SEDIMENTARY AND METAMORPHIC ROCKS AND AGE DETERMINATION

The photo shows rock layering or stratification which is a typical feature associated with sedimentary rocks--natural gas and petroleum are formed and found in sedimentary rocks--fossils which occur in sedimentary rocks show the life forms present on Earth during geologic time

I. Sedimentary rock family

A. Definition and explanation

• are rocks comprised of sediments accumulated from physical or chemical processes and consolidated through lithification---the accumulation of sediments occurs primarily in ocean waters or other large bodies of water

B. Important factors and concepts related to the formation of sedimentary rocks

1. Weathering

• is the "breaking down" of rock materials at or near the surface of the Earth
  • chemical weathering is the decomposition of materials resulting in the formation of new minerals or substances--new materials form through removal of or additions to the weather substance

  • physical or mechanical weathering is the disintegration of materials in which no new minerals or substances are formed

• both chemical and physical weathering processes can affect a rock material to contribute to the overall breakdown

2. Erosion

• is the removal or transportation of materials by stream water, glacial ice, wind, and gravity---these materials eventually may become deposited in large amounts

3. Lithification

• is the process(es) by which accumulated sediments become rock material---compaction of sediments by its own weight and cementation of the particles by a natural gluing agent are two important lithification processes

4. Stratification

• is the occurrence of sedimentary rocks in layers (strata or beds) and can be considered the most common feature related to all sedimentary rocks---it is easily seen why sediments depositing in a medium would form in layers
• click here to see stratified layers in the Grand Canyon, Arizona

5. Law of superposition

• states that during the formation of sedimentary rocks, each layer is older than the one above and younger than the one below---this statement reflects the relative age of sedimentary rocks.

C. Sedimentary rock types

• there are three types of sedimentary rocks based on the way they form; clastic, chemical and organic---a rock table summarizing the descriptions of these rock types will be given below

1. Detrital or clastic rocks

• are rocks formed from the accumulation of sediments by primarily physical or mechanical processes---sediments are formed from the erosion of preexisting rock materials and deposited by agents such as stream water, wind, glacial ice, and gravity---these sediments then become rocks through lithification
• detrital sediment classification (Wentworth's particle size) is based on the size of the sedimentary particle---the largest size called a boulder, the smallest called clay.
• click here to see the sedimentary particle sizes

• Kinds of clastic rocks---based primarily on sediment (particle) size

  • conglomerate (comprised of large rounded particles), breccia (comprised of large sharp-edged shaped particles), graywacke sandstone (comprised of dark sand sized particles), arkose sandstone ( sand sized particles with alot of feldspar especially orthoclase), quartz sandstone ( sand sized particles comprised largely of quartz particles), siltstone (a fine grained rock with a gritty feel), claystone (a very fine grained rock which feels slick when wet), shale ( a very fine grained rock which is fissile or breaks into sheets)

  • siltstone and claystone can be undifferentiated and called mudstone

click here to see some photos of clastic sedimentary rocks

2. Organic or biological rocks

• are rocks formed from the accumulation of animal shells or plant materials and lithified---size of sedimentary particle is not important in naming the rock

• Kinds of organic rocks
  • limestones (effervesce (bubble) with addition of dilute acid)

    • fossiliferous limestone--animal shells in an opaque mud
    • coquina--comprised essentially of 100% seeable animal shells
    • chalk--comprised essentially of 100% invisible animal shells--can be scratched with the fingernail---the White Cliffs of Dover in England is an example of a large chalk deposit
    • click here to see the "White Cliffs Of Dover"
  • coal
    • the coal series explains the sequence of changes of buried plant materials to coal---the series is as follows: buried plants--to--peat (recognizable non-fossilized remains of plants)--to--lignite (a crude burning material)--to--bituminous coal
  • chert
    • is a rock consisting of invisible animal shells and comprised of silica (quartz-like material)

3. Chemical rocks

• sediments formed from chemical precipitation, accumulation and lithification---size of sedimentary particle is not important in the naming of the rock

• evaporites--rocks formed from the evaporation of ocean or fresh water in which solubilized substances become supersaturated and precipitate forming deposits---rock gypsum (softer than a fingernail) and rock salt are examples of evaporite rocks---the Great Salt Lake in Utah and the Dead Sea are examples of bodies of water which have produced and are producing evaporite rocks
• click here to see evaporite rocks in Death Valley, California

• limestones
  • non-bioclastic fossiliferous limestones, crystalline limestones, micrite (very fined grained), and oolite (comprised of small spherical particles)

