Ch. 5 Rocks, Fossils, and Time

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Ch. 5 Rocks, Fossils, and Time

• ESCI 102

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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
• We will learn to interpret the geologic record
  using uniformitarianism

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Geologic Record

• Fossils in these rocks provide a record of
  climate change and biological events
• The rocks themselves help reconstruct the
  environment

John Day Fossil Beds National Monument, Oregon
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Stratigraphy

• Stratigraphy deals with the study of any layered
  (stratified) rock, but primarily with sedimentary
  rocks and their
• composition
• origin
• age relationships
• geographic extent
• Sedimentary rocks are almost all stratified
• Many igneous rocks and metamorphic rocks are also
  stratified

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Stratified Igneous Rocks

• Stratification in a succession of lava flows in
  Oregon

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Stratified Metamorphic Rocks

• Stratification in Siamo Slate, in Michigan

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Stratified Sedimentary Rocks

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

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Vertical Stratigraphic Relationships

• Surfaces known as bedding planes
• separate individual strata from one 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

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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
• sedimentary structures, such as cross-bedding,
  and fossils
• allow geologists to resolve these kinds of
  problems
• more later in term

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Principle of Inclusions

• According to the principle of inclusions
• inclusions or fragments in a rock are older than
  the rock itself

• Light-colored granite showing basalt inclusions
  (dark)
• Which rock is older?

basalt, because the granite includes it
northern Wisconsin
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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

• How can you determine whether a layer of basalt
  within a sequence of sedimentary rocks is a
  buried lava flow or a sill?

• sill will heat the rocks above and below

• 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
• 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
• millions to hundreds of millions of years
• The rock record is incomplete
• interval of time not represented by strata is a
  hiatus

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Origins of 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

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Types of Unconformities

• Three types of surfaces can be unconformities
• disconformity
• separates younger from older rocks
• both of which are parallel to one another
  (implies sed rx)
• nonconformity
• cuts into metamorphic or intrusive rocks
• is covered by sedimentary rocks
• angular unconformity
• 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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Lateral Relationships

• In 1669, Nicolas Steno proposed the principle of
  lateral continuity
• layers of sediment extend outward in all
  directions until they terminate
• terminations may be abrupt
• at the edge of a depositional basin, and

• where eroded
• where truncated by faults

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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

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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

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Sedimentary Facies

• On a continental shelf, sand may accumulate in
  the high-energy nearshore environment

• Mud and carbonate deposition takes place at the
  same time in offshore low-energy environments

? Different Facies
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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
• 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

• 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
• ages vary from place to place

younger shale
older shale
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A Marine Transgression in the Grand Canyon

• Three formations deposited in a widespread marine
  transgression are exposed in the walls of the
  Grand Canyon
• What is the sea level history recorded?

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Marine Regression

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

• and 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 facies and
  lithostratigraphic rock units become younger in
  the seaward direction

older shale
younger shale
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Walthers Law

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

adapted from Van Wagoner et al., 1990
http//www.uga.edu/strata/sequence/parasequences.
html
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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 sequence, 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
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Causes of Transgressions and Regressions

• Uplift of continents causes local regression
• Subsidence causes local transgression
• Widespread glaciation causes regression

due to the amount of water frozen in glaciers
Rapid seafloor spreading causes transgression

expands the mid-ocean ridge system, displacing
seawater onto the continents
Diminishing seafloor-spreading rates increase the
volume of the ocean basins and causes
regression
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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
• geologists also use them to ascertain
  environments of deposition
• Fossils provide some of the evidence for organic
  evolution
• many fossils are of organisms now extinct

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How do Fossils Form?

• Remains of organisms are called body fossils
• mostly durable skeletal elements such as bones,
  teeth and shells

• rarely we might find entire animals preserved by
  freezing or mummification

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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

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Trace Fossils

• A land-dwelling beaver, 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
  or structure by being permineralized,
  recrystallized, replaced, carbonized

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Unaltered Remains

• Insects in amber

• Preservation in tar

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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

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

• The fossil record is the record of ancient life
  preserved as fossils in rocks
• The fossil record is very incomplete because of
• bacterial decay
• physical processes
• scavenging
• metamorphism
• In spite of this, fossils are quite common

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Fossils and Telling Time

• William Smith
• 1769-1839, an English civil engineer
• independently discovered Stenos 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 locations

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Fossils from Different Areas

• Compare the ages of rocks from different
  localities

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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

• 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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Matching Rocks Using Fossils
youngest
oldest

• The youngest rocks are in column B
• Whereas the oldest are in column C

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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
• and the construction of a composite geologic
  column that 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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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

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Lithostratigraphic Correlation

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

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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
• for 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 (THAT'S MILLIONS OF YEARS!)

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Time Equivalence

• 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
• 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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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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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

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