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Rocks, Fossils and Time


Chapter 5 Rocks, Fossils and Time Making Sense of the Geologic Record – PowerPoint PPT presentation

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Title: Rocks, Fossils and Time

Chapter 5
Rocks, Fossils and TimeMaking Sense of the
Geologic Record
Geologic Record
  • The fact that Earth has changed through time is
    apparent from evidence in the geologic record
  • The geologic record is the record of events
    preserved in rocks
  • Although all rocks are useful in deciphering the
    geologic record, sedimentary rocks are especially
  • The geologic record is complex and requires
    interpretation, which we will try to do

  • Stratigraphy deals with the study of any layered
    (stratified) rock, but primarily with sedimentary
    rocks and their
  • composition
  • origin
  • age relationships
  • geographic extent
  • Many igneous rocks
  • such as a succession of lava flows or ash beds
    are stratified and obey the principles of
  • Many metamorphic rocks are stratified

Stratified Igneous Rocks
  • Stratification in a succession of lava flows in

Stratified Sedimentary Rocks
  • Stratification in sedimentary rocks consisting of
    alternating layers of sandstone and shale, in

Stratified Metamorphic Rocks
  • Stratification in Siamo Slate, in Michigan

Vertical Stratigraphic Relationships
  • Surfaces known as bedding planes separate
    individual strata from one another
  • or the strata grade vertically from one rock type
    to another
  • Rocks above and below a bedding plane differ in
    composition, texture, color or a combination of
    these features
  • The bedding plane signifies
  • a rapid change in sedimentation
  • or perhaps a period of nondeposition

  • Nicolas Steno realized that he could determine
    the relative ages of horizontal (undeformed)
    strata by their position in a sequence
  • In deformed strata, the task is more difficult
  • some sedimentary structures, such as
    cross-bedding, and some fossils allow geologists
    to resolve these kinds of problems
  • we will discuss the use of sedimentary structures
    more fully later in the term

Principle of Inclusions
  • According to the principle of inclusions, which
    also helps to determine relative ages, inclusions
    or fragments in a rock are older than the rock
  • Light-colored granite in northern Wisconsin
    showing basalt inclusions (dark)
  • Which rock is older?
  • Basalt, because the granite includes it

Age of Lava Flows, Sills
  • Determining the relative ages of lava flows,
    sills and associated sedimentary rocks uses
    alteration by heat and inclusions
  • How can you determine whether a layer of basalt
    within a sequence of sedimentary rocks is a
    buried lava flow or a sill?
  • A lava flow forms in sequence with the
    sedimentary layers.
  • Rocks below the lava will have signs of heating
    but not the rocks above.
  • The rocks above may have lava inclusions.

  • A sill will heat the rocks above and below.
  • The sill might also have inclusions of the rocks
    above and below,
  • but neither of these rocks will have inclusions
    of the sill.

  • So far we have discussed vertical relationships
    among conformable strata, which are sequences of
    rocks in which deposition was more or less
  • Unconformities in sequences of strata represent
    times of nondeposition and/or erosion that
    encompass long periods of geologic time, perhaps
    millions or tens of millions of years
  • The rock record is incomplete.
  • The interval of time not represented by strata is
    a hiatus.

The origin of an unconformity
  • The process of forming an unconformity
  • deposition began 12 million years ago (MYA),
  • continues until 4 MYA
  • For 1 million years erosion occurred and removed
    2 MY of rocks
  • and giving rise to a 3 million year hiatus
  • The last column
  • is the actual stratigraphic record
  • with an unconformity

Types of Unconformities
  • Three types of surfaces can be unconformities
  • A disconformity is a surface separating younger
    from older rocks, both of which are parallel to
    one another
  • A nonconformity is an erosional surface cut into
    metamorphic or intrusive rocks and covered by
    sedimentary rocks
  • An angular unconformity is an erosional surface
    on tilted or folded strata over which younger
    rocks were deposited

