Sedimentary Rocks

ABOUT THREE-FOURTH OF THE SURFACE OF THE CONTINENTS ARE COVERED BY SEDIMENTARY ROCKS. THEY ARE ALSO IMPORTANT, BECAUSE THEY HAVE a huge ECONOMICAL IMPORTANCE because resources SUCH AS COAL, LIMESTONE, OIL, GAS ARE stored SEDIMENTARY ROCKS.

Sedimentary Processes

1.Sediment is forming by physical and chemical weathering.

What is the major difference between Physical and chemical weathering?

Physical:

Fragmentation of the original rocks by physical processes such as heat difference, water freezing, roots and so on. We will be able to describe the original, parent material.

Chemical:

The original rock composition will go through essential chemical changes, and new sedimentary rocks are forming. No way to recognize parent material.

Sediment is the collective name for loose, solid particles originate from:

          1.weathering and erosion of preexisting rocks

2.chemical precipitation from solution, including organic secretion from water

          Sediment is unconsolidated, loose material.

2. Transportation

Second, very important process in forming sedimentary rocks is transportation.

          while transported sediment particles became rounded

          Slide 1

After Annabelle Foos

Sediment can be transported by:

          water, good sorting

          wind, perfect sorting

          ice, very poor sortingSlide 1

                                            After Annabelle Foos

 

3. Deposition

When transported material settles down or come to rest deposition occurs. It is in an other word Accumulation

 

4.Lithification (diagenesis)

Group of processes that turn sediment into sedimentary rock

       Compaction (weight of overlying sediment)

       Cementation (by precipitating minerals, such as calcite, quartz, hematite)

Sedimentary structures

Primary sedimentary structures

              stratification

              ripple marks

              cross-bedding

              mud-cracks

              graded bedding

They formed by sedimentary processes, such as currents (flow, eolian, glacier), during sedimentation.

Sedimentary layers:  strata, or bed

The plane separating the layers are:  bedding plane

       visible bec. color change

       grain size change

 Slide 1

After Annabelle Foos

Stratification: larger scale with lots of changes

Cross Bedding

Inclined layers compare to the bedding plane. Their thickness is very different (from some cm, to 100’s of m) depending from the material and current speed.

Slide 1

After Annabelle Foos

Current can be wind or river:

 Slide 1

Wind made crossbedding:After Annabelle Foos

Can be symmetrical (herring bone cross bedding (tidal current), or

Asymmetrical Unidirectional flow

Ripple marks,

Slide 1

After Annabelle Foos

Slide 1

Typical current ripples:After Annabelle Foos

Mudcracks,

 Slide 1

Ancient mudcracks

Graded bedding

Slide 1

After Annabelle Foos

Progressive upward decrease in grain size

Deep ocean floor by turbidity currents (from continental slope to the adjacent ocean basin)

Slide 1

 After Annabelle Foos

Trace fossils, fossils

 Slide 1

After Annabelle Foos

 Slide 1

After Annabelle Foos

Sedimentary Environments

Terrestrial

Marine

The shore zone is transitional environment

Classification of sedimentary rocks

Clastic sedimentary rocks:

Most common minerals: Clay min+ Quartz, because they are extremely  durable.

Classification based on grain size:

grain chart

Gravel: >2 mmm

Sand: 2 mm-0.063 mm

Silt: 0.063-0.005mm

Clay: < 0.005mm

       shape

 Slide 1

Conglomerate

       Rounded gravel size particles. They are large enough to identify them separately

       Usually poorly sorted: pebbles with sand and mud

  

From:(http://www.otago.ac.nz/geology/features/rocks-minerals/conglomerate.jpg)

Breccia

      Angular gravel size particles. They are large enough to identify them separately

       Usually poorly sorted: gravel with sand and mud

  

Photo by Ray Nordeen, NPS

Sandstone

       20% of all sedimentary rocks

       There are a lot of different types of sandstone. They are very and useful for geologists because they give information about the processes they went through, therefore we cand find information about ancient mountains, climate, plate tectonic setting. Characteristics we can use:

    1.sorting (indicates transportation distance and material)

    2. Grain shape (indicate distance)

    3. Mineral composition (indicate original rock composition, and the geology of the area )

Some important types:

Arkose: Contains k-feldspar, quartz

from:http://www2.vscc.cc.tn.us/svinson/geo100/library/sedimentary/large/arkose.jpg

Graywacke: contains lithic(rock) fragments, clay. Usually have poor sorting.

