Rocks, Fossils and Rocks, Fossils and Time—Time—

Making Sense of the Making Sense of the Geologic RecordGeologic Record

Chapter 5Chapter 5

► The fact that Earth has changed through The fact that Earth has changed through time is apparent from evidence in the time is apparent from evidence in the geologic recordgeologic record

► The The geologic recordgeologic record is the record of events is the record of events preserved in rockspreserved in rocks Although all rocks are useful in deciphering the Although all rocks are useful in deciphering the

geologic record, sedimentary rocks are especially geologic record, sedimentary rocks are especially usefuluseful

The geologic record is complex and requires The geologic record is complex and requires interpretation, which we will try to dointerpretation, which we will try to do

Geologic RecordGeologic Record

► StratigraphyStratigraphy deals with the study of any deals with the study of any layered (stratified) rock, but primarily with layered (stratified) rock, but primarily with sedimentary rocks and their sedimentary rocks and their

►compositioncomposition►originorigin►age relationshipsage relationships►geographic extentgeographic extent

► Many igneous rocks Many igneous rocks such as a succession of lava flows or ash beds such as a succession of lava flows or ash beds

are stratified and obey the principles of are stratified and obey the principles of stratigraphystratigraphy

► Many metamorphic rocks are stratifiedMany metamorphic rocks are stratified

StratigraphyStratigraphy

►Stratification in a succession of Stratification in a succession of lava flows in Oregon.lava flows in Oregon.

Stratified Igneous RocksStratified Igneous Rocks

► Stratification in sedimentary rocks Stratification in sedimentary rocks consisting of alternating layers of consisting of alternating layers of sandstone and shale, in California.sandstone and shale, in California.

Stratified Sedimentary Stratified Sedimentary RocksRocks

► Stratification in Siamo Slate, in MichiganStratification in Siamo Slate, in Michigan

Stratified Metamorphic Stratified Metamorphic RocksRocks

► Surfaces known as Surfaces known as bedding planesbedding planes separate individual strata from one anotherseparate individual strata from one another or the strata grade vertically from one rock or the strata grade vertically from one rock

type to anothertype to another► Rocks above and below a bedding Rocks above and below a bedding

plane differ in composition, texture, plane differ in composition, texture, color or a combination of these color or a combination of these featuresfeatures

► The bedding plane signifies The bedding plane signifies a rapid change in sedimentation a rapid change in sedimentation or perhaps a period of nondepositionor perhaps a period of nondeposition

Vertical Stratigraphic Vertical Stratigraphic RelationshipsRelationships

► Nicolas StenoNicolas Steno realized that he could determine realized that he could determine the relative ages of horizontal (undeformed) the relative ages of horizontal (undeformed) strata by their position in a sequencestrata by their position in a sequence

► In deformed strata, the task is more difficult In deformed strata, the task is more difficult some sedimentary structures, such as cross-some sedimentary structures, such as cross-

bedding, and some fossils allow geologists to bedding, and some fossils allow geologists to resolve these kinds of problemsresolve these kinds of problems

► we will discuss the use of sedimentary structures more fully we will discuss the use of sedimentary structures more fully later in the termlater in the term

SuperpositionSuperposition

► According to the According to the principle of inclusionsprinciple of inclusions, , which which also helps to determine relative ages, inclusions or also helps to determine relative ages, inclusions or fragments in a rock are older than the rock itselffragments in a rock are older than the rock itself

Principle of InclusionsPrinciple of Inclusions

► Light-colored granite in Light-colored granite in northern Wisconsin northern Wisconsin showing basalt showing basalt inclusions (dark)inclusions (dark)

► Which rock is older?Which rock is older? Basalt, because the Basalt, because the

granite includes itgranite includes it

► Determining the relative ages of lava flows, sills Determining the relative ages of lava flows, sills and associated sedimentary rocks uses alteration and associated sedimentary rocks uses alteration by heat and inclusionsby heat and inclusions

Age of Lava Flows, SillsAge of Lava Flows, Sills

► How can you determine whether a layer of basalt How can you determine whether a layer of basalt within a sequence of sedimentary rocks is a within a sequence of sedimentary rocks is a buried lava flow or a sill?buried lava flow or a sill?

