Chapter 5
Rocks, Fossils and Time—Making Sense of
the Geologic Record
-Basic Laws: Superposition, Horizontality, Inclusions, lateral continuity
-Unconformities: angular, disconformity, non conformity
-Sea Level changes: transgression (rise) or regression (fall)
-Sedimentary facies
-FOSSILS: what are they?...how do they form?...importance…
-use of fossils and rocks to tell ‘relative time’…
-use of fossils and rocks together to correlate rock outcrops..
-importance of ‘guide fossils’
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
• Uniformitarianism is useful for this activity
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 – from volcanoes
– such as a succession of lava flows or ash beds
– are stratified and obey the principles of stratigraphy
• Many metamorphic rocks are stratified
– metamorphic rocks- formed by igneous intrusions
that heat and change minerals in original rocks
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
Law of Superposition
• Nicolas Steno realized that he could determine
– the relative ages of horizontal (undeformed) strata
– by their position in a sequence:
…youngest rocks are on top, oldest on bottom…
• In deformed strata, the task is more difficult
– but 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
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
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
• In the process of forming 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
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 in Colorado
– between steeply dipping Pennsylvanian rocks
– and overlying Cenozoic-aged conglomerate
A Nonconformity
• A nonconformity in South Dakota separating
– Precambrian metamorphic rocks from
– the overlying Cambrian-aged Deadwood Formation
Sea Level Change- 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
• 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
younger
– meaning the ages vary from place to place
shale
older shale
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
– and the environments
paralleling the
shoreline
– migrate seaward
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
– which 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
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
– which may provide information about the size
– and diet of the animal that produced it
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
– or altered remains,
• with some change in composition or structure
• permineralized, recrystallized, replaced, carbonized
Unaltered Remains
• Insects in
amber
• Preservation
in tar
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
Molds and Casts
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
Fossil Record
• The fossil record is very incomplete because
–
–
–
–
–
bacterial decay,
physical processes,
scavenging,
and metamorphism
destroy organic remains
• In spite of this, fossils are quite common
Rocks, Fossils and Time—
Making Sense of the Geologic Record
Fossils have many usesa. Give an indication of relative time in comparison to other rocks
and fossils above, below and laterally to a particular layer..
b. Some fossils can be used as indicators of paleoenvironmenti.e. they are indicative of certain environments that they lived in.
Benthic forams =bottom dwellers, different forams lived in different
water depths. Planktonic forams- float near surface in ocean. Recognize
differences between low to mid to high latitude forms (morphology).
Pollen- indicative of swamps, forests, humid vs dry
environments.
c. Preservation of calcareous vs siliceous fossils are indicative of
certain depositional or post depositional processes.
Fossils and Telling Time
• William Smith
• 1769-1839, 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
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
Geologic Column and the
Relative Geologic Time Scale
Absolute
ages (the
numbers)
were
added
much
later.
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
– which is 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
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
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
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
• a 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 time-stratigraphic correlation,
because of their short ranges
• They are guide fossils
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
Classification of
Stratigraphic Units
Lithostratigraphic
Units
• Supergroup
– Group
• Formation
– Member
» Bed
TimeTimestratigraphic
Units
Units
• Eonothem
• Eon
– Erathem
• System
– Series
» Stage
– Era
• Period
– Epoch
» Age
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
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
Indirect Dating
• Absolute ages of sedimentary rocks
– are most often found
– by determining radiometric ages
