Sedimentology MIDTERM 2 Flashcards
(87 cards)
1
Q
Alluvial vs Bedrock Rivers
A
Alluvial Rivers: Flow across a bed of sediment they have deposited
Bedrock Rivers: Flow directly on bedrock
2
Q
“Fluvial” means…
A
RIVER
3
Q
The 2 Main Fluvial Environments
A
- Channels: Deposits of sand or gravel
- Floodplain/Overbank: Fine-grained sediments (deposited on floodplain due to flood)
4
Q
Types of River (Classifications)
A
- Straight - a stick (single-thread)
- Meandering - squiggly (single thread)
- Braided - inter-tangled (multi thread)
- Anastomosing - vegetated islands in between (multi thread)
5
Q
2 Main Ways of Sedimentation/Accretion
A
- Lateral Accretion –> on point bars
- Vertical Accretion –> in floodplains
6
Q
What is a channel thalweg?
A
An imaginary line drawn through the LOWEST points in the channel, which marks the path of FASTEST flow.
7
Q
Point bar migration creates…
Chutes (cutoffs) result in…
A
Scroll bars (inner scroll scars)
Oxbow lakes (C shape river)
8
Q
What is a floodplain? What are natural levees? What is the whole region called?
A
Floodplain: The WIDE PLAIN of a river, finer-grained material (silt and clay) deposits dropped from floodwaters.
Natural Levees: Fine sands dropped NEAR to the channel due to a sudden loss of competence.
The whole region is called the “Overbank Region”.
9
Q
Crevasse Splays
A
Fan-shaped material formed by breaching a levee during flood. Exhbits the finig upwards of a fan.
10
Q
Describe Meandering River DEPOSITS
A
- Fining upwards
- Evidence of lateral accretion/point bars
- Possible mud-filled channel plugs
- Mud/floodplain development
- Thin sands = crevasse splays and/or flood layers
11
Q
Describe Braided Rivers
A
“Overfed” –> more sediment than what can be transported away, and many channels are active at the same time.
Constant channel shifts, steep gradients, and more bedload than suspended load.
Coarse grained bars.
12
Q
Bedloads vs Suspended Loads
A
Bedload: Coarser material rolling, sliding and bouncing along the bed.
Suspended Load: Finer material moving in the water through turbulence.
13
Q
Describe Anastamosing Rivers
A
Multiple channel systems separated by vegetated islands.
- Many fine-grained overbank deposits
- Mostly suspended load
- Form in areas of rapid aggradation (deposition) and high bank stability
14
Q
What is the difference between the three main river types, sedimentology-wise?
A
Braided = proximal, coarse-grained, mostly bedload
Meandering = Distal, low gradient & fine-grained, bedload & suspended load
Anastomosing = Distal, low gradient & high clay content, mostly suspended load
15
Q
Proximal vs Distal
A
Proximal = channels
Distal = floodplains
16
Q
Why do rivers become finer downstream?
A
Due to abrasion, breakage and deposition due to slowing.
17
Q
The 3 Coastal Processes & their Directions
A
- River input (out to coast)
- Tidal currents (out & in)
- Waves (in to land)
18
Q
How do we classify coastal environments?
A
Based of dominating coastal process:
1. River dominated
2. Tide dominated
3. Wave dominated
And tide vs wave dominance is controlled by TIDAL RANGE
- Microtidal (0-2m)
- Mesotidal (2-4m)
These first two are mostly wave-dominated.
- Macrotidal (>4m)
Mostly tide-dominance.
19
Q
Wave heights and energy depend on…
A
- Duration of wind
- Velocity of wind
- Fetch (aka distance of water across which wind blows)
20
Q
_________________ causes wave orbitals to decrease with depth.
A
Internal friction
In shallow water, friction causes orbitals to flatten, which creates horizontal oscillation which creates sand ripples.
21
Q
Wave base
A
The depth to which surface waves affect a water body.
