Week 10: Dynamic Planet

Question 1

What 2 categories do landscapes fall into?

Answer 1

1. Erosional landscapes
2. Depositional Landscapes

Question 2

How are depositional landscapes generated?

Answer 2

generated by subsidence and deposition of sediment

Question 3

Differences between an erosional and depositional landscape?

Answer 3

Erosional:
-uplift
-topographic relief
-dominated surfaces covered in weathering products (“regloith”)
-Or dominated by bare eroding bedrock

Depositional:
-subsidence
-low areas
-either flooded by lakes or sea if they haven’t been filled with sediment
-very flat if they have been filled up with sediment
-characterised by bedrock buried 10 or 100s m of sediment

Question 4

Why does deposition happen?

Answer 4

Deposition happens because the capacity of the transporting medium (usually water but also wind) to carry the sediment decreases

this can happen locally for short periods of time, anywhere:

1. weathering and erosion generate clastic sediment
2. it is transported
3. ultimately deposited somewhere else

Question 5

How does long-term storage sediment happen?

Answer 5

need to generate space for that sediment to be deposited in:
- this space is often referred to as accommodation and is generated by subsidence of the Earth’s crust (WS)

Question 6

Where does long-term storage of sediment happen?

Answer 6

-occurs in sedimentary basins
-the subsidence of the sedimentary basin also ensures burial of the sediment with later sediment
-this burial takes the sediment to higher pressures and temperatures, where it can also be converted from sediment into a sedimentary rock.

Question 7

What is deposition governed by?

Answer 7

dynamic equilibrium
-between the amount of sediment delivered, and the amount of space made available to deposit the sediment by subsidence.

Question 8

What happens as transporting river systems enter a sedimentary basin? How does volume influence this?

Answer 8

it will begin to deposit its sediment load to “fill” the available space.

if the volume of space being generated ≥ volume of sediment being delivered, then all the sediment will be deposited.

if the volume of space being generated < volume of sediment being delivered, then the excess sediment will continue its journey down system to a place where there is sufficient space.

Question 9

What are shallow marine systems?

Answer 9

Rivers:
-lobate coastlines
-deltas

Tides:
-embayed coastlines
-estuaries and tidal flats

Waves:
-linear coastlines
-strandplains, spits, beaches and lagoons

Question 10

What are continental shelves?

Answer 10

flat areas of continental crust that are flooded by the sea.
They are separated from low-lying Oceanic Crust by steeper slopes.

Question 11

What processes start to happen where rivers, deltas or shorefaces are able to cross the shelf and deliver sediment to the continental slope?

Answer 11

mass wasting and transport processes

Question 12

How are landslides caused in canyons?

Answer 12

Commonly, canyons cut back from the Ocean floor through the shelf and can “tap” sediment directly from deltas or shorefaces.

Earthquakes, or oversteepening of the sediment deposited there causes landslides.

Question 13

Why does debris flow and turbidity currents occur?

Answer 13

Debris flows and turbidity currents occur because the mix of sediment and water is denser than the ambient medium (seawater).
So they flow because of their gravitational potential.
Both processes happen on land and underwater, if the gradient is great enough, and the concentration of the flow is high enough.
-Turbidity currents on land are usually Volcanic, and are called Pyroclastic Flows (W3).

Question 14

What are submarine (“deep water” fans?

Answer 14

-Submarine (“deep water”) fans are the largest sedimentary deposits on Earth.

-They are the final resting place for any particle that has been eroded from a “high area”.

-The processes occurring in these deep water conditions were a mystery until very recently

-They pose a major risk to submarine infrastructure.

Question 15

What are evaporites?

Answer 15

chemical sediments
Chemical sediments are precipitated directly from sea and lake water
-without being mediated by biogenic activity
-it is wholly inorganic.

Direct precipitation of dissolved minerals from seas and lakes occurs because the water becomes supersaturated with respect to that mineral.

Question 16

How does supersaturation occur?

Answer 16

In areas of excess evaporation, versus inflow of fresh (rain) water from rivers, dissolved minerals become hyperconcentrated.
These minerals are predominantly minerals of common Earth surface metals
-Calcite (CaCO3) and dolomite (CaMg(CO3)2)
-The salt group (Halogens) – NaCl (halite), KCl (sylvite), MgCl, etc.
-Gypsum (CaSO4·2H2O) and anhydrite (CaSO4).