– of associated igneous or metamorphic rocks
Geologic Time Scale
• Combining thousands of
absolute ages
– associated with
sedimentary rocks
– of known relative age
– gives the numbers
– on the geologic time
scale
Summary
• The first step in deciphering the geologic
history of a region
– is determining relative ages of the rocks
• First ascertain the vertical relationships
– among the rock layers
– even if they have been complexly deformed
• The geologic record
– is an accurate chronicle of ancient events,
– but it has many discontinuities or unconformities
– representing times of nondeposition, erosion or
both
Summary
• Simultaneous deposition
–
–
–
–
in adjacent but different environments
yields sedimentary facies,
which are bodies of sediment or sedimentary rock
with distinctive lithologic and biologic attributes
• According to Walther’s law,
– the facies in a conformable vertical sequence
– replace one another laterally
• During a marine transgression,
– a vertical sequence of facies results
– with offshore facies superposed over nearshore
facies
Summary
• During a marine regression,
–
–
–
–
a vertical sequence of facies results
with nearshore facies superposed
over offshore facies,
the opposite of transgression
• Marine transgressions and regressions result
from:
– uplift and subsidence of continents
– the amount of water in glaciers
– rate of seafloor spreading (volume of ridges)
Summary
• Most fossils are found in sedimentary rocks
– although they might also be in volcanic ash,
– volcanic mudflows, but rarely in other rocks
• Fossils are actually quite common,
–
–
–
–
but the fossil record is strongly biased
toward those organisms
that have durable skeletons
and that lived where burial was likely
• Law of fossil succession (William Smith)
– holds that fossil assemblages succeed one another
– through time in a predictable order
Summary
• Superposition and fossil succession
– were used to piece together
– a composite geologic column
– which serves as a relative time scale
• To bring order to stratigraphic terminology,
– geologists recognize units based entirely on content
• lithostratigraphic and biostratigraphic units
– and those related to time
• time-stratigraphic and time units
• Lithostratigraphic correlation involves
– demonstrating the original continuity
– of a presently discontinuous rock unit over an area
Summary
• Biostratigraphic correlation of range zones,
–
–
–
–
and especially concurrent range zones,
demonstrates that rocks in different areas
are of the same relative age,
even with different compositions
• The best way to determine absolute ages
– of sedimentary rocks and their contained fossils
– is to obtain absolute ages
– for associated igneous and metamorphic rocks
Chapter 6
Sedimentary Rocks—
The Archives of Earth History
History from Sedimentary Rocks
• How do we know whether sedimentary rocks
were deposited on
– continents—river floodplains or desert sand dunes?
– at the water's edge?
– in the sea?
• Sedimentary rocks
– preserve evidence of surface depositional processes
– also, many contain fossils
– These things give clues to the depositional
environment
• Depositional environments are specific areas
– or environments where sediment is deposited
Investigating Sedimentary Rocks
• Observation and data gathering
– visit rock exposures (outcrops)
– carefully examine
•
•
•
•
•
textures
composition
fossils (if present)
thickness
relationships to other rocks
• Preliminary interpretations in the field
– For example:
• red rocks may have been deposited on land
• whereas greenish rocks are more typical of marine
deposits
• (caution: exceptions are numerous)
Investigating Sedimentary Rocks
• More careful study of the rocks
–
–
–
–
microscopic examination
chemical analyses
fossil identification
interpretation of vertical and lateral facies
relationships
– compare with present-day sediments
• Make environmental interpretation
Composition of Detrital Rocks
• Very common minerals in detrital rocks:
– quartz, feldspars, and clay minerals
• Only calcite is very common in limestones
• Detrital rock composition tells
– about source rocks,
– not transport and deposition
• Quartz sand may have been deposited
– in a river system
– on a beach or
– in sand dunes
Composition of
Chemical Sedimentary Rocks
• Composition of chemical sedimentary rocks
– is more useful in revealing environmental
information
• Limestone is deposited in warm, shallow seas
– although a small amount also originates in lakes
• Evaporites such as rock salt and rock gypsum
– indicate arid environments
– where evaporation rates were high
• Coal originates in swamps and bogs on land
Grain Size
• Detrital grain size gives some indication
– of the energy conditions
– during transport and deposition
• High-energy processes
– such as swift-flowing streams and waves
– are needed to transport gravel
