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Sedimentology > Sedimentology MIDTERM 1 > Flashcards

Sedimentology MIDTERM 1 Flashcards

1
Q

Why is sedimentology important?

A
  • Sed rocks tell us how earth looked in the past
  • Sediments record Earth’s history
  • Fossil fuels, agriculture, water, etc.
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2
Q

What controls rates of weathering?

A

Surface Area: As mechanical weathering breaks rocks into smaller bits, more surface area is exposed to chemical weathering– faster weathering)

Mineral Resistance: Harder minerals with fewer planes of weakness will resist weathering. Note the most physically stable mineral is quartz!

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3
Q

What are the end results/products of weathering?

A

Source-rock residues (the more chemically resistant minerals) -> Quartz

Secondary minerals formed -> Clays

Solubles released from source rock -> Ions in solution

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4
Q

How does weathering play into a source to sink framework?

A

It breaks up the rock and stuff to be transported down to the sink.

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5
Q

How are sedimentary rocks formed?

A

Erosion, transportation, deposition, and re-deposition of rocks.

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6
Q

Erosional vs Depositional Systems

A

Erosional: A high source where sediment is produced and transported down.

Depositional: A low sink where sediment deposition creates sedimentary deposits.

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7
Q

Weathering vs Erosion

A

Weathering: Breaks down the rock

Erosion: Transports that material away

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8
Q

Two Types of Weathering

A
  1. Physical Weathering
  2. Chemical Weathering

They work at the same time and together!

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9
Q

Types of Physical Weathering

A
  1. Stress Release (me): Overlying rock erodes to uncover balloon-like pluton that “rebounds” up.
  2. Volume Changes: Bunch of types…
  3. Bio Agents: Little animals digging holes, tree roots breaking rocks!
  4. Abrasion (key): Transported grains (by one method or another) bash against each other, become more rounded, and break down.
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10
Q

Types of Physical Weathering: Volume Changes

A

A. Insolation: Repeated heating and cooling over daily cycles to break apart rock.

B. Freeze-Thaw: Water seeps into cracks, freezes and expands cracks, melts, and the process repeats until these cracks break the rock apart.

C. Salt: Rock disintegration by salts crystallizing in cracks during salt water evaporation, breaking the rock apart.

D. Wetting/Drying: Clays expand when wet and contract when dry- these cycles lead to cracks.

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11
Q

Types of Chemical Weathering

A
  1. Simple Solution: Mineral dissolves completely (ie halite).
  2. Hydrolysis: Hydrogen ion replaces other positive ions to dissolve ions and solid products (ie k-feldpsar). Clays (ie kaolinite) are main product.
  3. Redox: Reduction (addition of electrons) and oxidation (removal of electrons) of a substance (ie pyrite).
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12
Q

How can we tell how resistant or physically strong a mineral is?

A

It’s the opposite of Bowen’s reaction series! The bottom minerals like Quartz are the most chemically resistant.

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13
Q

Compositional Maturity

A

The amount of a sediment’s resistant minerals, like quartz. Mature is a lot of quartz.

High maturity/quartz indicates warm/humid source region, long transport distances, or the source rock was already mature.

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14
Q

Textural Maturity

A

The amount of clay removal, sorting on non-clays, and roundness of grains.

Textural maturity indicates longer transport or higher energy transport processes.

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15
Q

Provenance– What does this mean?

A

Place of origin; the source!

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16
Q

What indicates provenance?

A

Grain size and composition indicate:
- composition of source
- transport distance and processes
- local climate

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17
Q

What are some techniques for provenance analysis?

A
  1. QFL (Quartz, Feldspar, Lithics) Plots
  2. Sr Isotopes
  3. Detrital Zircons
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18
Q

Sedimentary Basin

A

Depressions that can trap sediment– sinks!

Tectonics control size, shape and location of basins.

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19
Q

What is basin accommodation?

A

Available space for sediment in the sink!

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20
Q

If basins are controlled by tectonic activity, that means they are…

A

Dynamic entities!

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21
Q

What is subsidence?

A

It’s the tectonic “downlift” of Earth’s crust in response to a nearby uplift. It creates the sink.

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22
Q

Mechanisms of subsidence include…

A
  • Crustal thinning
  • Mantle-lithosphere thickening
  • Sedimentary and volcanic loading
  • Tectonic loading
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23
Q

Isostasy

A

An equilibrium between the Earth’s crust and its upper mantle– its accommodating flexibility.

Adding a load = subsidence, removing a load = uplift.

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24
Q

Two Types of Basins

A
  1. Rift-Drift: Lithosphere stretching by thermal effects -> divergent and intraplate settings
  2. Flexural: Lithosphere flexure from a load -> convergent plate settings
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25
Q

Basins Associated with Divergent vs Convergent Plate Boundaries

A

Divergent Basins: Rift valley extends out to create a passive margin, where sediment accumulates until the crust undergoes “flexural subsidence”.

