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 – 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

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

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

• Sedimentary rocks are almost all stratified• Many igneous rocks

– such as a succession of lava flows or ash beds – are stratified and obey the principles of stratigraphy

• Many metamorphic rocks are stratified

Stratigraphy

• Stratification in a succession of lava flows in Oregon.

Stratified Igneous Rocks

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

Stratified Sedimentary Rocks

• Stratification in Siamo Slate, in Michigan

Stratified Metamorphic 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

• 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

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 alteration by heat – 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.– 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

• 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 California, with conglomerate deposited upon – an erosion surface in the underlying rocks

A Disconformity

• An angular unconformity in Colorado – between steeply dipping Pennsylvanian rocks – and overlying Cenozoic-aged conglomerate

An Angular Unconformity

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

A Nonconformity

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

be abrupt• at the edge of a

depositional basin

Lateral Relationships

• where eroded• where 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 intertonging

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

• 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

Sedimentary Facies

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

• 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 intertonging

Extent and Rates 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

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, one needs 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– older than in B

• Fossils could solve this problem

Relative Ages between Separate Areas

• 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

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

• Skeleton of a 2.3-m-long marine reptile – in the museum at Glacier Garden in Lucerne,

Switzerland

Body Fossil

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

• 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

• Paleontologists think – that a land-dwelling

beaver– called Paleocastor– made this spiral

burrow in Nebraska

Trace Fossils

• Fossilized feces (coprolite) – of a carnivorous mammal

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

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• permineralized, recrystallized, replaced, carbonized

Body Fossil Formation

• Insects in amber

Unaltered Remains

• Preservation in tar

Unaltered Remains• 40,000-

year-old frozen baby mammoth

• found in Siberia in 1971

• It is 1.15 m long and 1.0 m tall

• and it had a hairy coat

• Hair around the feet is still visible

• 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

Altered Remains

• Carbon film of an insect

• Molds form – when buried remains leave a cavity

• Casts form – if material fills in the cavity

Molds and Casts

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

• Fossil turtle – showing some of the original shell material

• body fossil

– and a cast

Cast of a Turtle

• 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

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 discovering a method – of relatively dating sedimentary rocks at different

locations

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 – lead 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– including carefully described fossils– His system could be identified elsewhere

Example of the Development of Systems

• Ordovician System– Lapworth assigned the overlap – between the two to a new system, – the Ordovician

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 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 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 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

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 farther east

Time Equivalence

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

• But other methods are useful

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• take place in a matter of hours or days • 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?

• 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 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

Indirect Dating

• 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