• Conglomerate must have been deposited
– in areas where these processes prevail
• Sand transport also requires vigorous currents
• Silt and clay are transported
– by weak currents and accumulate
– only under low-energy conditions
– as in lakes and lagoons
Sorting and Rounding
• Sorting and rounding are two textural features
– of detrital sedimentary rocks
– that aid in determining depositional processes
• Sorting refers to the variation
– in size of particles
– making up sediment or sedimentary rocks
• It results from processes
– that selectively transport and deposit
– sediments of particular sizes
Sorting
• If the size range is not very great,
– the sediment or rock is well sorted
• If they have a wide range of sizes,
– they are poorly sorted
• Wind has a limited ability to transport sediment
– so dune sand tends to be well sorted
• Glaciers can carry any sized particles,
– because of their transport power,
– so glacier deposits are poorly sorted
Rounding
• Rounding is the degree to which
– detrital particles have their sharp corners and edges
– warn away by abrasion
• Gravel in transport is rounded very quickly
– as the particles collide with one another
• Sand becomes rounded
– with considerably more transport
Rounding and Sorting
• A deposit
– of well rounded
– and well sorted
gravel
• Angular, poorly
sorted gravel
Sedimentary Structures
• Sedimentary structures are
– features visible at the scale of an outcrop
– that formed at the time of deposition or shortly
thereafter
– and are manifestations of the physical and
biological processes
– that operated in depositional environments
• Structures
–
–
–
–
–
seen in present-day environments
or produced in experiments
help provide information
about depositional environments of rocks
with similar structures
Bedding
• Sedimentary rocks generally have bedding or
stratification
– Individual layers
less than 1 cm
thick are
laminations
• common in
mudrocks
– Beds are thicker
than 1 cm
• common in rocks
with coarser grains
Graded Bedding
• Some beds show an upward gradual decrease
– in grain size, known as graded bedding
• Graded bedding is
common in
turbidity current
deposits
– which form when
sediment-water
mixtures flow
along the seafloor
– As they slow,
– the largest
particles settle out
– then smaller ones
Cross-Bedding
• Cross-bedding forms when layers come to rest
– at an angle to the surface
– upon which they accumulate
– as on the downwind side of a sand dune
• Cross-beds result from transport
– by either water or wind
• The beds are inclined or dip downward
– in the direction of the prevailing current
• They indicate ancient current directions,
– or paleocurrents
• They are useful for relative dating
– of deformed sedimentary rocks
Cross-Bedding
• Tabular crossbedding forms by
deposition on sand
waves
• Tabular crossbedding in the Upper
Cretaceous Two
Medicine Formation
in Montana
Cross-Bedding
• Trough cross-bedding
formed by migrating
dunes
• Trough cross-beds in
the Pliocene Six Mile
Creek Formation,
Montana
Ripple Marks
• Small-scale alternating ridges and troughs
– known as ripple marks are common
– on bedding planes, especially in sandstone
• Current ripple marks
–
–
–
–
form in response to water or wind currents
flowing in one direction
and have asymmetric profiles allowing geologists
to determine paleocurrent directions
• Wave-formed ripple marks
– result from the to-and-fro motion of waves
– tend to be symmetrical
• Useful for relative dating of deformed
sedimentary rocks
Current Ripple Marks
• Ripples with an
asymmetrical shape
• In the close-up of
one ripple,
– the internal structure
– shows small-scale
cross-bedding
• The photo shows
current ripples
– that formed in a
small stream channel
– with flow from right
to left
Wave-Formed Ripples
• As the waves
wash back
and forth,
– symmetrical
ripples form
• The photo
shows waveformed ripple
marks
– in shallow
seawater
Mud Cracks
• When clay-rich sediments dry, they shrink
– and crack into polygonal patterns
– bounded by fractures called mud cracks
• Mud cracks require wetting and drying to form,
– as along a
lakeshore
– or a river flood
plain
– or where mud is
exposed at low
tide along a
seashore
Ancient Mud Cracks
• Mud cracks in
ancient rocks
– in Glacier
National
Park,
Montana
• Mud cracks
typically fill in
– with sediment
– when they are
preserved
– as seen here
Biogenic Sedimentary Structures
• Biogenic sedimentary structures include
– tracks
– burrows
– trails
• called trace fossils
• Extensive burrowing by organisms
– is called bioturbation
• It may alter sediments so thoroughly
– that other structures are disrupted or destroyed
Bioturbation
• U-shaped burrows
• Vertical burrows
Bioturbation
• Vertical, dark-colored areas in this rock are
sediment-filled burrows
– Could you use burrows such as these to relatively
date layers in deformed sedimentary rocks?