Example= The Dead Sea

Question 17

What is necessary for the development of hypersalinity?

Answer 17

low/limited amount of mixing of currents:
-seaways (and especially lakes) that have poor/no connection to the open ocean,
and experience an excess of evaporation versus inflow of fresh water.

since If Oceans were perfectly mixed, these pools of hyperconcentrated water would be mixed around.

Question 18

What is the global distribution of dissolved salts in oceans?

Answer 18

two subtropical belts of elevated salinity tell you that the Atlantic is not fully mixed

the Mediterranean has elevated salinity because evaporation>rainfall and it is poorly connected to the Atlantic

the Bay of Bengal is very fresh because of the high freshwater discharge of the Ganges- we would usually expect it to be high because it is in a relatively arid tropical zone.

This tells you that the oceans are not fully mixed

Question 19

How are evaporite deposits good paleoclimate and paleogeographic indicators?

Answer 19

If they are preserved in the sedimentary record, they tell you that conditions were arid, and that the body of water was closed off or had restricted connection to the open ocean.

Question 20

Why are evaporite rocks economically important?

Answer 20

Evaporite rocks are a major source of salt – an historic and modern means of preserving and enhancing the flavour of food.
-Rock salt – ancient salt deposits preserved as layers of rock – is mined.

Salts are also a source of K, which is a globally important ingredient of fertiliser.

Gypsum is a key ingredient of Plaster-of-Paris and cement

Question 21

How is clastic sediment derived?

Answer 21

from the erosion of pre-existing rock.

Question 22

Where are biogenic sediments derived from?

Answer 22

those derived from the soft parts (“organic”) and hard parts (“skeletal”) of living organisms.

Question 23

What are chemical sediments derived from?

Answer 23

those precipitated directly out of water without the aid of organisms.

Question 24

What is marine organic matter?

Answer 24

Marine organic matter is largely the biomass of planktonic fauna and flora that live in the water column lakes and oceans.
-Diverse chemical composition because derived from complex organisms
-Proteins: animal tissue and enzymes
-Carbohydrates: animal tissue
-Lipids: algae, pollen, spores
-Lignin: plant tissue (plant material can be washed into lakes and seas from land)

But these are typically dilute in most rocks:
-Mudstones: 0.99 wt%
-Carbonates: 0.33 wt%
-Sandstones: 0.28 wt%

Question 25

What 2 fundamental factors control the occurrence of sedimentary deposits?

Answer 25

1. The rate of organic productivity in the first place
   -Fundamentally relies on photosynthetic rate
   -Without photosynthesis, there never will be organic matter.
2. The rate at which the organic matter decays after death
   -If this rate is greater than the rate at which organic material is produced, you will never preserve the organic material.

Question 26

Where does high primary productivity occur?

Answer 26

occurs on land, and in the upper 200 m (the “photic zone”) of the ocean/lake

also promoted by upwelling of cold, nutrient-rich ocean currents at continental margins

Concentrated along the west coasts of continents in the present day- increased phyto-/zooplankton productivity

Question 27

What is ocean stratification and organic preservation promoted by?

Answer 27

1 promoted by high productivity rates
2 promoted by poor mixing

Question 28

How is ocean stratification and organic preservation promoted by high productivity rates?

Answer 28

1. Dead microorganisms settle on the seafloor, and decay according to photosynthesis
2. High supply of dead organisms to seafloor leads to O2 demand > O2 supply
3. Anaerobic layer forms in lower ocean and preservation of organic matter is possible.

Question 29

How is stratification and organic preservation promoted by poor mixing?

Answer 29

Development of anoxic bottom waters are also promoted in restricted, poorly connected seaways and lakes

Dead organisms settle on sea/lake floor, and consume oxygen during decomposition.

Absence of major currents inhibit mixing of oxygen downwards, and an O2 concentration gradient quickly develops.

Question 30

What sedimentation must be absent to make rocks enriched in organic matter?

Answer 30

To make rocks that are enriched in organic matter, an absence of clastic or carbonate sedimentation is preferable, since these “dilute” the organic matter.
Referred to as “source rocks”, because the organic matter degrades to kerogens, which are the source of oil and gas

Question 31

What are carbonates? What elements make it up?

