Even more sedimentary rocks

Today’s Lecture:Sedimentary structures: • Inferring depositional processes from sedimentary rocks •Sea-level changes & the facies conceptChapter 7: Sedimentary Rocks... Contacts between

Today’s Lecture:

Sedimentary structures:

• Inferring depositional

processes from sedimentary rocks

•Sea-level changes

& the facies conceptChapter 7: Sedimentary Rocks

Fig 7.26aStephen Marshak

bed to the top

Tops of beds: Finer

Bed 1 Bed 2

Graded Bedding

Thus, graded

beds tell us

how flow velocity changed during deposition!

Lower Velocity

Sedimentary structures: Cross-Bedding

First we need to distinguish between

primary bedding vs internal layering

Contacts between

sedimentary beds

Internal, inclined layers

Primary bedding

vs internal layering

Contacts between

sedimentary beds

Internal inclined layers

Primary bedding

vs internal layering

More cross-bedding

Bed contacts”

Fig 7.25abc

W W Norton

More cross-bedding

Bed contacts”

Cross beds

Tilt-direction of cross beds indicates the

Transport direction

What a geologist sees.

What a geologist sees.

Sedimentary Structures: Ripple Marks

Ripple marks form when moving wind or water causes sedimentary grains to “hop” along the bottom

Fig 7.27a Stephen Marshak

Ripple marks can be either symmetrical (formed by waves

Ripple formation movie

Transport and Deposition in Running Water

Sedimentary Structures: Ripple Marks

Sedimentary Structures: Ripple Marks

Sedimentary Structures: Ripple Marks

Study these ripple carefully Are they symmetrical or asymmetrical? What do

Look at these!

Sedimentary structures: Ripple Marks

Oscillation ripples(back and forth)

Sedimentary Structures: Mud Cracks

Fig 7.27dStephen MarhsakAncient mud cracks (cross-sectional view)

Sedimentary Structures: Mud Cracks

Mud cracks form

Margin of a dry lake

with mud cracks

Sedimentary structures: Mud Cracks

Fig 7.27c Stephen Marhsak

Say you find mud cracks in

an ancient sedimentary rock

What does that suggest about the environment where the rock formed?

What a geologist sees.

Describe what you see in this outcrop and interpret the geologic history and conditions

of deposition

In class exercise:

Contains

“clasts” of older rocks

Igneous clast Metamorphic clast

Fine layeringWavy basal contact

Ripple bedding

cross-Bed shows size grading.

Finer top

Single bed

Uplift of a deep-seated igneous pluton (granitic), with subsequent erosion by running water which transported igneous and

metamorphic clasts to a river which then carried them some distance from the source area,

to a site of deposition (stream channel)

Transport by running is inferred

by the rounding of the clasts, size grading, and sorting

Erosional contacts at the bases of beds indicate initial turbulent

What a geologist sees:

“Facies” are representations of

Sedimentary environments

Concept of Sedimentary “Facies”