1

Sedimentary RocksWeathering, Erosion, Deposition,

and Lithification

Layered Rock on Mars may indicate sedimentary rock

Red Beds indicateoxygenation of early Earth

Importance of sedimentary rocks.

• Source of many construction materials.

• Sediments can store information on past climate.

• Type of rock can indicate environment of deposition.

• The only rock type to preserve fossils.

2

How to make a sedimentary rock.

• Weathering = the breaking down of rock into smaller pieces (sediment).

• Erosion = the movement of sediment from one place to another.

• Deposition = the placement of sediment at a new location.

• Lithification = turning loose sediment into a sedimentary rock.

Weathering• Chemical

– Rock dissolved (in water) into individual molecules.

– Especially effective if the water is acidic or hot.

• Mechanical– Rock is physically broken into smaller pieces.– Happens from impact, thermal expansion, or

frost wedging.

Agents of erosion1.Water2.Wind3.Gravity4.Ice

Erosion by water causes sediment to be transported as:

1. Bed load– Rolls or bounces along the bottom

2. Suspended load– Turbulence in water keeps smaller sized

particles in the water and off the bottom3. Dissolved load

– Rock material is dissolved into individual molecules and transported in solution.

Deposition1. Chemical deposition

– Dissolved molecules dissolved in water reaches saturation and begins to precipitate out of water.

– Makes a chemical sedimentary rock.

2. Clastic (detrital) deposition– Rock pieces being eroded without

being dissolved in water get deposited.– Makes a clastic sedimentary rock.

3

Lake Siltation

Clastic sediment size• Clay (mud)• Silt• Sand• Pebble• Cobble• Boulder

Clastic sedimentary rocksMud sized = mudstone or shaleSilt sized = siltstoneSand sized = sandstoneRounded pebbles = conglomerateSharp angular pebbles = breccia

4

5

Environments of depositionclastic

• Need high energy to move large particles, and need low energy to deposit small particles.

• Pebble sized and larger deposited in floods, debris flows, and mountain streams

• Sand sized found at the base of mountainous areas, fast moving stream beds, and beaches.

• Silt must have slow moving or still water to deposit, usually near shore.

• Clay must have still water to drop out, usually deep calm water far from shore.

6

Lake Bonneville flood gravels underlying Lake Missoula Flood Deposits

7

Chemical Sedimentary Rocks• Carbonates

– CaCO3 Limestone– (Ca,Mg) Dolostone

• Silicates– SiO2 Chert (including flint, jasper, and (including flint, jasper, and

agate) agate) •• EvaporitesEvaporites

–– Rock SaltRock Salt–– Rock GypsumRock Gypsum

Environments of depositionchemical

• Evaporites require a body of water with high evaporation rate.– Usually need a “closed basin” where water

can run in but can’t run out.• Limestone forms very slowly and will get

overwhelmed by any clastic particles– Need water with no clastics either far from

shore or in areas with no clastic input.

8

9

What happened to Lake Peigneur?

+

+

+ + + + ++

++ +

++

++ +

++ + +

++

+ +

Sedimentary particle texture

• Sorting–Poorly sorted, moderately sorted, well

sorted• Size

–Clay, silt, sand, pebble, cobble, boulder• Roundness

–Angular, subangular, subrounded, rounded, well-rounded

Particle sortingDegree to which particles are of the same

size.• Well sorted = particles all of the same size• Moderately sorted = particles within a

relatively narrow range of sizes• Poorly sorted = particles of very different

sizes.– Bimodal sorting = particles of two distinct

sizes.

10

Information from sorting• Well sorted = constant energy level• Moderately sorted = energy

fluctuates within a certain range• Poorly sorted = energy levels

fluctuates drastically over time or the material has been dropped all at once (as by melting ice).

Information from rounding and size

• Rock clasts usually start out large and angular.

• As they are eroded, sharper edges get worn down quickly.

• The farther clasts have been eroded from the site of weathering, the rounder smaller they tend to be.

Lithification/Diagenesis

1.Compaction2.Cementation3.Recrystallization

11

Sedimentary patterns and structures

• Rhythmic layers = repeated sequences of sedimentary rock

• Cross bedding = lineation inside layers of rock that go across the rock

• Ripples• Graded bedding = layer of sedimentary

rock with coarse clasts at the bottom fining upward

• Raindrop impressions• Mudcracks

Sandstone

Shale

Limestone

Shale

Sandstone

ShaleLimestone

Conglomerate

12

13

14

15