Today’s Lecture:

Sedimentary structures:

• Inferring depositional processes from sedimentary rocks •Sea-level changes & the facies concept

Chapter 7: Sedimentary Rocks

Sedimentary structures: Features observed within a single bed.

Within sedimentary beds, distinctive structures can

usually be seen. These include

systematic variations in grain size and sorting, internal bedding

features, etc. that are diagnostic of

particular depositional processes.

Sedimentary Structures

Graded beds:

Show a gradual change in particle size as you move from the bottom of a bed to the top.

Bed 1

Bed 2

Fig. 7.26a

Stephen Marshak

Sedimentary Structures

Graded beds:

Show a gradual change in particle size as you move from the bottom of a bed to the top.

Bottoms of beds: Coarser

Bed 1

Bed 2

Sedimentary Structures

Graded beds:

Show a gradual change in particle size as you move from the bottom of a bed to the top.

Tops of beds: Finer

Bed 1

Bed 2

Graded Bedding

The bed to the right shows a change fromlarge grains at the bottom, to small at the top. This is called “normal” grading.

Coarser

Finer

Graded Bedding

As transportvelocity declines, coarser particlessettle out first

(see videoon turbidity

currents).

Higher Velocity

Lower Velocity

Graded Bedding

Thus, graded beds tell us

how flow velocity changed during

deposition!

Higher Velocity

Lower Velocity

Sedimentary structures: Cross-Bedding

Cross-bedding is internal bedding

that is tilted at an angle to the

primary bedding. Cross beds are

formed by a scour and fill transport process involving

either wind or water (see

ripple movie).

First we need to distinguish between

primary bedding vs. internal layering.

Contacts betweensedimentary beds

Primary bedding vs. internal layering

Primary Bed

Contacts betweensedimentary beds

Internal, inclined layers

Primary bedding vs. internal layering

Contacts betweensedimentary 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

Cross-bedding

Tilt-direction of cross beds indicates the direction of transport (e.g., wind direction or

direction of water flow).

Transport direction

Large-scale cross-beds like these are formed by sand dune migration

Which way did the wind blow?

What a geologist sees.

Paleowinds

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 sloshing back and forth), or symmetrical (formed by water

or wind flowing in one direction).

Ripple formation movie

Transport and Deposition in Running Water

Sedimentary Structures: Ripple Marks

Look closely at the ripples on this surface.

Are they symmetrical, or asymmetrical?

Which way did the water

flow?

Sedimentary Structures: Ripple Marks

Look closely at the ripples on this surface.

Are they symmetrical, or asymmetrical?

Which way did the water

flow?

Paleocurrent direction

Sedimentary Structures: Ripple Marks

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

they suggest aboutpaleocurrent direction?

Look at these!

Sedimentary structures: Ripple Marks

Oscillation ripples(back and forth)

Interpretation: Paleoshoreline

Sedimentary Structures: Mud Cracks

Fig. 7.27d

Stephen Marhsak

Ancient mud cracks (cross-sectional view)

Sedimentary Structures: Mud Cracks

Mud cracks form when mud-

covered shorelines orlake bottoms, dry up. This produces an irregularly-

cracked surface.

Margin of a dry lake with mud cracks.

Note ripple-marked sand dunes at top

of picture.

Sedimentary Structures

Note ripple-marked

sand dunes at top

of picture.

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

Lower contacts cut into unitsunderneath. Erosional!

Fine layering

Wavy basal contact

Wavy basal contact

Ripple cross-bedding

Bed shows size grading.

Coarser base

Finer top

Single bed

Uplift of a deep-seated igneous pluton (granitic), with subsequent erosion by running water which transported igneous andmetamorphic 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 transport, followed by declining flow velocity (flood event?).

What a geologist sees:

“Facies” are representations of sedimentary environments defined by the overall association of featurespreserved in rocks.

Sedimentary environments

Concept of Sedimentary “Facies”