From:http://www.drexel.edu/coe/enggeo/rocks3/graywacke.JPG

Quartz sandstone: contains almost completely quartz

from:http://www.radford.edu

Siltstone

Silt-sized sediment grains. 

from:www.coconino.edu/kmullins/ rocks/Sedimentary/

Shale (Mudstone)

Clay sized particles

       Half of all the sedimentary rocks

       Very fine grain-size indicate quiet environment, mostly below wave base (even there they would deposit as aggregates). Their color gives important clues of the environment they formed in.Black color indicates: organic matter (anaerobic environment). Light color means it was formed in oxygen rich environment.

Originally clay has huge pore space filled with water. When it drays later it might form    mudcracks, if close to the surface.(mudcrack)

Small grainsize cause tight packing (water can not penetrate)

Fissility forms when clay splits into thin layers with well defined, closely spaced plains.

      

from:http://easyweb.easynet.co.uk/~gcaselton/fossil/images/rock-04.gif

The rock which lack fissility is termed mudstone.

      

from:http://www2.vscc.cc.tn.us/svinson/geo100/library/sedimentary/large/mudstone.jpg

Shale will never make cliff outcrops, because when wet it  erodes easily. Gentle slope is formation is cheracteristic.

Chemical and organic rocks

Carbonates

Limestone (CaCO3)

They effervescences with hydrochloric (HCl) acid

       10% of all the sedimentary rocks

       Tropical Marine environment.

       coral reefs

       other animals

Coquina

Composed of shell fragments . Formed in high energy environment; close to the shore.

Oolitic limestone

 Composed of thousands of tiny balls (concentric calcareous ooliths). Their presence indicate high energy arid environment; close to the shore.

Fossiliferous limestone

 

from:http://www.gc.maricopa.edu/earthsci/imagearchive/NACO_HORN_CORAL_big.jpg

Fine grained limestone with visible fossils.

 

from:http://www.eos.ubc.ca/courses/eosc221/images/sed/carb/pic/mud2.jpg

Lime mudstones are composed of clay sized carbonate particles.

Chalk

Contains microscopic organism (white cliff of Dover England)

http://www.bbc.co.uk/kent/places/towns/dover/images/white_cliff_270.jpg             

Travertine (in caves.And freshwater)

 

from:http://userpage.fu-berlin.de/~voelker/Vorlesung_Chemische/Abbildungen/sinter.jpg

Forms in freshwater around plants, or in caves.            

 

DOLOMITE Ca,Mg(CO3)2

Forms from preexisting limestone under arid condition. Will not effervescence.

Chert

mycroscrystalline silica

        flint Black

        jasper red

        banded  agate

Irregular bodies or layers in limestone

It can be inorganic and organic (radiolaria, diatoms)

 

from:www.beg.utexas.edu/.../ graphics/chert.htm

Evaporites

They form in desert environment. They precipitate in order of their solubility;

Gypsum

Anhydrate

halite

sylvite

Coal

Coal forms gradually by burial from undecayed plant material. There are four basic type along  the evolution of coal: peat, lignite, bituminous coal, and anthracite. 

Peat (brown spongy mass of plants)

65% carbon

from:www.msnucleus.org/.../ lesson2/mining2b.html

Lignite

75% carbon

www.msnucleus.org/.../ lesson2/mining2b.html

Bitomenous coal

85% carbon

from:http://go.owu.edu/~jbkrygie/krygier_html/geog_111/geog_111_lo/geog_111_lo07_gr/bituminous.jpg

Antracite

95% carbon

from:http://go.owu.edu/~jbkrygie/krygier_html/geog_111/geog_111_lo/geog_111_lo07_gr