No Single Structure Is Unique
• Sedimentary structures are important
– in environmental analyses
– but no single structure is unique to a specific
environment
• Example:
– Current ripples are found
• in stream channels
• in tidal channels
• on the sea floor
• Environmental determinations
– are usually successful with
– associations of a groups of sedimentary structures
– taken along with other sedimentary rock properties
Geometry of Sedimentary Rocks
• The three-dimensional shape or geometry
–
–
–
–
–
–
–
–
of a sedimentary rock body
may be helpful in environmental analyses
but it must be used with caution
because the same geometry may be found
in more than one environment
can be modified by sediment compaction
during lithification
and by erosion and deformation
• Nevertheless, it is useful in conjunction
– with other features
Blanket or Sheet Geometry
• Some of the most extensive sedimentary rocks
– in the geologic record result from
– marine transgressions and regressions
• The rocks commonly cover
– hundreds or thousands of square kilometers
– but are perhaps only
– a few tens to hundreds of meters thick
• Their thickness is small compared
– to their length and width
• Thus, they are said to have
– blanket or sheet geometry
Elongate or Shoestring Geometry
• Some sand deposits have an elongate or
shoestring geometry
– especially those deposited in
• stream channels
• or barrier islands
Other Geometries
• Delta deposits tend to be lens shaped
– when viewed in cross profile or long profile
– but lobate when observed from above
• Buried reefs are irregular
– but many are long and narrow
– or rather circular
Fossils—The Biological Content
of Sedimentary Rocks
• Fossils
– are the remains or traces of prehistoric organisms
– can be used in stratigraphy for relative dating and
correlation
– are constituents of rocks, sometimes making up the
entire rock
– and provide evidence of depositional environments
• Many limestones are composed
– in part or entirely of shells or shell fragments
• Much of the sediment on the deep-seafloor
– consists of microscopic shells of organisms
Fossils Are Constituents of
Sedimentary Rocks
• This variety of
limestone,
– known as
coquina,
– is made entirely
of shell
fragments
Fossils in
Environmental Analyses
• Did the organisms in question live where they
were buried?
• Or where their remains or fossils transported
there?
• Example:
–
–
–
–
–
Fossil dinosaurs usually indicate deposition
in a land environment such as a river floodplain
But if their bones are found in rocks with
clams, corals and sea lilies,
we assume a carcass was washed out to sea
Environmental Analyses
• What kind of habitat did the organisms
originally occupy?
• Studies of a fossil’s structure
– and its living relatives, if any,
– help environmental analysis
• For example: clams with heavy, thick shells
– typically live in shallow turbulent water
– whereas those with thin shells
– are found in low-energy environments
• Most corals live in warm, clear,
– shallow marine environments where
– symbiotic bacteria can carry out photosynthesis
Depositional Environments
• A depositional environment
–
–
–
–
is anywhere sediment accumulates
especially a particular area
where a distinctive kind of deposit originates
from physical, chemical, and biological processes
• Three broad areas of deposition include
–
–
–
–
continental
transitional
marine
each of which has several specific environments
Depositional Environments
Continental environments
Transitional environments
Marine
environments
Continental Environments
• Deposition on continents (on land) might take
place in
– fluvial systems – rivers and streams
– deserts
– areas covered by and adjacent to glaciers
• Deposits in each of these environments
– possess combinations of features
– that allow us to differentiate among them
Fluvial
• Fluvial refers to river and stream activity
– and to their deposits
• Fluvial deposits accumulate in either of two
types of systems
• One is a braided stream system
–
–
–
–
with multiple broad, shallow channels
in which mostly sheets of gravel
and cross-bedded sand are deposited
mud is nearly absent
Braided Stream
• The deposits of braided streams are mostly
– gravel and cross-bedded sand with subordinate mud
Braided Stream Deposits
• Braided stream
deposits consist of
– conglomerate
– cross-bedded
sandstone
– but mudstone is rare
or absent
Fluvial Systems
• The other type of system is a meandering
stream
