Ch3_SedimentaryRx_students

CH. 3 – SEDIMENTARY RX

Sediment

1) weathering products (gravel, sand, clay minerals)

2) chemical precipitates3) organic remains

Sedimentary Rocks

“sedimentum” = “settling”Sediment settles out of wind or

water- forms layers at the surface

Importance of Sedimentary Rx

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

2) Clues to past environments3) Fossil record4) Economic value

Sedimentary Rx Classification

1) Detrital = weathering products (mechanical or chemical)

2) Chemical

Detrital Sedimentary Rocks

Named by particle sizeParticle Size NameGravel, pebbles Conglomerate

Detrital Sedimentary Rocks

Particle Size NameSand Sandstone

(quartz sand)

Clay-size Shale (red, green, black)

Detrital Rock Process

1) Weathering2) 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 SandstoneStarved Rock State Park, IL

b) CementationMinerals dissolved in groundwater

precipitate around sedimentEx: silica, calcite, hematite

Lithification Processes

Sedimentary Rx Classification

2) ChemicalNamed based on compositionEx: calcite = limestone

halite = rock saltplant remains = coal

Chemical Sedimentary Rocks

a) Inorganic – formed by chemical reactions in environment

Ex: Evaporites - rock gypsum, (Fig. 3.23) rock salt

Ex: Travertine (limestone)

Chemical Sedimentary Rx

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

Detective Analogy

Geologists are solving a mystery

Scene of Clues Mysterythe Crime Solved

Mystery Solved (Sed. Rx)

Depositional environment- any area on surface where

sediment accumulatesEx:

Principle of Uniformitarianism

“Present is the key to the past”Study modern depositional

environments for cluesEx: Coquina limestone

Clues

1) Sediment sizeDistance deposited from source

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

Clues (sediment size)

High vs. low energy environmentLarge particles = (high, low)

Ex: ___________Small particles = (high, low) Ex: ___________

Clues

2) Sediment shapeDistance deposited from source

area: close to or far from?angular = ______rounded = ______

Breccia

Clues

3) Sedimentary Structures- features produced in sediment

BEFORE lithificationEx: mudcracks, ripple marks,

trace fossils

Mudcracks

Environmental conditions:a)b)

Ex: river floodplain

Ripple Marks

Indicates current direction in shallow water

Trace Fossils

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

Ex: footprints, coprolites

Clues

4) 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: DeathStep 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: Deep marine shale vs. muddy river shale