More sedimentary rocks

Today’s Lecture: Types of sedimentary rocksConcepts of textural & mineralogical maturity Nature and classification of detrital sedimentary rocks Turning sediments into sedimentary r

Today’s Lecture: Types of sedimentary rocks

Concepts of textural & mineralogical maturity

Nature and classification

of detrital sedimentary rocks

Turning sediments into sedimentary rocks

Stratification features

of sedimentary rocksChapter 7: Sedimentary Rocks

◆ Detrital (or “clastic”)

Sedimentary Rocks

❖ Made up of fragments (clasts) of pre-existing rocks.

❖ In terms of composition, the most mature contain:

- Clay minerals (weathering of feldspar)

- Quartz (resistant to weathering)

❖ Presence of unstable minerals (mafics, feldspars)

indicates immaturity:

- limited weathering

- rapid transport and deposition near source

❖ For detrital rocks, particle (clast) size:

- most basic distinguishing factor for naming

rocks.

Sedimentary rock classification

is based on texture and composition.

Classification of

detrital (or “clastic”)

sedimentary rocks

Based on size of clasts (particles):

❖ Gravel larger than 2 mm Conglomerate

(1/10 inch) or Breccia

❖ Sand sand-sized Sandstone

❖ Silt, Mud, Clay very fine-grained Shale or Mudstone

Sediment name Size Rock name

Decrease in grain size with increasing transport

Transport Distance

of clasts: Tell us about the nature of the source rocks.

Inferring Depositional Processes

Proximity to Source

Deposited Nearby

Deposited Far Away

Clast Size

conglomerate sandstone siltstone shale

clay silt

sand gravel

Loose Sediments

Sedimentary rocks

Detrital Sedimentary

Rocks

❖ Other important textural features:

Grain shape (Roundness, sphericity)

Shapes of sand grains

Angular Well-rounded

Detrital Sedimentary

Rocks

❖ Other Important Textural Properties:

Grain size sorting

Range of particle sizes in a sediment or rock

Very poorly-sorted moderately sorted Very well-sorted

Fig 7.18

Upstream, nearer the

source, clasts are

larger& more angular

Fig 7.29b

Fig 7.28c

© Martin Miller

Products of long-term weathering and erosion:

Quartz and clay

Detrital Sedimentary

Rocks

Breccia

❖ Course like a conglomerate,

but with angular grains

❖ Short transport!

❖Deposited close to the source

area for sediment.

◆ Detrital sedimentary

rocks

Sandstone

❖ Composed of sand grains

❖ 2nd most abundant sedimentary rock

❖ Deposited by moderately active transport processes:

- Running water (rivers & deltas)

- Along shorelines (beaches)

- By the wind (sand dunes)

❖ Mostly quartz (chemically stable), feldspar, micas, &

rock fragments.

Australia Hawaii

Mineralogy of sands reflects their source

Example:

◆ Detrital Sedimentary

Rocks

Shale & Siltstone

❖ Silt & clay-sized particles (mud, clay, silt)

❖ Over 1/2 of all sedimentary rocks.

❖ Particles too small to identify w/ naked eye.

❖ Deposited in quiet (slow moving) water.

- deep ocean & continental slope

- lakes

- floodplains of rivers

❖ Tends to form slopes when eroded.

❖ Raw material for making brick, tile, pottery, china.

Name this rock and describe the degree

of sorting.

Two minute in-class

exercise:

Describe the grain sorting and rounding

of the quartz sand above What does

it take to get a sediment like this?

Two minute in-class exercise

Turning sediment into rock

How does this happen?

Process of transforming unconsolidated

sediment into rock is called “lithification”

Sand to sandstone: How?

◆ Compaction

Two important processes:

Sediments accumulate, one layer on

top of another to great thincknesses

Weight of overlying sediments

compresses deeper buried sediments

Dissolved minerals in groundwater,

Precipitate in the spaces between grains,

“cementing” them together

The most common cements are calcite, silica, & iron oxide

Turning sediment into rock

beds are better

cemented and

more resistant

Nature of Sedimentary Deposits

Most sedimentary deposits are

layered or “stratified”

Horizontal layering

Color variation of layers

So what happened here?

Nature of Sedimentary Rocks

Each bed or stratum is unique in showing differences in color, texture (grain size &

sorting) and internal structures

Classic example of layered

sedimentary rocks: Grand

Canyon of Arizona

Nature of Sedimentary Rocks

Each layer (bed, or “stratum”)

is bounded by an upper

& lower bedding plane.

Nature of Sedimentary Rocks

Each layer (bed, or “stratum”)

is separated by bedding planes.

Nature of Sedimentary Rocks

Each layer (bed, or “stratum”)

is bounded by bedding planes.

The shape of the beds and nature of bedding planes provides important information clues about the processes that originally deposited the sediments

How does the shape of this bed differ from

the others?

Why is this bed different?

It was deposited

in a channel!