Rocks, Fossils, and Time—Making Sense of the

Geologic Record

Chapter 5

• 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 – sedimentary rocks are especially useful – in deciphering the geologic record,

• The geologic record is complex – and requires interpretation

• Uniformitarianism offers a useful approach

Geologic Record

• Stratigraphy deals with the study – of any layered (stratified) rock,– but primarily with sedimentary rocks and their

• composition• origin• age relationships• geographic extent

• Almost all sedimentary rocks are stratified• Many volcanic rocks

– such as lava flows or ash beds – as well as many metamorphic rocks – are stratified and obey the principles of stratigraphy

Stratigraphy

• Although these rocks in South Dakota• are deeply eroded• stratification is still clearly visible

Stratified Sedimentary Rocks

• Stratified rocks in California are• deformed so that they are no longer in their original

position

Stratified Rocks

• Surfaces known as bedding planes – separate individual strata from

one another

Vertical Stratigraphic Relationships

– 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

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

• In deformed strata, the task is more difficult– but some sedimentary structures– and some fossils – allow geologists to resolve these kinds of problems

Superposition

• 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

Principle of Inclusions

• Light-colored granite – in northern Wisconsin – showing basalt

inclusions (dark)• Which rock is older?

– Basalt, because the granite includes it

• Determining the relative ages – of lava flows, sills and associated sedimentary rocks– uses contact metamorphism effects – and inclusions

Age of Lava Flows, Sills

• 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.

– A sill will heat the rocks above and below.

Sill

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

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

• 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 at this location– The interval of time not represented by strata is a

hiatus.

Unconformities

– For 1 million years erosion occurred

– removing 2 MY of rocks

The origin of an unconformity• In the process of forming an unconformity,

– deposition began 12 million years ago (MYA), – continuing until 4 MYA

• The last column – is the actual

stratigraphic record – with an unconformity

– and giving rise to – a 3 million year

hiatus

• Three types of surfaces can be unconformities:– A disconformity is a surface in sedimentary rocks

• 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

Types of Unconformities

• A disconformity between sedimentary rocks:– in Montana, Jurassic-age rocks rest unconformably on

top of Mississippian-age strata– an erosion surface separates the two.

A Disconformity

• A nonconformity between Precambrian granite– and the Cambrian Formation – in Bighorn Mountains, Wyoming

A Nonconformity

• An angular unconformity between the flat-lying Medial Jurassic Entrada Sandstone and underlying Upper Jurassic red beds in New Mexico.

An Angular Unconformity

• In 1669, Nicolas Steno proposed – the principle of lateral continuity, – meaning that layers of sediment extend outward – in all directions until they terminate– Terminations may

be abrupt• at the edge of a

depositional basin

Lateral Relationships

• where they are eroded• where they are truncated by faults

Gradual Terminations– or they may be gradual

• where a rock unit • becomes progressively thinner • until it pinches out

• or where it splits into • thinner units • each of which pinches out, • called intertonguing

• where a rock unit changes • by lateral gradation • as its composition and/or texture • becomes increasingly different

• Both intertonguing 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

• 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 Transgressions

• The rocks of each facies become younger – in a landward direction during a marine

transgression

Marine Transgression

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

older shale

younger shale

• Three formations deposited – in a widespread

marine transgression

– exposed in the walls of the Grand Canyon, Arizona

A Marine Transgression in the Grand Canyon

• During a marine regression, – sea level falls – with respect – to the continent

Marine Regression

– and the environments paralleling the shoreline

– migrate seaward

Marine Regression

• A marine regression – is the opposite of a marine transgression

• It yields a vertical sequence – with nearshore facies – overlying offshore facies– and rock units become younger – in the seaward direction

younger shale

older shale

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

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

• 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 intertonguing

Extent, Rates of Transgressions and Regressions

• Uplift of continents causes regression• Subsidence causes transgression• Widespread glaciation causes regression

– because of the amount of water frozen in glaciers • Rapid seafloor spreading,

– expands the mid-ocean ridge system, – displacing seawater and causing transgression

• Diminishing seafloor-spreading rates – increases the volume of the ocean basins – and causes regression

Causes of Transgressions and Regressions

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

• However, you need more information – to determine the ages of

rocks – in one section relative to – those in the other

Relative Ages between Separate Areas

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

• Fossils can help to solve this problem

Relative Ages between Separate Areas

• Fossils are the remains or traces of past life forms

• They are most common in sedimentary rocks– but can be found– iIn volcanic ash and volcanic mudflows

• 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

Fossils

• Remains of organisms are called body fossils.– and consist mostly of durable skeletal elements – such as bones, teeth and shells

How do Fossils Form?