Types of Unconformities
  • Unconformities of regional extent may change from
    one type to another
  • They may not represent the same amount of
    geologic time everywhere

A Disconformity
  • A disconformity between sedimentary rocks in
    California, with conglomerate deposited upon an
    erosion surface in the underlying rocks

An Angular Unconformity
  • An angular unconformity, Santa Rosa

A Nonconformity
  • A nonconformity in South Dakota separating
    Precambrian metamorphic rocks from the overlying
    Cambrian-aged Deadwood Formation

Lateral Relationships
  • In 1669, Nicolas Steno proposed his principle of
    lateral continuity, meaning that layers of
    sediment extend outward in all directions until
    they terminate
  • Terminations may be
  • Abrupt at the edge of a depositional basin where
  • where truncated by faults

  • or they may be gradual
  • where a rock unit becomes progressively thinner
    until it pinches out
  • or where it splits into thinner units each of
    which pinches out,
  • called intertonging
  • where a rock unit changes by lateral gradation as
    its composition and/or texture becomes
    increasingly different

Sedimentary Facies
  • Both intertonging and lateral gradation indicate
    simultaneous deposition in adjacent environments
  • A sedimentary facies is a body of sediment with
    distinctive physical, chemical and biological
    attributes deposited side-by-side with other
    sediments in different environments

Sedimentary Facies
  • On a continental shelf, sand may accumulate in
    the high-energy nearshore environment
  • while mud and carbonate deposition takes place at
    the same time in offshore low-energy environments

Marine Transgressions
  • A marine transgression occurs when sea level
    rises with respect to the land
  • During a marine transgression,
  • the shoreline migrates landward
  • the environments paralleling the shoreline
    migrate landward as the sea progressively covers
    more and more of a continent

Marine Transgressions
  • Each laterally adjacent depositional environment
    produces a sedimentary facies
  • During a transgression, the facies forming
    offshore become superposed upon facies deposited
    in nearshore environments

Marine Transgression
Marine Transgression
  • The rocks of each facies become younger in a
    landward direction during a marine transgression
  • One body of rock with the same attributes (a
    facies) was deposited gradually at different
    times in different places so it is time
  • meaning the ages vary from place to place

A Marine Transgression in the Grand Canyon
  • Three formations deposited in a widespread marine
    transgression exposed in the walls of the Grand
    Canyon, Arizona

Marine Regression
  • During a marine regression, sea level falls with
    respect to the continent
  • the environments paralleling the shoreline
    migrate seaward

Marine Regression
  • A marine regression
  • is the opposite of a marine transgression
  • It yields a vertical sequence with nearshore
    facies overlying offshore facie sand rock units
    become younger in the seaward direction

Walthers Law
  • Johannes Walther (1860-1937) noticed that the
    same facies he found laterally were also present
    in a vertical sequence, now called Walthers Law
  • holds that
  • the facies seen in a conformable vertical
    sequence will also replace one another laterally
  • Walthers law applies to marine transgressions
    and regressions

Extent and Rates of Transgressions and
  • Since the Late Precambrian, 6 major marine
    transgressions followed by regressions have
    occurred in North America
  • These produce rock sequences, bounded by
    unconformities, that provide the structure for
    U.S. Paleozoic and Mesozoic geologic history
  • Shoreline movements are a few centimeters per
  • Transgression or regressions with small reversals
    produce intertonging

Causes of Transgressions and Regressions
  • Uplift of continents causes regression
  • Subsidence causes transgression
  • Widespread glaciation causes regression
  • due to the amount of water frozen in glaciers
  • Rapid seafloor spreading,
  • expands the mid-ocean ridge system,
  • displacing seawater onto the continents
  • Diminishing seafloor-spreading rates
  • increases the volume of the ocean basins
  • and causes regression

Relative Ages between Separate Areas
  • Using relative dating techniques, it is easy to
    determine the relative ages of rocks in Column A
    and of rocks in Column B
  • However, one needs more information to determine
    the ages of rocks in one section relative to
    those in the other

