More sedimentary rocks

Today’s Lecture:

Types of sedimentary
rocks

Chapter 7: Sedimentary Rocks

Concepts of textural &
mineralogical maturity
Nature and classification
of detrital sedimentary
rocks
Turning sediments into
sedimentary rocks.
Stratification features
of 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.
Rock type

Main criterion

Detrital

◆ Particle size

Chemical

◆ Mineral composition


Classification of
detrital (or “clastic”)
sedimentary rocks

Based on size of clasts (particles):
Sediment name
❖ Gravel

Size
larger than 2 mm
(1/10 inch)

❖ Sand

sand-sized

❖ Silt, Mud, Clay very fine-grained

Rock name
Conglomerate
or Breccia
Sandstone

Shale or Mudstone


Decrease in grain size with increasing transport


Inferring Depositional Processes
Sedimentary rocks contain many clues
about the processes that formed them.
Clast Size

Current
Velocity

Transport
Distance

Proximity to
Source

◆ Large clasts

Stronger

Shorter

Deposited
Nearby

◆ Small clasts

Weaker

Composition
of clasts:

Longer

Deposited
Far Away

Tell us about the nature of
the source rocks.


Loose Sediments

gravel

sand

silt

clay

Sedimentary
rocks

conglomerate

sandstone

siltstone

shale


Detrital Sedimentary
Rocks

❖ Other important textural features:
Grain shape (Roundness, sphericity)

angular

intermediate

rounded

Angular

Well-rounded

Shapes of sand grains


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
(texturally immature).
They also contain a
greater proportion

of unstable minerals
& rock fragments
(compositionally
Immature).


Fig. 7.29b


Fig. 7.28c
© Martin Miller


Products of long-term weathering and erosion:
Quartz and clay


Detrital Sedimentary
Rocks

Conglomerate
❖ Composed mostly of
pebble to boulder-sized clasts
❖ Clasts well-rounded.
(angular clasts form a
rock called “breccia”).
❖ Usually poorly-sorted.
❖ Deposited by high energy currents:
- flooding rivers
- streams near mountain source

areas (steep)
- mudlfows


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.

Mineralogy of sands reflects their source

skeletal
Tahiti

quartz
Wisconsin

obsidian
Hawaii

reef carbonate
Australia

“Oolites”
Cancun, Mexico

skeletal
Oregon


Example:

Well-sorted

Poorly-sorted


◆

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.


Two minute in-class
exercise:

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?

Turning sediment into rock

Process of transforming unconsolidated
sediment into rock is called “lithification”.

How does this happen?


Sand to sandstone: How?