(S4) Rivers Flashcards
1
Q
What are the three zones found in fluvial and alluvial systems?
A
Erosional zone, transfer zone and depositional zone
2
Q
Give three locations you would likely find a depositional zone?
A
River channel, flood plains, alluvial fan
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.
4
Q
What does the term “fluvial” involve?
A
Channels and overbank systems
5
Q
What does the term “alluvial” involve?
A
Channels, overbank, alluvial fans - all general processes which involve river systems
6
Q
What are the two main factors on water supply control in a drainage basin?
A
Catchment area, Climate
7
Q
What does “perennial” mean?
A
Year around
8
Q
What does “ephemeral” mean?
A
Peroidic
9
Q
How does catchment area have an effect on the water supply in a drainage basin?
A
Small catchments have limited storage
10
Q
What is the name for flow which is below the channel banks?
A
Low flow stage
11
Q
What is the name for flow which is just under or at the channel banks?
A
High flow stage
12
Q
What is the name for flow which is over channel bank onto the floodplain?
A
Overbank flow
13
Q
What is the name for the cross-sectional area of a river with the highest flow rate?
A
Thalweg
14
Q
What are the three main variables of river forms?
A
Straight/sinuous Presence/absence of depositional bars Number of separate channels present
15
Q
What is the simplest form of river?
A
Straight, single channel, no bars - in reality: uncommon
16
Q
What is the technical definition for a braided river?
A
Mid-channel bars which are covered at high flow stage
17
Q
What is an anastomosing river?
A
Contains multiple channels which are interconnected, separated by areas of floodplain
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
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
20
Q
What is a common feature in a bedload river?
A
Mid-channel bars, i.e. a braided river
21
Q
What is “coarse lag”?
A
When coarsest material is deposited to form an accumulation of larger clasts
22
Q
What is the name for bars which run along the axis of a river?
A
Longitudinal bars
23
Q
What is the name for bars which spread across the river?
A
Transverse bars
24
Q
What is the name for bars which have their apex pointing downstream?
A
Linguoid bars
25
What is the name for bars which consist of sand and gravel mixtures?
Compound bars
26
What are two common features of longitudinal bars?
Low relief, poorly defined low angle cross-strata
27
What are two common features of transverse bars?
High relief, well defined cross-strata dipping downstream
28
What is a common feature of a sandy bar which is not usually found in a gravel bar?
Superimposed dune formations on the bar, which tend to be linguoid (trough-cross)
29
What is likely to be found in a compound bar?
Cross-strata of gravel with lenses of sand, or vice versa
30
What is usually the demise of a migrating bar in a channel?
Migrate until the channel moves sideways - migrates out of the main flow, and becomes covered in overbank deposits or bars of another channel
31
What are the five stages which would be expected in a sedimentary succession of a braided river?
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
32
Does the succession in a braided river usually fine or coarsen upwards?
Fining upwards
33
What does the thickness of a braided river succession usually represent?
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
34
What is a "braid plane"?
A broad extensive region of gravelly bar deposits many times larger than the channel
35
Where are braid planes commonly found?
Glacial outwash areas
36
What are the three common features of a braid plane?
Wed regions, poor vegetation, river banks are less stable
37
Why is it difficult to recognise individual successions in a braid plane stratagraphic record?
Difficult to recognise scour marks at the base of a channel, which distinguish individual channels from one another
38
What are channel fill-successions and when are they preserved?
Lateral migration of the channel - only preserved when this occurs
39
What is the technical definition of a sinuous river?
Distance along stretch of channel / direct straight line distance = \> 1.5
40
What is the technical definition of a meandering river?
Erosion on the outside of the bends and deposition on the inside
41
What do the deposits on a meander bank look like?
Coarser on at the base and finer further up
42
Where are sub-aqueous dunes found in meandering rivers?
Faster/deeper parts of the channel
43
Where are ripples found in rivers?
Higher up on the inside of bends where there is slower flow and finer sand
44
What is the name of the deposit on the inside of a meander?
Point bar
45
What is a "braid plane"?
A broad extensive region of gravelly bar deposits many times larger than the channel
46
Where are braid planes commonly found?
Glacial outwash areas
47
What are the three common features of a braid plane?
Wed regions, poor vegetation, river banks are less stable
48
Why is it difficult to recognise individual successions in a braid plane stratagraphic record?
Difficult to recognise scour marks at the base of a channel, which distinguish individual channels from one another
49
What are channel fill-successions and when are they preserved?
