Intro to Sedimentary Rocks

8/6/2019 Intro to Sedimentary Rocks

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GEOL110

Introduction to Sedimentary Rocks


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Sedimentary rocks

Sedimentary rocks are

essentially the consolidation(or lithification) of sediments.

Commonly composed ofquartz, feldspars, calcite &

clay minerals. Can also contain other rock

fragments & fossils.

Contain evidence of past

environments. Economic importance, as they

may contain: coal, petroleum &natural gas, ore deposits, etc.


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Sedimentary Rocks

Are the most common rock type on Earths land surface

Deposited in a large variety of settings terrestrial & marine

Contain many clues to the character of the sediments source area and

environment of deposition

~5% (by volume) of theEarths crust

~75% of rock outcropson the continents are

sedimentary


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Sediments and Sedimentary Rocks

Sedimentary rocks are composed of: particles (mineral grains & rock fragments) derived from weathering &

erosion of pre-existing rocks clastic

precipitates from solution chemical

consolidated fossil fragments/remains of plants & animals biogenic

volcanic ejecta pyroclastic

Most sedimentary rocks form from loose grains of sediment,for example:

Sand on beaches

Mud on a lake bottom Gravel in streams

Boulders frozen in ice (glaciers)

Dust settling out of the air

Volcanic ash and debris from volcanoes


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Turning sediment into rock

Sediment is unconsolidated grains are separate

(unattached) from one another.

Many changes occur to sediment after it is deposited.

Diagenesis = chemical, physical & biological changes thattake place after sediments are deposited.

Diagensis includes:- Recrystallisation: development of more stable minerals from less stable

ones, (common in limestones)

- Lithification: sediments are transformed into solid rock by compaction &cementation


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Lithification Compaction:

- As sediment accumulates, the weight of overlying materialcompresses the deeper sediments

- As the grains within the deeper, buried sediment arecompressed, there is a considerable reduction in pore space(the open space between particles) [For e.g., the volume of buriedclay-rich sediments can be reduced by as much as 40%]

- Decreasing pore space drives out any remaining water within

the sediment

- Coarser sediments (e.g. sand & gravel) are lesscompressible, thus compaction is a more common lithificationprocess in fine-grained sedimentary rocks (e.g. mudstones).


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Lithification

Cementation:- Chemical diagenetic change that involves the precipitation of

minerals among individual sediment grains.

- Saturated solutions (i.e., dissolved minerals in water)percolate through pore spaces between the sediment grains.

- Over time, the mineral/s precipitate out of solution, fills theopen pore spaces, thus cementing the grains together.

- The most common cements include quartz (silica), calcite(calcium carbonate) and iron oxide.


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Sedimentary strata

Beds series of visible layers in the rock

normally horizontal (cross bedding is the exception)

Bedding plane is the relatively flat surface of deposition

surface often displays sedimentary structures and fossils

Laminations thin (


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Bedding (strata)

beds

bedding planes


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Classification of Sedimentary Rocks

Sedimentary rocks are classified on the basis ofthe type of sediment they contain.

Four major groups of sedimentary rocks:

Clastic or detrital (terrigenous = terrestrial source rocks) Biogenic

Chemical Pyroclastic


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From Weathering to Clastic Sedimentary Rock


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Components of Clastic Sedimentary Rocks

Grains (also called clasts) Mineral, fossil shell or plant fragments Particles of rock

Matrix or Cement Finer grained material that binds the grains together


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Classification of clastic sedimentary rocks

(0.004)

(0.004-0.06)


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Sedimentary structures Most common in clastic

sedimentary rocks.

Important indicators ofenvironmental conditions.

Form whilst the sediment isunconsolidated.

Common structures include: cross

bedding, ripple marks, mud cracks,etc


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Biogenic (or organic) Sedimentary Rocks

Composed (substantially) of the fossilised remains of animals or plants.

Sediment with a high organic component (biogenic sediment) that hassubsequently lithified.

Chert, coal, limestones, chalk, etc.

CoquinaLimestone containing brachiopod fossils


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Chemical SedimentaryRocks

Chemical precipitation ofminerals from a saturatedsolution.

Examples include:

Evaporites (deposits that formfrom a saline solution as a result ofextensive water evaporation)

Banded iron formations (BIFs)

Inorganic limestones (e.g. ooliticlimestone)

Banded iron formation (BIF)

www.pibweb.com


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Pyroclastic Rocks

Composed of: volcanic fragments ejected

during an eruption

felsic minerals, typicallyquartz and feldspar, but also

glassy material such aspumice

Highly viscous magmacontaining volatiles (e.g., gas

under high pressure) resultsin explosive eruptions

[Analogous to a shaken softdrink bottle]

Mt Pinatubo, Philippines - 1991

Pumice


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Fossils Remains of once living organisms.

Important indicator of the depositional environment.

Different species usually inhabit specific environments.

Important for relative dating biostratigraphy.

Can also be important indicators of past climates & continentalreconstruction.

Can only be preserved & found in sedimentary rocks!!!

TrilobiteFossil leaves - GlossopterisBrachiopod


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Fossils Not all sedimentary rocks

contain fossils.

Preservation of fossilsdepends on a number offactors.

Organisms with high

preservation potential:- Live in a marine environment

(relatively constant & quicksedimentation avoiding physical& biological destruction)

- Deposited under low energy & lowoxygen levels leading toextraordinary preservation (softparts)

- Morphology: possess hard parts(e.g., shell, bone, etc.)


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Examples of extraordinary preservationFish from Solnhofen, Germany (Jurassic: 150 Ma) Insect in amber (Canada; Cretaceous: ~100 Ma)

Ichthyosaur giving birth (Germany; Jurassic: 175 Ma)


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Fossils

Organisms withlow preservationpotential:

- Live in the terrestrial realm,which has fewer suitablesedimentary environments greater chance of

physical & biologicaldestruction

- Morphology: no hard parts(i.e., no biomineralisedstructures)

aydin.net


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Fossilisation example

1. A dinosaur dies at a

stream's edge.

2. The carcass is quicklysubmerged by seasonal

flooding.The river carries sediment,which begins to bury thecarcass.


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Fossilisation example (cont.)

3. The flesh decomposes and

the skeleton is buried in alayer of sediment. (Oftendisarticulated prior to thisstage).

4. Over millions of years thesediment lithifies and bones

are slowly replaced byminerals percolatingthrough the surroundingrock (permineralisation).


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Fossilisation example (cont.)

5. Eventually the rocksabove the skeleton are

eroded, and the fossil isexposed (and hopefullyfound!).

6. Specimen is prepared,studied/described and

restored to providemore information aboutancient life on Earth.


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Upcoming lectures on sedimentary rocks

Clastic sedimentary rocks & sedimentary structures

Biogenic sedimentary rocks

Chemical sedimentary rocks

Pyroclastic rocks

Sedimentary environments

Essential reading:TEXTBOOK Earth: Portrait of a Planet (3rdEdition):

- Chapter 7: A Surface Veneer: Sediments, Soils, andSedimentary Rocks, pages 183-192 + 198-227 only.

- Chapter 9: The Wrath of Vulcan: Volcanic Eruptions, pages269-281 only.

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