Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Sedimentology and Stratigraphy > (S4) Rivers > Flashcards

(S4) Rivers Flashcards

1
Q

What are the three zones found in fluvial and alluvial systems?

A

Erosional zone, transfer zone and depositional zone

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Give three locations you would likely find a depositional zone?

A

River channel, flood plains, alluvial fan

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Why do the fluvial/alluvial zones vary with specific systems?

A

Some systems may only have erosional zones into the sea, some may not have transfer zones, etc.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What does the term “fluvial” involve?

A

Channels and overbank systems

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What does the term “alluvial” involve?

A

Channels, overbank, alluvial fans - all general processes which involve river systems

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What are the two main factors on water supply control in a drainage basin?

A

Catchment area, Climate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What does “perennial” mean?

A

Year around

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What does “ephemeral” mean?

A

Peroidic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

How does catchment area have an effect on the water supply in a drainage basin?

A

Small catchments have limited storage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the name for flow which is below the channel banks?

A

Low flow stage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the name for flow which is just under or at the channel banks?

A

High flow stage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the name for flow which is over channel bank onto the floodplain?

A

Overbank flow

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the name for the cross-sectional area of a river with the highest flow rate?

A

Thalweg

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What are the three main variables of river forms?

A

Straight/sinuous Presence/absence of depositional bars Number of separate channels present

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the simplest form of river?

A

Straight, single channel, no bars - in reality: uncommon

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is the technical definition for a braided river?

A

Mid-channel bars which are covered at high flow stage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is an anastomosing river?

A

Contains multiple channels which are interconnected, separated by areas of floodplain

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is the name for a river which contains multiple channels which are interconnected, separated by areas of floodplain?

A

An anastomosing river

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is a bedload river?

A

A river which contains a high proportion of sediment which is being transported by rolling or saltation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What is a common feature in a bedload river?

A

Mid-channel bars, i.e. a braided river

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What is “coarse lag”?

A

When coarsest material is deposited to form an accumulation of larger clasts

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What is the name for bars which run along the axis of a river?

A

Longitudinal bars

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What is the name for bars which spread across the river?

A

Transverse bars

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What is the name for bars which have their apex pointing downstream?

A

Linguoid bars

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

What is the name for bars which consist of sand and gravel mixtures?

A

Compound bars

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What are two common features of longitudinal bars?

A

Low relief, poorly defined low angle cross-strata

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

What are two common features of transverse bars?

A

High relief, well defined cross-strata dipping downstream

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

What is a common feature of a sandy bar which is not usually found in a gravel bar?

A

Superimposed dune formations on the bar, which tend to be linguoid (trough-cross)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What is likely to be found in a compound bar?

A

Cross-strata of gravel with lenses of sand, or vice versa

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

What is usually the demise of a migrating bar in a channel?

A

Migrate until the channel moves sideways - migrates out of the main flow, and becomes covered in overbank deposits or bars of another channel

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

What are the five stages which would be expected in a sedimentary succession of a braided river?

A
  1. Erosional surface at base 2. Basal lag of coarse clasts on channel floor 3. Bar deposits - sand (stacked subaqueous dunes) or gravel (granules, pebbles & cobbles in cross-strata) 4. Larger dunes lower in succession (stronger current at bottom of channel as sand builds up) 5. Finer sands/silts on top of bar succession - abandonment
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

Does the succession in a braided river usually fine or coarsen upwards?

A

Fining upwards

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

What does the thickness of a braided river succession usually represent?

A

It may represent overall thickness of the channel, although it is possible that the top has been eroded due to scour of a later channel

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

What is a “braid plane”?

A

A broad extensive region of gravelly bar deposits many times larger than the channel

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

Where are braid planes commonly found?

A

Glacial outwash areas

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

What are the three common features of a braid plane?

A

Wed regions, poor vegetation, river banks are less stable

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

Why is it difficult to recognise individual successions in a braid plane stratagraphic record?