Lateral migration of the channel - only preserved when this occurs
50
What is the technical definition of a sinuous river?
Distance along stretch of channel / direct straight line distance = \> 1.5
51
What is the technical definition of a meandering river?
Erosion on the outside of the bends and deposition on the inside
52
What happens to the mud clasts which are eroded away from the inside of a meander?
The are deposited in the deeper parts of the channel due to their cohesive properties
53
Where are sub-aqueous dunes found in meandering rivers?
Faster/deeper parts of the channel
54
Where are ripples found in rivers?
Higher up on the inside of bends where there is slower flow and finer sand
55
What is the name of the deposit on the inside of a meander?
Point bar
56
What does the cross-strata look like in a point bar?
Larger scale cross-bedding at the base and smaller sets of cross-lamination at the top
57
Describe what occurs to a point bar with time
Lateral accretion surfaces so the point bar migrates laterally as sediment layers are deposited on the inside
58
When are lateral accretion surfaces most distinct?
When there are periods of low flow which allow the slower velocity for the deposition of finer silts in the layers
59
What is "epsilon cross-stratification"?
Another name for the lateral accretion surfaces found from point bar migration
60
What is the typical angle for a lateral accretion surface?
\<15 degrees
61
What is the relationship between flow direction and epsilon cross-stratification?
The point bar surface is perpendicular to flow
62
What is the outer bend of a meander made up of?
Flood plain deposits (muddy) as it is cutting into the flood plain
63
What happens to the mud clasts which are eroded away from the inside of a meander?
The are deposited in the deeper parts of the channel due to their cohesive properties
64
What is the stratification of an ephemeral river usually like?
Horizontal deposits, may be graded as flow decreases in strength with time Some low angle cross-strata may be found with longitudinal bars
65
What is a chute channel?
A channel which cuts across the inner bank of a meander during high stage flow
66
What does the development of a chute channel usually lead to?
An oxbow lake formation
67
How are oxbow lakes commonly recognised in ancient deposits?
Channel fills of fine-grained (sometimes) carbonaceous sediment
68
What are the two main variables which affect the velocity of flow of a river?
Slope and discharge
69
What is an example of avulsion?
Oxbow lakes - although other additional tracks may be involved
70
What is the full process of avulsion?
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.
71
What other alluvial feature may also be ephemeral, aside from rivers?
Alluvial fans
72
Describe an ephemeral alluvial fan
Detritus weathered away, not able to be carried until a significant flood event or spring runoff
73
What do typical deposits in ephemeral rivers usually look like?
Poorly sorted, angular to sub-angular, gravel in a sand/mud matrix
74
What paleocurrent indicator may occur in ephemeral lakes?
Imbrication
75
What is the stratification of an ephemeral river usually like?
Horizontal deposits, may be graded as flow decreases in strength with time Some low angle cross-strata may be found with longitudinal bars
76
What is a "wadi"?
A river/stream in a desert system with ephemeral flow
77
What is the name for a channel with minor channels splitting off?
A bifurcating pattern
78
What is the difficulty in determining anastomosing rivers in ancient deposits?
Impossible to tell if two or more channels were active at the same time - possible that it is a single channel repeatedly changing position
79
What are the two main variables which affect the velocity of flow of a river?
Slope and discharge
80
What type of rivers have the steepest depositional gradient and what is that gradient?
Gravelly braided rivers, \<0.5 degrees
81
What is a common formation in a sandy bedload?
Bars
82
What gradients do meanders tend to form at?
1/100th of a degree
83
What are the five main depositional features on a flood plain?
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
84
What type of river facies preserves flood plain deposits the best and which the worst?
Meandering rivers = best preservation of flood plain deposits, braided rivers = worst
85
What is an endoheic basin?
A basin which is a closed system - does not have an outlet to the open ocean
86
What is the name for a splay of water/sediment at the end of a channel (mostly found in arid environments with high evaporation)
Terminal fan
87
What is another name for a river with a terminal fan?
A fluvial distributary system
88
What is the name for a channel with minor channels splitting off?
A bifurcating pattern
89
What are the six main ways to classify soils?
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
How do levee's form?
Suspended load dumped really quickly due to rapid loss in energy
91
What usually forms when a levee breaks?
Crevasse splay - low cone of sediment
92
Is a breach in the levee instantaneous?
No - small conduit gets wider and deeper with each event
93
What is the process of a crevasse splay formation?