A lava flow forms in sequence with A lava flow forms in sequence with the sedimentary layers.the sedimentary layers.

► Rocks below the lava will have signs of Rocks below the lava will have signs of heating but not the rocks above.heating but not the rocks above.

► The rocks above may have lava The rocks above may have lava inclusions.inclusions.

A sill will heat the rocks above and below.A sill will heat the rocks above and below.

SillSill

The sill might also have inclusions of the rocks above and The sill might also have inclusions of the rocks above and below, below,

but neither of these rocks but neither of these rocks will have inclusions of will have inclusions of the sill.the sill.

► So far we have discussed vertical relationships among So far we have discussed vertical relationships among conformableconformable strata, which are sequences of rocks in strata, which are sequences of rocks in which deposition was more or less continuouswhich deposition was more or less continuous

► UnconformitiesUnconformities in sequences of strata represent in sequences of strata represent times of nondeposition and/or erosion that encompass times of nondeposition and/or erosion that encompass long periods of geologic time, perhaps millions or tens long periods of geologic time, perhaps millions or tens of millions of yearsof millions of years

► The rock record is incomplete.The rock record is incomplete. The interval of time not represented by strata is a The interval of time not represented by strata is a hiatushiatus..

UnconformitiesUnconformities

For 1 million years For 1 million years erosion occurred and erosion occurred and removed 2 MY of rocksremoved 2 MY of rocks

and giving rise to a 3 and giving rise to a 3 million year hiatusmillion year hiatus

The origin of an The origin of an unconformityunconformity

► The process of forming an unconformityThe process of forming an unconformity deposition began 12 million years ago (MYA), deposition began 12 million years ago (MYA), continues until 4 MYAcontinues until 4 MYA

► The last column The last column is the actual is the actual

stratigraphic record stratigraphic record with an unconformitywith an unconformity

► Three types of surfaces can be unconformities:Three types of surfaces can be unconformities: A A disconformitydisconformity is a surface separating younger is a surface separating younger

from older rocks, both of which are parallel to one from older rocks, both of which are parallel to one anotheranother

A A nonconformitynonconformity is an erosional surface cut into is an erosional surface cut into metamorphic or intrusive rocks and covered by metamorphic or intrusive rocks and covered by sedimentary rockssedimentary rocks

An An angular unconformityangular unconformity is an erosional surface is an erosional surface on tilted or folded strata over which younger rocks on tilted or folded strata over which younger rocks were depositedwere deposited

Types of UnconformitiesTypes of Unconformities

► Unconformities of regional extent may change Unconformities of regional extent may change from one type to anotherfrom one type to another

► They may not represent the same amount of They may not represent the same amount of geologic time everywheregeologic time everywhere

Types of UnconformitiesTypes of Unconformities

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

A DisconformityA Disconformity

An Angular UnconformityAn Angular Unconformity► An angular An angular

unconformityunconformity, Santa Rosa, Santa Rosa

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

A NonconformityA Nonconformity

► In 1669, Nicolas Steno proposed his In 1669, Nicolas Steno proposed his principle principle of lateral continuityof lateral continuity, meaning that layers of , meaning that layers of sediment extend outward in all directions until sediment extend outward in all directions until they terminatethey terminate Terminations may Terminations may

bebe► Abrupt at the edge of a Abrupt at the edge of a

depositional basin where erodeddepositional basin where eroded► where truncated by faultswhere truncated by faults

Lateral RelationshipsLateral Relationships

or they may be gradual or they may be gradual ► where a rock unit becomes where a rock unit becomes

progressively thinner until it progressively thinner until it pinches outpinches out

► or where it splits into thinner units or where it splits into thinner units each of which pinches out, each of which pinches out,

called called intertongingintertonging

► where a rock unit changes by where a rock unit changes by lateral lateral gradationgradation as its composition and/or as its composition and/or texture becomes increasingly differenttexture becomes increasingly different

► Both intertonging and lateral gradation indicate Both intertonging and lateral gradation indicate simultaneous deposition in adjacent simultaneous deposition in adjacent environmentsenvironments

► A A sedimentary faciessedimentary facies is a body of sediment is a body of sediment with distinctive physical, chemical and biological with distinctive physical, chemical and biological attributes deposited side-by-side with other attributes deposited side-by-side with other sediments in different environmentssediments in different environments

