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