22
Q
What happens at the coastline depends on 2 major factors. What are they?
A
- Sediment Supply: Sediment delivered to the depositional system (coastline)
- Accommodation: Space available to be filled with sediment
23
Q
Transgression/Retrogradation
A
During sea level rise, when accommodation is created faster than sediment supply can keep up with –> shoreline moves LANDWARD.
24
Q
Regression/Progradation
A
When sediment supply outpaces accommodation creation (sea-level rise with high sediment input) OR when accommodation is lost (sea-level fall) –> shoreline moves SEAWARD.
25
Explain Storm Deposits
During a storm the wave base gets deeper before returning to shallower water during normal weather. this causes deposition of sand beds during storms in areas that usually accumulate mud.
26
Wave- vs Current-Generated Ripples
Wave Ripples: Symmetrical, long straight crests and "tuning fork bifurcations".
Current Ripples: Shorter and more sinuous (curvy) crests.
27
What is typically the first bedform to appear as wave-orbital speed exceeds the threshold of motion?
Wave ripple lamination
If onshore and offshore wave motion are equal, ripples are symmetrical and properly stacked.
If onshore or offshore direction is dominant over the other, the ripples are still symmetrical but waves are stacked randomly.
28
Hummocks vs Swales
Occur in the transition between ripples and upper plane-beds during a storm.
Hummocks = Rounded mounds of sand on seafloor
Swales = Low points between hummocks
29
Hummocky Cross-Stratification vs Swaley Cross-Stratification
HCS = Both hummocks and swales are preserved
SCS = Only scoop-shaped part is preserved; forms in shallower water
30
Tempesites
The products of a storm that produce waves and current that extent to just below the wave base in shallow shelf settings.
31
What does regression/progradation look like in outcrop?
Shallow upward from offshore to beach facies.
32
Progradation vs Retrogradation in cross-section
Progradation = SHALLOWING
- Increasing energy
- Getting coarser
Retrogradation = DEEPENING
- Decreasing energy
- Getting finer
33
Decreasing wave orbital energy. leads to deposits varying by water depth...
A. Deposits below storm wave base are mainly mud
B. Offshore transition (mid-shelf) deposits comprise mud punctuated by tempesites (storm beds) of sandstone or siltstone with HCS
C. Shoreface deposits are dominated by CS and SCS in lower shoreface
D. Foreshore deposits are dominated by parallel-laminated sandstone
34
Where do aeolian features form?
Where vegetation is sparse or absent and wind can pick up sand or dust.
35
Desert Pavement
A pavement of rocks that form because sand and silt are blown away (aeolian processes).
36
How does wind move grains?
1. Saltation: Key mode of transport; sand-sized grains
2. Creep: Grains rolling along the surface under pressure of wind or impacts from saltating grains; larger grains
3. Suspension: Smaller grains light enough to be carried by turbulent air, scattered as dust
37
Saltation creates __________ grains. Explain.
FROSTED grains -- grain-to-grain collisions in the air create grains that look like frosted glass (chemical weathering also breaks down weaker grains, leaving aeolian deposits of mostly quartz).
38
Describe Aeolian Grains
- Very well sorted
- Very well rounded
- Commonly frosted and iron-stained
39
Aeolian dunes are characterized by huge ____________.
HUGE CROSS-BEDS
40
Explain Dune Structure
Grain Fall = Grains settle from the air (saltation)
Grain Flow = Gravity from down the slipface --> deposits in reverse grading (coarsest at top)
41
Dune morphology is controlled by...
1. Sand supply
2. Wind variability
42
Yardangs vs Ventifacts
Yardangs: Linear ridges formed by wind erosion (seen on Mars too!).
Ventifacts: Rocks abraded by wind; not streamlined like yardangs (more resistance = strange forms)
43
Alluvial Fans
Fan-shaped accumulations of sediment that form at steep mountain fronts from large breaks in slope and loss of confinement.
