Sedimentary Structures (Chap 4) Flashcards
What does the lower line on the Hjulstrom Diagram represent?
The relationship between flow velocity and particles that are already in motion
What does the upper line on the Hjulstrom Diagram represent?
The flow velocity required to move a particle at rest
Why does it take more flow velocity to erode a clay particle than it does a larger, medium-sized sand
grain?
Clay minerals are moer cohesive and once they have been deposited they tend to stick together.
Bedforms are initiated by processes that involve the viscous sublayer. What is this? What are it’s
properties? How does grain sizes affect it?
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.
Flow separation, defined by streamlines, is required for the initiation of ripple mark formation. How
does flow separation occur?
The Bernoulli effect
T or F – a requirement of ripple formation is for grains to be smaller than the thickness of the viscous
sublayer
True
How do cross laminations form in a ripple mark?
The transport of sediments through migrating crest advances and ripple troughs leave behind thin inclined layers of sand
What information can be estimated using the shape of ripple mark crests as viewed from above?
Direction of movement
What is the value of the bedform stability diagram?
It visualizes the relationship between grain size and flow velocity
Scour Marks and how they form
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.
Tool marks and how they form
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
Flaser bedding
thin, isolated drapes of mud seperated by layers of sand along cross laminae
Lenticular bedding
isolated ripples of sand surrounded by mud
wavy bedding
intermediate form with equal amounts mud and sand
T/F a requirement of ripple formation is for the grains to be smaller than the thickness of the viscous sublayer
True
What characteristics are unique to a wave ripple? How are they distinguished from a current ripple?
They are symmetrical and cross-laminae dipping in both directions
What criteria must be met to label a rock unit as a formation?
It must be distinguishable from the surroundings and thick enough to map
Ripples with sinuous crests create what kind of cross lamination?
Trough
T/F the formation of ripples is independent to depth
True
When is it adventitious to divide a formation up into members?
when you have relative and distinguishable homogeneity
Why does it take more flow velocity to erode clay?
Clay compacts and tends to stick together
What does flaser bedding tell you about current flow conditions?
Shows variable flow conditions
What are the requirements to form dunes?
Large grain size
Higher flow rates
Why can planar beds form at high velocities?
High velocities washout bed forms