– with winding channels
– mostly fine-grained sediments on floodplains
– cross-bedded sand bodies with shoestring
geometry
– point-bar deposits consisting of a sand body
– overlying an erosion surface
– that developed on the convex side of a meander
loop
Meandering Stream
• Meandering
stream
deposits
– are mostly fine-grained floodplain
– sediments with subordinate sand bodies
Meandering Stream Deposits
• In meandering stream
deposits,
– mudstone deposited in a
floodplain is common
– sandstones are point bar
deposits
– channel conglomerate is
minor
Desert Environments
• Desert environments contain an association of
features found in
– sand dune deposits,
– alluvial fan deposits,
– and playa lake deposits
• Windblown dunes are typically composed
– of well-sorted, well-rounded sand
– with cross-beds meters to tens of meters high
– land-dwelling plants and animals make up any
fossils
Associations in Desert Basin
• A desert basin showing the
association
– of alluvial fan,
– sand dune,
– and playa lake deposits
• In the photo,
– the light colored area in the
distance
– is a playa lake deposit in
Utah
Dune Cross-Beds
• Large-scale crossbeds
– in a Permian-aged
– wind-blown dune
deposit in Arizona
Alluvial Fans and Playa Lakes
• Alluvial fans form best along the margins of
desert basins
–
–
–
–
where streams and debris flows
discharge from mountains onto a valley floor
They form a triangular (fan-shaped) deposit
of sand and gravel
• The more central part of a desert basin
– might be the site of a temporary lake, a playa lake,
– in which laminated mud and evaporites accumulate
Glacial Environments
• All sediments deposited in
– glacial environments are collectively called drift
• Till is poorly sorted, nonstratified drift
– deposited directly by glacial ice
– mostly in ridge-like deposits called moraines
• Outwash is sand and gravel deposited
– by braided streams issuing from melting glaciers
• The association of these deposits along with
– scratched (striated) and polished bedrock
– is generally sufficient to conclude
– that glaciers were involved
Moraines and Till
• Origin of glacial drift
• Moraines and poorly sorted till
Glacial Varves
• Glacial lake deposits show
– alternating dark and light laminations
• Each dark-light couplet is a varve,
– representing one year’s accumulation of sediment
– light layers accumulate in summer
– dark in winter
• Dropstones
– liberated from
icebergs
– may also be
present
– Varves with a
dropstone
Transitional Environments
• Transitional environments include those
– with both marine and continental processes
• Example:
–
–
–
–
Deposition where a river or stream (fluvial system)
enters the sea
yields a body of sediment called a delta
with deposits modified by marine processes,
especially waves and tides
• Transitional environments include
–
–
–
–
deltas
beaches
barrier islands and lagoons
tidal flats
Transitional Environments
Transitional environments
Marine Deltas
• Marine deltas rarely conform precisely
– to this simple threefold division because
– they are strongly influenced
– by one or more modifying processes
• When fluvial processes prevail
– a stream/river-dominated delta results
• Strong wave action
– produces a wave dominated delta
• Tidal influences
– result in tide-dominated deltas
Stream/River-Dominated Deltas
• Stream/riverdominated
deltas
– have long
distributary
channels
– extending far
seaward
– Mississippi
River delta
Wave-Dominated Deltas
• Wavedominated
deltas
– such as the Nile
Delta of Egypt
– also have
distributary
channels
– but their
seaward margin
– is modified by
wave action
Tide-Dominated Deltas
• Tide-Dominated Deltas,
– such as the Ganges-Brahmaputra delta
– of Bangladesh
– have
tidal
sand
bodies
– along the
direction
of tidal
flow
Barrier Islands
• On broad continental margins
– with abundant sand, long barrier islands lie offshore
– separated from the mainland by a lagoon
• Barrier islands are common along the Gulf
– and Atlantic Coasts of the United States
• Many ancient deposits formed in this
environment
• Subenvironments of a barrier island complex:
– beach sand grading offshore into finer deposits
– dune sands contain shell fragments
• not found in desert dunes
– fine-grained lagoon deposits
– with marine fossils and bioturbation
Barrier Island Complex
• Subenvironments of a barrier island complex
Tidal Flats
• Tidal flats are present
– where part of the shoreline is periodically covered
