Sedimentology & Stratigraphy

Thanks to Rob Viens for slides

SedimentologyThe study of the processes that erode,

transport and deposit sediments

Sedimentary PetrologyThe study of the characteristics and origin of

sedimentary rocks.

StratigraphyThe study of the origin, relationship, and

extent of rock layers (strata).

Sedimentary Rocks on Earth

Shale Sandstone Siltstone LimestoneConglom.

What do Sedimentary Rocks Record?

Magma

Igneous Rocks

Sedimentary Rocks

Metamorphic Rocks

Sediment

Crystallization Melting

Metamorphism

Lithification

Weathering

Erosion

Transport

Deposition

What do Sedimentary Rocks Record?

•Source (Provenance) of sediment•Erosion and Transport Agent•Depositional Environment•Paleogeography/Tectonic Setting•Diagenesis (what happened after deposition)

Rock Identification is based on:

• Composition

What minerals make up the rock?

• Texture

What is the shape, size and orientation of the mineral grains that make up the rock?

Major Difference:Crystalline vs. Clastic

Types of Sedimentary RocksDetrital Chemical

OrganicClastic

Texture CrystallineTexture

Detrital Rock Composition

Detrital Rocks• Lithic Fragments• Quartz• Clay Minerals• Fossil Fragments• Iron Minerals• Orthoclase (K-spar)• Muscovite / Biotite

Note About Sediment Composition:Weathering

The sediments that make up sedimentary rocks are produced by:

Mechanical & Chemical Weathering

Mechanical Weathering• Frost action

– Mechanic effect of freezing (and expanding) water on rocks

• Pressure release– Removal of overlying rock allows

expansion and fracturing

• Plant growth– Growing roots widen fractures

• Burrowing animals• Thermal cycling

– Large temperature changes fracture rocks by repeated expansion and contraction

Mechanical Weathering

But mostly physical weathering is a matter of things just falling down. So in a sense, gravity, is the primary cause of physical weathering.

Chemical Weathering• Oxidation

– Chemically active oxygen from atmosphere– Iron oxides are common result

• Soil and sedimentary rocks often stained with iron oxides

• Acid dissolution– Hydrogen cations replace others in minerals– Carbonic acid from atmospheric CO2

dissolved in water– Sulfuric, hydrofluoric acids emitted by

volcanic eruptions– Some minerals, such as calcite, may be

totally dissolved– Human activity, such as mining and

burning of fossil fuels, produces acids

Chemical Weathering

Chemical Weathering• Feldspars

– Most common minerals in crust– Slightly acidic rain water

attacks feldspar– Clay minerals produced

• K+, Na+, Ca++ ions released into water

• Other minerals– Ferromagnesian minerals

• Clays, iron oxides, Mg++ ions produced– More complex silicate bonds lead to lower weathering susceptibility

• Olivine most susceptible, quartz least

• Warm, wet climatic conditions maximize weathering

MineralStability

Products of Weathering

• Lithic (Rock) Fragments(granite, basalt, schist, etc.)

• Dissolved Ions(Calcium, Potassium, Sodium, etc.)

• Rust Minerals (Hematite, Goethite, etc.)• Clay Minerals

(Bentonite, Montmorillonite, etc.)• Residual Minerals

(Quartz, Orthoclase, Muscovite, etc.)

Sediment• Sediment - loose, solid particles originating from:

– Weathering and erosion of pre-existing rocks– Chemical precipitation from solution, including

secretion by organisms in water

• Classified by particle size – Boulder - >256 mm– Cobble - 64 to 256 mm– Pebble - 2 to 64 mm– Sand - 1/16 to 2 mm– Silt - 1/256 to 1/16 mm– Clay - <1/256 mm

Gravel

Distinguishing Characteristics of Clastic Sediments:Grain Size - mud/clay, silt (<0.06mm), fine sand (0.06-0.25mm),

coarse sand (0.25-2.0mm), pebbles (2-64mm), cobbles (64-256mm), boulders (>256mm); particle size reflects energy(velocity) of the transport and depositional system.

From Sediment to Sedimentary Rock

• Transportation– Movement of sediment away from its source, typically by

water, wind, or ice– Rounding of particles occurs due to abrasion during transport– Sorting occurs as sediment is separated according to grain size

by transport agents, especially running water– Sediment size decreases with increased transport distance

Distinguishing Characteristics of Clastic Sediments (cont.): Sorting - Well-sorted sediment indicates prolonged reworking by wind or water; poorly sorted sediment may indicate rapid deposition, or deposition by ice or mass movement.

Angularity/Roundness and Shape – Well rounded sediment also indicate prolonged reworking by transporting agent; the shape of grains often indicates the transport system, but also may be related to the type of mineral or rock fragment

Interpretation of Composition

Detrital Rocks:• Source of Sediment (Provenance)• Exposure to Weathering(Distance of Transport)(Type of Climate)

Note About What Causes Detrital Rock Color

• Grain SizeSmaller is often darker

• CompositionCarbon - black or brownQuartz - tan, clear, whiteOrthoclase - orange

• Depositional EnvironmentIron on land - redIron in deep, oxygen poor water - green

Chemical & Organic Sedimentary Rocks

(Names based primarily on composition)

Basic Rock Name• Limestone• Dolostone• Chert• Rock Salt• Rock Gypsum• Peat or Coal

Composition• Calcite• Dolomite• Quartz (silica)• Halite• Gypsum• “Carbon” / Plant

Remains

Limestone

CrystallineLimestone

Chalk

Fossiliferous Limestone

Formation of LimestoneCalcite Precipitates inWarm WaterWater Under Low Pressure (Shallow)

Or

It is secreted by biological organismsClams, Mussels, Scallops, etc.CoralsForaminifera

Limestone Represents:

Shallow Tropical Ocean

Or relatively ‘shallow’ open ocean, i.e. near shore or mid ocean ridges

http://geosci.uchicago.edu/~archer/classes/GeoSci238/

Chalk(Coccolithophores)

Black Sea

Oolitic Limestone

Travertine(Limestone)

Dolostone

Chert (Flint, Jasper, Agate…)

Diatom Skeletons

Chert Represents

Highly productive areas of the

Ocean (diatoms, radiolarians as beds) or near

shore (sponges as nodules)

Evaporites:Bonneville Salt Flats, Utah

RockGypsum

Rock Salt

MediterraneanGeology

Isolated,Arid,Salty

Sea/Lake

Peat and Coal

Swamp, Bog