A

Difficult to recognise scour marks at the base of a channel, which distinguish individual channels from one another

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

What are channel fill-successions and when are they preserved?

A

Lateral migration of the channel - only preserved when this occurs

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

What is the technical definition of a sinuous river?

A

Distance along stretch of channel / direct straight line distance = > 1.5

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
Q

What is the technical definition of a meandering river?

A

Erosion on the outside of the bends and deposition on the inside

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
Q

What do the deposits on a meander bank look like?

A

Coarser on at the base and finer further up

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
42
Q

Where are sub-aqueous dunes found in meandering rivers?

A

Faster/deeper parts of the channel

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
43
Q

Where are ripples found in rivers?

A

Higher up on the inside of bends where there is slower flow and finer sand

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
44
Q

What is the name of the deposit on the inside of a meander?

A

Point bar

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
45
Q

What is a “braid plane”?

A

A broad extensive region of gravelly bar deposits many times larger than the channel

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
46
Q

Where are braid planes commonly found?

A

Glacial outwash areas

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
47
Q

What are the three common features of a braid plane?

A

Wed regions, poor vegetation, river banks are less stable

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
48
Q

Why is it difficult to recognise individual successions in a braid plane stratagraphic record?

A

Difficult to recognise scour marks at the base of a channel, which distinguish individual channels from one another

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
49
Q

What are channel fill-successions and when are they preserved?

A

Lateral migration of the channel - only preserved when this occurs

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
50
Q

What is the technical definition of a sinuous river?

A

Distance along stretch of channel / direct straight line distance = > 1.5

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
51
Q

What is the technical definition of a meandering river?

A

Erosion on the outside of the bends and deposition on the inside

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
52
Q

What happens to the mud clasts which are eroded away from the inside of a meander?

A

The are deposited in the deeper parts of the channel due to their cohesive properties

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
53
Q

Where are sub-aqueous dunes found in meandering rivers?

A

Faster/deeper parts of the channel

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
54
Q

Where are ripples found in rivers?

A

Higher up on the inside of bends where there is slower flow and finer sand

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
55
Q

What is the name of the deposit on the inside of a meander?

A

Point bar

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
56
Q

What does the cross-strata look like in a point bar?

A

Larger scale cross-bedding at the base and smaller sets of cross-lamination at the top

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
57
Q

Describe what occurs to a point bar with time

A

Lateral accretion surfaces so the point bar migrates laterally as sediment layers are deposited on the inside

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
58
Q

When are lateral accretion surfaces most distinct?

A

When there are periods of low flow which allow the slower velocity for the deposition of finer silts in the layers

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
59
Q

What is “epsilon cross-stratification”?

A

Another name for the lateral accretion surfaces found from point bar migration

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
60
Q

What is the typical angle for a lateral accretion surface?

A

<15 degrees

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
61
Q

What is the relationship between flow direction and epsilon cross-stratification?

A

The point bar surface is perpendicular to flow

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
62
Q

What is the outer bend of a meander made up of?

A

Flood plain deposits (muddy) as it is cutting into the flood plain

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
63
Q

What happens to the mud clasts which are eroded away from the inside of a meander?

A

The are deposited in the deeper parts of the channel due to their cohesive properties

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
64
Q

What is the stratification of an ephemeral river usually like?

A

Horizontal deposits, may be graded as flow decreases in strength with time Some low angle cross-strata may be found with longitudinal bars

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
65
Q

What is a chute channel?

A

A channel which cuts across the inner bank of a meander during high stage flow

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
66
Q

What does the development of a chute channel usually lead to?

A

An oxbow lake formation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
67
Q

How are oxbow lakes commonly recognised in ancient deposits?

A

Channel fills of fine-grained (sometimes) carbonaceous sediment

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
68
Q

What are the two main variables which affect the velocity of flow of a river?

A

Slope and discharge

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
69
Q

What is an example of avulsion?

A

Oxbow lakes - although other additional tracks may be involved

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
70
Q

What is the full process of avulsion?

