Sediments, Sedimentary Rocks and the Geological Record Flashcards
Lectures 4.1-4.5
What are the three materials sediment % is measured on?
Gravel, mud and sand.
What is the meaning of a Gaussian curve?
Bell shaped.
What does a large standard deviation mean in terms of soil sorting and size?
- Many different sizes, meaning not very well sorted. Probably immature.
Describe the amount of sorting occurring down down the rock cycle.
Top of river/lake: Increasing sorting, ongoing chemical weathering
Middle of river: Slower flow, coarser materials left behind. Only silt/clay left.
Mouth: Load deposition, well sorted. Flocculation removes clay.
What are the five stages of sediment maturity?
Immature, submature, mature, supermature. Maturity measures grain size (mature = more even) and grain shape (mature = more round)
How are sedimentary rocks arranged?
- Layered or stratified in planar horizontal beds
- Sequence of beds is called bedding and stratification
What is the boundary between two sedimentary beds called?
Bedding plane.
What are strata?
Several sedimentary beds together.
Why does bedding form?
- Reflects changing depositional conditions
- Changing transporting speed, velocity, volume
- Changing sediment source: Sediment composition, grain size, sorting
What are clastic sediments?
- Mineral grains are lithified (the process of transforming loose sediment into solid rock)
- Compacted through burial and cemented (Precipitate gluing sediments together) , often by silica or calcite
What are biochemical sediments?
- Skeletal parts (opal)
- Calcite and aragonite carbonate
What are chemical sediments?
- Non biogenic precipitates such as evaporites like gypsum
How is dolomite formed?
- Interaction of calcite with Mg-bearing groundwater
What does clast mean?
Grain.
What do asymmetric ripples show about the flow directions?
- Used to indicate current direction in ancient sediments
What are cross beds created by?
- Sediment moves up gentle side of dune
- Piles up, slips down steep face
- Slip face continuously moves down current
- Added sediment forms cross beds
What is glacial till?
Sediment deposited by a glacier.
What is a breccia?
A rock made of angular rock fragments cemented together. Deposited close to clast source.
What is an arkose?
Sand and gravel with abundant feldspar (indicates short transport and arid weather)
- Common in alluvial fans
What is conglomerate sediment?
- Rounded rock clasts
- Rounded from flowing water and deposited rather than breccia
What is a delta?
Where sediment accumulates where a river enters the sea. Creates many sub environments present.
What forms down slope turbidity currents?
- Sediment moves on a slop as turbid water
- As it slows, grains settle
- Coarsest material settles first, then medium and fine.
What is chert?
Rocks made of cryptocrystalline quartz.
- After burial opaline silica in bottom sediment dissolves
- Silica in pores forms a gel and precipitates chert
What algae is calcite made of?
Coccolithophorids.
Name the replacement cherts (other materials silica replaces).
Flint
Jasper
Petrified wood
Agate
Describe the formation of Travertine.
- CaCO₃ precipitated from ground water
- Dissolved calcium reacts with bicarbonate
- Co2 expelled into air causes CaCO₃ to precipitate
How do rift basins form?
- Divergent plate boundaries
- Crust thins, stretches and subsides
- Sediment fills down dropped basin
What are foreland basins?
- Craton side of a collision mountain belt. The flexure of the crust from loading creates a downwarp.
- Fills with eroded mountain debris
What is a relative age?
- Based on order of formation
- Qualitative
- Determines older vs younger relationships
What is numerical age?
Actual number of years since an event.
What is uniformitarianism?
The theory that the processes observed today were the same in the past.
What is the principle of original horizontality?
Sediments accumulate horizontally so tilted sediment rocks must be deformed.
What are formations in stratigraphic columns called?
Mapable rock units.
What are the two types of stratigraphic correlation?
Lithologic: Based on rock type
Fossil: Based o fossils within the rocks
When and what was the Phanerozoic?
- 542 Mya to present
- Visible life. Starts at Pre Cambrian - Cambrian boundary.
- First hard shells
When and what was the Proterozoic?
- 2.5 to 0.542 Ga
- Development of tectonic plates and atmospheric O²
- Multicellular life
When and what was the Archean?
- 3.8 to 2.5 Ga
- Birth of continents and initiation of plate tectonics
When and what was the Hadean?
- 4.6 to 3.8 Ga
- Internal differentiation
- Formation of the oceans and secondary atmosphere
What is numerical dating called? What is it based on?
Geochronology. Based on radioactive decay using U-238.