Happens on Mars too!
44
Key Features of Alluvial Fans
1. Channel networks originate from a single node
2. Have abandoned surfaces- different areas of the fan are active at different times
45
What are the differences between alluvial fans are rivers?
Alluvial Fans:
- Steeper slopes
- Supercritical flow regime
Rivers:
- Less steep slopes
- Subcritical
46
Types of Alluvial Fan Flow/Deposits
1. Stream Flow: Downstream fining & clast-supported.
2. Debris Flow: Found apex to mid-fan, poorly sorted, angular, maxtrix-supported.
3. Sheet Flood: Thin flow on fan surface, shallow and fast flow, upper plane beds at mid-fan to toe.
47
Gravity flow fan vs fluvial fan facies models
Gravity Flow Fans:
- Dry environment
- Dominated by debris flow
- Also a lot of stream flow and sheet flood
Fluvial Fans:
- Humid environment
- Dominated by stream flow
- Form really large fans
48
The formation of alluvia fans require...
RAPID UPLIFT
49
Aggradation vs Progradation
Aggradation = Vertical accumulation of sediment
Progradation = Horizontal/basinward accumulation (creates proximal facies stacked on distal facies)
50
Deltas are part of _____________ coastlines
REGRESSIVE
51
Deltas
Form where a river supplies sediments FASTER than basinal processes can redistribute them away.
Thin, fast water flows into larger calmer water.
52
Parts of a Delta
Delta Plain: Subaerial part of the delta (above main water body). Contains
- Distributing channels
- Floodplains
- Marshes, swamps, shallow bays & lagoons
Delta Front: Steeply sloping "front" of the delta, extending from shoreline to the deeper prodelta.
Prodelta: Most distal part of the delta of mostly muddy sediment below wave base.
53
Briefly explain delta mouth bars
Mouth bars are the sediment a delta deposits from the river mouth-- tides and waves then redistribute it.
54
Hyperpycnal Flows
Turbulent gravity flow triggered by river floods. Associated with prodeltas.
55
Delta Clinoforms
Surfaces curved like the letter C.
Topset = delta plain (middle)
Foreset = delta front (steepest)
Bottomset = prodelta (flattest)
56
Types of Deltas
1. River-dominated = broccoli. Commonly have sticking-out distributary channels bounded by levees.
2. Wave-dominated = curves. Relatively "clean" delta front deposits.
3. Tide-dominated = spaghetti. Have sand bars and tidal flats.
57
The key signature of deltas is the ___________________ succession of coastal and shallow marine facies as a result of ____________________.
COARSENING-UPWARD (& shallowing upwards)
PROGRADATION
58
Avulsion
Rapid abandonment of a river channel and the formation of a new one.
Leads to cycles of "lobe" (delta) growth and abandonment.
59
Types of Tidal Environments
1. Flood Tide: Incoming tide (landward direction)
2. Ebb Tide: Outgoing tide (seaward direction)
3. Slackwater: Period between flood and ebb tide where flow velocity is almost zero
60
Sand is deposited during _______ flow velocity. Mud is deposited as currents ____________________.
Sand = high flow velocity
Mud = currents slow and stop
61
Fluvial vs Tidal Flow
Fluvial Flow = Unimodal
Tidal Flow = Tidal flow stops and reverses
62
Tidal Sedimentary Structures...
1. Flaser bedding
2. Wavy bedding
3. Lenticular bedding
63
Herringbone Cross-Stratification
Flow direction reverses between flood and ebb tides.
64
Tidal Flats
Form when there's no strong wave activity -- exposed at low tide.
On edges of a channel between high tide and low tide level.
65
Tidal Flats in Cross Section Have...
- Bimodal current directions
- Reactivation surfaces
- Flaser bedding
- Tidal "rhythmites"
66
Barrier islands move __________. Breifly explain.
LANDWARD.
During storms, washover erodes channels through the barrier islands, erode outer sand and carry it to the backside through the breaking channels.
