Rocks, Fossils and Time

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.

11
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.

12
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.

13
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

14
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

15
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

16
A Disconformity

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

17
An Angular Unconformity

• An angular unconformity, Santa Rosa

18
A Nonconformity

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

19
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

20

• 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

24
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

25
Marine Transgression
26
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

27
A Marine Transgression in the Grand Canyon

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

28
Marine Regression

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

• the environments paralleling the shoreline
  migrate seaward

29
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

31
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

32
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

34
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

35
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

36
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

37
Body Fossil

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

38
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

39
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

40
Trace Fossils

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

41
Trace Fossils

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

42
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

45
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

46
Altered Remains

• Carbon film of a palm frond

• Carbon film of an insect

47
Molds and Casts

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

48
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
49
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

53
Fossils from Different Areas

• To compare the ages of rocks from two different
  localities

• Smith used fossils

54
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

55
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

56
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.

57
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

58
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

59
Geologic Column and the Relative Geologic Time
Scale
Absolute ages (the numbers) were added much
later.
60
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

61
Dispute of Systems

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

62
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

63
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

64
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

65
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

66
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

67
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

68
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

69
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

70
Lithostratigraphic Correlation

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

71
Lithostratigraphic Correlation

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

72
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

73
Time Equivalence

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

74
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

75
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

76
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

77
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

78
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

79
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

81
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