Convergent Basins: “Foreland basins” that form parallel to mountain ranges. Subsidence driven by flexure of underlying plate from weight of overlying plate at a subducting plate boundary– a basin just before a volcanic arc.

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26
Q

Types of Sedimentary Rocks

A
  1. Silici-clastic: Made from clasts (fragments) of older rocks (ie sandstone)
  2. Carbonaceous: Accumulation of organic debris (ie coal)
  3. Chemical/Biochemical: Precipitation of minerals from water (ie limestone)
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27
Q

What are the ingredients for our “Carbonate Factory”?

A
  • Water clarity
  • Sunlight
  • Nutrient levels
  • Salinity
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28
Q

Iron Formation vs Ironstone

A

Iron Formation: Cherty, banded, iron-rich sediments

Ironstone: Noncherty, non-banded, iron-rich rocks

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29
Q

What do we use to classify SILICICLASTIC rocks?

A

Grain size, shape, and sorting.
- Transport distances
- Transport processes

Grain composition.
- Source area
- Transport processes

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30
Q

Ways to estimate grain size

A
  1. Visially: hand lens and grain size comparator.
  2. Sieving (beach activity)
  3. Particle size analyzers
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31
Q

Rounding and sorting increases with ________________ and ______________________.

A

Transport DISTANCE and ENERGY of transport (processes).

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32
Q

Fissility

A

Ease of platy breakage. Used to categorize mudstones.

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33
Q

Steps to Classifying Conglomerates & Breccia.

A

Step 1: Angularity of large blobs.
a. Conglomerate: Rounded
b. Braccia: Angular

Step 2: Are blobs supported or floating in matrix?
a. Ortho-conglomerate: Supported
b. Para-conglomerate: Floating

Step 3: Composition + Grain Size + Texture-determined in steps above
Ex: “Quartz Pebble Orthoconglomerate”

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34
Q

Implications of orthoconglomerate vs paraconglomerate vs breccia

A

Ortho-conglomerate = Transported by moving water

Para-conglomerate = Transported by ice or mass flow

Breccia = Generated in place or minimal transport

35
Q

Classifying Mudstones vs Sandstones vs Conglomerates

A

Mudstones: Fissility + grain size

Sandstones: QFL + matrix content

Conglomerates: Composition + grain size + texture

36
Q

What factors determine the threshold of motion for grains? What forces caused a grain to move?

A

When the fluid force / shear overcomes the gravity and frictional forces.

37
Q

What is bed shear stress and why is it important?

A

The stress imparted on a sediment bed; the difference in force from the top to bottom of a grain.

It dictates how good a substance is at moving sediment (ie water vs air).

38
Q

Reynold’s Number

A

The ratio of inertial forces to viscous forces.

High R = turbulent flow
R = 500 is transitional (wavy but parallel)
Low R = laminar flow

39
Q

What is stress? What is shear?

A

Stress = Force per unit area

Shear = Lateral shift induced by pressure

40
Q

Shields Stress

A

Uses the balance of DRIVING vs RESISTING forces to tell us if a grain will move or not.

41
Q

Critical Shear Stress

A

The shear stress at the moment of particle entrainment.

42
Q

Why is water better at moving sediment than air?

A

More flow, and more turbulent flow to overcome resisting forces.

43
Q

Froude Number

A

The ratio of flow inertia to external field.

High Fr = supercritical (rapid)
Low Fr = subcritical (tranquil)

44
Q

Hydraulic Jump

A

When a flow goes from supercritical to subcritical.

Spillways create hydraulic jumps to dissipate energy.

45
Q

Why is the Froude Number important?

A

We use it to predict the type of bed form that will develop on a bed of mobile sediment (in phase with water surface/supercritical vs out of phase/subcritical).

46
Q

Types of Grain Movement

A

Bedload: Rolling, sliding, saltation

Suspended Load: Irregular intervals, continuous suspension

47
Q

When a current interacts with sediment, it creates a ____________.

A

Bedform

48
Q

Which bedforms develop from currents depends on…

A
  1. Velocity of flow
  2. Depth of flow
  3. Size of sediment

controls height, wavelength, etc of bedforms.

49
Q

As velocity increases, we move through different bedforms. Explain this.

A

From lowest to highest flow velocity…
1. Lower plane beds: flat
2. Current ripples: mini dunes
3. Dunes
4. Upper plane beds: dunes are wiped out
5. Antidunes: dunes in opp direction!
6. Chutes and pools: short-lived and unstable forms

50
Q

Lower vs Upper Flow Regime in terms of Fr Number

A

Lower = Fr less than 1 = ripples and dune = downstream migration

Upper = Fr greater than 1 = planes, antidunes, chutes and pools = upstream migration

51
Q

Types of Sediment Gravity Flows

A
  1. Debris Flow: Laminar flow, matrix supported, angular clasts.
  2. Grain Flow: Grains supported by grain-to-grain collisions, common with dunes, reverse grading (coarse on top) deposits.
  3. Turbidity Current: Fluid turbulence, primary transport of sand to deep marine environments.
52
Q

What are beds? What are bedding planes?