Initial upward coarsening of particle size Lenticular in three dimentions May develop into new river channels and carry more water until avulsion occurs
94
What are the features of histisols? Climate, Soil type (4)
Waterlogged, peaty - bedded sediments overlain by carbonaceous layer - root systems
95
What are the features of entisols? Climate, Soil type (3)
Immature, little soil development - mostly bedded sediments with roots in the upper layer
96
What are the features of inceptisols? Climate, Soil type (3)
Mature, soil development, less mature than some other types - bedded sediments with some roots
97
What are the features of spodosols? Climate, Soil type (5)
Dense vegetation forests - layers of bedded sediments overlain by iron rich and carbonaceous horizons, deep roots
98
What are the features of alfisols? Climate, Soil type (4)
Dense vegetation forests - layers of bedded sediments overlain by iron rich and calcareous nodule beds, deep roots
99
What are the features of utilistols? Climate, Soil type (4)
Dense vegetation forests - bedded sediments, reddened iron rich horizons and deep roots
100
What are the six main ways to classify soils?
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
What are the features of gelisols? Climate, Soil type (2)
Cold climate formation - clasic sediments
102
What are the features of aridsols? Climate, Soil type (5)
Arid conditions - Bedded sediments overlain by gypsum/calcareous nodules/roots
103
What are the features of oxisols? Climate, Soil type (4)
Humid, tropical conditions - Bedded sediments overlain by reddened iron rich soils, overlain by root systems
104
What are the features of vertisols? Climate, Soil type (4)
Sub-humid to semi arid with pronounced seasonality - bedded sediments overlain by carbonaceous layer at surface with root systems
105
What are the features of histisol? Climate, Soil type (4)
Waterlogged, peaty - bedded sediments overlain by carbonaceous layer - root systems
106
What are the features of entisol? Climate, Soil type (3)
Immature, little soil development - mostly bedded sediments with roots in the upper layer
107
What are the features of inceptisol? Climate, Soil type (3)
Mature, soil development, less mature than some other types - bedded sediments with some roots
108
What are "glabules"?
Small nodules of calcium carbonate
109
How can we use paleosols to determine the length of time that an ancient profile took to form?
Stages of development of calcrete can be measured, and thus the development of a mature profile can be measured
110
What are the features of utilistol?
Dense vegetation forests - bedded sediments, reddened iron rich horizons and deep roots
111
What are the features of mollisols? Climate, Soil type (6)
Grasslands - bedded sediments, gypsum and calcareous nodules, carbonaceous top layer with deep roots
112
What are the features of andisols? Climate, Soil type (3)
Volcanic bedrock - roots and a carbonaceous top layer
113
What are five common features found in paleosols?
Fossilized roots Burrows Vertical cracks Layers enriched in certain minerals Layers depleated in certain minerals
114
What are three things that paleosols can tell us?
Climate Vegetation type Time period that land was exposed
115
What are "seatearths"? (3 examples)
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
What is the process of formation of "calcrete"?
Water movement through soils Calcium carb as root encrustations (rhizocretions) or small nodules (glabules) Nodules grow and coalesce - forms fully developed CaCO3 horizon
117
How is it possible to determine if an eruption was subareal?
Existance of laterites between basalts
118
What are "rhizocretions"?
Root encrustations of calcium carbonate precipitation - fossilised roots
119
What are "glabules"?
Small nodules of calcium carbonate
120
How can we use paleosols to determine the length of time that an ancient profile took to form?
Stages of development of calcrete can be measured, and thus the development of a mature profile can be measured
121
What does paleo-entisols and paleo-inceptisols tell us about the length of time exposed at the surface?
Relatively short as these profiles are relatively immature paleosols
122
Under what alluvial conditions would it be expected to find an entisol?
Floodplain conditions - where a flood deposits sediment, and relatively little time was allowed for pedogenisis before a further flood covered the profile
123
What can make the recognition of a paleosol more difficult?
Destruction by diagenetic alteration
124
What are the variables which affect time for soils to develop? (3)
Temperature, rainfall, availability of different minerals
125
What two factors complicate paleosols in the geological record?
Time estimates are all relative, not absoloute Soil profiles can be further complicated by superposition of a younger profile over an older one
126
What are "seatearths"? (3 examples)
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
What are "laterites"?
Oxisols, product of extensive weathering of bedrock to form rock mainly of aluminium/iron oxides
128
How is it possible to determine if an eruption was subareal?
