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Sedimentology MIDTERM 1 Flashcards

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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
Basins Associated with Divergent vs Convergent Plate Boundaries
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.
26
Types of Sedimentary Rocks
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)
27
What are the ingredients for our "Carbonate Factory"?
- Water clarity - Sunlight - Nutrient levels - Salinity
28
Iron Formation vs Ironstone
Iron Formation: Cherty, banded, iron-rich sediments Ironstone: Noncherty, non-banded, iron-rich rocks
29
What do we use to classify SILICICLASTIC rocks?
Grain size, shape, and sorting. - Transport distances - Transport processes Grain composition. - Source area - Transport processes
30
Ways to estimate grain size
1. Visially: hand lens and grain size comparator. 2. Sieving (beach activity) 3. Particle size analyzers
31
Rounding and sorting increases with ________________ and ______________________.
Transport DISTANCE and ENERGY of transport (processes).
32
Fissility
Ease of platy breakage. Used to categorize mudstones.
33
Steps to Classifying Conglomerates & Breccia.
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"
34
Implications of orthoconglomerate vs paraconglomerate vs breccia
Ortho-conglomerate = Transported by moving water Para-conglomerate = Transported by ice or mass flow Breccia = Generated in place or minimal transport
35
Classifying Mudstones vs Sandstones vs Conglomerates
Mudstones: Fissility + grain size Sandstones: QFL + matrix content Conglomerates: Composition + grain size + texture
36
What factors determine the threshold of motion for grains? What forces caused a grain to move?
When the fluid force / shear overcomes the gravity and frictional forces.
37
What is bed shear stress and why is it important?
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
Reynold's Number
The ratio of inertial forces to viscous forces. High R = turbulent flow R = 500 is transitional (wavy but parallel) Low R = laminar flow
39
What is stress? What is shear?
Stress = Force per unit area Shear = Lateral shift induced by pressure
40
Shields Stress
Uses the balance of DRIVING vs RESISTING forces to tell us if a grain will move or not.
41
Critical Shear Stress
The shear stress at the moment of particle entrainment.
42
Why is water better at moving sediment than air?
More flow, and more turbulent flow to overcome resisting forces.
43
Froude Number
The ratio of flow inertia to external field. High Fr = supercritical (rapid) Low Fr = subcritical (tranquil)
44
Hydraulic Jump
When a flow goes from supercritical to subcritical. Spillways create hydraulic jumps to dissipate energy.
45
Why is the Froude Number important?
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
Types of Grain Movement
Bedload: Rolling, sliding, saltation Suspended Load: Irregular intervals, continuous suspension
47
When a current interacts with sediment, it creates a ____________.
Bedform
48
Which bedforms develop from currents depends on...
1. Velocity of flow 2. Depth of flow 3. Size of sediment controls height, wavelength, etc of bedforms.
49
As velocity increases, we move through different bedforms. Explain this.
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
Lower vs Upper Flow Regime in terms of Fr Number
Lower = Fr less than 1 = ripples and dune = downstream migration Upper = Fr greater than 1 = planes, antidunes, chutes and pools = upstream migration
51
Types of Sediment Gravity Flows
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
What are beds? What are bedding planes?
Beds: Layers that have unity to distinguish them from layers above or below. Bedding Planes: Surfaces separating these beds.
53
What creates bedding?
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
Name the Three Basic Principles of Sedimentology (first year stuff)
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
What is graded bedding?
Coarse grains on the bottom, fine on the top. Indicates deposition from a waning current.
56
How is cross bedding created?
Migration of dunes.
57
2D vs 3D Dunes
2D: Tabular (flat) cross-bedding 3D: Trough (smiley face) cross-bedding (from the back)
58
Sinuous vs linguoid ripples
Sinuous: Diverging lines Linguoid: Bumps
59
Flaser vs Lenticular Bedding
Flaser: Thin mud streaks between layers Lenticular: Sand lenses in mud
60
Upper plane beds generate ___________________________, top surfaces shows ________________________.
Parallel laminations Parting lineations
61
Tides cause ______________, resulting in...
Flow reversal, resulting in changing current direction and flow energy.
62
What causes high high tides and low low tides?
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
What's the difference between fluvial and tidal flow?
Fluvial: Unimodal Tidal: Stops and reverses (tides cause flow reversal!)
64
Bimodal dips of dune and ripple foresets due to tidal/flow reversal is called...
Herringbone Cross-Stratification
65
What causes wave ripples to get larger?
- As waves get larger - Water gets shallower, or - Grain size increases
66
"Massive" means what?
Structureless- indicates rapid deposition.
67
What do sole markings tell us?
Paleo flow direction.
68
Convolute bedding forms...
During or shortly after deposition from de-watering and destabilization in rapidly deposited beds. The squiggles.
69
What are trace fossils?
Structures made by organisms on bedding surfaces or within beds; tracks, trails, burrows, etc.
70
Ichnology
The study of trace fossils
71
Ichnogenera
Classifications of trace fossils based on major BEHAVIOURAL traits. Their names apply to behaviour, not specific organisms. One organism can make multiple ichnogenera.
72
Ichnofacies
Assemblages of trace fossils (ichnogenera) that can reflect environmental conditions.
73
Skolithos Ichnofacies
- Vertical/cylindrical burrows - Shallow marine, sandy high energy environment - Typical suspension feeder burrows
74
Cruziana Ichnofacies
- Slightly deeper than skolithos; mid and outer continental shelf - Vertical, horizontal AND inclined burrows - Below fair weather line, above storm wave base
75
Zoophycos Ichnofacies
- Quiet water environments - Muddy sediment, low oxygen - 3D feeding traces
76
Nereites Ichnofacies
- Deep water, common in turbidite environments - Complex horizontal crawling traces
77
Bioturbation Index (BI)
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
If a bed is "structureless", it could jus be...
Heavily bioturbated!!
79
How can we interpret a depositional environment from a series of sedimentary rocks?
FACIES! YAY!
80
Facies
A particular combination of lithology (general characteristics) and physical/biological structures. Different from rock bodies above or below!
81
Types of Facies
1. Lithofacies: Facies subdivided on the basis of LITHOLOGIC characteristics 2. Biofacies: Facies subdivided on the bases of FOSSIL content
82
What is facies association? What is a facies model?
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
Walther's Law
A vertical succession of facies represents migration LATERALLY ADJACENT depositional environments (unless there is evidence of erosion!)

Sedimentology (4 decks)

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