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. Walther’s Law
Johannes Walther ( ) noticed that the same facies he found laterally were also present in a vertical sequence
Walther’s Law: the facies seen in a conformable vertical sequence will also replace one another laterally
Walther’s law applies to marine transgressions and regressions
adapted from Van Wagoner et al., 1990;

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
Skeleton of a 2.3-m-long marine reptile
in the museum at Glacier Garden in Lucerne, Switzerland
Shells of Mesozoic invertebrate animals
known as ammonoids and nautiloids
on a rock slab
in the Cornstock Rock Shop in Virginia City Nevada
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
, an English civil engineer
independently discovered Steno’s 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
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