Sedimentary FaciesSedimentary Facies

► On a continental shelf, sand may accumulate in the high-On a continental shelf, sand may accumulate in the high-energy nearshore environmentenergy nearshore environment

Sedimentary FaciesSedimentary Facies

while mud and carbonate deposition takes place at while mud and carbonate deposition takes place at the same time in offshore low-energy environmentsthe same time in offshore low-energy environments

► A A marine transgressionmarine transgression occurs when sea level occurs when sea level rises with respect to the landrises with respect to the land

► During a marine transgression, During a marine transgression, the shoreline migrates landward the shoreline migrates landward the environments paralleling the shoreline migrate the environments paralleling the shoreline migrate

landward as the sea progressively covers more and landward as the sea progressively covers more and more of a continentmore of a continent

Marine TransgressionsMarine Transgressions

► Each laterally adjacent depositional environment Each laterally adjacent depositional environment produces a sedimentary faciesproduces a sedimentary facies

► During a transgression, the facies forming During a transgression, the facies forming offshore become superposed upon facies offshore become superposed upon facies deposited in nearshore environmentsdeposited in nearshore environments

Marine TransgressionsMarine Transgressions

Marine Transgression

Marine TransgressionMarine Transgression

► The rocks of each facies become younger The rocks of each facies become younger in a in a landward direction during a marine transgressionlandward direction during a marine transgression

► One body of rock with the same attributes (a facies) was One body of rock with the same attributes (a facies) was deposited gradually at different times in different places so it is deposited gradually at different times in different places so it is time transgressivetime transgressive meaning the ages vary from place to placemeaning the ages vary from place to place

► Three formations Three formations deposited in a deposited in a widespread marine widespread marine transgression exposed transgression exposed in the walls of the in the walls of the Grand Canyon, Grand Canyon, ArizonaArizona

A Marine Transgression in A Marine Transgression in the Grand Canyonthe Grand Canyon

► During a During a marine regressionmarine regression, sea level falls , sea level falls with respect to the continentwith respect to the continent

Marine RegressionMarine Regression

the environments paralleling the environments paralleling the shoreline migrate seawardthe shoreline migrate seaward

Marine RegressionMarine Regression

► A marine regression A marine regression is the opposite of a marine transgressionis the opposite of a marine transgression

► It yields a vertical sequence with nearshore It yields a vertical sequence with nearshore facies overlying offshore facie sand rock facies overlying offshore facie sand rock units become younger in the seaward units become younger in the seaward directiondirection

► Johannes Walther (1860-1937) noticed that the Johannes Walther (1860-1937) noticed that the same facies he found laterally were also present same facies he found laterally were also present in a vertical sequence, now called in a vertical sequence, now called Walther’s Walther’s LawLaw

Walther’s LawWalther’s Law

► holds that holds that the facies seen in a the facies seen in a

conformable vertical conformable vertical sequence will also replace sequence will also replace one another laterallyone another laterally

Walther’s law applies to Walther’s law applies to marine transgressions marine transgressions and regressionsand regressions

► Since the Late Precambrian, 6 major marine transgressions Since the Late Precambrian, 6 major marine transgressions followed by regressions have occurred in North Americafollowed by regressions have occurred in North America

► These produce rock sequences, bounded by unconformities, that These produce rock sequences, bounded by unconformities, that provide the structure for U.S. Paleozoic and Mesozoic geologic provide the structure for U.S. Paleozoic and Mesozoic geologic historyhistory

► Shoreline movements are a few centimeters per yearShoreline movements are a few centimeters per year

► Transgression or regressions with small reversals produce Transgression or regressions with small reversals produce intertongingintertonging

Extent and Rates of Extent and Rates of Transgressions and Transgressions and

RegressionsRegressions

► Uplift of continents causes regressionUplift of continents causes regression► Subsidence causes transgressionSubsidence causes transgression► Widespread glaciation causes regression Widespread glaciation causes regression

due to the amount of water frozen in glaciers due to the amount of water frozen in glaciers ► Rapid seafloor spreading, Rapid seafloor spreading,

expands the mid-ocean ridge system, expands the mid-ocean ridge system, displacing seawater onto the continentsdisplacing seawater onto the continents

