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Chapter 5. Rocks, Fossils, and Time— Making 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

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Rocks fossils and time making sense of the geologic record

Chapter 5

Rocks, Fossils, and Time—Making Sense of the Geologic Record

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

    • sedimentary rocks are especially useful

    • in deciphering the geologic record,

  • The geologic record is complex

    • and requires interpretation

  • Uniformitarianism offers a useful approach


  • Stratigraphy deals with the study

    • of any layered (stratified) rock,

    • but primarily with sedimentary rocks and their

      • composition

      • origin

      • age relationships

      • geographic extent

  • Almost all sedimentary rocks are stratified

  • Many volcanic rocks

    • such as lava flows or ash beds

    • as well as many metamorphic rocks

    • are stratified and obey the principles of stratigraphy

Stratified sedimentary rocks
Stratified Sedimentary Rocks

  • Although these rocks in South Dakota

  • are deeply eroded

  • stratification is still clearly visible

Stratified rocks
Stratified Rocks

  • Stratified rocks in California are

  • deformed so that they are no longer in their original position

Vertical stratigraphic relationships
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

  • Superposition

    • 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

      • but some sedimentary structures

      • and some fossils

      • allow geologists to resolve these kinds of problems

    Principle of inclusions
    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 itself

    • 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
    Age of Lava Flows, Sills

    • Determining the relative ages

      • of lava flows, sills and associated sedimentary rocks

      • uses contact metamorphism effects

      • 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.

    Rocks fossils and time making sense of the geologic record

    • The sill might also have inclusions of the rocks above and below,

    • but neither of these rocks will have inclusions of the sill.

    • A sill will heat the rocks above and below.


    • So far we have discussed vertical relationships

      • among conformable strata,

        • which are sequences of rocks

        • in which deposition was more or less continuous

    • 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 at this location

      • The interval of time not represented by strata is a hiatus.

    The origin of an unconformity
    The origin of an unconformity

    • In the process of forming an unconformity,

      • deposition began 12 million years ago (MYA),

      • continuing until 4 MYA

    • For 1 million years erosion occurred

    • removing 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
    Types of Unconformities

    • Three types of surfaces can be unconformities:

      • A disconformity is a surface in sedimentary rocks

        • 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 unconformities1
    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

    • A disconformity between sedimentary rocks:

      • in Montana, Jurassic-age rocks rest unconformably on top of Mississippian-age strata

      • an erosion surface separates the two.

    A nonconformity
    A Nonconformity

    • A nonconformity between Precambrian granite

      • and the Cambrian Formation

      • in Bighorn Mountains, Wyoming

    An angular unconformity
    An Angular Unconformity

    • An angular unconformity between the flat-lying Medial Jurassic Entrada Sandstone and underlying Upper Jurassic red beds in New Mexico.

    Lateral relationships
    Lateral Relationships

    • In 1669, Nicolas Steno proposed

      • the 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 they are eroded

    • where they are truncated by faults

    Gradual terminations
    Gradual Terminations

    • 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 intertonguing

    • where a rock unit changes

    • by lateral gradation

    • as its composition and/or texture

    • becomes increasingly different

    Sedimentary facies
    Sedimentary Facies

    • Both intertonguing 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

    Marine transgressions
    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 transgressions1
    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 transgressive

      • meaning the ages vary from place to place

    younger shale

    older shale

    A marine transgression in the grand canyon
    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
    Marine Regression

    • During a marine regression,

      • sea level falls

      • with respect

      • to the continent

    • and the environments paralleling the shoreline

    • migrate seaward

    Marine regression1
    Marine Regression

    • A marine regression

      • is the opposite of a marine transgression

    • It yields a vertical sequence

      • with nearshore facies

      • overlying offshore facies

      • and rock units become younger

      • in the seaward direction

    older shale

    younger shale

    Walther s law
    Walther’s Law

    • Johannes Walther (1860-1937) noticed that

      • the same facies he found laterally

      • were also present in a vertical sequence,

      • now called Walther’s Law

    • which holds that

      • the facies seen in a conformable vertical sequence

      • will also replace one another laterally

    • Walther’s law applies

      • to marine transgressions and regressions

    Extent rates of transgressions and regressions
    Extent, Rates of Transgressions and Regressions

    • 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 year

    • Transgression or regressions

      • with small reversals produce intertonguing

    Causes of transgressions and regressions
    Causes of Transgressions and Regressions

    • Uplift of continents causes regression

    • Subsidence causes transgression

    • Widespread glaciation causes regression

      • because of the amount of water frozen in glaciers

    • Rapid seafloor spreading,

      • expands the mid-ocean ridge system,

      • displacing seawater and causing transgression

    • Diminishing seafloor-spreading rates

      • increases the volume of the ocean basins

      • and causes regression

    Relative ages between separate areas
    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, you need more information

      • to determine the ages of rocks

      • in one section relative to

      • those in the other

    Relative ages between separate areas1
    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 can help to solve this problem


    • Fossils are the remains or traces of past life forms

    • They are most common in sedimentary rocks

      • but can be found

      • iIn volcanic ash and volcanic mudflows

    • 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

    How do fossils form
    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

    Trace fossils
    Trace Fossils

    • Indications of organic activity

      • including tracks, trails, burrows, and nests

      • are called trace fossils

    • A coprolite is a type of trace fossil

      • consisting of fossilized feces

      • that may provide information about the size

      • and diet of the animal that produced it

    Trace fossils1
    Trace Fossils

    • This slab of rock

      • formed over the actual tracks of birds,

      • so it is a cast of the tracks

    Trace fossils2
    Trace Fossils

    • Fossilized feces (coprolite)