A

Water channel changes during high stage flow. Water supply to old channel becomes sluggish and slow limiting carrying capacity. New river floods taking fine sediment to the old river. Fills with mud - may be difficult to distinguish from flood plane.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
71
Q

What other alluvial feature may also be ephemeral, aside from rivers?

A

Alluvial fans

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
72
Q

Describe an ephemeral alluvial fan

A

Detritus weathered away, not able to be carried until a significant flood event or spring runoff

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
73
Q

What do typical deposits in ephemeral rivers usually look like?

A

Poorly sorted, angular to sub-angular, gravel in a sand/mud matrix

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
74
Q

What paleocurrent indicator may occur in ephemeral lakes?

A

Imbrication

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
75
Q

What is the stratification of an ephemeral river usually like?

A

Horizontal deposits, may be graded as flow decreases in strength with time Some low angle cross-strata may be found with longitudinal bars

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
76
Q

What is a “wadi”?

A

A river/stream in a desert system with ephemeral flow

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
77
Q

What is the name for a channel with minor channels splitting off?

A

A bifurcating pattern

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
78
Q

What is the difficulty in determining anastomosing rivers in ancient deposits?

A

Impossible to tell if two or more channels were active at the same time - possible that it is a single channel repeatedly changing position

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
79
Q

What are the two main variables which affect the velocity of flow of a river?

A

Slope and discharge

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
80
Q

What type of rivers have the steepest depositional gradient and what is that gradient?

A

Gravelly braided rivers, <0.5 degrees

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
81
Q

What is a common formation in a sandy bedload?

A

Bars

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
82
Q

What gradients do meanders tend to form at?

A

1/100th of a degree

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
83
Q

What are the five main depositional features on a flood plain?

A
  1. Very thin beds - graided from sand to mud 2. Initial rapid flow (plane parallel lamination) quick waning then rapid deposition (climbing ripple cross-lamination) 3. Thin sheets of sediment cm’s deep, hundreds meters wide 4. Erosion at base of overbank sheet sandstone beds - flow most vigorous near channel 5. Soil formation
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
84
Q

What type of river facies preserves flood plain deposits the best and which the worst?

A

Meandering rivers = best preservation of flood plain deposits, braided rivers = worst

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
85
Q

What is an endoheic basin?

A

A basin which is a closed system - does not have an outlet to the open ocean

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
86
Q

What is the name for a splay of water/sediment at the end of a channel (mostly found in arid environments with high evaporation)

A

Terminal fan

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
87
Q

What is another name for a river with a terminal fan?

A

A fluvial distributary system

88
Q

What is the name for a channel with minor channels splitting off?

A

A bifurcating pattern

89
Q

What are the six main ways to classify soils?

A
  1. Degree of alteration (weathering) 2. Precipitation of soluble minerals (calciate/gypsum etc) 3. Redox conditions 4. Development of layering (horizonation) 5. Illuvation - distribution of clays, iron and organic material 6. Amount of organic matter which is preserved
90
Q

How do levee’s form?

A

Suspended load dumped really quickly due to rapid loss in energy

91
Q

What usually forms when a levee breaks?

A

Crevasse splay - low cone of sediment

92
Q

Is a breach in the levee instantaneous?

A

No - small conduit gets wider and deeper with each event

93
Q

What is the process of a crevasse splay formation?

A

Initial upward coarsening of particle size Lenticular in three dimentions May develop into new river channels and carry more water until avulsion occurs

94
Q

What are the features of histisols? Climate, Soil type (4)

A

Waterlogged, peaty - bedded sediments overlain by carbonaceous layer - root systems

95
Q

What are the features of entisols? Climate, Soil type (3)

A

Immature, little soil development - mostly bedded sediments with roots in the upper layer

96
Q

What are the features of inceptisols? Climate, Soil type (3)

A

Mature, soil development, less mature than some other types - bedded sediments with some roots

97
Q

What are the features of spodosols? Climate, Soil type (5)

A

Dense vegetation forests - layers of bedded sediments overlain by iron rich and carbonaceous horizons, deep roots

98
Q

What are the features of alfisols? Climate, Soil type (4)

A

Dense vegetation forests - layers of bedded sediments overlain by iron rich and calcareous nodule beds, deep roots

99
Q

What are the features of utilistols? Climate, Soil type (4)

A

Dense vegetation forests - bedded sediments, reddened iron rich horizons and deep roots

100
Q

What are the six main ways to classify soils?

