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Sedimentary Rocks






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Sedimentary Rocks. John Day Fossil Beds Natl. Mon.,Oregon. Sedimentary rocks  composed of sediment (particles derived by weathering). Detrital sediments  particles derived by physical weathering
Sedimentary Rocks

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Sedimentary Rocks

John Day Fossil Beds Natl. Mon.,Oregon

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Sedimentary rocks composed of sediment

(particles derived by weathering)

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  • Detrital sediments particles derived by physical weathering

  • Non-detrital sediments minerals precipitated from solution by inorganic chem. processes or activities of organisms

    Classification of Sedimentary Particles

    SizeSediment Name

    >2 mm gravel

    1/16-2 mm sand

    1/256-1/16 mm silt

    <1/256 mm clay

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Why Should You Study Sedimentary Rocks?

  • Sediments are the most common Earth materials.

  • Rock Features record environmental conditions at the time sediment was deposited. (impt. in deciphering Earth history)

  • Fossils knowledge of pre-existent life preserved in sedimentary rocks

  • Some sed. rocks contain resources (i.e. water, petroleum, natural gas, etc.).

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Transport & Deposition

Ice

-- carries particles of any size.

Wind

-- transports sand & smaller particles.

Water (most prolific agent)

Larger particles require more vigorous current for transport.

  • Depositional environment geographic area where sediment accumulates

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well-rounded, poor-sorted gravel

angular, poor-sorted gravel

  • Rounding -- abrasion during transport reduces particle size & smoothes sharp corners

  • Sorting -- variety of particle sizes present in sediment or sed. rock (influenced by transport & depositional processes)

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Processes (Physical, chemical, & biological) operating in the depositional environment impart distinctive charac. to accumulating sediment.

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How Are Sediments Transformed into Sedimentary Rocks?

Compaction

  • due to weight of overlying sediment

  • particles pack more closely & pore space is reduced

  • deposit’svolume reduced

    Cementation

  • binds one particle to anotherby chemical precipitation of minerals in sediment pore space.

  • {Common cements include quartz, calcite, & hematite.}

    Lithification

  • involves compaction & cementation

  • converts sediments to sedimentary rocks

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Detrital sedimentary rocks

comprised of solid particles derived from parent material by phys. weathering

Classification of Detrital Sed. Rocks

Based on:

Sediment Size

Composition

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Conglomerate = rounded gravel (>2 mm)

Breccia = angular gravel

A

B

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Sandstone = sand-size (1/16 - 2 mm) particles

  • classified by mineralogy

    Quartz sandstone most common

    Arkose sandstone 25% feldspar

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Mudrock any combination of silt & clay

Siltstone silt (1/16 – 1 / 256 mm)

Claystone clay (< 1 / 256 mm)

  • Shale is fissile clay. (splits along many closely spaced planes)

  • Mudrocks = most common sed. rock (quiet-water environ)

shale

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Chemical sedimentary rocks

 ions taken into soln by weathering of parent material **crystalline texture (interlocking mineral grains)

Biochemical sedimentary rocks Organisms aid in the precipitation of minerals.

Classification of Chemical Sed. Rocks

Based on:

Mineral composition

Texture

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Evaporites formed by precipitation of minerals from evaporating water

\

Rock salt{halite (NaCl)} (A) & Rock gypsum{gypsum (CaSO4. 2H2O)}(B) are most common.

B

A

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Limestones

composed of Calcite (CaCO3)

contains much pore space

Coquina

Clastic texture

 shell fragments

Fossiliferouslimestone

skeletal fragments of marine invertebrates

-- pores filled w/ cement & mud --

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Coal

crystalline texture

carbon

(compressed, altered plant remains that occupied swamps & bogs)

A

B

Chert

crystalline texture

silica

(layers of microscopic shells of marine organisms)

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Strata (beds)layers differ in color, texture, & comp. from rock layers above & below.

Sedimentary Facies

-- sets of sed. rock w/ distinct attributes imparted by their depositional environment

Coastal areas facies accumulate simultaneously on various areas of seafloor (w/ unique charac.).

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1

Regression(seaward shift in the shoreline)

  • Drop in sea level

    Nearshore deposits overlie offshore deposits.

2

3

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Transgression (landward shift in shoreline)

  • Rise in sea level

    Nearshore seds overlie old land surfaces, & offshoreseds are stacked on top.

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Determining the Depositional Environment

Grand Canyon

Muav Limestone (offshore)

Bright Angel Shale

Tapeats Sandstone (onshore)

(deposition record of laterally adjacent environ.)

Transgression Sed structures, fossils & ripple marks

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Sedimentary Structures

- formed by physical processes at the time of deposition

  • Cross bedding

  • Graded bedding

  • Oscillation ripple marks

  • Current ripple marks

  • Mudcracks

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  • Cross bedding

  • inclined layers w/in a bed

  • Formed by wind or water

  • slopes downward in flow direction

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Graded bedding

  • upward decrease in grain size w/in a bed

    {deposits of turbidity currents}

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Ripple marks

-- small ridges separated by intervening troughs

Oscillation ripple marks(A) symmetric cross-sections generated by the back-&-forth motion of waves.

A

B

Current ripple marks(B) asymmetriccross- sections record flow in one direction (i.e., streams).

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Mudcracks

 shrinkage (polygonal patterns of intersecting fractures)

  • clay-rich sediment

  • depositional environ. periodic drying (i.e., a river floodplain, lake shore, or tidal flat)

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Fossilsare NOTSedimentary Structures.  remains or traces of ancient organisms

Morphology of organisms reveals info about environment the organisms lived in.

  • Heavy-shelled clams = shallow-water, turbulent

  • Thin, fragile-shelled clams = low-energy

  • Filter-feeding organisms (corals) = clear seawater (suspended sediment clogs their feeding organs.)

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Resources in Sedimentary Rocks?

  • Sand & gravel building & road construction

  • Limestone cement

  • Gypsum wallboard/plaster

  • Phosphate-bearing sedimentary rock fertilizer

  • Sand-sizedquartz glass

  • Carnotite(uranium mineral) fuels nuclear reactors [assoc w/ plant remains in sandstones formed in ancient stream channels]

  • Hematite & magnetite (banded iron formations)  iron ores


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