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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
– sedimentary rocks are especially useful
– in deciphering the geologic record,
• The geologic record is complex
– and requires interpretation
• Uniformitarianism offers a useful approach
Stratigraphy
• 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
• Although these rocks in South Dakota
• are deeply eroded
• stratification is still clearly visible
Stratified Rocks
• Stratified rocks in California are
• deformed so that they are no longer in their original
position
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
• 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 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.
Sill
– 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.
Unconformities
• 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
• 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
• 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 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 Montana, Jurassic-age rocks rest unconformably on
top of Mississippian-age strata
– an erosion surface separates the two.
A Nonconformity
• A nonconformity between Precambrian granite
– and the Cambrian Formation
– in Bighorn Mountains, Wyoming
An Angular Unconformity
• An angular unconformity between the flatlying Medial Jurassic Entrada Sandstone and
underlying Upper Jurassic red beds in New
Mexico.
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
– 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
• 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
• 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
• 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
younger
– meaning the ages vary from place to place
shale
older shale
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
– and 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 facies
and rock units become younger
in the seaward direction
younger shale
older
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
sequence
• will also replace one
another laterally
– Walther’s law applies
• to marine 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
• 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
• 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 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
• 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?
• 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
• 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 Fossils
• This slab of rock
– formed over the actual
tracks of birds,
– so it is a cast of the
tracks
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
• permineralization, replacement, carbonization
Unaltered Remains
• Insects in
amber
Unaltered Remains
• Frozen
baby
mammoth
• found in
Russia in
1989
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 Remains
• Carbon film of
a palm frond
• Carbon film of an insect
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
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
• 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
• 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
• 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
France
– also recognized this
relationship
• Their observations
– led 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
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
youngest
oldest
• 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
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
Absolute
ages (the
numbers)
were
added
much
later.
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
• The two systems partially overlapped!
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 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 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
• 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 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
Lithostratigraphic
Units
• Supergroup
– Group
• Formation
– Member
» Bed
TimeTimestratigraphic
Units
Units
• Eonothem
• Eon
– Erathem
• System
– Series
» Stage
– Era
• Period
– Epoch
» Age
Correlation
• 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 further east
Time Equivalence
• The most effective way
– to demonstrate time equivalence
– is time-stratigraphic correlation
– using biozones
• But several other methods are useful as well
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
• 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
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
• 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
• 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
• 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
• 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
• These sedimentary rocks
– are bracketed by metamorphic and
– igneous rocks for which
– absolute ages are known
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 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
Summary
• 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
both
Summary
• 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
facies
Summary
• 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
from
– uplift and subsidence of continents
– the amount of water in glaciers
– rate of seafloor spreading (volume of ridges)
Summary
• Most fossils are found in sedimentary rocks
– although they might also be in volcanic ash,
– volcanic mudflows, but rarely in other rocks
• Fossils of some organisms are 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
Summary
• Superposition and fossil succession
– were used to piece together
– a composite geologic column
– which serves as the basis for 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 rock over a given area
Summary
• 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