a sed rock produced by its environment. Allows it to be distinguished from rock in adjacent environment
What is Walthers law of facies?
The concept that a vertical succession of facies reflects lateral changes in environment
What are lithofacies?
Anything associated with a rock that isn’t alive.
E.g. sorting, colour, grain size, reaction with HCl.
All physical and chemical characteristics.
Sphericity, sed structures
What are biofacies?
Include the paleontological characteristics of a rock.
Fossils, traces
What are facies associations?
Specific sequence of rock associated with a specific type of environmental change: transgression and regression
What is a graphic log?
Graphical representation of recording a sequence of rock types and their characteristics
What do you need to draw a graphic log?
Beds/rock types
Grain sizes
Scale
Sed structures
Fossils
Colours
When drawing a graphic log, where does the oldest bed go?
At the bottom
What is lithostratigraphic correlation?
Relies on recognising rock types or a sequence or succession of rock types.
Marker horizons e.g. iridium
What are the methods of lithostratigraphic correlation?
Downhole logging
similar results = match up
What are some problems with lithostratigraphic correlation?
Diachronus beds (sometimes) - same lithology but different age (e.g. transgression/regression)
What is biostratigraphic correlation?
Uses assemblages of fossils to find rock of same age (incl. traces).
If rock contains same fossil groups, can be assumed to have similar age (esp. zone fossils)
What makes a good zone fossil?
Evolved rapidly, short stratigraphic range.
Abundant
Easily preserved
Found in numerous rock types.
Wide distribution
E.g. trilobites and ammonites
What are the problems with biostratigraphic correlation?
Many organisms restricted to one environment.
Some are long range
Some are delicate
Not all sed rocks have fossils.
Some took significant time to distribute.
Fossils can be eroded and move elsewhere
