Fluvial Systems - alluvial fans and braided streams

Question 1

Cone shaped deposit of coarse stream sediments, sheet-flood deposits, and debris flows that forms where a narrow canyon stream suddenly disgorges into a flat valley.

Answer 1

Alluvial fan

Question 2

In terms of fluvial systems, what is a short system?

Answer 2

Something like a mountain by the coast line, contributing sediment factor is close by.

Question 3

What are some ways you can identify a fluvial deposit?

Answer 3

a. few fossils with no marine fossils
b. poor to moderate sorting
c. red color due to hematite
d. unidirectional paleocurrents
e. strong downstream decrease in particle size

Question 4

In terms of fluvial systems, what is a long system?

Answer 4

The source of the sediment is far from the basin where it eventually accumulates. Far transport

Question 5

What are the different kinds of river systems?

Answer 5

straight channel, alluvial fans, braided stream, meandering stream.

Question 6

In an alluvial fan system, the coarse/fine sediment gets deposited at the beginning of the system and coarse/fine sediment gets deposited the farther away you are from the mountains.

Answer 6

coarse, fine

Question 7

What do you call an accumulation of alluvial fans?

Answer 7

bajada

Question 8

T/F: at the edge of the alluvial fan, you can get evaporation, forming halite.

Answer 8

T

Question 9

Why do you get clastic wedges?

Answer 9

Clastic wedges form near mountainous areas as the mountains erode and collect at the basin.

Question 10

What are some indicators of a proximal system for alluvial fans?

Answer 10

lots of gravel, no real bedding but a little coarser material higher up.

Question 11

T/F: An indicator of an alluvial fan is seeing a debris flow structure, meaning a sand matrix.

Answer 11

F, mud matrix

Question 12

T/F: Braided streams are closer to the source than alluvial fans.

Answer 12

F, farther

Question 13

What type of rock can you expect in braided streams?

Answer 13

Conglomerates, and then as you get farther in the braided stream you will see sandstone.

Question 14

What sediment size dominates longitudinal bars (“L-bar”)in braided streams?

Answer 14

gravel.

Question 15

On the side of braided streams, the fluid flow may slow down so, instead of gravel, you get _____-sized dominated transverse bars.

Answer 15

sand

Question 16

How do you tell, for braided streams, if you are proximal, medial and distal?

Answer 16

Prox - gravel (L-bar) dominated, conglomerates, breccias
Med - a mix of L-bars and transverse bars
Dist - transverse bars, dominated by sand

Question 17

What is an ephemeral flow?

Answer 17

An ephemeral flow is a stream or water channel that only flows for a short duration, typically after a precipitation event. Episodic.

Question 18

An alluvial fan is also gravel dominated, so how can you tell if an area was an alluvial fan v. a braided stream?

Answer 18

Alluvial fans are mud matrix, and also more of a slurry of sediment. The braided stream has a constant flow so there really isn’t mud. Even if it dries up, there isn’t mud.

Question 19

T/F: Transverse bars wouldn’t have any cross-bedding or moving ripples.

Answer 19

F, they can

Question 20

In braided streams, the sediments are more angular/rounded, and there is a higher/lower diversity of material

Answer 20

rounded, higher

Question 21

What is an unconformity?

Answer 21

A gap in the rock record due to erosion or non-deposition.

Question 22

What are the different types of unconformities?

Answer 22

Disconformity - parallel beds above and below the unconformity

Angular - tectonic plates collide, creating an uplift, so you have bedding at an angle

Non-conformity - unconformity sitting directly on crystalline basement rock

Question 23

How would you have a non-depositional event?

Answer 23

As sea level rises, a river can flood and become an estuary with brackish water. The river is then flowing into the estuary. The sediment gets trapped in the estuary instead of going out to the ocean. So the rock record where the ocean begins is a non-depositional area. (This non-depositional area can then form glauconite!)

Question 24

As soon as you get _______-sized particles, you have moved out of a braided stream environment.

Answer 24

mud

Question 25

In meandering streams, you have a lot/no mud.

Answer 25

a lot

Question 26

Meandering streams tend to be sinuous channels that are constantly flowing, that goes faster/slower on the outside edges of the stream.

Answer 26

faster

Question 27

Do meandering streams want flat or hilly areas?

Answer 27

flat plain

Question 28

When there's a single channel in a meandering stream, the bend changes velocity. Which part of the channel has erosion and which has deposition?

Answer 28

Erosion on outter, deposition on inner.

Question 29

What is a point bar in terms of meandering stream?

Answer 29

A point bar is a deposit of sediment, typically sand or gravel, found on the inside bend of a meandering stream. As a river meanders, the slower-moving water on the inside of the bend causes sediment to be deposited, building up a crescent-shaped feature called a point bar. The opposite side of the meander, where the water flows faster, experiences erosion, forming a cut bank.

Question 30

What is an oxbow lake?

Answer 30

An oxbow lake is a U-shaped lake or pool that forms when a wide meander of a river is cut off, creating a free-standing body of water.

Question 31

In oxbow lakes, what kind of sediment do you typically see?

Answer 31

Commonly clay, and organic rich

Question 32

What is a Crevasse splay?

Answer 32

Crevasse splay is a deposit that looks like an alluvial fan. From a when a levee breaks due to a very high flood. The sediment from that levee gets deposited in a sheet like fashion

Question 33

Study this fluvial system graphic

Answer 33

Question 34

study this order of fluvial environments

Answer 34

Question 35

study this alluvial fan graphic

Answer 35

Question 36

Are fossils common in alluvial fans?

Answer 36

No they are extremely rare bc alluvial deposits are usually highly oxidized, and high energy floodwaters with deposition will destroy fossils

Question 37

Alluvial fan grain sizes vary from boulders to clay particles. Are these particles mature (rounded) or immature (angular)?

Answer 37

Immature, unsorted and angular sed

Question 38

Look at this braided stream bar sequences that garb really likes

Answer 38

Question 39

What are the only types of fossils you might see in a braided stream?

Answer 39

root casts

Question 40

What sediment structures do you generally see in meandering streams.

Answer 40

a. generally strong upward fining trnds from lateral facies shifts b. hydrologic shifts can also cause reverese grading c. coarse basal channel lag gravels d. parallel-laminated to trough crossbedding (dunes) sands e. ripple cross-laminated sands f. alternating sand and mud overbank and levee deposits g. produce shoe-string sand bodies

Question 41

Because there is little silt or clay, the water in an alluvial can can pass through the porous gravels on the fan without blocking the pores. Thus, the lobe-shaped deposit, which is called a __________ deposit becomes progressively coarser toward the front of the lobe, where porous gravel accumulates. This form is usually seen in the proximal fan or upper mid-fan.

Answer 41

Sieve

Question 42

What's another name for a transverse bar?

Answer 42

Linguiod bc of lobate in shape

Question 43

What are some low velocity features you can find in a transverse bar?

Answer 43

ripples, dunes, trough cross-bedding

Question 44

Alluvial fans are best known from _______ environments, although they also occur in humid environments.

Answer 44

desert

Question 45

What is sheet flow in the context of alluvial fans?

Answer 45

Sheet flow is the unconfined, shallow movement of water across the surface of an alluvial fan. It spreads out over a broad area, depositing sediments in thin, widespread layers, and typically occurs during high-intensity rainfall or flood events.

Question 46

How does sheet flow contribute to sediment distribution on alluvial fans?

Answer 46

Sheet flow helps distribute sediment evenly across the surface of an alluvial fan by transporting material in broad, thin flows. This process results in the formation of gently sloping, fan-shaped deposits composed of poorly sorted sediments.