1. Rocks, Fossils and Time— Making Sense of the Geologic Record
Chapter 5
Rocks, Fossils and Time— Making Sense of the Geologic Record

2. 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

3. Stratified Igneous Rocks
Stratification in a succession of lava flows in Oregon.

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

5. Stratified Metamorphic Rocks
Stratification in Siamo Slate, in Michigan

6. 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

7. 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.

8. 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.

9. 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.

10. 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

11. 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

12. 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

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

14. An Angular Unconformity
An angular unconformity, Santa Rosa

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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. Marine Transgression

23. 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

24. A Marine Transgression in the Grand Canyon
Three formations deposited in a widespread marine transgression exposed in the walls of the Grand Canyon, Arizona

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

26. 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

27. Walther’s Law
Johannes Walther ( ) noticed that the same facies he found laterally were also present in a vertical sequence, now called Walther’s Law
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

28. 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

29. 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

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 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

31. 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

32. Body Fossil
Skeleton of a 2.3-m-long marine reptile in the museum at Glacier Garden in Lucerne, Switzerland

33. 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

34. 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

35. Trace Fossils
Paleontologists think that a land-dwelling beaver called Paleocastor made this spiral burrow in Nebraska

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

37. 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

38. Unaltered Remains
Insects in amber
Preservation in tar

39. Unaltered Remains
40,000-year-old frozen baby mammoth found in Siberia in It is 1.15 m long and 1.0 m tall and it had a hairy coat.
Hair around the feet is still visible

40. 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

41. Altered Remains
Carbon film of a palm frond
Carbon film of an insect

42. Molds and Casts Molds form when buried remains leave a cavity
Casts form if material fills in the cavity

43. 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

44. Cast of a Turtle
Fossil turtle showing some of the original shell material
body fossil
and a cast

45. 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

46. WHY is the fossil record incomplete
WHY is the fossil record incomplete??? Why are there large gaps of time and biological strata?

47. 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

48. Fossils and Telling Time
William Smith
, an English civil engineer independently discovered Steno’s 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

49. Fossils from Different Areas
To compare the ages of rocks from two different localities
Smith used fossils

50. 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

51. 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

52. 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.

53. 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

54. 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

55. 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

56. 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

57. 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

58. 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

59. 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

60. 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

61. Lithostratigraphic Correlation
Correlation of lithostratigraphic units such as formations traces rocks laterally across gaps

62. Lithostratigraphic Correlation
We can correlate rock units based on
composition
position in a sequence
and the presence of distinctive key beds

63. 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

64. Time Equivalence
The most effective way to demonstrate time equivalence is time-stratigraphic correlation using biozones

65. 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

66. 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

67. 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

68. 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

69. 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

70. 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

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

72. 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

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