Chapter 6

Sediments and Sedimentary Rocks

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Introductory Geology

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Sediments

Collective name for loose, solid particles originated from weathering and erosion, thus Unconsolidated.

Classification of sediments based on sizes

Gravel > 2mm

Sand between 1/16 and 2 mm

Silt between 1/256 mm and 1/16 mm

Clay < 1/256 mm

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Change from Sediments to Sed Rxs

Transportation

Roundness: round – distance from source

Sorting: well sorted - distance from source

Deposition: settle of particles

Preservation

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Change from Sediments to Sed Rxs

LithificationCompaction – reduction in pore space

Cementation – seal out the pores

Crystallization – growth of crystals from solution.

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Sedimentation Process

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What is Sedimentary Rock?Sedimentary rocks are products of mechanical and chemical weathering

Account for about 5 % (by volume) of Earth’s upper crust

Contain evidence of past environments• Provide information about sediment transport• Often contain fossils

Economic importance• Coal

• Petroleum and natural gas

• Sources of iron, aluminum, and manganese

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Sedimentary Rocks

Rock types

Classification

Sedimentary Environments

Fossils

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Classified according to material type• Clastic• Chemical

Two major textures are used in the classification of sedimentary rocks

• Clastic– Discrete fragments and particles– All detrital rocks have a clastic texture

• Nonclastic– Pattern of interlocking crystals– May resemble an igneous rock

Sedimentary Rock Classification

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Sedimentary Rocks Types

Sediment originates from mechanical and/or chemical weathering

Rock types are based on the source of the material

• Clastic 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 minerals• Quartz• Feldspars• Micas

Particle size is used to distinguish among the various types of detrital rocksParticle shape and sorting give indication of transport distance (maturity)

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Particle Size Classification

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Sorting, Shape, Setting

Sorting

SphericityRoundness

DepositionalSetting

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Clastic Sedimentary RocksCommon Clastic sedimentary rocks (in order of increasing particle size)

• Shale– Mud-sized particles in thin layers that are

commonly referred to as laminea– Most common sedimentary rock

• Sandstone– Composed of sand-sized particles– Forms in a variety of environments– Sorting, shape, and composition of the grains

can be used to interpret the rock’s history– Quartz is the predominant mineral

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Detrital Sedimentary Rocks

• Conglomerate and breccia– Both are composed of particles greater than

2mm in diameter

– Conglomerate consists largely of rounded gravels

– Breccia is composed mainly of large angular particles

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Shale containing plant remains

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Sandstone

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Conglomerate

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Breccia

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Chemical Sedimentary RocksConsist of precipitated material that was once in solution

Precipitation of material occurs in two ways

• Inorganic processes• Organic processes (biochemical origin)

Common chemical sedimentary rocks

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Chemical Sedimentary RocksCommon Types

Limestone

Dolostone

Chert

Evaporites

Coal

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Limestone• Most abundant chemical rock

• Composed of calcite carbonate CaCO3

• Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms)

• Inorganic limestones include travertine and oolitic limestone

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Coquina

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Fossiliferous Limestone

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Chemical Sedimentary Rocks• Dolostone

– Composed of calcite, magnesium carbonate CaMgCO3

– Typically formed secondarily from limestone

• Chert– Made of microcrystalline quartz

– Varieties include flint , jasper and agate (banded form)

• Evaporites– Evaporation triggers deposition of chemical

precipitates

– Examples include rock salt and rock gypsum

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ChertAgate – Banded Chert

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CoalCommon chemical sedimentary rocks

Different from other rocks because it is composed of organic material

Stages in coal formation (in order)• Plant material• Peat• Lignite• Bituminous

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Coal

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Coal Formation Stages

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Sedimentary Environments

A geographic setting where sediment is accumulating

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

Identified by sedimentary facies, fossils, and structures

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Sedimentary Environments• Continental

– Dominated by stream erosion and deposition– Glacial– Wind (eolian)

• Marine– Shallow (< 200 meters)– Deep (seaward of continental shelves)

• Transitional (shoreline)– Tidal flats

– Lagoons

– Deltas

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Depositional Environments

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Depositional Environments

Continental Marine

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Turbidity Currents

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Turbidity Currents

Graded Bedding

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Evaporite Formation

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Sedimentary FaciesDifferent sediments often accumulate adjacent to one another at the same time

Facies: possesses a distinctive set of characteristics reflecting the conditions in a particular environment

Merging of adjacent facies tends to be a gradual transition

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Sedimentary Facies

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Sedimentary Structures

Useful for interpreting sedimentary environment

• Strata: or beds, layers of similar sediment (most characteristic of sedimentary rocks)

• Lamina: thin layers (< 1cm)• Bedding planes separate strata• Cross-bedding• Graded beds• Ripple marks• Mud cracks

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Bedding

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Bedding Structures

Lamina Graded bedding

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Cross Beds

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Ripple Marks

unconsolidated sediment lithified rock

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Mud Cracks

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Grand Canyon

o

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Fossils: Evidence of past life

By definition, fossils are the traces or remains of prehistoric life now preserved in rock

Fossils are generally found in sediment or sedimentary rock (rarely in metamorphic and never in igneous rock)

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Dinosaur footprint in limestone

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Fossils: Evidence of past life

Geologically fossils are important for several reasons

• Aid in interpretation of the geologic past• Serve as important time indicators• Allow for correlation of rocks from

different places

Studied in detail in Historical Geology (GEOL 102), offered in Spring 2004

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Chalk Cliffs of Dover, England

Tiny planckon shells

Deposited on sea floorLater uplifted