      • of a carnivorous mammal

    • Specimen measures about 5 cm long

      • and contains small fragments of bones

    Body fossil formation
    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

      • or altered remains,

        • with some change in composition or structure

        • permineralization, replacement, carbonization

    Unaltered remains
    Unaltered Remains

    • Insects in amber

    Unaltered remains1
    Unaltered Remains

    • Frozen baby mammoth

    • found in Russia in 1989

    Altered remains
    Altered Remains

    • The bones of this mammoth

      • on display at the Museum of Geology and Paleontology in Florence, Italy

    • have been permineralized

      • with minerals added to the pores and cavities of the bones

    Altered remains1
    Altered Remains

    • Carbon film of a palm frond

    • Carbon film of an insect

    Molds and casts
    Molds and Casts

    • Molds form

      • when buried remains dissolve and leave a cavity

    • Casts form

      • if minerals or sediments fill in the cavity

    Mold and cast
    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

    Fossil record
    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

    Fossils and telling time
    Fossils and Telling Time

    • William Smith

      • 1769-1839, an English civil engineer

    • independently discovered

    • Steno’s principle of superposition

  • He also realized

    • that fossils in the rocks followed the same principle

  • He discovered that sequences of fossils,

    • especially groups of fossils

    • are consistent from area to area

  • Thereby he discovered a method

    • whereby relative ages of sedimentary rocks at different locations could be determined

  • Fossils from different areas
    Fossils from Different Areas

    • Smith used fossils

    • To compare the ages of rocks from two different localities

    Principle of fossil succession
    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

      • led to the principle of fossil succession

    Principle of fossil succession1
    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
    Distinct Aspect

    • An assemblage of fossils

      • has a distinctive aspect

      • compared with younger

      • or older fossil assemblages

    Matching rocks using fossils
    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

    Matching rocks using fossils1
    Matching Rocks Using Fossils


    • The youngest rocks are in column B

      • whereas the oldest ones are in column C


    Relative geologic time scale
    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

      • and the construction of a composite geologic column

      • that is the basis for the relative geologic time scale

    Geologic column and the relative geologic time scale
    Geologic Column and the Relative Geologic Time Scale

    Absolute ages (the numbers) were added much later.

    Example of the development of systems
    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 area

    • Silurian System

      • Murchinson described the Silurian System in South Wales

      • and carefully described fossils

      • His system could be identified elsewhere

    Dispute of systems
    Dispute of Systems

    • The two systems partially overlapped!

    System dispute
    System Dispute

    • The dispute was settled in 1879

      • when Lapworth proposed the Ordovician

    Stratigraphic terminology
    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

      • rocks—defined by their content

        • lithostratigraphic unit – rock content

        • biostratigraphic unit – fossil content

      • and time—expressing or related to geologic time

        • time-stratigraphic unit – rocks of a certain age

        • time units – referring to time not rocks

    Lithostratigraphic units
    Lithostratigraphic Units

    • Lithostratigraphic units are based on rock type

      • with no consideration of time of origin

    • The basic lithostratigraphic element is the formation

      • which is a mappable rock body

      • 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 units1
    Lithostratigraphic Units

    • Representation of the lithostratigraphic units in Capital Reef National Park, Utah

      • Notice that come formations

      • Are further divided into members,

      • And some are parts of more inclusive groups.

    Biostratigraphic units
    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

    • 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

    • 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 an 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
    Classification of Stratigraphic Units

    Time-stratigraphic Units

    • Eonothem

      • Erathem

        • System

          • Series

            • Stage


    • Eon

      • Era

        • Period

          • Epoch

            • Age

    Litho-stratigraphic Units

    • Supergroup

      • Group

        • Formation

          • Member

            • Bed


    • Correlation is the process

      • of matching up rocks in different areas

    • There are two types of correlation:

      • Lithostratigraphic correlation

        • simply matches up the same rock units

        • over a larger area with no regard for time

      • Time-stratigraphic correlation

        • demonstrates time-equivalence of events

    Lithostratigraphic correlation
    Lithostratigraphic Correlation

    • Correlation of lithostratigraphic units such as formations

      • traces rocks laterally across gaps

    Lithostratigraphic correlation1
    Lithostratigraphic Correlation

    • We can correlate rock units based on

      • composition

      • position in a sequence

      • and the presence of distinctive key beds

    Time equivalence
    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 further east

    Time equivalence1
    Time Equivalence

    • The most effective way

      • to demonstrate time equivalence

      • is time-stratigraphic correlation

      • using biozones

    • But several other methods are useful as well


    • 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

        • a 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
    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 range

    • 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
    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
    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

        • 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 scale
    Absolute Dates and the Relative Geologic Time Scale

    • 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?

    • Absolute ages determined for minerals

      • in sedimentary rocks

      • give only the ages of the source

      • that supplied the minerals

      • and not the age of the rock itself

    Absolute dates for sedimentary rocks are indirect
    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 relationships,

      • 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
    Indirect Dating

    • Absolute ages of sedimentary rocks

      • are most often found

      • by determining radiometric ages

      • of associated igneous or metamorphic rocks

    Indirect dating1
    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 dating2
    Indirect Dating

    • These sedimentary rocks

      • are bracketed by metamorphic and

      • igneous rocks for which

      • absolute ages are known

    Indirect dating3
    Indirect Dating

    • Accurate radiometric dates are now available

      • for many ash falls, plutons, lava flows and metamorphic rocks

      • with associated fossil-bearing sedimentary rocks

    • These absolute ages have been added to the geologic time scale

    • Additionally, we know when a particular organism lived

    Indirect dating4
    Indirect Dating

    • Baculites reesidei

      • biostratigraphic zone

      • in the Bearpaw Formation

      • Saskatchewan, Canada

      • is about 72-73 million years old

      • because absolute ages have been determined

      • for associated ash layers