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Sediments & Sedimentary RocksSediments & Sedimentary Rocks

Bryce Canyon, Utah

07_02a.jpgGrand Canyon

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Weathering: the processes that break up and corrode Weathering: the processes that break up and corrode solid rock solid rock -- two end members: physical and chemicaltwo end members: physical and chemical

Physical (or mechanical) Weathering - results in unconnected chunks, classified by size:

Boulders >256 mm

Cobbles 64-256 mm

Pebbles 2-64 mm

Sand 1/16 - 2 mm

Silt 1/256 - 1/16

Mud < 1/256

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Sierra Nevada - exfoliation joints

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joint

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talus

07_06a.jpgFrost wedging -

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root wedging salt wedging

07_07ab.jpgChemical Weathering

Dissolution - halite (NaCl) in H2O and

limestone (CaCO3) in acidic water

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Dissolution - in

limestone (CaCO3)

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Hydrolysis - chemical reaction of water with minerals to loosen crystal lattice - more efficient in acidic water - example potassicfeldspar in granite breaks down into kaolinite, a clay

Oxidation - chemical reaction in which an element looses an electron (often by combing with oxygen) - one effect is a change in size of atoms and loosening them in crystal lattice

e.g. 4Fe2+ + 3O2 --> 2(Fe3+)2O2 -- “rusting”

iron oxygen hematite

Hydration - absorbing water into the crystal lattice, especially clay causes expansion

Relative stability of minerals

Minerals that crystallize at high temperatures are less stable at surface temperatures than low temperatures minerals Olivine < pyroxene

Minerals with fewer links between silica tetrahedra have weaker crystal structures

Minerals containing Fe, Mg Na, K and Al weather faster than those without them

Thus quartz (pure SiO2) with strong 3D network bonds in all directions & a low temperature of crystallization is one of the most stable minerals - ends up as “mineral of choice” in beaches

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Positive feed-back cycle: mechanical weathering increase rate of chemical weathering

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Spheroidal blocks of granite in Joshua Tree National Monument California

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Variable resistance to weathering

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granite marble

07_12.jpgSoils

leaching

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Soil Soil horizons horizons ((layerslayers))

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Clastic Sedimentary Rocks 07_17.jpgLithification

Describing and Classifying Clastic Sedimentary Rocks

Clast size

Sorting (similarity in size)

Angularity and sphericity

Clast composition

Character of cement

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Mixed grain size

One grain size

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Sedimentary breccia

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Stream gravel

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Conglomerate made from stream gravel

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Arkose (light clasts are feldspar)

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Photomicrograph of sandstone - quartz grains

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Sandstones - Mesa Verde, Colorado

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Hand specimen of sandstoneRainbow Bridge - Navajo Sandstone

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07_19j.jpgsandstone

shale

07_19k.jpgScanning electron microscope image of clay flakes ~ 2 microns across

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Coral reef --->limestone

Biochemical & Organic Sedimentary Rocks

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Lime mud (micrite) & fossiliferous (reef) limestone fossiliferous limestone

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Biochemical chert (SiO2) (- San Francisco -formed from plankton (radiolaria and diatoms) - folded 145 m.y. ago during subduction of part of Pacific Plate

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Coal seam >50% carbon from plant material

Chemical Sedimentary Rocks

Oil - carbon from protein and fats of plankton buried in marine sediments

07_23ac.jpgChemical

Sedimentary RocksFormed by precipitation of

minerals out of water solutions

Evaporation removes only water

Evaporation:

~80% of seawater - gypsum

>90% of seawater - halite

Evaporate all of the seawater -get a total of 80% halite, 13% gypsum

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Playa lake

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Salt mine

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Travertine - CaCO3 - cave in Blue Ridge Mtns, Virginia

Sedimentary Structures

Layers

Markings on surface

Arrangement of grains within layers

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Single layer with recognizable top & bottom = bedBoundary between layers = bedding plane

Several beds together = strata Arrangement of beds = stratification

Study of strata = stratigraphy One who studies earth history as preserved in strata = stratigrapher

07_27.jpgIn this photo of the Grand Canyon, we can see five formations. Formations and groups are examples of stratigraphicunits. Note that each formation consists of many beds, and that beds range greatly in thickness. The boundaries between units are called “contacts.”

07_28ab.jpgFeatures within layers

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Modern ripples on beach

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1.5 billion year old ripples (Wisconsin)

Migrating dune, near Lee’s Ferry Arizona07_29a.jpg

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Graded bedding

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Mud cracks

07_32a.jpgSedimentary Environments

Glacial till, New Zealand

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Mountain stream -deposit conglomerate

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Alluvial fan - California

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07_33b.jpg 07_33c.jpgRiver environment - channel-fill deposit at Sugarloaf, Sunderland, Ma

07_34a.jpgSunderland Delta

Topset beds

Foreset beds

Junction of the topsetand foreset beds at deltas like the Sunderland, up and down the valley tell us the lake level of Glacial Lake Hitchcock 16,000 years ago.

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Sedimentary deposits of deltas

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Plankton shells

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White cliffs of Dover, England - made up of chalk

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Transgressive sequence (sea level rises)

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Transgression - regression

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Sedimentary Basins (sediment-filled depressions)

Rift basins - Continental rifts - stretched lithosphere

Amherst

Passive-margin basins - e.g. east coast of North America

Intracontinental basins - e.g. Michigan basin (Devonian, salt, hydrocarbons)

Foreland basins - on continental side of mountain belts

All provide environment for diagenesis - bioturbation, growth of minerals in pore spaces, replacement of crystals, pressure, compaction and growth of cement to complete lithification

When blowing sand builds into sand dunes in a desert, the sand tumbles up the windward side of the dune, and settles in quieter air on the leeward side. This animation shows how cross beds develop during the deposition of sediment.

PC version Mac version

Formation of Cross Beds

As sea level rises the coast migrates inland (transgression) and retreats seaward (regression), and a record of this movement is preserved in the strata of the sedimentary basin. View 1 shows how this sedimentary sequence is formed; View 2 examines a segment of the landscape millions of years later, after the land has been uplifted and erosion has occurred.

PC version Mac version