Existance of laterites between basalts
129
What are "ferricretes"?
Horizontal paleosol layers of cemented iron-rich oxides
130
What is a "duricrust"? (3 factors)
A hardened soil profile which includes ferricetes - highly resistance and develop over long time periods - found associated with major unconformities
131
What are "silcretes"?
Horizontal paleosol layers of cemented silica rich minerals
132
What is the most reliable indicator of a terrestrial environment?
Paleosol identification
133
Why is the identification of a channel not a reliable indicator of the terrestrial environment?
Can be found in deltas, tidal settings, deep marine environments
134
What can make the recognition of a paleosol more difficult?
Destruction by diagenetic alteration
135
What used to determine the "architecture" of fluvial successions?
Arrangement of channel vs overbank deposits in 3D
136
How is the architecture of fluvial successions described (2)
Proportion of "in-channel" deposits to finer overbank facies Size and shape of sand/gravel beds deposited in channels
137
What is the width of channel-fill determined by?
Process of avulsion and lateral migration
138
What does the archetecture of more frequent avulsion as opposed to more lateral migration look like?
139
What does the archetecture of more lateral migration as opposed to more frequent avulsion look like?
140
What does the archetecture of a slower subsidence rate look like?
141
What does the archetecture of a faster subsidence rate look like?
142
What are the two settings in which avulsion is frequent?
Rivers with high tectonic activity
Rivers with frequent flood events
143
When is lateral migration slowed? (2)
When river banks are stable - muddy (cohesive) floodplains, dense vegetation (particulary grass with fiborous root system)
144
Aside from slowing lateral migration, what else does vegetation effect in the alluvial system?
Slows overland flow
145
Under what conditions is the floodplain particularly unstable, where flows are faster and more likely to erode?
Arid and cold conditions which result in a lack of vegetation
146
What is the main use of determining paleocurrents in a fluvial system?
Determines direction of mouth/shore and source
147
What are the three main ways to determine paleocurrent in a fluvial system?
Channel margins, cross-bedding, imbrication
148
Why is care needed when determining the paleocurrent direction of floodplane deposits?
Often perpendicular to flow
149
What is the possible variance of paleoflow in braided river deposits?
60 degrees
150
What is the possible variance of paleoflow in meandering river deposits?
90 degrees
151
What is required for accurate field measurements of paleoflow of meandering and braided rivers?
Large numbers of measurements
152
How is it possible to detect changes in paleogeography in sucession?
Changes in facies - changes in fluvial deposition style plus grain size
153
What does a change from braided characteristics to meandering up through a sucession tell us? (2)
Decrease in gradient or reduction in discharge in a system
154
What can the dimensions of channel fill deposits infer? (2)
Size of river, size of drainage basin
155
How is the lithology of channel-fill deposits important?
Tells of bedrock lithologies in the surrounding catchment prior to the deposition
156
How can paleoclimate be determined from the character of fluvial deposits?
Ephemeral and perennial flow
157
What archetecture is best for a potential oil/gas reservoir?
Braidplane with moderate subsidence
158
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
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
What is an "alluvial fan"
A cone of detritus which forms at the break in slope at the edge of an alluvial plane
160
Give a location where alluvial fans are found?
Death Valley, CA
161
What is the name for a sediment body which consists of a mixture of talus deposits and debris-flow deposits?
Colluvial fans
162
At what setting do alluvial fans form?
Where the is a destinct break between high ground drainage and flatter basin floor
163
What is the "feeder canyon"
Funnels drainage to the basin margin where the valley opens out
164
What is the fan apex?
The highest most proximal point - adjacent to the feeder canyon
165
What is the fan-head canyon?
A small canyon incised into the surface near the fan apex
166
Where is the slope the steepest on an alluvial fan?
Near the apex of the fan
167
What is the fan toe?
A break in the slope of the alluvial fan
168
Where are the deposits thickest on an alluvial fan?
At apex - a conical wedge towards the toe
169
How is deposition on an alluvial fan determined? (4)
Availability of water, Amount/type of sediment and gradient of fan
170
What is the name for a dense mixture of water and sediment on an alluvial fan?
Debris flow
171
How does debris flow tend to travel?
In a lobe - doesn't travel far, small and relatively steep
172
What is the name for the process by which water is more dilute in sediments traveling down an alluvial fan?
Unconfined sheetfloods
173
What is a "stream-channel fan"?
An alluvial fan which contains a channel of braided streams
174
What is the closest material which can describe the viscosity of subaerial debris flow?
