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Earth Materials: Sedimentary Rocks

• What we can learn from sedimentary rocks

• Types of sediment

• Classification of sedimentary rocks

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• Sedimentary processes: weathering, erosion, transport, deposition, etc.

• Depositional environments & sedimentary structures

Sediments and Sedimentary rocks

Why study sediments and sedimentary rocks?• Most of the earth’s surface is covered with sediments.• They record past environmental conditions and much of the

history of the earth.• They contain fossil evidence of past life.

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• Some natural resources are of sedimentary origin or are foundprimarily in sedimentary rocks: i.e. coal, oil, natural gas, etc.

Three main categories of sediments and sedimentary rocks:• Clastic (also called detrital)• Chemical• Biochemical

Rock composed of clastic sediment

Rock composed of chemical sediment

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Chapter 7 Understanding Earth

Rock containing biochemical sediment

Sediments and Sedimentary rocks

Classification of clastic sedimentary rocks:

Texture

Maximum grain size: fine, medium, coarse

S ti ll t d l t d

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Sorting: well-sorted, poorly-sorted

Grain shape: rounded, angular

Composition

what the grains are composed of:

i.e. quartz, feldspar, clay, rock fragments, etc.

Well-sorted, angular, quartz sand (magnified) Fig 7.2 Understanding Earth

Poorly sorted sediment (mixture of sand and gravel) with rounded fragmentsGeology - Chernicoff

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Sediments and Sedimentary rocks

Classification of clastic sedimentary rocks by texture:

Texture Rock Type

coarse rounded Conglomerate

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coarse, rounded Conglomerate

coarse, angular Breccia

medium Sandstone

fine Siltstone, Mudstone,

Claystone, or Shale

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The four major types of sandstones:an example of classification by grain composition

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Fig 7.16 Understanding Earth

Classification of chemical and biochemical sedimentary rocks by composition

Composition Rock Type

calcite (may also contain some mud) limestone (several types)

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plant remains coal or peat

silica chert

halite rock salt

gypsum rock gypsum

Three examples of limestone

Chalk - a type of fossil-bearing limestone

Fossiliferous limestone

Fig 7.19 Understanding Earth 9

Geology - Chernicoff

Rock salt (halite) - a chemical sedimentary rock formed by evaporation of water

Rock gypsum -a chemical sedimentary rock formed by evaporation of water

Chert - a sedimentary rock formed from silica sediment. Chert may be chemical or biochemical in origin

Fig 7.19 Understanding Earth

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Fig 7.19 Understanding Earth

Fig 7.19 Understanding Earth

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Relative abundance of major sedimentary rock typesFig 7.14 Understanding Earth

Sedimentary processes:

Weathering•physical & chemical

•rates of weathering

•importance of minerals and climate

E i d T t ti

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Erosion and Transportation

Deposition

Burial and compaction

Diagenesis and lithification

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Processes involved in the formation of sedimentary rocks:Weathering, Erosion, Transportation, Deposition, Burial, and DiagenesisFig 7.1 Understanding Earth

The Grand Canyon, a landscape shaped by weathering and erosionand the source of a huge amount of sediment

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S.

Weathering and Erosion

Weathering

- includes all processes which break down rocks at the earth’s surface

- there are two general types of weathering:

• physical (mechanical) weathering

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p y ( ) g

• chemical weathering

Erosion

- includes processes which remove weathered material from its source

- water, wind, and ice, for example, can erode weathered material

Weathering and Erosion

Physical (mechanical) weathering

- breaks rocks into smaller pieces

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Examples:

• abrasion by particles carried by wind, water, or ice

• fractures created or widened by changes in pressureor temperature or by root action

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Abrasion by glaciersGlaciers typically have many fragments of rock embedded in the ice. When the ice slides along, these rock fragments grind against the bedrock beneath the glacier. This wears down underlying rock and creates smoothed and grooved surfaces. S. Kuehn

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Abrasion by flowing waterWater typically carries sand and gravel. These particles bump into each other and larger rocks wearing them down over time.

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Mechanical weathering caused by growing rootsPlant roots can grow into tiny fractures in rocks. As the roots grow over time, the fractures become wider.Fig 6.12 Understanding Earth

Mechanical weathering by frost actionFreezing and thawing widens fractures over time. Pieces can then fall off the cliff and move down the slope.

Mount Rainier andesite

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lava flow at Burroughs Mountain, Mt. Rainier National ParkS. Kuehn

Animation:

Frost Wedging

Mechanical weathering by exfoliation (sheeting)Exfoliation occurs as rocks expand when brought to the surface from deep in the crust where pressures are much higher than at the surface.

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Animation:

Uplift and erosion of a volcanic arc

.Photos: exfoliating granite in Yosemite National Park -

Fig 6.14 Understanding Earth (right) & S. Kuehn (left)

Weathering and Erosion

Chemical weathering

- breakdown of minerals by chemical reactions toform dissolved material and/or other minerals

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such as clay

Examples:

Dissolution - dissolves in water

Oxidation - oxygen is added

Hydration - water is added

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The brown stain on the surface of this sandstone is called desert varnish. It is produced by chemical weathering and evaporation. S. Kuehn

Rates of Weathering and Erosion

type of mineral or rock

surface area

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climate

acids

time

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Understanding Earth

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Understanding Earth

Time

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Different rates of weathering of slate and marble. Marble is composed of calcite which dissolves easily in acidic water. The faster weathering of the marble headstone is apparent in the blurring of the text. Fig 6.1 Understanding Earth

Weathering along natural fractures in rocks

Photos of weathered granite at Yosemite National Park

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Effects of sediment transport: Grain shape related to the duration of transport

Grains transported by water or wind become rounded over time

Angular grains Rounded grainsFig 7.3 Understanding Earth

Sediments and Sedimentary rocks

Depositional environments:

-where

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- what / how

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Some common sedimentary environments31

Fig 7.5 Understanding Earth

Interpreting the depositional environment

Grain size: fine, medium, coarse

Sorting: uniform size or mixed sizes

S di t t t

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Sedimentary structures:

- sedimentary layering – expressed as changes in sediment size and/or type

- ripples

- cross-bedding

- mud cracks

- burrows and tracks (bioturbation, trace fossils)

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Sedimentary layersCh 7 Understanding Earth

Modern ripples on a beach (below) and ancient ripples preserved in sandstone (left)Understanding Earth

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Cross-bedding in sandstoneThese sediments were deposited in desert dunes.Understanding Earth

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Mud cracks: modern (left) and ancient (right)Understanding Earth

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Sediments and Sedimentary rocks

From sediment to rock, changes after deposition:

- dissolution

- re-crystallization

- compaction

- formation of new minerals

- cementation

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Fig 7.13 Understanding Earth