– by seawater at high tide and then exposed at low tide
• Many tidal flats build or prograde seaward
– and yield a sequence of rocks grading upward
– from sand to mud
• One of their most distinctive features
– is sets of cross-beds that dip in opposite directions
Marine Environments
• Marine environments include:
–
–
–
–
continental shelf
continental slope
continental rise
deep-seafloor
• Much of the detritus eroded from continents
– is eventually deposited in marine environments
• but sediments derived from chemical
– and organic activity are found here as well, such as
• limestone
• evaporites
• both deposited in shallow marine environments
Marine Environments
Marine
environments
Detrital Marine Environments
• The gently sloping area adjacent to a continent
– is a continental shelf
• It consists of a high-energy inner part that is
– periodically stirred up by waves and tidal currents
• Its sediment is mostly sand,
– shaped into large cross-bedded dunes
• Bedding planes are commonly marked
– by wave-formed ripple marks
• Marine fossils and bioturbation are typical
Slope and Rise
• The low-energy part of the shelf
– has mostly mud with marine fossils,
– and interfingers with inner-shelf sand
• Much sediment derived from the continents
– crosses the continental shelf
– and is funneled into deeper water
– through submarine canyons
• It eventually comes to rest
– on the continental slope and continental rise
– as a series of overlapping submarine fans
Detrital Marine Environments
• Shelf, slope and rise environments
• The main avenues of sediment transport
– across the shelf are submarine canyons
Turbidity currents
carry sediment
to the
submarine fans
Sand with
graded bedding
and mud settled
from seawater
Deep Sea
• Beyond the continental rise, the seafloor is
– nearly completely covered by fine-grained deposits
• no sand and gravel
– or no sediment at all
• near mid-ocean ridges
• The main sources of sediment are:
– windblown dust from continents or oceanic islands
– volcanic ash
– shells of microorganisms dwelling in surface
waters of the ocean
Deep Sea
• Types of sediment are:
– pelagic clay,
• which covers most of the deeper parts
• of the seafloor
– calcareous (CaCO3) and siliceous (SiO2) oozes
• made up of microscopic shells
Carbonate Environments
• Carbonate rocks are
– limestone, which is composed of calcite
– dolostone, which is composed of dolomite
• most dolostone is altered limestone
• Limestone is similar to detrital rock in some
ways
– Many limestones are made up of
• gravel-sized grains
• sand-sized grains
• microcrystalline carbonate mud called micrite
– but the grains are all calcite
– and are formed in the environment of deposition,
– not transported there
Limestone Environments
• Some limestone form in lakes,
–
–
–
–
but most limestone by is deposited
in warm shallow seas
on carbonate shelves and
on carbonate platforms rising from oceanic depths
• Deposition occurs where
– little detrital sediment, especially mud, is present
• Carbonate barriers form in high-energy areas
and may be
– reefs
– banks of skeletal particles
– accumulations of spherical carbonate grains known
as oolites
• which make up the grains in oolitic limestone
Evaporite Environments
• Evaporites consist of
– rock salt
– rock gypsum
• They are found in environments such as
– playa lakes
– saline lakes
– but most of the extensive deposits formed in the
ocean
• Evaporites are not nearly as common
– as sandstone, mudrocks and limestone,
– but can be abundant locally
Evaporites
• Large evaporite deposits
– lie beneath the Mediterranean Seafloor
• more than 2 km thick
– in western Canada, Michigan, Ohio, New York,
– and several Gulf Coast states
• How some of these deposits originated
– is controversial, but geologists agree
– that high evaporation rates of seawater
– caused minerals to precipitate from solution
• Coastal environments in arid regions
– such as the present-day Persian Gulf
– meet the requirements
Environmental Interpretations
and Historical Geology
• Present-day gravel
deposits
– by a swiftly-flowing stream
– Most transport and
deposition takes place
when the stream is higher
• Nearby gravel deposit
probably less than a few
thousand years old
Environmental Interpretations
and Historical Geology
• Conglomerate
more than 1 billion
years old
– shows similar
features
• We infer that it too was deposited
–
–
–
–
by a braided stream in a fluvial system
Why not deposition by glaciers or along a seashore?