– rarely we might find entire animals preserved by freezing or mummification

• Indications of organic activity – including tracks, trails, burrows, and nests – are called trace fossils

• A coprolite is a type of trace fossil – consisting of fossilized feces– that may provide information about the size – and diet of the animal that produced it

Trace Fossils

• This slab of rock– formed over the actual

tracks of birds,– so it is a cast of the

tracks

Trace Fossils

• Fossilized feces (coprolite) – of a carnivorous mammal

• Specimen measures about 5 cm long – and contains small fragments of bones

Trace Fossils

• 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• permineralization, replacement, carbonization

Body Fossil Formation

• Insects in amber

Unaltered Remains

Unaltered Remains• Frozen

baby mammoth

• found in Russia in 1989

• The bones of this mammoth– on display at the

Museum of Geology and Paleontology in Florence, Italy

• have been permineralized– with minerals

added to the pores and cavities of the bones

Altered Remains

• Carbon film of a palm frond

Altered Remains

• Carbon film of an insect

• Molds form – when buried remains dissolve and leave a cavity

• Casts form – if minerals or sediments fill in the cavity

Molds and Casts

Mold and CastStep a: burial of a shell

Step b: dissolution leaving a cavity, a mold

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

• 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

• 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 he discovered a method – whereby relative ages of sedimentary rocks at

different locations could be determined

Fossils and Telling Time

• To compare the ages of rocks from two different localities

Fossils from Different Areas• Smith used fossils

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

Principle of Fossil Succession

• Alexander Brongniart in France – also recognized this

relationship• Their observations

– led to the 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

Principle of Fossil Succession

• An assemblage of fossils – has a distinctive aspect – compared with younger – or older fossil assemblages

Distinct Aspect

• 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

Matching Rocks Using Fossils

• The youngest rocks are in column B – whereas the oldest ones are in column C

Matching Rocks Using Fossilsyoungest

oldest

• 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

Relative Geologic Time Scale

Geologic Column and the Relative Geologic Time Scale

Absolute ages (the numbers) were added much later.

• 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

– and carefully described fossils– His system could be identified elsewhere

Example of the Development of Systems

• The two systems partially overlapped!

Dispute of Systems

• The dispute was settled in 1879 – when Lapworth proposed the Ordovician

System Dispute

• 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

Stratigraphic Terminology

• Lithostratigraphic units are based on rock type – with no consideration of time of origin

• The basic lithostratigraphic element is the formation– which is a mappable rock body – 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

• Representation of the lithostratigraphic units in Capital Reef National Park, Utah– Notice that come

formations– Are further divided

into members,– And some are parts of

more inclusive groups.

Lithostratigraphic 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

Biostratigraphic 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-Stratigraphic 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 an 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

Time Units

Litho-stratigraphic Units

• Supergroup– Group

• Formation– Member

» Bed

Classification of Stratigraphic Units

Time-stratigraphic Units

• Eonothem– Erathem

• System– Series

» Stage

Time-Units

• Eon– Era

• Period– Epoch

» Age

• 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

Correlation

Lithostratigraphic Correlation

• Correlation of lithostratigraphic units such as formations – traces rocks laterally across gaps

• We can correlate rock units based on – composition– position in a sequence – and the presence of distinctive key beds

Lithostratigraphic Correlation

• 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 further east

Time Equivalence

• The most effective way – to demonstrate time equivalence – is time-stratigraphic correlation – using biozones

• But several other methods are useful as well

Time Equivalence

• For all organisms now extinct, – their existence marks two points in time

• their time of origin• their time of extinction

• One type of biozone, the range zone, – is defined by the geologic range

• total time of existence– of a particular fossil group

• a species, or a group of related species called a genus

• Most useful are fossils that are – easily identified, geographically widespread– and had a rather short geologic range

Biozones

• 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

Guide Fossils

• A concurrent range zone is established – by plotting the overlapping ranges – of two or more fossils – with different

geologic ranges

Concurrent Range Zones

• This is probably the most accurate method – of determining

time equivalence

• 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• may cover large areas• are not restricted to a specific

environment

Short Duration Physical Events

• Absolute ages may be obtained for igneous events – using radiometric dating

• 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?

• Absolute ages determined for minerals– in sedimentary rocks– give only the ages of the source– that supplied the minerals– and not the age of the rock itself

Absolute Dates and the Relative Geologic Time Scale

• 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

Absolute Dates for Sedimentary Rocks Are Indirect

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

Indirect Dating

• The absolute dates obtained – from regionally metamorphosed rocks – give a maximum age – for overlying sedimentary rocks

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

• Both provide time-equivalent surfaces– giving a maximum age for any rocks above – and a minimum age for any rocks below

Indirect Dating

• These sedimentary rocks – are bracketed by metamorphic and – igneous rocks for which– absolute ages are known

Indirect Dating

Indirect Dating

• Accurate radiometric dates are now available– for many ash falls, plutons, lava flows and

metamorphic rocks– with associated fossil-bearing sedimentary rocks

• These absolute ages have been added to the geologic time scale

• Additionally, we know when a particular organism lived

Indirect Dating

• Baculites reesidei – biostratigraphic zone– in the Bearpaw Formation– Saskatchewan, Canada– is about 72-73 million years

old– because absolute ages have

been determined– for associated ash layers