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Ch. 5 Rocks, Fossils, and Time

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Title: Ch. 5 Rocks, Fossils, and Time


1
Ch. 5 Rocks, Fossils, and Time
  • ESCI 102

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
  • We will learn to interpret the geologic record
    using uniformitarianism

3
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
4
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

5
Stratified Igneous Rocks
  • Stratification in a succession of lava flows in
    Oregon

6
Stratified Metamorphic Rocks
  • Stratification in Siamo Slate, in Michigan

7
Stratified Sedimentary Rocks
  • Stratification in sedimentary rocks consisting of
    alternating layers of sandstone and shale, in
    California

8
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

9
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

10
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
11
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

12
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

13
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

14
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

15
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

16
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

17
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

18
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

19
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

20
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
21
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

22
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
23
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?

24
Marine Regression
  • During a marine regression, sea level falls with
    respect to the continent
  • and the environments paralleling the shoreline
    migrate seaward

25
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
26
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
27
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

28
Causes of Transgressions and Regressions
29
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
30
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

31
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

32
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

33
Trace Fossils
  • A land-dwelling beaver, Paleocastor, made this
    spiral burrow in Nebraska

34
Trace Fossils
  • Fossilized feces (coprolite) of a carnivorous
    mammal
  • specimen measures about 5 cm long and contains
    small fragments of bones

35
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

36
Unaltered Remains
  • Insects in amber
  • Preservation in tar

37
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

38
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

39
Altered Remains
  • Carbon film of a palm frond
  • Carbon film of an insect

40
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
41
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
42
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

43
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

44
Fossils from Different Areas
  • Compare the ages of rocks from different
    localities

45
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

46
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
47
Matching Rocks Using Fossils
youngest
oldest
  • The youngest rocks are in column B
  • Whereas the oldest are in column C

48
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

49
Geologic Column and the Relative Geologic Time
Scale
Absolute ages (the numbers) were added much
later.
50
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

51
Lithostratigraphic Correlation
  • Correlation of lithostratigraphic units such as
    formations
  • traces rocks laterally across gaps

52
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!)

53
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

54
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

55
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

56
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

57
Indirect Dating
  • Absolute ages of sedimentary rocks are most often
    found by determining radiometric ages of
    associated igneous or metamorphic rocks

58
Indirect Dating
  • Combining thousands of absolute ages associated
    with sedimentary rocks of known relative age
    gives the numbers on the geologic time scale
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