► Diminishing seafloor-spreading rates Diminishing seafloor-spreading rates increases the volume of the ocean basins increases the volume of the ocean basins and causes regressionand causes regression

Causes of Causes of Transgressions and Transgressions and

RegressionsRegressions

► Using relative dating techniques, Using relative dating techniques, it is easy to determine the it is easy to determine the relative ages of rocks in Column relative ages of rocks in Column A and of rocks in Column BA and of rocks in Column B

► However, one needs more However, one needs more information to determine the information to determine the ages of rocks in one section ages of rocks in one section relative to those in the otherrelative to those in the other

Relative Ages between Relative Ages between Separate AreasSeparate Areas

► Rocks in A may be younger Rocks in A may be younger than those in B, the same than those in B, the same age as in B or older than in Bage as in B or older than in B

► Fossils could solve this Fossils could solve this problemproblem

Relative Ages between Relative Ages between Separate AreasSeparate Areas

► FossilsFossils are the remains or traces of prehistoric organisms are the remains or traces of prehistoric organisms

► They are most common in sedimentary rocks and in some They are most common in sedimentary rocks and in some accumulations of pyroclastic materials, especially ashaccumulations of pyroclastic materials, especially ash

► They are extremely useful for determining relative ages of They are extremely useful for determining relative ages of strata but geologists also use them to ascertain strata but geologists also use them to ascertain environments of depositionenvironments of deposition

► Fossils provide some of the evidence for organic evolution Fossils provide some of the evidence for organic evolution and many fossils are of organisms now extinctand many fossils are of organisms now extinct

FossilsFossils

► Remains of organisms are called Remains of organisms are called body fossilsbody fossils. and . and consist mostly of durable skeletal elements such as consist mostly of durable skeletal elements such as bonesbones, , teethteeth and and shellsshells

How do Fossils Form?How do Fossils Form?

rarely we might find entire animals rarely we might find entire animals preserved by freezing or preserved by freezing or mummificationmummification

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

Body FossilBody Fossil

Body FossilsBody Fossils

► Shells of Mesozoic Shells of Mesozoic invertebrate animals invertebrate animals known as ammonoids known as ammonoids and nautiloids on a rock and nautiloids on a rock slab in the Cornstock slab in the Cornstock Rock Shop in Virginia Rock Shop in Virginia City NevadaCity Nevada

► Trace fossilsTrace fossils are indications of organic activity are indications of organic activity including including tracks, tracks, trails, trails, burrows, burrows, nests nests

► A A coprolitecoprolite is a type of trace fossil consisting of is a type of trace fossil consisting of fossilized feces which may provide information about fossilized feces which may provide information about the size and diet of the animal that produced itthe size and diet of the animal that produced it

Trace FossilsTrace Fossils

► Paleontologists think that Paleontologists think that a land-dwelling beaver a land-dwelling beaver called called PaleocastorPaleocastor made made this spiral burrow in this spiral burrow in NebraskaNebraska

Trace FossilsTrace Fossils

► Fossilized feces (coprolite) Fossilized feces (coprolite) of a carnivorous of a carnivorous mammalmammal

► Specimen measures about 5 cm long Specimen measures about 5 cm long and and contains small fragments of bonescontains small fragments of bones

Trace FossilsTrace Fossils

► The most favorable conditions for preservation of The most favorable conditions for preservation of body fossils occurs when the organism body fossils occurs when the organism possesses a durable skeleton of some kind and possesses a durable skeleton of some kind and lives in an area where burial is likelylives in an area where burial is likely

► Body fossils may be preserved as Body fossils may be preserved as unaltered remainsunaltered remains, meaning they retain their , meaning they retain their

original composition and structure,original composition and structure,► by freezing, mummification, in amber, in tar by freezing, mummification, in amber, in tar

altered remainsaltered remains, with some change in composition, with some change in composition► permineralizedpermineralized► recrystallizedrecrystallized ► replacedreplaced ► carbonizedcarbonized

Body Fossil FormationBody Fossil Formation

► Insects Insects in in amberamber

Unaltered RemainsUnaltered Remains

►PreservatioPreservation in tarn in tar

Unaltered RemainsUnaltered Remains

► 40,000-year-old 40,000-year-old frozen baby frozen baby mammoth found in mammoth found in Siberia in 1971. It Siberia in 1971. It is 1.15 m long and is 1.15 m long and 1.0 m tall and it 1.0 m tall and it had a hairy coat. had a hairy coat.