Relative Ages between Separate Areas
  • Rocks in A may be younger than those in B, the
    same age as in B or older than in B
  • Fossils could solve this problem

  • Fossils are the remains or traces of prehistoric
  • They are most common in sedimentary rocks and in
    some accumulations of pyroclastic materials,
    especially ash
  • They are extremely useful for determining
    relative ages of strata but geologists also use
    them to ascertain environments of deposition
  • Fossils provide some of the evidence for organic
    evolution and many fossils are of organisms now

How do Fossils Form?
  • Remains of organisms are called body fossils. and
    consist mostly of durable skeletal elements such
    as bones, teeth and shells
  • rarely we might find entire animals preserved by
    freezing or mummification

Body Fossil
  • Skeleton of a 2.3-m-long marine reptile in the
    museum at Glacier Garden in Lucerne, Switzerland

Body Fossils
  • Shells of Mesozoic invertebrate animals known as
    ammonoids and nautiloids on a rock slab in the
    Cornstock Rock Shop in Virginia City Nevada

Trace Fossils
  • Trace fossils are indications of organic activity
  • tracks,
  • trails,
  • burrows,
  • nests
  • A coprolite is a type of trace fossil consisting
    of fossilized feces which may provide information
    about the size and diet of the animal that
    produced it

Trace Fossils
  • Paleontologists think that a land-dwelling beaver
    called Paleocastor made this spiral burrow in

Trace Fossils
  • Fossilized feces (coprolite) of a carnivorous
  • Specimen measures about 5 cm long and contains
    small fragments of bones

Body Fossil Formation
  • The most favorable conditions for preservation of
    body fossils occurs when the organism possesses a
    durable skeleton of some kind and lives in an
    area where burial is likely
  • Body fossils may be preserved as
  • unaltered remains, meaning they retain their
    original composition and structure,
  • by freezing, mummification, in amber, in tar
  • altered remains, with some change in composition
  • permineralized
  • recrystallized
  • replaced
  • carbonized

Unaltered Remains
  • Insects in amber
  • Preservation in tar

Unaltered Remains
  • 40,000-year-old frozen baby mammoth found in
    Siberia in 1971. It is 1.15 m long and 1.0 m tall
    and it had a hairy coat.
  • Hair around the feet is still visible

Altered Remains
  • Petrified tree stump in Florissant Fossil Beds
    National Monument, Colorado
  • Volcanic mudflows 3 to 6 m deep covered the lower
    parts of many trees at this site

Altered Remains
  • Carbon film of a palm frond
  • Carbon film of an insect

Molds and Casts
  • Molds form when buried remains leave a cavity
  • Casts form if material fills in the cavity

Mold and Cast
Step a burial of a shell Step b dissolution
leaving a cavity, a mold Step c the mold is
filled by sediment forming a cast
Cast of a Turtle
  • Fossil turtle showing some of the original shell
  • body fossil
  • and a cast

Fossil Record
  • The fossil record is the record of ancient life
    preserved as fossils in rocks
  • Just as the geologic record must be analyzed and
    interpreted, so too must the fossil record
  • The fossil record is a repository of prehistoric
    organisms that provides our only knowledge of
    such extinct animals as trilobites and dinosaurs

Fossil Record
  • The fossil record is very incomplete because of
    destruction to organic remains
  • bacterial decay
  • physical processes
  • scavenging
  • metamorphism
  • In spite of this, fossils are quite common

Fossils and Telling Time
  • William Smith
  • 1769-1839, an English civil engineer
    independently discovered Stenos principle of
  • Realized that fossils in rocks followed the same
  • He discovered that sequences of fossils,
    especially groups of fossils, are consistent from
    area to area
  • Thereby discovering a method of relatively dating
    sedimentary rocks at different locations

Fossils from Different Areas
  • To compare the ages of rocks from two different
  • Smith used fossils