A

Beds: Layers that have unity to distinguish them from layers above or below.

Bedding Planes: Surfaces separating these beds.

53
Q

What creates bedding?

A
  1. Physical Processes: Changes in sediment transport conditions.
  2. Chemical Processes: Changes in temp, CO2, ion concentrations.
  3. Biological Processes: Changes in biota.
  4. Diagenesis: Post-depositional changes.
54
Q

Name the Three Basic Principles of Sedimentology (first year stuff)

A
  1. Original Horizontality: Sediment is deposited horizontally.
  2. Original Continuity: Sediment layers extent outwards in all directions.
  3. Superposition: Oldest layers are at the bottom.
55
Q

What is graded bedding?

A

Coarse grains on the bottom, fine on the top. Indicates deposition from a waning current.

56
Q

How is cross bedding created?

A

Migration of dunes.

57
Q

2D vs 3D Dunes

A

2D: Tabular (flat) cross-bedding
3D: Trough (smiley face) cross-bedding (from the back)

58
Q

Sinuous vs linguoid ripples

A

Sinuous: Diverging lines
Linguoid: Bumps

59
Q

Flaser vs Lenticular Bedding

A

Flaser: Thin mud streaks between layers

Lenticular: Sand lenses in mud

60
Q

Upper plane beds generate ___________________________, top surfaces shows ________________________.

A

Parallel laminations

Parting lineations

61
Q

Tides cause ______________, resulting in…

A

Flow reversal, resulting in changing current direction and flow energy.

62
Q

What causes high high tides and low low tides?

A

The moon isn’t always exactly above the equator– which can create extra high tides at this angle in some regions, and extreme low tides in others.

63
Q

What’s the difference between fluvial and tidal flow?

A

Fluvial: Unimodal
Tidal: Stops and reverses (tides cause flow reversal!)

64
Q

Bimodal dips of dune and ripple foresets due to tidal/flow reversal is called…

A

Herringbone Cross-Stratification

65
Q

What causes wave ripples to get larger?

A
  • As waves get larger
  • Water gets shallower, or
  • Grain size increases
66
Q

“Massive” means what?

A

Structureless- indicates rapid deposition.

67
Q

What do sole markings tell us?

A

Paleo flow direction.

68
Q

Convolute bedding forms…

A

During or shortly after deposition from de-watering and destabilization in rapidly deposited beds.

The squiggles.

69
Q

What are trace fossils?

A

Structures made by organisms on bedding surfaces or within beds; tracks, trails, burrows, etc.

70
Q

Ichnology

A

The study of trace fossils

71
Q

Ichnogenera

A

Classifications of trace fossils based on major BEHAVIOURAL traits. Their names apply to behaviour, not specific organisms.

One organism can make multiple ichnogenera.

72
Q

Ichnofacies

A

Assemblages of trace fossils (ichnogenera) that can reflect environmental conditions.

73
Q

Skolithos Ichnofacies

A
  • Vertical/cylindrical burrows
  • Shallow marine, sandy high energy environment
  • Typical suspension feeder burrows
74
Q

Cruziana Ichnofacies

A
  • Slightly deeper than skolithos; mid and outer continental shelf
  • Vertical, horizontal AND inclined burrows
  • Below fair weather line, above storm wave base
75
Q

Zoophycos Ichnofacies

A
  • Quiet water environments
  • Muddy sediment, low oxygen
  • 3D feeding traces
76
Q

Nereites Ichnofacies

A
  • Deep water, common in turbidite environments
  • Complex horizontal crawling traces
77
Q

Bioturbation Index (BI)

A

The degree of bioturbation (disturbance of sedimentary deposits by organisms).

6 = high disturbance by organisms. We can’t get this without high sedimentation rates.

This also tells us about stability of environmental conditions.

78
Q

If a bed is “structureless”, it could jus be…

A

Heavily bioturbated!!

79
Q

How can we interpret a depositional environment from a series of sedimentary rocks?

A

FACIES! YAY!

80
Q

Facies

A

A particular combination of lithology (general characteristics) and physical/biological structures.

Different from rock bodies above or below!

81
Q

Types of Facies

A
  1. Lithofacies: Facies subdivided on the basis of LITHOLOGIC characteristics
  2. Biofacies: Facies subdivided on the bases of FOSSIL content
82
Q

What is facies association? What is a facies model?

A

Association: Associations of facies that help us organize our facies and interpret depositional environments.

Model: A summary of depositional environments and can help us to interpret what we see in the field.

83
Q

Walther’s Law

A

A vertical succession of facies represents migration LATERALLY ADJACENT depositional environments (unless there is evidence of erosion!)

Sedimentology (4 decks)

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