► Hair around the Hair around the feet is still visiblefeet is still visible

► PetrifiedPetrified tree stump tree stump in Florissant Fossil in Florissant Fossil Beds National Beds National Monument, ColoradoMonument, Colorado

► Volcanic mudflows 3 Volcanic mudflows 3 to 6 m deep covered to 6 m deep covered the lower parts of the lower parts of many trees at this site many trees at this site

Altered RemainsAltered Remains

► Carbon film of a Carbon film of a palm frondpalm frond

Altered RemainsAltered Remains

► Carbon film of an insectCarbon film of an insect

►MoldsMolds form when buried remains leave form when buried remains leave a cavitya cavity

►CastsCasts form if material fills in the cavity form if material fills in the cavity

Molds and CastsMolds and Casts

Mold and CastMold and Cast

Step a: burial of a shellStep a: burial of a shell

Step b: dissolution leaving a Step b: dissolution leaving a cavity, a moldcavity, a mold

Step c: the mold is filled by Step c: the mold is filled by sediment forming a castsediment forming a cast

►Fossil turtle showing some of Fossil turtle showing some of the original shell material the original shell material

►body fossilbody fossil►and a castand a cast

Cast of a TurtleCast of a Turtle

► The The fossil recordfossil record is the record of ancient life is the record of ancient life preserved as fossils in rockspreserved as fossils in rocks

► Just as the geologic record must be analyzed and Just as the geologic record must be analyzed and interpreted, so too must the fossil recordinterpreted, so too must the fossil record

► The fossil record is a repository of prehistoric The fossil record is a repository of prehistoric organisms that provides our only knowledge of organisms that provides our only knowledge of such extinct animals as trilobites and dinosaurssuch extinct animals as trilobites and dinosaurs

Fossil RecordFossil Record

► The fossil record is very incomplete The fossil record is very incomplete because of destruction to organic remainsbecause of destruction to organic remains bacterial decay bacterial decay physical processesphysical processes scavengingscavenging metamorphism metamorphism

► In spite of this, fossils are quite commonIn spite of this, fossils are quite common

Fossil RecordFossil Record

► William Smith William Smith ► 1769-1839, an English civil engineer independently 1769-1839, an English civil engineer independently

discovered Steno’s principle of superpositiondiscovered Steno’s principle of superposition

► Realized that fossils in rocks followed the same principleRealized that fossils in rocks followed the same principle

► He discovered that sequences of fossils, especially He discovered that sequences of fossils, especially groups of fossils, are consistent from area to areagroups of fossils, are consistent from area to area

► Thereby discovering a method of relatively dating Thereby discovering a method of relatively dating sedimentary rocks at different locationssedimentary rocks at different locations

Fossils and Telling TimeFossils and Telling Time

► To compare the ages To compare the ages of rocks from two of rocks from two different localitiesdifferent localities

Fossils from Different AreasFossils from Different Areas

► Smith used fossilsSmith used fossils

► Using superposition, Smith was able to predict Using superposition, Smith was able to predict the order in which fossils would appear in the order in which fossils would appear in rocks not previously visitedrocks not previously visited

Principle of Fossil Principle of Fossil SuccessionSuccession

Alexander Brongniart in France Alexander Brongniart in France also recognized this relationshipalso recognized this relationship

► Their observations Their observations lead to the lead to the principle of fossil principle of fossil successionsuccession

► Principle of fossil successionPrinciple of fossil succession holds that fossil holds that fossil assemblages (groups of fossils) succeed one assemblages (groups of fossils) succeed one another through time in a regular and determinable another through time in a regular and determinable orderorder

► Why not simply match up similar rocks types?Why not simply match up similar rocks types? Because the same kind of rock has formed repeatedly Because the same kind of rock has formed repeatedly

through timethrough time

► Fossils also formed through time, Fossils also formed through time, but because different organisms existed at different times, but because different organisms existed at different times, fossil assemblages are uniquefossil assemblages are unique

Principle of Fossil Principle of Fossil SuccessionSuccession

►An assemblage of fossils An assemblage of fossils has a distinctive aspect compared with younger or has a distinctive aspect compared with younger or

older fossil assemblagesolder fossil assemblages Rocks that contain similar fossil assemblages Rocks that contain similar fossil assemblages

had to have been deposited at about the same had to have been deposited at about the same time.time.

