ch 8 web notes

7/30/2019 Ch 8 Web Notes

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What is a sedimentary rock?Sedimentary rocks are products of mechanical and chemical

weathering

They account for about 5 percent (by volume) of Earths outer

10 miles, but 75% of the outcrops

They contain evidence of past environmentsClues in these rocks provide a window into the history of the

earths surface

Sedimentary rocks are important for economic considerations

because they may containCoal

Petroleum and natural gas

Sources of iron, aluminum, and manganese

Turning sediment into rockMany changes occur to sediment after it is deposited

Diagenesis all of the chemical, physical, and biological

changes that take place after sediments are depositedOccurs within the upper few kilometers of Earths crust

Diagenesis

IncludesRecrystallization development of more stable minerals from less stable

ones

Lithification unconsolidated sediments are transformed into solid

sedimentary rock by

Compaction

Cementation by calcite, silica, and iron oxide

Types of sedimentary rocks

Sediment originates from mechanical and/or chemicalweathering

Rock types are based on the source of the materialDetrital rocks transported sediment as solid particles

Chemical rocks sediment that was once in solution


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Detrital sedimentary rocksThe chief constituents of detrital rocks include

Clay mineralsQuartz

Feldspars

Micas

Particle size is used to distinguish among the various types of

detrital rocks

Common detrital sedimentary rocks (in order of increasing

particle size)Shale

Mud-sized particles in thin layers that are commonly referred to aslaminea

Most common sedimentary rock

SandstoneComposed of sand-sized particles

Forms in a variety of environments

Sorting, shape, and composition of the grains can be used to interpret the

rocks history

Quartz is the predominant mineral

Conglomerate and brecciaBoth are composed of particles greater than 2mm in diameter, larger

pieces are called clasts which are embedded in groundmass

The clasts provide clues to provenance

Conglomerate consists of large rounded clasts

Breccia consists of large angular clasts

Chemical sedimentary rocksConsist of precipitated material that was once in solution

Precipitation of material occurs in two ways

Inorganic processesOrganic processes (biochemical origin)

Common chemical sedimentary rocksLimestone

Most abundant chemical rock

Composed chiefly of the mineral calcite


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Marine biochemical limestones form as coral reefs, coquina (broken

shells), and chalk (microscopic organisms)

Inorganic limestones include travertine and oolitic limestone

Common chemical sedimentary rocksDolostone

Typically formed secondarily from limestone, consists of the mineraldolomite

ChertMade of microcrystalline quartz

Varieties include flint and jasper (banded form is called agate)

Common chemical sedimentary rocksEvaporites

Evaporation triggers deposition of chemical precipitates

Examples include rock salt and rock gypsum

Common chemical sedimentary rocks

CoalDifferent from other rocks because it is composed of organic material

Stages in coal formation (in order)

1. Plant material

2. Peat

3. Lignite

4. Bituminous

Classification of

sedimentary rocksSedimentary rocks are classified ac-cording to the type of

material

Two major groupsDetrital (transported as solid particles)

Chemical (transported as solutions, produced primarily by

chemical weathering)

Two major textures are used in the classification of

sedimentary rocksClastic

Discrete fragments and particlesAll detrital rocks have a clastic texture

NonclasticPattern of interlocking crystals

May resemble an igneous rock


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Sedimentary environmentsA geographic setting where sediment is accumulating

Determines the nature of the sediments that accumulate (grainsize, grain shape, etc.)

Types of sedimentary environmentsContinental

Dominated by erosion and deposition associated with fluvial (stream)

environments

Glacial

Eolian (wind)

MarineShallow (to about 200 meters)

Deep (seaward of continental shelves)Types of sedimentary environments

Transitional (shoreline)Deltas

Tidal flats

Lagoons

Transgression and RegressionTransgression (Rise in Sea Level) results in a sequence of deeper

environments upward in section

Regression (Lowering Sea Level) results in a sequence ofshallower environments upward in section

Sedimentary structuresProvide information useful in the interpretation of Earth

history

Types of sedimentary structuresStrata, or beds (most characteristic of sedimentary rocks)

Bedding planes that separate strata

Cross-bedding

Rock Beds

Sedimentary structuresTypes of sedimentary structures

Graded beds and Turbidites

Ripple marks


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Mud cracks, raindrop prints

Bioturbation

Fossils

Fossils include remnants of the organisms (Body Fossils) andtraces of their lives (Trace Fossils)Types of body fossils

Unaltered remains (freezing, tar, amber, mummification)

Recrystallization (eg aragonite to calcite)

Permineralization (filling of pores)Petrification (replacement)

Carbonization

Molds (original dissolved) and Casts (mold fill)

Trace Fossils include tracks, trails, burrows, borings, and feces

Fossil PreservationPreservation of individuals rare requires just the right sequence

of eventsQuick burial

Hard parts

However fossils are common and extremely important in

determining earth history

Nonmetallic mineral resourcesUse of the word mineral is very broad

Two common groupsBuilding materials

Natural aggregate (crushed stone, sand, and gravel)

Gypsum (plaster and wallboard)

Clay (tile, bricks, and cement)

Two common groupsIndustrial minerals

Corundum

Garnet

Energy resources from sedimentary rocksCoal


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Formed mostly from plant material

Along with oil and natural gas, coal is commonly called a fossil

fuel

The major fuel used in power plants to generate electricity

Potential environmental problems from mining and air pollution

Energy resources from sedimentary rocksOil and natural gas

Derived from the remains of marine plants and animals (mostly

not dinosaurs)

Both are composed of various hydro-carbon compounds and

found in similar environments

Oil trap geologic environment that allows significant amounts of

oil and gas to accumulate

Oil and natural gasTwo basic conditions for an oil trap

Porous, permeable reservoir rock

Impermeable cap rock, such as shale

Cap rock keeps the mobile oil and gas from escaping at the

surface

Consumption of energy in the US, 1998

Key Terms Chapter 8DepositionClastic (detrital) sediment

Chemical sediment

Biogenic sedimentLithification

Bedding (lamina)

Bedding surface (plane)Compaction

Cementation

Recrystalization

DiagenisisComglomerate

Sandstone

SiltstoneMudstone

Shale

ClayEvaporite


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Banded iron formation

Limestone (carbonite)

PeatCoal (lignite, bituminous, anthracite)

Delta

Eolian, lacustrine, fluvial sedimentsEstuary

Turbidity current (turbidite)

Rift valleyDepositional environment

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