A
  1. Degree of alteration (weathering) 2. Precipitation of soluble minerals (calciate/gypsum etc) 3. Redox conditions 4. Development of layering (horizonation) 5. Illuvation - distribution of clays, iron and organic material 6. Amount of organic matter which is preserved
101
Q

What are the features of gelisols? Climate, Soil type (2)

A

Cold climate formation - clasic sediments

102
Q

What are the features of aridsols? Climate, Soil type (5)

A

Arid conditions - Bedded sediments overlain by gypsum/calcareous nodules/roots

103
Q

What are the features of oxisols? Climate, Soil type (4)

A

Humid, tropical conditions - Bedded sediments overlain by reddened iron rich soils, overlain by root systems

104
Q

What are the features of vertisols? Climate, Soil type (4)

A

Sub-humid to semi arid with pronounced seasonality - bedded sediments overlain by carbonaceous layer at surface with root systems

105
Q

What are the features of histisol? Climate, Soil type (4)

A

Waterlogged, peaty - bedded sediments overlain by carbonaceous layer - root systems

106
Q

What are the features of entisol? Climate, Soil type (3)

A

Immature, little soil development - mostly bedded sediments with roots in the upper layer

107
Q

What are the features of inceptisol? Climate, Soil type (3)

A

Mature, soil development, less mature than some other types - bedded sediments with some roots

108
Q

What are “glabules”?

A

Small nodules of calcium carbonate

109
Q

How can we use paleosols to determine the length of time that an ancient profile took to form?

A

Stages of development of calcrete can be measured, and thus the development of a mature profile can be measured

110
Q

What are the features of utilistol?

A

Dense vegetation forests - bedded sediments, reddened iron rich horizons and deep roots

111
Q

What are the features of mollisols? Climate, Soil type (6)

A

Grasslands - bedded sediments, gypsum and calcareous nodules, carbonaceous top layer with deep roots

112
Q

What are the features of andisols? Climate, Soil type (3)

A

Volcanic bedrock - roots and a carbonaceous top layer

113
Q

What are five common features found in paleosols?

A

Fossilized roots Burrows Vertical cracks Layers enriched in certain minerals Layers depleated in certain minerals

114
Q

What are three things that paleosols can tell us?

A

Climate Vegetation type Time period that land was exposed

115
Q

What are “seatearths”? (3 examples)

A

Bed of organic matter underlain by a leached horizon of white sandstone from which iron has been washed out. Common in coal measures of NW Europe and N America E.g.’s: histisols, argillisols, spodosols.

116
Q

What is the process of formation of “calcrete”?

A

Water movement through soils Calcium carb as root encrustations (rhizocretions) or small nodules (glabules) Nodules grow and coalesce - forms fully developed CaCO3 horizon

117
Q

How is it possible to determine if an eruption was subareal?

A

Existance of laterites between basalts

118
Q

What are “rhizocretions”?

A

Root encrustations of calcium carbonate precipitation - fossilised roots

119
Q

What are “glabules”?

A

Small nodules of calcium carbonate

120
Q

How can we use paleosols to determine the length of time that an ancient profile took to form?

A

Stages of development of calcrete can be measured, and thus the development of a mature profile can be measured

121
Q

What does paleo-entisols and paleo-inceptisols tell us about the length of time exposed at the surface?

A

Relatively short as these profiles are relatively immature paleosols

122
Q

Under what alluvial conditions would it be expected to find an entisol?

A

Floodplain conditions - where a flood deposits sediment, and relatively little time was allowed for pedogenisis before a further flood covered the profile

123
Q

What can make the recognition of a paleosol more difficult?

