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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
– in deciphering the geologic record,
– sedimentary rocks are especially useful
• The geologic record is complex
– and requires interpretation, which we will try to do
• Uniformitarianism is useful for this activity
Geologic Record
• for nearly 14
million years of
Earth history
– preserved at Sheep
– in John Day Fossil
Beds National
• Fossils in these
– provide a record
– of climate change
– and biological
• Stratigraphy deals with the study
– of any layered (stratified) rock,
– but primarily with sedimentary rocks and their
age relationships
geographic extent
• Sedimentary rocks are almost all stratified
• Many igneous rocks
– such as a succession of lava flows or ash beds
– are stratified and obey the principles of stratigraphy
• Many metamorphic rocks are stratified
Stratified Igneous Rocks
• Stratification in a succession of lava flows
in Oregon.
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
– 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
– but 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 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
• 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
– 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 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.
– The interval of time not represented by strata is a
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
– and giving rise to
– a 3 million year
• 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 in Colorado
– between steeply dipping Pennsylvanian rocks
– and overlying Cenozoic-aged conglomerate
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 eroded
• where truncated by faults
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 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
– produces a sedimentary facies
• During a transgression,
the facies forming offshore
become superposed
upon facies deposited
in nearshore environments
Marine Transgression
• The rocks of each facies become younger
– in a landward direction during a marine
• 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
older shale
A Marine Transgression in the
Grand Canyon
• Three
– in a widespread
– exposed in the
walls of the
Grand Canyon,
Marine Regression
• During a marine regression,
– sea level falls
– with respect
– to the continent
– and the environments
paralleling the
– migrate seaward
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
younger shale
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
• will also replace one
another laterally
– Walther’s law applies
• to marine transgressions
and regressions
Extent and 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 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
it is easy to determine
the relative ages of rocks
in Column A
and of rocks in Column B
• However, one needs more
– to determine the ages of
– 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
– older than in B
• Fossils could solve this
• 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 extinct
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
Body Fossil
• Skeleton of a 2.3-m-long marine reptile
– in the museum at Glacier Garden in Lucerne,
Body Fossils
• Shells of Mesozoic
invertebrate animals
– known as
ammonoids and
– on a rock slab
• in the Cornstock
Rock Shop in
Virginia City
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
– which may provide information about the size
– and diet of the animal that produced it
Trace Fossils
• Paleontologists think
– that a land-dwelling
– called Paleocastor
– made this spiral
burrow in Nebraska
Trace Fossils
• Fossilized feces (coprolite)
– of a carnivorous mammal
• 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
– or altered remains,
• with some change in composition or structure
• permineralized, recrystallized, replaced, carbonized
Unaltered Remains
• Insects in
• Preservation
in tar
Unaltered Remains
• 40,000year-old
frozen baby
• found in
Siberia in
• 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
– in Florissant
Fossil Beds
• Volcanic
– 3 to 6 m deep
– covered the lower
– 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 material
• 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
bacterial decay,
physical processes,
and metamorphism
destroy organic remains
• In spite of this, fossils are quite common
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 discovering a method
– of relatively dating sedimentary rocks at different
Fossils from Different Areas
• To compare the ages
of rocks from two
different localities
• 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
– also recognized this
• Their observations
– lead to the principle of fossil
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
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 Fossils
• The youngest rocks are in column B
– whereas the oldest ones are in column C
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
– and the construction of a composite geologic
– that is the basis for the relative geologic time scale
Geologic Column and the
Relative Geologic Time Scale
ages (the
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
– including carefully described fossils
– His system could be identified elsewhere
Dispute of Systems
• Ordovician System
– Lapworth assigned the overlap
– between the two to a new system,
– the Ordovician
System Dispute
• The dispute was settled in 1879
– when Lapworth proposed 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
– 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 are based on rock type
– with no consideration of time of origin
• The basic lithostratigraphic element is a
– which is 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, Utah
• For example: The
Chinle Formation is
divided into
– Springdale Sandstone
– Petrified Forest
– 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
• Two or more periods may be designated as an
• 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
• Supergroup
– Group
• Formation
– Member
» Bed
• Eonothem
• Eon
– Erathem
• System
– Series
» Stage
– Era
• Period
– Epoch
» Age
• 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
• 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 east
Time Equivalence
• The most effective way
– to demonstrate time equivalence
– is time-stratigraphic correlation
– using biozones
• But other methods are useful
• 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
• 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
• 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
– of determining
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
• Absolute ages may be obtained
for igneous events
– using radiometric dating
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?
• 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
• The first step in deciphering the geologic
history of a region
– is determining relative ages of the rocks
• First ascertain the vertical relationships
– among the rock layers
– even if they have been complexly deformed
• The geologic record
– is an accurate chronicle of ancient events,
– but it has many discontinuities or unconformities
– representing times of nondeposition, erosion or
• Simultaneous deposition
in adjacent but different environments
yields sedimentary facies,
which are bodies of sediment or sedimentary rock
with distinctive lithologic and biologic attributes
• According to Walther’s law,
– the facies in a conformable vertical sequence
– replace one another laterally
• During a marine transgression,
– a vertical sequence of facies results
– with offshore facies superposed over nearshore
• During a marine regression,
a vertical sequence of facies results
with nearshore facies superposed
over offshore facies,
the opposite of transgression
• Marine transgressions and regressions result
– uplift and subsidence of continents
– the amount of water in glaciers
– rate of seafloor spreading (volume of ridges)
• Most fossils are found in sedimentary rocks
– although they might also be in volcanic ash,
– volcanic mudflows, but rarely in other rocks
• Fossils are actually quite common,
but the fossil record is strongly biased
toward those organisms
that have durable skeletons
and that lived where burial was likely
• Law of fossil succession (William Smith)
– holds that fossil assemblages succeed one another
– through time in a predictable order
• Superposition and fossil succession
– were used to piece together
– a composite geologic column
– which serves as a relative time scale
• To bring order to stratigraphic terminology,
– geologists recognize units based entirely on content
• lithostratigraphic and biostratigraphic units
– and those related to time
• time-stratigraphic and time units
• Lithostratigraphic correlation involves
– demonstrating the original continuity
– of a presently discontinuous rock unit over an area
• Biostratigraphic correlation of range zones,
and especially concurrent range zones,
demonstrates that rocks in different areas
are of the same relative age,
even with different compositions
• The best way to determine absolute ages
– of sedimentary rocks and their contained fossils
– is to obtain absolute ages
– for associated igneous and metamorphic rocks