Because evidence is lacking for either
glacial activity or transitional environment
Interpretation
• Jurassic-aged Navajo Sandstone
– of the Southwestern United states
– has all the features of wind-blown sand dunes:
•
•
•
•
•
•
•
•
the sandstone is mostly well-sorted, well-rounded quartz
measuring 0.2 to 0.5 mm in diameter
tracks of land-dwelling animals,
including dinosaurs, are present
cross-beds up to 30 m high have current ripple marks
like those produced on large dunes by wind today
cross-beds dip generally southwest
indicating a northeast prevailing wind
Navajo Sandstone
Checkerboard Mesa,
Zion National Park,
Utah
– Vertical
fractures
– intersect
cross beds
of desert
dunes
– making the
checkerboard
pattern
Paleogeography
• Paleogeography deals with
– Earth’s geography of the past
• Using interpretations
– of depositional environment
– such as the ones just discussed
• we can attempt to reconstruct
– what Earth’s geography was like
– at these locations at various times in the past
• For example,
– the Navajo Sandstone shows that a vast desert
– was present in what is now the southwest
– during the Jurassic Period
Paleogeography
– and from Late Precambrian to Middle Cambrian
– the shoreline migrated inland from east and west
– during a marine transgression
Paleogeography
• Detailed studies of various
rocks
–
–
–
–
–
in several western states
allow us to determine
with some accuracy
how the area appeared
during the Late Cretaceous
• A broad coastal plain
– sloped gently eastward
– from a mountainous region
– to the sea
Paleogeography
• Later, vast lakes,
– river floodplains, alluvial fans
– covered much of this area
– and the sea had withdrawn
from the continent
• Interpretations the geologic
record
– we examine later
– will be based on similar
– amounts of supporting
evidence
Summary
• The physical and biological features
– of sedimentary rocks reveal something about
– the depositional processes that form them
• Environmental analysis
–
–
–
–
of sedimentary rocks uses
mainly sedimentary structures and fossils
but also textures, rock body geometry
and even composition
• Geologists recognize
– three primary depositional areas
– continental, transitional, and marine
– each with several specific environments
Summary
• Fluvial systems might be braided or
meandering
– Braided streams deposit mostly sand and
gravel,
– whereas deposits of meandering streams are
mostly mud and subordinate sand bodies with
shoestring geometry
• An association of alluvial fan, sand dune,
– and playa lake deposits
– is typical of desert depositional environments
• Glacial deposits consist mostly of till
– in moraines and outwash
Summary
• The simplest deltas, those in lakes,
– consist of a three-part sequence of rocks
– grading from finest at the base,
– upward to coarser-grained rocks
• Marine deltas dominated by
– fluvial processes, waves, or tides
– are much larger and more complex
• A barrier island system includes beach,
– dune, and lagoon subenvironments,
– each characterized a unique association
– of rocks, sedimentary structures, and fossils
Summary
• Inner shelf deposits are mostly sand,
– whereas those of the outer shelf are mostly mud;
– both have marine fossils and bioturbation
• Much of the sediment from land
– crosses the shelves and is deposited
– on the continental slope and rise as submarine fans
• Either pelagic clay or oozes
– derived from the shells of
– microscopic floating organisms cover
– most of the deep seafloor
Summary
• Most limestone originates in shallow,
– warm seas where little detrital mud is present
• Carbonate rocks (just as detrital rocks)
–
–
–
–
may possess cross-beds, ripple marks,
mud cracks, and fossils
that provide information
about depositional processes
• Evaporites form in several environments,
– but the most extensive ones were deposited
– in marine environments
• In all cases, though, they formed
– in arid regions with high evaporation rates
Summary
• With information from sedimentary rocks,
– as well as other rocks,
– geologists determine the past distribution
– of Earth's surface features
– Determine the environment of deposition of a
particular package of rocks
- If fossils are present, make some relative age
determination based upon the fossil content
Phosphorous
• For instance, phosphorous
– from phosphorous-rich sedimentary rocks
– is used in
•
•
•
•
•
•
metallurgy
preserved foods
ceramics
matches
chemical fertilizers
animal-feed supplements
Beach Environment
• Sand deposition
– on a beach along the
Pacific coast
– of the United States
• Many ancient sandstones
– possess features
– that indicate they were