Answer 31

The major biological sediment, and sedimentary rock type generated in marine (and lake) settings

mainly limestone (CaCO3) and dolomite (CaMg(CO3)2) (others exist).
-precipitation largely mediated biologically, by the extraction of Ca, Mg, C and O dissolved in water.

Question 32

What do some organisms like radiolaria form shells from?

Answer 32

silica (SiO2)
(much less commonly than carbonate)

Question 33

What does production need?

Answer 33

since it is largely biologically driven it needs:
-Light (the photic zone, absence of suspended mud)
-Nutrients (usually land derived)

Organic growth promoted by:
- low pH
-unusual salinities
So carbonate is primarily produced in shallow marine environments.

Question 34

Where do carbonate “platforms” and “ramps” mainly form?

Answer 34

shallow water shelves, where water is warm, has normal salinity, and and away from sources of clastic sediment input.

Question 35

What are the 2 ways in which carbonate material is formed?

Answer 35

1. In situ growth
2. Plankton , nekton and benthos

Question 36

How does in situ growth form carbonates?

Answer 36

This refers to carbonate that is precipitated by organisms which are fixed to the sediment substrate.

In the shallow, photic zone, these organisms constitute a “reef”, and when the organism dies, it may not move very far from the site in which it lived.

Question 37

What are some common in-situ reef-builders?

Answer 37

1 Corals
2. Algal mats
3. stromatoporoids
4. sponges
5. bryozoa
6. crinoids

Question 38

How do plankton, nekton and benthos form carbonates?

Answer 38

they are free moving organisms that could potentially contribute carbonate skeletal material to sediment

Question 39

What are plankton?

Answer 39

those that drift (microorganisms are common: e.g. Algae, some foraminifera)

Question 40

What are nekton?

Answer 40

those that swim (macroorganisms are common: e.g. Cuttlefish in the modern day)

Question 41

What are benthos?

Answer 41

those that live on or in the sediment (many micro-types, e.g. some foramenifera, and macro-types, e.g. Gastropods, Bivalves)

Question 42

What are “whitings”?

Answer 42

Inorganic precipitation of carbonate in the water column
Not an overall major contributor of carbonate material

Question 43

What are “coated grains”?

Answer 43

Carbonate can sometimes precipitate directly on to prexisting grain surfaces.

Question 44

What is the environment of carbonate production and deposition known as? What classes do they fall into?

Answer 44

Carbonate platforms
3 main classes:
-rimmed shelves
-isolated/ unattached platforms (aka “atolls”)
-ramps

Question 45

What are rimmed shelves and how do they occur?

Answer 45

Morphologically they are:
-attached to land
-a flat top, with a distinct barrier reef,
-a lagoon, behind, protected from waves by the reef
-a steep slope in front of the reef into true “deep water”.

Occur when:
-carbonate production is HIGH
-water is warm, clear and of a normal salinity

Question 46

What are isolated platforms and how do they occur?

Answer 46

Morphologically, they
-not attached to land, (they are an island)
-a flat top, with a distinct barrier reef,
-a lagoon, behind, protected from waves by the reef
-a steep slope in front of the reef into true “deep water”.

Occur when:
-carbonate production is HIGH
-water is warm, clear and normal salinity

Where:
Often, they form in the shallow water around a marine volcanic island
The land mass is not big enough to produce much clastic sediment, so they are good sites for carbonate productivity.

Question 47

Under what conditions are lagoons within isolated platforms maintained?

Answer 47

High carbonate productivity means that the reefs can keep up with sea-level, if the volcano subsides, or sea-levels rise.

The lagoon is maintained, because the reef inhibits mixing of sea-water.

Question 48

What are carbonate ramps? How do they occur?

Answer 48

Morphologically:
-ramp that dips uniformly towards much deeper water
-no barrier reef, just local patches of reef in places
-lower carbonate productivity means that the sediment is more readily reworked and moved away from shallow water by wave and tidal processes.

Occur when:
-carbonate production is LOWER
-water is cooler, contaminated by clastics, or the water is brackish or hypersaline

Example= Arabian Gulf

Question 49

Why are there carbonate ramps in the Arabian Gulf?

Answer 49

Here, carbonate productivity is limited by poor circulation and high temperatures, in the Arabian Gulf
-promote hypersalinity in the water.

Carbonate production is limited to a few reefs, and bacterial and algal mats forming in intertidal areas.

Lack of good barrier means that waves and tidal processes break up material and transport it away to deeper water.