Rocks, Fossils and Time Making Sense of the Geologic Record

1
Chapter 5
Rocks, Fossils and TimeMaking Sense of the
Geologic Record
2
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

3
Stratigraphy

• 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
  stratigraphy
• Many metamorphic rocks are stratified

4
Stratified Igneous Rocks

• Stratification in a succession of lava flows in
  Oregon.

5
Stratified Sedimentary Rocks

• Stratification in sedimentary rocks consisting of
  alternating layers of sandstone and shale, in
  California.

6
Stratified Metamorphic Rocks

• Stratification in Siamo Slate, in Michigan

7
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

8
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
• 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

9
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

10
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.

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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.

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

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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

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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

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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

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A Disconformity

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

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An Angular Unconformity

• An angular unconformity, Santa Rosa

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A Nonconformity

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

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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

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• 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

21
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

22
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

23
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

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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

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Marine Transgression
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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

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A Marine Transgression in the Grand Canyon

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

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Marine Regression

• During a marine regression, sea level falls with
  respect to the continent

• the environments paralleling the shoreline
  migrate seaward

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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

30
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

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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

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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

33
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

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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

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Fossils

• Fossils are the remains or traces of prehistoric
  organisms
• 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

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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

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Body Fossil

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

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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

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Trace Fossils

• Trace fossils are indications of organic activity
  including
• 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

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Trace Fossils

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

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Trace Fossils

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

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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

43
Unaltered Remains

• Insects in amber

• Preservation in tar

44
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

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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

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Altered Remains

• Carbon film of a palm frond

• Carbon film of an insect

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Molds and Casts

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

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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
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Cast of a Turtle

• Fossil turtle showing some of the original shell
  material
• body fossil
• and a cast

50
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

51
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

52
Fossils and Telling Time

• William Smith
• 1769-1839, an English civil engineer
  independently discovered Stenos principle of
  superposition
• Realized that fossils in 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 locations

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Fossils from Different Areas

• To compare the ages of rocks from two different
  localities

• Smith used fossils

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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

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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

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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.

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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

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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

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Geologic Column and the Relative Geologic Time
Scale
Absolute ages (the numbers) were added much
later.
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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 including carefully described fossils
• His system could be identified elsewhere

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Dispute of Systems

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

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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

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Lithostratigraphic Units

• Lithostratigraphic units are based on rock type
• with no consideration of time of origin
• The basic lithostratigraphic element is a
  formation
• 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

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Lithostratigraphic Units

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

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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

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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

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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

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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

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Correlation

• 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

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Lithostratigraphic Correlation

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

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Lithostratigraphic Correlation

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

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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

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Time Equivalence

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

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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

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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

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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

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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

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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

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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

80
Indirect Dating

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

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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

82
Indirect Dating

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