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Sedimentation and Sedimentary Rocks

GLY 2010 - Summer 2014 - Lecture 10

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Sediment

• Unconsolidated material that accumulates at the earth’s surface

• Minerals and organic remains of plants and animals are the major components of soil

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Sedimentary Rock

• As sediment accumulates, pressure, and often temperature, increases

• Changes occur which convert the sediment from unconsolidated form to a consolidated form, sedimentary rock

• Sedimentary rock makes up 5% of the crust of the earth, but accounts for 75% of the rock exposed at the surface

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Importance of Sedimentary Rock

• Provide clues to the earth’s past – examples: Including erosion of mountain ranges Transgressions of the sea over the land

Fossils

• Often contain fossils, which provide clues to: Types of life

living in the past The environment

they lived in

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Types of Sediment

• Detrital

• Chemical

• Biogenic

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Sediment Transport

• Water, glaciers, or wind moves the sediment from high elevations to lower elevations, where it may accumulate

• Annual transport of detrital sediments to the oceans is about 10 billion tons

• During transportation, sediment size is often reduced

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Transport of Sediment

By rivers By glaciers

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Formation of Detrital Rock

• Deposition

• Sorting

• Shape

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Clastic Rock and

Matrix

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Lithification

• Literally means creation of stone

• Involves three possible processes Compaction Cementation Recrystallization

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Compaction

• Fragments will be compacted by the weight of accumulating sediment

• Air and water are expelled from spaces between grains

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Cementation

• Dissolved substances in water may precipitate solids which act as cements

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Recrystallization

• Unstable minerals may reorganize due to heat, pressure, and fluid interaction into more stable minerals

• Process must occur at low temperatures Ex: Aragonite Calcite

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Sedimentary Structures

• Bedding - sediments are ordinarily deposited in horizontal units called beds Graded bedding Cross-bedding

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Graded Bedding

Graded Bedding Animation

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Cross Bedding Animation

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Cross-bedding, Zion National

Park

Photos: Duncan Heron

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Principle of Original Horizontality• Most sediments settle through bodies of

water• They will be deposited in horizontal, or

very nearly horizontal, layers• Beds which are not horizontal have often

had their position changed by post-depositional processes

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Right Side Up

• Beds may occasionally be completely overturned, so we need ways to tell if beds are right side up

• Selected indicators: Ripple Marks Mudcracks Raindrop impressions Salt crystals

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Aeolian Ripple Marks

• Aeolian (wind) ripples at White Sands, New Mexico (Photo Yamato Sato)

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Fossilized Ripple Marks

• Parallel ripple marks preserved on a slab of sandstone

• The rocks contain the fossilized remains of marine animals - these ripples were formed in shallow sea water by gentle currents

• Fossilized ripple marks. Capitol Reef National Park, Utah

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Mudcracks

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Mudcracks

Mudcracks form by desiccation of mud or clay

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Raindrop Impressions

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Salt Crystals

• If the deposition occurs in the ocean, and the water is quite saline, salt crystals may precipitate and settle on the sediment

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Size Range of Detrital Particles

• Clay < 0.004 millimeters

• Silt 0.004 to 0.063 millimeters

• Sand 0.063 to 2 millimeters

• Granule 2 to 4 millimeters

• Pebble 4 to 64 millimeters

• Cobble 64 to 256 millimeters

• Boulder >256 millimeters

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Sediment Types

• Mud is composed of clay or silt

• Sand is composed exclusively of sand sized particles

• Gravel includes granules, pebbles, cobbles, and boulders

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Detrital Rock Types

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Shale

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Siltstone

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Sandstone with Silica Cement

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Sandstone with Hematite Cement

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Conglomerate

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Quartz Pebble Conglomerate

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Breccia

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Formation of Chemical Sedimentary Rocks

• Precipitation is the process of converting materials dissolved in a fluid (water or air) to another form

• Water dissolved in the atmosphere may precipitate as rain, or as some solid form such as snow, sleet, hail, etc

• Solids dissolved in water precipitate as solids

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Saturated Solution

• In order for precipitation to occur, the fluid must be saturated

• A saturated solution holds as much as it can of a particular substance A fluid may be saturated with respect to one

substance (e.g. lime) and undersaturated with respect to another substance (e.g. halite)