A

Destruction by diagenetic alteration

124
Q

What are the variables which affect time for soils to develop? (3)

A

Temperature, rainfall, availability of different minerals

125
Q

What two factors complicate paleosols in the geological record?

A

Time estimates are all relative, not absoloute Soil profiles can be further complicated by superposition of a younger profile over an older one

126
Q

What are “seatearths”? (3 examples)

A

Bed of organic matter underlain by a leached horizon of white sandstone from which iron has been washed out. Common in coal measures of NW Europe and N America E.g.’s: histisols, argillisols, spodosols.

127
Q

What are “laterites”?

A

Oxisols, product of extensive weathering of bedrock to form rock mainly of aluminium/iron oxides

128
Q

How is it possible to determine if an eruption was subareal?

A

Existance of laterites between basalts

129
Q

What are “ferricretes”?

A

Horizontal paleosol layers of cemented iron-rich oxides

130
Q

What is a “duricrust”? (3 factors)

A

A hardened soil profile which includes ferricetes - highly resistance and develop over long time periods - found associated with major unconformities

131
Q

What are “silcretes”?

A

Horizontal paleosol layers of cemented silica rich minerals

132
Q

What is the most reliable indicator of a terrestrial environment?

A

Paleosol identification

133
Q

Why is the identification of a channel not a reliable indicator of the terrestrial environment?

A

Can be found in deltas, tidal settings, deep marine environments

134
Q

What can make the recognition of a paleosol more difficult?

A

Destruction by diagenetic alteration

135
Q

What used to determine the “architecture” of fluvial successions?

A

Arrangement of channel vs overbank deposits in 3D

136
Q

How is the architecture of fluvial successions described (2)

A

Proportion of “in-channel” deposits to finer overbank facies Size and shape of sand/gravel beds deposited in channels

137
Q

What is the width of channel-fill determined by?

A

Process of avulsion and lateral migration

138
Q

What does the archetecture of more frequent avulsion as opposed to more lateral migration look like?

A
139
Q

What does the archetecture of more lateral migration as opposed to more frequent avulsion look like?

A
140
Q

What does the archetecture of a slower subsidence rate look like?

A
141
Q

What does the archetecture of a faster subsidence rate look like?

A
142
Q

What are the two settings in which avulsion is frequent?

A

Rivers with high tectonic activity

Rivers with frequent flood events

143
Q

When is lateral migration slowed? (2)

A

When river banks are stable - muddy (cohesive) floodplains, dense vegetation (particulary grass with fiborous root system)

144
Q

Aside from slowing lateral migration, what else does vegetation effect in the alluvial system?

A

Slows overland flow

145
Q

Under what conditions is the floodplain particularly unstable, where flows are faster and more likely to erode?

A

Arid and cold conditions which result in a lack of vegetation

146
Q

What is the main use of determining paleocurrents in a fluvial system?

A

Determines direction of mouth/shore and source

147
Q

What are the three main ways to determine paleocurrent in a fluvial system?

A

Channel margins, cross-bedding, imbrication

148
Q

Why is care needed when determining the paleocurrent direction of floodplane deposits?

A

Often perpendicular to flow

149
Q

What is the possible variance of paleoflow in braided river deposits?

A

60 degrees

150
Q

What is the possible variance of paleoflow in meandering river deposits?

A

90 degrees

151
Q

What is required for accurate field measurements of paleoflow of meandering and braided rivers?

A

Large numbers of measurements

152
Q

How is it possible to detect changes in paleogeography in sucession?

A

Changes in facies - changes in fluvial deposition style plus grain size

153
Q

What does a change from braided characteristics to meandering up through a sucession tell us? (2)

A

Decrease in gradient or reduction in discharge in a system

154
Q

What can the dimensions of channel fill deposits infer? (2)

A

Size of river, size of drainage basin

155
Q

How is the lithology of channel-fill deposits important?

A

Tells of bedrock lithologies in the surrounding catchment prior to the deposition

156
Q

How can paleoclimate be determined from the character of fluvial deposits?