Principle of Fossil Succession
  • Using superposition, Smith was able to predict
    the order in which fossils would appear in rocks
    not previously visited
  • Alexander Brongniart in France also recognized
    this relationship
  • Their observations lead to the principle of
    fossil succession

Principle of Fossil Succession
  • Principle of fossil succession holds that fossil
    assemblages (groups of fossils) succeed one
    another through time in a regular and
    determinable order
  • Why not simply match up similar rocks types?
  • Because the same kind of rock has formed
    repeatedly through time
  • Fossils also formed through time,
  • but because different organisms existed at
    different times,
  • fossil assemblages are unique

Distinct Aspect
  • An assemblage of fossils
  • has a distinctive aspect compared with younger or
    older fossil assemblages
  • Rocks that contain similar fossil assemblages had
    to have been deposited at about the same time.

Matching Rocks Using Fossils
  • Geologists use the principle of fossil succession
    to match ages of distant rock sequences
  • Dashed lines indicate rocks with similar fossils
    thus having the same age

Relative Geologic Time Scale
  • Investigations of rocks by naturalists between
    1830 and 1842 based on superposition and fossil
    succession resulted in the recognition of rock
    bodies called systems
  • the construction of a composite geologic column
    is the basis for the relative geologic time scale

Geologic Column and the Relative Geologic Time
Absolute ages (the numbers) were added much
Example of the Development of Systems
  • Cambrian System
  • Sedgwick studied rocks in northern Wales and
    described the Cambrian System without paying much
    attention to the fossils
  • His system could not be recognized beyond the
  • Silurian System
  • Murchinson described the Silurian System in South
    Wales including carefully described fossils
  • His system could be identified elsewhere

Dispute of Systems
  • The dispute was settled in 1879
  • Ordovician System
  • Lapworth assigned the overlap between the two to
    a new system, the Ordovician

Stratigraphic Terminology
  • Because sedimentary rock units are time
    transgressive, they may belong to one system in
    one area and to another system elsewhere
  • At some localities a rock unit
  • straddles the boundary between systems
  • We need terminology that deals with both
  • rocksdefined by their content
  • lithostratigraphic unit rock content
  • biostratigraphic unit fossil content
  • and timeexpressing or related to geologic time
  • time-stratigraphic unit rocks of a certain age
  • time units referring to time not rocks

Lithostratigraphic Units
  • Lithostratigraphic units are based on rock type
  • with no consideration of time of origin
  • The basic lithostratigraphic element is a
  • a mappable rock unit with distinctive upper and
    lower boundaries
  • It may consist of a single rock type
  • such as the Redwall limestone
  • or a variety of rock types
  • such as the Morrison Formation
  • Formations may be subdivided
  • into members and beds
  • or collected into groups and supergroups

Lithostratigraphic Units
  • Lithostratigraphic units in Zion National Park,
  • For example The Chinle Formation is divided into
  • Springdale Sandstone Member
  • Petrified Forest Member
  • Shinarump Conglomerate Member

Biostratigraphic Units
  • A body of strata recognized only on the basis of
    its fossil content is a biostratigraphic unit
  • the boundaries of which do not necessarily
    correspond to those of lithostratigraphic units
  • The fundamental biostratigraphic unit
  • is the biozone

Time-Stratigraphic Units
  • Time-stratigraphic units
  • also called chronostratigraphic units
  • consist of rocks deposited during a particular
    interval of geologic time
  • The basic time-stratigraphic unit is the system

Time Units
  • Time units simply designate certain parts of
    geologic time
  • Period is the most commonly used time designation
  • Two or more periods may be designated as an era
  • Two or more eras constitute and eon
  • Periods can be made up of shorter time units
  • epochs, which can be subdivided into ages
  • The time-stratigraphic unit, system, corresponds
    to the time unit, period