67
In front of a barrier island, ____________________ is the most important process. Behind the barrier, __________________ are the most important process.
Wave action
Tidal currents
68
Estuary
BANANA!
An embayed coastline with fluvial AND marine sediment sources.
- Commonly occupy a drowned valley
- Lagoons are an end-member
69
The 3 Zones of an Estuary
1. Outer Zone: Dominated by marine processes (waves + tides)
2. Low-Energy Central Zone: Marine energy = river current strength
3. Inner Zone: River-dominated
70
Tripartite Zonation
Beyhead delta at the head, central basin in the middle, barrier with inlets at the mouth.
Sand --> mud --> sand pattern
71
Wave- vs Tide- Dominated Estuaries
Wave-Dominated Estuaries
- Higher fluvial energy
- Landward = high fluvial energy, low energy in central basin, near barrier = high marine energy
- Sediment from rivers is trapped in the estuaries and lagoons
Tide-Dominated Estuaries
- Tidal flats can flank the central basin
- Less energy than wave-dominated because flow is "channelized"
- Less defined tripartite facies (because less energy)
72
Landward of elongate bars are _________________________________________________.
UPPER FLOW REGIME SAND FLATS
Corresponds to zone of strongest tidal currents
73
Transgressive deposits are typically very ________ compared to regressive deposits.
Thin
74
In stratigraphy, the turnaround from progradation to
retrogradation is marked by a sharp...
FLOODING SURFACE
75
Transgressing Coasts: In Detail
They're caused by rising sea level, tectonic subsidence, or compaction.
- Shoreline moves landward & erodes the coast
- Low-lying areas including river valleys become flooded
- Forms estuaries, lagoons and barrier islands in low-lying areas
76
Epicontinental/epeiric seas
Broad, shallow seas of the geologic past
77
Continental shelves are ______________ and continental slopes are ___________ along active margins.
Narrower
Steeper
78
Types of Marine Deposits
1. Terrigenous: Land-derived
2. Mixture: Land + ocean derived
3. Pelagic: Ocean derived
79
Hemipelagic Sediment
Terrigenous mud (delivered by rivers) deposited far from the coast with other pelagic sediment.
80
Modes of Sediment Transport in Water
1. Gravity Flows: Transport sediment down continental slope to continental rise.
2. Contour Currents: Currents along sea bottom.
3. Pelagic Rain: Calcareous & siliceous shells of plankton settle to the seafloor after they die.
81
Types of Submarine Gravity Flow Deposits
1. Turbidity Currents (Turbidites): Driven downslope by gravity; sediment held in suspension by turbulence in flow. Can be initiated by large storms or earthquakes.
2. Debris Flow Deposits (Debrites): Cohesion from increased sed content increases the shear strength of flow --> laminar flow.
3. Slumps/Slides: Mass transport complexes.
82
What is a turbidite? What is the Bouma sequence?
Turbidite = deposit from a turbidity current
Bouma Sequence = idealized trubidite succession
83
What are sole markings? Give examples.
Casts created by the erosion of seabeds filled in with overlying sand.
Ex:
- Groove casts (elongate linears)
- Bounce, brush, prod, roll marks (weird stuff)
- Flute casts (bulbous clumps)
84
Low Density vs High Density Turbidity Currents
Low Density Turbidites:
- Thin-bedded
High Density Turbidites:
- Thicker & sandier
- More proximal deposits
85
Submarine fans are predominantly composed of...
Turbidites & debris flows
86
Debrites
Deposits from debris flows. Occurs when shear strength of matrix or frictional forces overcome the gravitational force --> "En Masse Deposition".
- Structureless
- Usually muddy
- Chaotic internal structure
87
Turbidites are _____________________________ in channels and near a fan axis. On the other hand, they're ___________________________ in fringes of submarine fan lobes.
Thicker and coarser-grained
Thinner-bedded