A

Ephemeral and perennial flow

157
Q

What archetecture is best for a potential oil/gas reservoir?

A

Braidplane with moderate subsidence

158
Q

Rank in terms of reliability of paleoflow in an alluvial system:

  • Bar surface cross-strata in braided rivers
  • Lateral accretion surfaces
  • Ripple lamination on crevasse splay
  • Ripple lamination on point bar
  • Cut of river bank
A
  1. Cut of river bank
  2. Lateral accretion surfaces
  3. Bar surface cross-strata in braided rivers
  4. Ripple lamination on point bar
  5. Ripple lamination on crevasse splay
159
Q

What is an “alluvial fan”

A

A cone of detritus which forms at the break in slope at the edge of an alluvial plane

160
Q

Give a location where alluvial fans are found?

A

Death Valley, CA

161
Q

What is the name for a sediment body which consists of a mixture of talus deposits and debris-flow deposits?

A

Colluvial fans

162
Q

At what setting do alluvial fans form?

A

Where the is a destinct break between high ground drainage and flatter basin floor

163
Q

What is the “feeder canyon”

A

Funnels drainage to the basin margin where the valley opens out

164
Q

What is the fan apex?

A

The highest most proximal point - adjacent to the feeder canyon

165
Q

What is the fan-head canyon?

A

A small canyon incised into the surface near the fan apex

166
Q

Where is the slope the steepest on an alluvial fan?

A

Near the apex of the fan

167
Q

What is the fan toe?

A

A break in the slope of the alluvial fan

168
Q

Where are the deposits thickest on an alluvial fan?

A

At apex - a conical wedge towards the toe

169
Q

How is deposition on an alluvial fan determined? (4)

A

Availability of water, Amount/type of sediment and gradient of fan

170
Q

What is the name for a dense mixture of water and sediment on an alluvial fan?

A

Debris flow

171
Q

How does debris flow tend to travel?

A

In a lobe - doesn’t travel far, small and relatively steep

172
Q

What is the name for the process by which water is more dilute in sediments traveling down an alluvial fan?

A

Unconfined sheetfloods

173
Q

What is a “stream-channel fan”?

A

An alluvial fan which contains a channel of braided streams

174
Q

What is the closest material which can describe the viscosity of subaerial debris flow?

A

Wet concrete mix

175
Q

What is the flow of subaerial debris flow like? (3)

A

High density, high viscosity, laminar flow

176
Q

When does debris flow retire? (2)

A

When it runs out of momentum due to gradient decrese or flow loses water content

177
Q

What is the resultant bedding on subaerial debris flow? (2 factors)

A

10’s cm to m’s thick with very little thinning downflow

178
Q

What does the lack of turbulence in debris flow mean for the deposit?

A

No sorting of grainsize within the flow and for the most part; randomly orientated

179
Q

What are the five main characteristics for a bed which is deposited by debris flow?

A
  1. Matrix supported
  2. Sorting usually very poor
  3. May show crude allignment but generally structureless and randomly orientated
  4. Outsize clasts may be m’s across
  5. Beds deposited 10’s cm to m’s thick
180
Q

What is the name for flow which spreads out over a portion of an alluvial fan?

A

Sheetflood

181
Q

What are the three features of sheetflood deposition?

A

Rapid, supercritical, turbulent flow

182
Q

What gradient does sheetflow usually occur on?

A

gradients of 3 to 5 degrees

183
Q

What size sediments would be expected to be carried in bedload and in suspension in a sheetflood event?

A

Bedload: pebbles, cobbles, boulders
Suspension: finer pebbles, smaller grains

184
Q

What bedform may occur during sheetflood events which are later washed out in lowering velocities?

A

Antidunes

185
Q

What is the most common style of bedding found in sheetflood deposits?

A

Depositional couplets

186
Q

How do depositional couplets form?