Wet concrete mix
175
What is the flow of subaerial debris flow like? (3)
High density, high viscosity, laminar flow
176
When does debris flow retire? (2)
When it runs out of momentum due to gradient decrese or flow loses water content
177
What is the resultant bedding on subaerial debris flow? (2 factors)
10's cm to m's thick with very little thinning downflow
178
What does the lack of turbulence in debris flow mean for the deposit?
No sorting of grainsize within the flow and for the most part; randomly orientated
179
What are the five main characteristics for a bed which is deposited by debris flow?
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
What is the name for flow which spreads out over a portion of an alluvial fan?
Sheetflood
181
What are the three features of sheetflood deposition?
Rapid, supercritical, turbulent flow
182
What gradient does sheetflow usually occur on?
gradients of 3 to 5 degrees
183
What size sediments would be expected to be carried in bedload and in suspension in a sheetflood event?
Bedload: pebbles, cobbles, boulders
Suspension: finer pebbles, smaller grains
184
What bedform may occur during sheetflood events which are later washed out in lowering velocities?
Antidunes
185
What is the most common style of bedding found in sheetflood deposits?
Depositional couplets
186
How do depositional couplets form?
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
How thick is a single typical depositional couplet?
5 to 20 cm thick
188
How wide/far may an individual sheetflow event stretch?
100's m wide, and stretching from apex to toe
189
Is there much difference between the deposits on the distal and proximal parts of the fan after a sheetflow event?
No
190
What are the five main characteristics of a sheetflood fan?
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
How do fluvial deposits form alluvial fans?
Confined channel on the fan, reduction in gradient gives deposition of gravel - braided rivers
192
What happens during periods of high discharge in stream-channel fans?
Forces lateral migration or avulsion due to choaking of sediment
193
How is the radius of a stream-channel fan determined?
The length at which the river carries gravel - smaller particles are carried off the fan
194
What characteristic allows stream-channel fans to be that much larger than their sheetflood and debris flow counterparts?
Not limited in length by sheetflood extent, only by the length that the river can carry gravel
195
How large can stream-channel fans grow to be?
\>10km in length
196
What is a typical scale for sheetflood fans?
1 to 10 km
197
Describe a typical stratigraphic record for a stream-channel fan
Discinct sharp base to channel, clast supported conglomerate fining upwards to sandstone
198
What typical sedimentary structures can be found in stream-channel fan deposits?
Same as a braided river: imbrication, cross-strata of gravels and cross-bedded sandstone
199
What would be a typical deposition regeme for an alluvial fan in an arid environment?
Hour long flood events every ten to hundred years (sheetflood/debris flow) with periodic rainfall events (channel-stream fan)
200
Why would an open-framework conglomerate be expected on an alluvial fan in an arid environment?
Winnowing of finer sediments between deposition events
201
What types of modification would be expected on the surface of an alluvial fan in an arid environment?
Pedogenisis, aeolian winnowing, desert varnish
202
Why are alluvial fan deposits so important in the geological record?
Very sensitive to tectonic changes due to their location at the margin of floodplain/orogeny
203
What may a sudden influx of coarse detritus onto an alluvial fan indicate?
Sudden tectonic uplift
204
What may a switch to predominantly sheetflood/stream-channel processes on an alluvial fan indicate?
An increase in rainfall and thus change in climate regeme
205
What are the typical lithologies associated with alluvial deposits?
Conglomerate, sandstone, mudstone
206
What are the typical mineralogies associated with alluvial deposits?
Variable, often compositionally immatre
207
What is the typical texture associated with alluvial deposits?
Very poor in debris flow, moderate in river sands
208
What are the typical bed geometries associated with alluvial deposits?
Sheets on fans, lens-shaped river channel units
209
What are the typical sedimentary structures associated with alluvial deposits?
Cross-bedding and laminations in channel deposits
210
What do the typical paleocurrents associated with alluvial deposits indicate?
Indicate direction of flow and depositional slope
211
What are the typical fossils associated with alluvial deposits?
Fauna uncommon, plant fossils may be common in floodplane deposits
212
What are the typical bed colours associated with alluvial deposits?
Yellow, red, brown due to oxidising
213
What are the typical facies associated with alluvial deposits?
Alluvial fan deposits: ephemeral lakes and aeolian dunes
Rivers: lakes delta or esturine faces
214
What is the problem with overbank sediments as potential reservoirs?
Too fine grained
215