Distinct AspectDistinct Aspect

► Geologists use the principle of fossil succession to Geologists use the principle of fossil succession to match ages of distant rock sequencesmatch ages of distant rock sequences

► Dashed lines indicate rocks with similar fossils thus having the Dashed lines indicate rocks with similar fossils thus having the same agesame age

Matching Rocks Using Matching Rocks Using FossilsFossils

► Investigations of rocks by naturalists between Investigations of rocks by naturalists between 1830 and 1842 based on superposition and 1830 and 1842 based on superposition and fossil succession resulted in the recognition of fossil succession resulted in the recognition of rock bodies called rock bodies called systemssystems the construction of a composite geologic column is the construction of a composite geologic column is

the basis for the relative geologic time scalethe basis for the relative geologic time scale

Relative Geologic Time Relative Geologic Time ScaleScale

Geologic Column and the Geologic Column and the Relative Geologic Time Relative Geologic Time

ScaleScale

Absolute ages (the numbers) were added much later.

► Cambrian SystemCambrian System Sedgwick studied rocks in northern Wales and described the Sedgwick studied rocks in northern Wales and described the

Cambrian System without paying much attention to the fossilsCambrian System without paying much attention to the fossils His system could not be recognized beyond the areaHis system could not be recognized beyond the area

► Silurian SystemSilurian System Murchinson described the Silurian System in South Wales Murchinson described the Silurian System in South Wales

including carefully described fossilsincluding carefully described fossils His system could be identified elsewhereHis system could be identified elsewhere

Example of the Example of the Development of SystemsDevelopment of Systems

► The dispute was settled in 1879 The dispute was settled in 1879 ► Ordovician SystemOrdovician System

Lapworth assigned the overlap between the two to Lapworth assigned the overlap between the two to a new system, the a new system, the OrdovicianOrdovician

Dispute of SystemsDispute of Systems

► Because sedimentary rock units Because sedimentary rock units are time are time transgressive, they may belong to one system in transgressive, they may belong to one system in one area and to another system elsewhereone area and to another system elsewhere

► At some localities a rock unit At some localities a rock unit straddles the boundary between systemsstraddles the boundary between systems

► We need terminology that deals with both: We need terminology that deals with both: rocks—defined by their contentrocks—defined by their content

► lithostratigraphic unitlithostratigraphic unit – rock content – rock content► biostratigraphic unitbiostratigraphic unit – fossil content – fossil content

and time—expressing or related to geologic timeand time—expressing or related to geologic time► time-stratigraphic unittime-stratigraphic unit – rocks of a certain age – rocks of a certain age► time unitstime units – referring to time not rocks – referring to time not rocks

Stratigraphic TerminologyStratigraphic Terminology

► Lithostratigraphic unitsLithostratigraphic units are based on rock type are based on rock type with no consideration of time of originwith no consideration of time of origin

► The basic lithostratigraphic element is a The basic lithostratigraphic element is a formationformation a mappable rock unit with distinctive upper and lower a mappable rock unit with distinctive upper and lower

boundariesboundaries It may consist of a single rock typeIt may consist of a single rock type

► such as the such as the Redwall limestoneRedwall limestone or a variety of rock typesor a variety of rock types

► such as the such as the Morrison FormationMorrison Formation

► Formations may be subdivided Formations may be subdivided into into membersmembers and and bedsbeds or collected into or collected into groupsgroups and and supergroupssupergroups

Lithostratigraphic UnitsLithostratigraphic Units

► Lithostratigraphic units Lithostratigraphic units in Zion National Park, in Zion National Park, UtahUtah

► For example: The For example: The Chinle Formation is Chinle Formation is divided into divided into Springdale Sandstone Springdale Sandstone