• dolostone--rock dolomite

• chert (flint)--often found in layers or in pods within other sedimentary rocks as limestones

click here to see sedimentary rock table(1) which includes rocks discussed above

D. Some special features in sedimentary rocks

1. cross-bedding

• relatively thin layers inclined at an angle to the main bedding
• formed by currents in water or wind
• click here to see cross bedding

2. graded bedding

• a sediment layer characterized by a decrease in sediment size from bottom to top

3. ripple marks

• small waves of sand that develop on surface of sedimentary layer by the action of moving water or air
• click here to see ripple marks

E. Significance of sedimentary rocks

1. The only family of rock containing an abundant record of life forms and the changes of life forms throughout geologic time

2. The only family of rock in which natural gas and petroleum form and from which these are extracted in abundance

3. Used for constructing buildings, tomb stones, and some limestones are used as drive way coverings and an important source of lime

Click below for important references for the sedimentary rock family

click here

click here

II. Metamorphic rock family

A. Definition and explanation

• rocks formed primarily through the action of heat and/or pressure on preexisting rocks in which there is essentially no melting during the process---time is also a factor in the metamorphic process

• rocks will loose any evidence of fossil remains and other features that were present before the metamorphic change

B. Types of metamorphism

1. Contact or thermal

• small igneous magma bodies as stocks intrude country rock "baking" the rock---this metamorphism or change of preexisting rock is caused primarily by temperature effects associated with the intruding magma

2. Regional

• larger igneous magma bodies as batholiths or laccoliths intrude country rock and form a much greater area of metamorphism---temperature and pressure are equally important as agents in this kind of metamorphism

C. Kinds of metamorphic rocks

• all but one kind (marble) of metamorphic rock are comprised of silicate minerals---a rock table outlining the types of rocks discussed below will be given later

1. Foliated metamorphic rocks

• minerals are aligned in a pattern
  • gneiss--visible minerals appear in straight parallel or nearly parallel bands---most gneisses resemble phaneritic textured rocks like granite because they often form from these rocks

  • schist--visible minerals appear in irregular or wavy bands and often contain much mica (biotite and/or muscovite)---the parent rock is usually shale

  • slate--invisible minerals are aligned and the rock appears to have a "rock cleavage" breaking along layers---the parent rock is usually shale

  • phyllite--visible but small minerals are aligned and the rock looks like a hybrid or cross between a schist and a slate---the parent rock is usually shale

2. Non foliated metamorphic rocks

• minerals are not aligned in a pattern but occur arranged "at random" in the rock
  • marble--comprised of CaCO3 and form from limestone or dolostone

  • quartzite--comprised of silicate minerals and formed from sandstone

  • metaconglomerate--comprised of large mineral fragments and form from conglomerate

  • amphibolite--comprised of seeable minerals and usually forms from a basalt---this rock looks like a gabbro

  • anthracite coal--a high grade of coal formed from bituminous coal

click here to see some photos of foliated and non-foliated metamorphic rocks

click here to review metamorphic rocks defined above and see the metamorphic rock table(2)

D. Important uses

1. Used as building materials, and tombstones

2. Marble can be used in art as a sculpturing material

click below for more information on metamorphic rocks

click here

click here

III. Age determination (geologic age)

A. Basics

• now that we have studied the principles of the 3 rock families in detail including the crosscutting and intrusive-extrusive nature of igneous rocks and the law of superposition pertaining to sedimentary rocks, and the absolute age determination method in the mineral section, the basic concept of age determination should be easy for you to comprehend---follow the interpretation of geologic events in page 179 below. Also the cd rom "Geode" is an excellence reference for this material
• click here to see relative age events

B. Age determination method

• relative and absolute age determination methods are used together to establish absolute age of rocks which cannot themselves be dated using the absolute age determination method--as mentioned in the mineral section igneous rocks are the best to determine absolute age dates
• click here to see figure 1 and the relative and absolute age methods used together
• in figure 1 determine the absolute age date of formation B: A is older than B which is older than C (from the law of superposition)--igneous intrusive, V was dated at 2.15 million years--lava flow, P was dated at 2.25 million years (both V and P were dated using an absolute age determination method)--since V bakes (metamorphoses) B, B is older than intrusive V ( B is greater than 2.15 million years)--since lava flow P does not bake (metamorphose) B, B is younger than P (B is less than 2.25 million years)--therefore B is between 2.15 and 2.25 million years old
• Index fossils are also important in the age dating of rocks.  Once a species of fossil is identified as being of a specific age, other sedimentary rocks having that same species must be of the same age.

click here to read more about relative and absolute age determinations

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table 1 click to return

table 2 click to return

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page 179 click to return

figure 1 click to return