A

Coarse gravel deposited as bedload when standing waves are forming - overlain by finer gravel and sand deposits from suspension as the wave is washed out

187
Q

How thick is a single typical depositional couplet?

A

5 to 20 cm thick

188
Q

How wide/far may an individual sheetflow event stretch?

A

100’s m wide, and stretching from apex to toe

189
Q

Is there much difference between the deposits on the distal and proximal parts of the fan after a sheetflow event?

A

No

190
Q

What are the five main characteristics of a sheetflood fan?

A
  1. Sheet geometry of beds 10’s cm to couple m thick
  2. Beds very well straitified, with distinct couplets of coarse gravel and finer sandy gravel
  3. Imbrication common upstream
  4. Sediment poorly sorted by silt/clay sized largely absent
  5. Normal grading due to waning flow
191
Q

How do fluvial deposits form alluvial fans?

A

Confined channel on the fan, reduction in gradient gives deposition of gravel - braided rivers

192
Q

What happens during periods of high discharge in stream-channel fans?

A

Forces lateral migration or avulsion due to choaking of sediment

193
Q

How is the radius of a stream-channel fan determined?

A

The length at which the river carries gravel - smaller particles are carried off the fan

194
Q

What characteristic allows stream-channel fans to be that much larger than their sheetflood and debris flow counterparts?

A

Not limited in length by sheetflood extent, only by the length that the river can carry gravel

195
Q

How large can stream-channel fans grow to be?

A

>10km in length

196
Q

What is a typical scale for sheetflood fans?

A

1 to 10 km

197
Q

Describe a typical stratigraphic record for a stream-channel fan

A

Discinct sharp base to channel, clast supported conglomerate fining upwards to sandstone

198
Q

What typical sedimentary structures can be found in stream-channel fan deposits?

A

Same as a braided river: imbrication, cross-strata of gravels and cross-bedded sandstone

199
Q

What would be a typical deposition regeme for an alluvial fan in an arid environment?

A

Hour long flood events every ten to hundred years (sheetflood/debris flow) with periodic rainfall events (channel-stream fan)

200
Q

Why would an open-framework conglomerate be expected on an alluvial fan in an arid environment?

A

Winnowing of finer sediments between deposition events

201
Q

What types of modification would be expected on the surface of an alluvial fan in an arid environment?

A

Pedogenisis, aeolian winnowing, desert varnish

202
Q

Why are alluvial fan deposits so important in the geological record?

A

Very sensitive to tectonic changes due to their location at the margin of floodplain/orogeny

203
Q

What may a sudden influx of coarse detritus onto an alluvial fan indicate?

A

Sudden tectonic uplift

204
Q

What may a switch to predominantly sheetflood/stream-channel processes on an alluvial fan indicate?

A

An increase in rainfall and thus change in climate regeme

205
Q

What are the typical lithologies associated with alluvial deposits?

A

Conglomerate, sandstone, mudstone

206
Q

What are the typical mineralogies associated with alluvial deposits?

A

Variable, often compositionally immatre

207
Q

What is the typical texture associated with alluvial deposits?

A

Very poor in debris flow, moderate in river sands

208
Q

What are the typical bed geometries associated with alluvial deposits?

A

Sheets on fans, lens-shaped river channel units

209
Q

What are the typical sedimentary structures associated with alluvial deposits?

A

Cross-bedding and laminations in channel deposits

210
Q

What do the typical paleocurrents associated with alluvial deposits indicate?

A

Indicate direction of flow and depositional slope

211
Q

What are the typical fossils associated with alluvial deposits?

A

Fauna uncommon, plant fossils may be common in floodplane deposits

212
Q

What are the typical bed colours associated with alluvial deposits?

A

Yellow, red, brown due to oxidising

213
Q

What are the typical facies associated with alluvial deposits?

A

Alluvial fan deposits: ephemeral lakes and aeolian dunes
Rivers: lakes delta or esturine faces

214
Q

What is the problem with overbank sediments as potential reservoirs?

A

Too fine grained

215
Q
A

Sedimentology and Stratigraphy (19 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions