Sedimentary Structures (Chap 4)

Question 1

What does the lower line on the Hjulstrom Diagram represent?

Answer 1

The relationship between flow velocity and particles that are already in motion

Question 2

What does the upper line on the Hjulstrom Diagram represent?

Answer 2

The flow velocity required to move a particle at rest

Question 3

Why does it take more flow velocity to erode a clay particle than it does a larger, medium-sized sand
grain?

Answer 3

Clay minerals are moer cohesive and once they have been deposited they tend to stick together.

Question 4

Bedforms are initiated by processes that involve the viscous sublayer. What is this? What are it’s
properties? How does grain sizes affect it?

Answer 4

It is a region of reduced turbulence where flow is 0. Sediments collect and cause flow separation and cause ripple marks.
It is typically thin
Grainsize defines the property of flow.

Question 5

Flow separation, defined by streamlines, is required for the initiation of ripple mark formation. How
does flow separation occur?

Answer 5

The Bernoulli effect

Question 6

T or F – a requirement of ripple formation is for grains to be smaller than the thickness of the viscous
sublayer

Answer 6

True

Question 7

How do cross laminations form in a ripple mark?

Answer 7

The transport of sediments through migrating crest advances and ripple troughs leave behind thin inclined layers of sand

Question 8

What information can be estimated using the shape of ripple mark crests as viewed from above?

Answer 8

Direction of movement

Question 9

What is the value of the bedform stability diagram?

Answer 9

It visualizes the relationship between grain size and flow velocity

Question 10

Scour Marks and how they form

Answer 10

Flute: Looks like the lee side of a ripple. Forms from the current
Obsticle: Forms when a current exavates sediment from the front and around an obstacle.

Question 11

Tool marks and how they form

Answer 11

Grooves: Sharply defined elongated marks created by an object being dragged along the bed
Skips: Marks made in the bed from where a sediment in saltation lands

Question 12

Flaser bedding

Answer 12

thin, isolated drapes of mud seperated by layers of sand along cross laminae

Question 13

Lenticular bedding

Answer 13

isolated ripples of sand surrounded by mud

Question 14

wavy bedding

Answer 14

intermediate form with equal amounts mud and sand

Question 15

T/F a requirement of ripple formation is for the grains to be smaller than the thickness of the viscous sublayer

Answer 15

True

Question 16

What characteristics are unique to a wave ripple? How are they distinguished from a current ripple?

Answer 16

They are symmetrical and cross-laminae dipping in both directions

Question 17

What criteria must be met to label a rock unit as a formation?

Answer 17

It must be distinguishable from the surroundings and thick enough to map

Question 18

Ripples with sinuous crests create what kind of cross lamination?

Answer 18

Trough

Question 19

T/F the formation of ripples is independent to depth

Answer 19

True

Question 20

When is it adventitious to divide a formation up into members?

Answer 20

when you have relative and distinguishable homogeneity

Question 21

Why does it take more flow velocity to erode clay?

Answer 21

Clay compacts and tends to stick together

Question 22

What does flaser bedding tell you about current flow conditions?

Answer 22

Shows variable flow conditions

Question 23

What are the requirements to form dunes?

Answer 23

Large grain size
Higher flow rates

Question 24

Why can planar beds form at high velocities?

Answer 24

High velocities washout bed forms