Classification of Stratigraphic Units
  • Litho-stratigraphic Units
  • Supergroup
  • Group
  • Formation
  • Member
  • Bed
  • Time-stratigraphic Units
  • Eonothem
  • Erathem
  • System
  • Series
  • Stage
  • Time-Units
  • Eon
  • Era
  • Period
  • Epoch
  • Age

  • Correlation is the process of matching up rocks
    in different areas
  • There are two types of correlation
  • Lithostratigraphic correlation
  • simply matching up the same rock units over a
    larger area with no regard for time
  • Time-stratigraphic correlation
  • demonstrates time-equivalence of events

Lithostratigraphic Correlation
  • Correlation of lithostratigraphic units such as
    formations traces rocks laterally across gaps

Lithostratigraphic Correlation
  • We can correlate rock units based on
  • composition
  • position in a sequence
  • and the presence of distinctive key beds

Time Equivalence
  • Because most rock units of regional extent are
    time transgressive we cannot rely on
    lithostratigraphic correlation to demonstrate
    time equivalence
  • Example
  • sandstone in Arizona is correctly correlated with
    similar rocks in Colorado and South Dakota
  • but the age of these rocks varies from Early
    Cambrian in the west to middle Cambrian farther

Time Equivalence
  • The most effective way to demonstrate time
    equivalence is time-stratigraphic correlation
    using biozones

  • For all organisms now extinct, their existence
    marks two points in time
  • their time of origin
  • their time of extinction
  • One type of biozone, the range zone, is defined
    by the geologic range (total time of existence)
    of a particular fossil group, species, or a group
    of related species called a genus
  • Most useful are fossils that are
  • easily identified
  • geographically widespread
  • and had a rather short geologic range

Guide Fossils
  • The brachiopod Lingula is not useful because,
    although it is easily identified and has a wide
    geographic extent, it has too large a geologic
  • The brachiopod Atrypa and trilobite Paradoxides
    are well suited for time-stratigraphic
    correlation, because of their short ranges
  • They are guide fossils

Concurrent Range Zones
  • A concurrent range zone is established by
    plotting the overlapping ranges of two or more
    fossils with different geologic ranges
  • This is probably the most accurate method of
    determining time equivalence

Short Duration Physical Events
  • Some physical events of short duration are also
    used to demonstrate time equivalence
  • distinctive lava flow
  • would have formed over a short period of time
  • ash falls
  • take place in a matter of hours or days
  • may cover large areas
  • are not restricted to a specific environment
  • Absolute ages may be obtained for igneous events
    using radiometric dating

Absolute Dates and the Relative Geologic Time
  • Ordovician rocks
  • are younger than those of the Cambrian
  • and older than Silurian rocks
  • But how old are they? When did the Ordovician
    begin and end?
  • Since radiometric dating techniques work on
    igneous and some metamorphic rocks, but not
    generally on sedimentary rocks, this is not so
    easy to determine

Absolute Dates for Sedimentary Rocks Are Indirect
  • Mostly, absolute ages for sedimentary rocks must
    be determined indirectly by dating associated
    igneous and metamorphic rocks
  • According to the principle of cross-cutting
  • a dike must be younger than the rock it cuts, so
    an absolute age for a dike gives a minimum age
    for the host rock and a maximum age for any rocks
    deposited across the dike after it was eroded

Indirect Dating
  • Absolute ages of sedimentary rocks are most often
    found by determining radiometric ages of
    associated igneous or metamorphic rocks

Indirect Dating
  • The absolute dates obtained from regionally
    metamorphosed rocks give a maximum age for
    overlying sedimentary rocks
  • Lava flows and ash falls interbedded with
    sedimentary rocks are the most useful for
    determining absolute ages
  • Both provide time-equivalent surfaces
  • giving a maximum age for any rocks above
  • and a minimum age for any rocks below

Indirect Dating
  • Combining thousands of absolute ages associated
    with sedimentary rocks of known relative age
    gives the numbers on the geologic time scale
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