Member Member Petrified Forest Petrified Forest

MemberMember Shinarump Shinarump

Conglomerate MemberConglomerate Member

Lithostratigraphic UnitsLithostratigraphic Units

► A body of strata recognized only on the basis of A body of strata recognized only on the basis of its fossil content is a its fossil content is a biostratigraphic unitbiostratigraphic unit

► the boundaries of which do not necessarily correspond to the boundaries of which do not necessarily correspond to those of lithostratigraphic unitsthose of lithostratigraphic units

► The fundamental biostratigraphic unit The fundamental biostratigraphic unit is the is the biozonebiozone

Biostratigraphic UnitsBiostratigraphic Units

► Time-stratigraphic unitsTime-stratigraphic units ► also called also called chronostratigraphic unitschronostratigraphic units

consist of rocks deposited during a particular consist of rocks deposited during a particular interval of geologic timeinterval of geologic time

► The basic time-stratigraphic unit The basic time-stratigraphic unit is the is the systemsystem

Time-Stratigraphic UnitsTime-Stratigraphic Units

► Time units simply designate certain parts of Time units simply designate certain parts of geologic timegeologic time

► PeriodPeriod is the most commonly used time designation is the most commonly used time designation ► Two or more periods may be designated as an Two or more periods may be designated as an eraera► Two or more eras constitute and Two or more eras constitute and eoneon► Periods can be made up of shorter time units Periods can be made up of shorter time units

epochsepochs,, which can be subdivided into which can be subdivided into agesages

► The time-stratigraphic unit, system, The time-stratigraphic unit, system, corresponds to the time unit, periodcorresponds to the time unit, period

Time UnitsTime Units

Litho-Litho-stratigraphic stratigraphic UnitsUnits

►SupergroupSupergroup GroupGroup

►FormationFormation MemberMember

► BedBed

Classification of Classification of Stratigraphic UnitsStratigraphic Units

Time-Time-stratigraphic stratigraphic UnitsUnits

►EonothemEonothem ErathemErathem

►SystemSystem SeriesSeries

► StageStage

Time-Time-UnitsUnits

►EonEon EraEra

►PeriodPeriod EpochEpoch

► AgeAge

► CorrelationCorrelation is the process of matching up rocks is the process of matching up rocks in different areasin different areas

► There are two types of correlation:There are two types of correlation: Lithostratigraphic correlation Lithostratigraphic correlation

► simply matching up the same rock units over a larger area simply matching up the same rock units over a larger area with no regard for timewith no regard for time

Time-stratigraphic correlation Time-stratigraphic correlation ► demonstrates time-equivalence of eventsdemonstrates time-equivalence of events

CorrelationCorrelation

Lithostratigraphic Lithostratigraphic CorrelationCorrelation

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

► We can correlate rock units based on We can correlate rock units based on compositioncomposition position in a sequence position in a sequence and the presence of distinctive key bedsand the presence of distinctive key beds

Lithostratigraphic Lithostratigraphic CorrelationCorrelation

► Because most rock units of regional extent are Because most rock units of regional extent are time transgressive we cannot rely on time transgressive we cannot rely on lithostratigraphic correlation to demonstrate lithostratigraphic correlation to demonstrate time equivalencetime equivalence

► Example:Example: sandstone in Arizona is correctly correlated with sandstone in Arizona is correctly correlated with

similar rocks in Colorado and South Dakotasimilar rocks in Colorado and South Dakota but the age of these rocks varies from Early but the age of these rocks varies from Early

Cambrian in the west to middle Cambrian farther Cambrian in the west to middle Cambrian farther easteast

Time EquivalenceTime Equivalence

►The most effective way to The most effective way to demonstrate time equivalence is demonstrate time equivalence is time-stratigraphic correlation using time-stratigraphic correlation using biozonesbiozones

Time EquivalenceTime Equivalence

► For all organisms now extinct, their existence marks For all organisms now extinct, their existence marks two points in timetwo points in time

► their time of origintheir time of origin► their time of extinctiontheir time of extinction

► One type of biozone, the One type of biozone, the range zonerange zone, is defined by the , is defined by the geologic geologic rangerange (total time of existence) of a particular fossil group, species, (total time of existence) of a particular fossil group, species, or a group of related species called a genusor a group of related species called a genus

