CH. 6 – SEDIMENTARY RX

Sediment:

- weathering products (gravel, sand, clay minerals)

- chemical precipitates

- organic remains

Sedimentary Rocks

“sedimentum” = “settling”

Sediment settles out of wind or water

- forms layers at the surface

Importance of Sedimentary Rx

1) Form 75% of exposed rx at surface (outcrops)

- compose only 5% of total crust

2) Contain clues to reconstruct past environments on Earth

Importance of Sedimentary Rx

3) Fossil record

4) Economic value

- coal, petroleum, construction

Sedimentary Rx Classification

1) Detrital = weathering products

(mechanical or chemical)

Named based on particle size

Detrital Sedimentary Rocks

Particle Size Name Gravel Breccia

(angular) Conglomerate (rounded)

Detrital Sedimentary Rocks

Particle Size Name Sand Quartz Sandstone

(quartz sand)

Arkose (K-spar)

Detrital Sedimentary Rocks

Particle Size Name

Sand Graywacke

(rock frags,

mafic minerals)

Detrital Sedimentary Rocks

Particle Size Name

Clay-size *Shale

(clay minerals)

*Shale is the most abundant detrital sedimentary rock

Detrital Rock Process

1) Weathering

2) Erosion (transportation)

3) Deposition (sediment settles)

4) Lithification – turning sediment into rock (“lithos” = rock)

(after sediment is buried)

Types of Lithification Processes

a) Compaction

- most effective on fine-grained rx (ex: shale)

Exception: St. Peters Sandstone

Starved Rock State Park, IL

b) Cementation

Minerals dissolved in groundwater precipitate around sediment

Ex: silica, calcite, hematite

Lithification Processes

Sedimentary Rx Classification

2) Chemical – named based on composition

Ex: calcite = limestone

halite = rock salt

plant remains = coal

Chemical Sedimentary Rocks

a) Inorganic – formed by chemical reactions in environment

Ex: evaporites – rock gypsum, rock salt

Ex: Travertine (limestone)

Inorganic Chemical Sed. Rx

Calcite:*Limestone = mostly marine origin Travertine = speleothems Oolitic Limestone = tidal flats* Most abundant chemical

sedimentary rock

Inorganic Chemical Sed. Rx

Silica (microcrystalline quartz):

Varieties: chert

agate

Silica (microcrystalline quartz):

Varieties:

Petrified Wood

Jasper

Chemical Sedimentary Rx

b) Organic (“biochemical”)Plant remains = coalMicroscopic sea shells = chalkBroken sea shells = coquina

limestone

Detective Analogy

Geologists are solving a mystery

Scene of Clues Mystery

the Crime Solved

Mystery Solved (Sed. Rx)

Depositional environment

- any area on surface where sediment accumulates

Ex:

Principle of Uniformitarianism

“Present is the key to the past”

Study modern depositional environments for clues

Ex: coquina limestone

Ex: ripple marks

Sedimentary Facies

Cumulative physical characteristics of a sedimentary rock which indicates its depositional environment

“Clues” in detective analogy

Principle of Lateral Continuity

Nicolaus Steno (1669)

Sediment is accumulated to edge of its depositional environment

Sedimentary Facies

Note: At the same time, each environment (facies) is accumulating its own sediment and characteristics!

Sedimentary Facies

Clues

1) Sediment sizeDistance deposited from source

areaLarge = (closer to, further from)Small = (closer to, further from)

Clues (sediment size)

High vs. low energy environment

Large particles = (high, low)

Ex: ___________

Small particles = (high, low)

Ex: ___________

Clues

2) Sediment shape

Distance deposited from source area: close to or far from?

angular = ______

rounded = ______

Clues

3) Composition – minerals present

Two most common minerals found in sedimentary rocks are _____ and ______ .

Variety of minerals:

Ex: quartz, K-spar, plagioclase feldspar, mafic minerals, rock fragments

- very little chemical weathering- indicates rapid deposition close

to source areaEx: arkose in alluvial fan

One mineral composition:

Well-sorted

- sediment traveled far from source area

Ex: Quartz sandstone, shale

Clues

4) Sedimentary Structures

- features produced in sediment BEFORE it’s lithified

Sedimentary Structures

a) Bedding – layering w/in rock

Bedding plane – flat surfaces along which rx tend to break or separate

- separates different episodes of deposition

Bedding

i) Laminar bedding – horizontal layers

- usually deposited in calm energy environment

Ex: shale, some sandstones

Bedding

ii) Graded bedding – sorted by size

- occurs w/sudden decrease in velocity

Ex: turbidity currents, alluvial fans

Bedding

iii) Cross-bedding (Figure 6.22)

- layers are angled in sets

- occurs with change of current direction (either wind or water)

Sedimentary Structures

b) Ripple marks

- wave action moves sediment

- shallow water

Types of Ripple Marks

i) Asymmetrical - also called “current ripple marks”

- used to determine paleocurrent directions

Types of Ripple Marks

ii) Oscillation (symmetrical)- wave action is back and forth- usually indicates shallow lagoon environment

Sedimentary Structures

c) Mudcracks

Environmental conditions:

a)

b)

Ex:

Sedimentary Structures

d) Trace Fossils

Any evidence an organism was in the environment but no physical remains of the organism exist

Trace Fossils

Examples:

i) Footprints

ii) Coprolites

iii) Bioturbation

Clues

5) Fossils – evidence of prehistoric life

Fossil record is incomplete- shows remarkable pattern of

change from simple to complex life forms

Steps to becoming a fossil

Step 1: Death

Step 2: Hard parts – bones, teeth, shells (Who you are!)

Step 3: Rapid burial (Where you die!)

Step 4: Time (prehistoric)

Fossils

Fossil record biased towards marine environment

- abundant life in oceans

- many critters have hard parts

- lots of sediment for quick burial

Fossils

Good evidence for reconstructing past environments

Ex: Beach sandstone vs. desert sandstone

Clues

6) Color

Black or green = unoxidized iron

- lack of oxygen in environment

Ex: deep ocean, deep lake, swamp

Color

Red = oxidized iron

Ex: river floodplain, tidal flats, desert deposits

Green/purple = volcanic ash mixed in with sediment

Clues

7) Geometry of rock unit

- shape & thickness of rock unit

Ex: beach sandstone vs. desert sandstone

Depositional Environments

Handout

Lithofacies map

For a particular moment in geologic time, rocks indicate the landscape

1) Map out the areas where each rock type is found & draw boundaries

Lithofacies map

2) Look at the lithologies (rock types) & compare them to:

- nearby rx- geometry of rock unit3) Determine the original

depositional environment

Cyclothems

Cyclical alternations of lithologies deposited as shorelines shift

Sedimentary Facies

Stratigraphic Sequences

1) Transgressions - sea level risesStrat column changes vertically

from coarser sediment to finer sediment

“Fining-upward” sequence

Stratigraphic Sequences

2) Regressions – sea level drops

Strat column changes vertically from finer sediment to coarser sediment

“Coarsening-upward” sequence

Color

Red = oxidized iron

Ex: river floodplain, tidal flats, desert deposits

Green/purple = volcanic ash mixed in with sediment