Chapter 5
Rocks, Fossils
and Time—
Making Sense
of the
Geologic
Record
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
Stratified Igneous Rocks
• Stratification in a succession of lava
flows in Oregon.
Stratified Sedimentary Rocks
• Stratification in sedimentary rocks consisting of
alternating layers of sandstone and shale, in
California.
Stratified Metamorphic Rocks
• Stratification in Siamo Slate, in Michigan
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
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.
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.
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.
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
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
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
A Disconformity
• A disconformity between sedimentary rocks in
California, with conglomerate deposited upon an
erosion surface in the underlying rocks
An Angular Unconformity
• An angular
unconformity,
Santa Rosa
A Nonconformity
• A nonconformity in South Dakota separating
Precambrian metamorphic rocks from the overlying
Cambrian-aged Deadwood Formation
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
– 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
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
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
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
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
Marine Transgression
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
A Marine Transgression in the
Grand Canyon
• Three formations
deposited in a
widespread marine
transgression exposed
in the walls of the
Grand Canyon, Arizona
Marine Regression
• During a marine regression, sea level falls with
respect to the continent
– the environments
paralleling the shoreline
migrate seaward
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
Walther’s Law
• Johannes Walther (1860-1937) 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
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
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
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
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
Body Fossil
• Skeleton of a 2.3-m-long marine reptile in the museum at
Glacier Garden in Lucerne, Switzerland
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
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
Trace Fossils
• Paleontologists think
that a land-dwelling
beaver called
Paleocastor made this
spiral burrow in
Nebraska
Trace Fossils
• Fossilized feces (coprolite) of a carnivorous
mammal
• Specimen measures about 5 cm long and contains
small fragments of bones
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
Unaltered Remains
• Insects
in amber
• Preservation
in tar
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
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
Altered Remains
• Carbon film of a
palm frond
• Carbon film of an insect
Molds and Casts
• Molds form when buried remains leave a
cavity
• Casts form if material fills in the cavity
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
Cast of a Turtle
• Fossil turtle showing some of the
original shell material
• body fossil
• and a cast
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
• WHY is the fossil record incomplete???
Why are there large gaps of time and
biological strata?
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
Fossils and Telling Time
• William Smith
• 1769-1839, 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
Fossils from Different Areas
• To compare the ages of
rocks from two different
localities
• Smith used fossils
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
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
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.
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
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
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
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
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
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
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
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
Lithostratigraphic Correlation
• Correlation of lithostratigraphic units
such as formations traces rocks laterally
across gaps
Lithostratigraphic Correlation
• We can correlate rock units based on
– composition
– position in a sequence
– and the presence of distinctive key beds
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
Time Equivalence
• The most effective way to demonstrate
time equivalence is time-stratigraphic
correlation using biozones
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
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 timestratigraphic correlation, because of
their short ranges
• They are guide fossils
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
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
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
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
Indirect Dating
• Absolute ages of sedimentary rocks are most often
found by determining radiometric ages of associated
igneous or metamorphic rocks
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
Indirect Dating
• Combining thousands of
absolute ages associated with
sedimentary rocks of known
relative age gives the numbers
on the geologic time scale