– also deposited on beaches
Sedimentary rocks
• Sedimentary rocks may be
–
–
–
–
–
detrital
or chemical, including biochemical
and all preserve evidence
of the physical, chemical and biological processes
that formed them
• Some sedimentary rocks are or contain
resources
– phosphorous
– liquid petroleum
– natural gas
Slope and Rise
• Once sediment passes the outer margin
– of the self, the shelf-slope break,
– turbidity currents transport it
• So sand with graded bedding is common
• Also common is mud that settled from
seawater
Simple Deltas
• The simplest deltas are those in lakes and
consist of
– topset beds
– foreset beds
– bottomset
beds
– As the delta
builds
outward it
progrades
–
–
–
–
and forms a vertical sequence of rocks
that becomes coarser-grained from the bottom to top
The bottomset beds may contain marine (or lake) fossils,
whereas the topset beds contain land fossils
Carbonate Subenvironments
• Reef rock tends to be
– structureless
– composed of skeletons of corals, mollusks, sponges
and other organisms
• Carbonate banks are made up of
– layers with horizontal beds
– cross-beds
– wave-formed ripple marks
• Lagoons tend to have
– micrite
– with marine fossils
– bioturbation
Microfossils
• Microfossils are particularly useful
– because many individuals can be recovered
– from small rock samples
• In oil-drilling operations, small rock chips
– called well cuttings are brought to the surface
• These cuttings rarely
– contain complete fossils of large organisms,
– but they might have thousands of microfossils
– that aid in relative dating and environmental
analyses
Trace Fossils In Place
• Trace fossils, too, may be characteristic of
particular environments
• Trace fossils, of course, are not transported
from their original place of origin
Tidal Flats
• Tidal-flat deposits showing a prograding
shoreline
– Notice the distinctive cross-beds
– that dip in opposite directions
– How could this happen?
Carbonate Shelf
• The
carbonate
shelf is
attached to
a continent
– Examples
occur in
southern
Florida
and the
Persian
Gulf
Carbonate Platform
• Carbonates may be deposited on a platform
– rising from oceanic depths
• This example shows a cross-section
– of the present-day Great Bahama Bank
– in the Atlantic Ocean southeast of Florida
Evaporites
• Evaporites
could form
• in an
environment
similar to this
• if the area
were in an
arid region,
– with restricted inflow of normal seawater
– into the lagoon
– leading to increased salinity and salt depositions
Trace Fossils
• Paleontologists think
– that a land-dwelling
beaver
– called Paleocastor
– made this spiral
burrow in Nebraska
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
Geologic Record
• for nearly 14
million years of
Earth history
– preserved at Sheep
Rock
– in John Day Fossil
Beds National
Monument,
Oregon
• Fossils in these
rocks
– provide a record
– of climate change
– and biological
events
Relative Ages between
Separate Areas
• Rocks in A may be
– younger than those in B,
– the same age as in B
– older than in B
• Fossils could solve this
problem
Matching Rocks Using Fossils
youngest
oldest
• The youngest rocks are in column B
– whereas the oldest ones are in column C
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
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
Dispute of Systems
• Ordovician System
– Lapworth assigned the overlap
– between the two to a new system,
– the Ordovician
System Dispute
• The dispute was settled in 1879
– when Lapworth proposed the Ordovician
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
Distinct Aspect
• An assemblage of fossils
– has a distinctive aspect
– compared with younger
– or older fossil assemblages
Matching Rocks Using Fossils
youngest
oldest
• 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
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
Time Equivalence
• The most effective way
– to demonstrate time equivalence
– is time-stratigraphic correlation
– using biozones
• But other methods are useful
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
• 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
Unaltered Remains
• 40,000year-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
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
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
Marine Regression
• A marine regression
– is the opposite of a marine transgression
• It yields a vertical sequence
–
–
–
–
with nearshore facies
overlying offshore facies
and rock units become younger
in the seaward direction
younger shale
older
shale
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
– Fossils can help….