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Chemical Precipitation

• Solutions which are saturated, or slightly supersaturated, may spontaneously form crystals, which settle in the solution

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Biochemical Precipitation

• Some organisms have the ability to concentrate an unsaturated solution internally to the point where precipitation occurs

This fossiliferous limestone contains carbonate shells, produced by biochemicalprecipitation

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Evaporites• When saline solutions (sea-water) evaporate, a

series of substances precipitate in a definite sequence, from the least to the most soluble

• Sequence is: Lime – a carbonate Gypsum, a sulfate, precipitates second Halite, common table salt, is next Potassium and magnesium salts are last

Bonneville Salt Flats

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Layered Gypsum

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Evaporite Nodules

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Chemical Sedimentary

Rocks• Limestone is

composed of calcium carbonate, CaCO3

• Dolostone is composed of dolomite, CaMg(CO3)2

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Oolitic Limestone

• In warm, saturated, and highly energetic environments, lime may precipitate around tiny bits of suspended matter

• They stick together to form an oolitic limestone, such as the Miami oolite formation

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Oolitic Limestone

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Biochemical Sedimentary Rock

• Coral reefs

• Coquina

Ancient Marine Reef

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• El Capitan reef, Guadalupe Mountains National Park, formed as part of a very large reef complex during the Permian period

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Florida Coquina

• Castillo de San Marcos, St. Augustine, Fl

• Anastasia Formation coquina, used to construct the fort

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Organic Sedimentary Rocks

• Coal always forms from fresh-water environments

• Petroleum, liquid organic mater, is typically formed in salt-water environments

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Peat to Lignite

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Bituminous Coal

Anthracite

• Anthracite is actually a metamorphic rock55

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Organic chert

• Chert is a form of silica, SiO2

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Fossiliferous chert

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Sedimentary Environments

• Careful study of sedimentary rocks can often reveal information about the type of environment in which the sediment was deposited

• Useful in studying the earth’s history

• May reveal information about past climate conditions (paleoclimatology)

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Continental Deposits• Lake

deposits

• River deposits

• Glacial sediments

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Sandstone and Shale

• Interbedded layers of sandstone and shale

Western California

Turbulent Stream Channel

• Only coarse particles are deposited

• Fines move further downstream before deposition

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Coastal Sediments • Shallow marine environments consist of: Detrital material Carbonate-rich

deposits Abundant fossils

of plants and animals, because light penetrates shallow water

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Coastal Sediments

• In a marine environment, different types of sediment are associated with varying depths of ocean water

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Shallow Marine Environments• Wave-action pulverizes soft minerals and fossils• Pulverized material swept out to sea• Remaining material is well-sorted, rounded,

sand-sized deposits• Most of these deposits will be durable minerals,

such as quartz• Dominant rock in Florida is limestone, so we

have soft carbonate grains on our beaches as well

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Deep Marine Deposits

• Mainly remains of carbonate and silica microorganisms which die and settle to the sea-floor

• Submarine landslides may carry material off the continental shelf, and sub-marine volcanoes may contribute

• Landslide deposits are poorly sorted

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Unraveling Sedimentary Sequences• Large exposures of sedimentary

rocks often contain rocks with more than one depositional environment

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Geologic History

• Geologists study entire sequences of rocks to attempt to unravel geologic history

• Sedimentary rocks provide evidence for ancient mountains, long since eroded away

• These mountains are often evidence of plate collisions

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Marine Rocks in Mountains

• Many mountains contain sedimentary strata deposited in marine environments

• The earth has not lost enough water so that these mountains would have once been flooded

• Therefore, we believe that plate tectonic forces have lifted these former sea-beds miles high!

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Bokkeveld Shales

• Fossiliferous shales from the Groot River area, South Africa• Shales were deposited in a quiet marine environment

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Marine Fossils in Mountains

• Fossil: Stephanoceras sp. with different shells and one small snail

• Location: Sommerau, Swiss Jura Mountains

• Geologic age: Humphriesian-layer, Bajocian, middle Jurassic