► Most useful are fossils that are Most useful are fossils that are easily identifiedeasily identified geographically widespreadgeographically widespread and had a rather short geologic rangeand had a rather short geologic range

BiozonesBiozones

► The brachiopod The brachiopod LingulaLingula is not is not useful because, although it is useful because, although it is easily identified and has a wide easily identified and has a wide geographic extent, it has too geographic extent, it has too large a geologic rangelarge a geologic range

► The brachiopod The brachiopod AtrypaAtrypa and and trilobite trilobite ParadoxidesParadoxides are well are well suited for time-stratigraphic suited for time-stratigraphic correlation, because of their correlation, because of their short rangesshort ranges

► They are They are guide fossilsguide fossils

Guide FossilsGuide Fossils

► A A concurrent range zoneconcurrent range zone is established is established by by plotting the overlapping ranges of two or more plotting the overlapping ranges of two or more fossils with different geologic rangesfossils with different geologic ranges

Concurrent Range ZonesConcurrent Range Zones

► This is probably the most This is probably the most accurate method of accurate method of determining time determining time equivalenceequivalence

► Some physical events Some physical events of short of short duration are also used to duration are also used to demonstrate time equivalence:demonstrate time equivalence: distinctive lava flowdistinctive lava flow

► would have formed over a short period would have formed over a short period of timeof time

ash fallsash falls► take place in a matter of hours or days take place in a matter of hours or days ► may cover large areasmay cover large areas► are not restricted to a specific are not restricted to a specific

environmentenvironment

Short Duration Physical Short Duration Physical EventsEvents

► Absolute ages may be obtained Absolute ages may be obtained for igneous events using for igneous events using radiometric datingradiometric dating

► Ordovician rocks Ordovician rocks are younger than those of the Cambrian are younger than those of the Cambrian and older than Silurian rocksand older than Silurian rocks

► But how old are they? When did the Ordovician But how old are they? When did the Ordovician begin and end?begin and end?

► Since radiometric dating techniques work on igneous Since radiometric dating techniques work on igneous and some metamorphic rocks, but not generally on and some metamorphic rocks, but not generally on sedimentary rocks, this is not so easy to determinesedimentary rocks, this is not so easy to determine

Absolute Dates and the Absolute Dates and the Relative Geologic Time Relative Geologic Time

ScaleScale

► Mostly, absolute ages for sedimentary rocks must Mostly, absolute ages for sedimentary rocks must be determined indirectly by dating associated be determined indirectly by dating associated igneous and metamorphic rocksigneous and metamorphic rocks

► According to the principle of cross-cutting According to the principle of cross-cutting relationships, relationships, a dike must be younger than the rock it cuts, so an a dike must be younger than the rock it cuts, so an

absolute age for a dike gives a minimum age for the absolute age for a dike gives a minimum age for the host rock and a maximum age for any rocks host rock and a maximum age for any rocks deposited across the dike after it was erodeddeposited across the dike after it was eroded

Absolute Dates for Absolute Dates for Sedimentary Rocks Are Sedimentary Rocks Are

IndirectIndirect

► Absolute ages of sedimentary rocks are most Absolute ages of sedimentary rocks are most often found by determining radiometric ages of often found by determining radiometric ages of associated igneous or metamorphic rocksassociated igneous or metamorphic rocks

Indirect DatingIndirect Dating

► The absolute dates obtained from regionally metamorphosed The absolute dates obtained from regionally metamorphosed rocks give a maximum age for overlying sedimentary rocksrocks give a maximum age for overlying sedimentary rocks

► Lava flows and ash falls interbedded with sedimentary rocks are Lava flows and ash falls interbedded with sedimentary rocks are the most useful for determining absolute agesthe most useful for determining absolute ages

► Both provide time-equivalent surfacesBoth provide time-equivalent surfaces giving a maximum age for any rocks above giving a maximum age for any rocks above and a minimum age for any rocks belowand a minimum age for any rocks below

Indirect DatingIndirect Dating

Indirect DatingIndirect Dating

► Combining thousands of Combining thousands of absolute ages associated absolute ages associated with sedimentary rocks of with sedimentary rocks of known relative age gives the known relative age gives the numbers on the geologic time numbers on the geologic time scalescale