Chapter 5 – Time and Stratigraphy
5.1 Introduction
 James Hutton was among the first to recognize the enormity of geologic time
o Considered by some to be geology’s greatest contribution to human
thought
o An important implication is the large amount of time for Earth’s systems
to evolve
 Understanding the timing of events in Earth history helps us understand cause and
effect relationships
 Age relations in geology are typically established by the discipline of stratigraphy
o Stratigraphy is the study of stratified (layered) rocks and their age
relationships
o Sedimentary rocks are generally studied because they are normally layered
5.2 Relative Ages
 The easiest way to establish the age of something is to establish its age relative to
something else
o Such ages are called relative ages
 Establishing relative ages involves recognizing sequences of events (and therefore
history)
o The Principle of Superposition states that younger sedimentary rocks lie
on top of older rocks
o The related Principle of Original Horizontality states that sedimentary
strata are deposited initially in horizontal layers
5.3 Absolute Ages
 Radiometric dates provide absolute ages (ages of rocks in years)
o They not only provide the ages of rocks and fossils, but also allow
calculation of approximate rates of processes (such as deposition, erosion,
and uplift)
o Are derived from the decay of radioactive isotopes (the same types of
isotopes that generate Earth’s internal heat)
 Radiometric dates are based on the concept of the half-life
o A half-life is the amount of time it takes for one half of an amount of a
radioactive (parent) isotope to decay to its daughter product
o Radioactive isotopes vary substantially in terms of their half-lives
 A number of factors can affect the use of radiometric dates
o These include the blocking temperature of minerals, closure of minerals to
gain or loss of patent or daughter, and weathering and leaching
o One way to determine the accuracy of dates is the method of cross-checks
(use of two different isotopic pairs)
 Radiometric dates usually come from igneous rocks
o In a sequence of sedimentary rocks, radiometric dates are from igneous
intrusions
o The Principle of Cross Cutting Relationships states that rock bodies
cutting across others must have come after them
5.4 Evolution of the Geologic Time Scale
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The earliest time scales were based on the Principle of Superposition
o Rock type was equated with time interval on these early time scales
o The term “primary” was used for all crystalline rock, “secondary” for
stratified rock with fossils, “tertiary” for rock above secondary, capped by
alluvium
o The significance of fossils was unknown, as was the theory of evolution
 In 1815 William Smith proposed the Principle of Fossil Succession
o He had noticed that fossil assemblages always occurred in the same
superpositional sequence
o He had developed and tested his hypothesis through his work as a
surveyor responsible for canal construction
o Smith’s principle is based on the fact that fossil biotas have changed
through time because of biologic evolution
 The main subdivisions of the time scale are the eons, eras, periods, and epochs
o The interval formerly called “primary: is now called the Precambrian; its
eons are the Hadean (which refers to Earth’s early molten state), the
Archean, and the Proterozoic.
o The former subdivisions called “secondary,” “tertiary,” and “quaternary”
are now placed in the Phanerozoic Eon (in reference to abundant fossils)
o The Phanerozoic is divided into three eras based on distinct groups of
fossils: Paleozoic (“ancient life”), Mesozoic (“middle life”) and Cenozoic
(“recent life”)
o Each era has its own distinct fossil assemblages that are used to subdivide
the eras into smaller units called periods (which are further subdivided
into epochs)
5.5 Correlation – the procedure that infers the age equivalence of rocks
 Lithocorrelation – based solely on lithology
o May be the only means possible for correlation if fossils are absent
o Distinctive rocks units are assumed to be contemporaneous
o In some cases, marker beds (such as volcanic ash layers) are used
o For subsurface rocks, distinctive shifts in well logs may be used for
correlation
o Another type of lithocorrelation uses oxygen isotopes, which are also used
to infer the past behavior of the hydrologic cycle (such as ice volume)
o The magnetic properties of rocks and Earth’s history of magnetic polarity
reversals are used for another lithocorrelation method (and may be
combined with oxygen isotopes studies)
o Lithocorrelation is complicated by the fact that unlike fossils, rocks repeat
themselves through time
 Formations and Facies
o Not only do the same kinds of rocks repeat themselves through geologic
time, but different types of rocks can occur at the same time
o “Walther’s Law” refers to this aspect of sedimentary rocks and the
environments in which they were deposited
o A formation is a mappable unit of rock that is recognized based on its
lithology; formations are named based on a type locality
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o The concept of facies recognizes that different sedimentary rocks (and
their depositional environments) can exist at the same time, and that the
same rocks (and environments) have existed at different times
 Biostratigraphy – the use of fossils to study stratigraphic relationships in
sedimentary rocks
o Allows correlation over distances and avoids erroneous reconstructions of
Earth history
o Once formations have been identified and mapped, they can be correlated
using their enclosed fossils
o Recognition of First Appearance Datums (evolution) and Last Appearance
Datums (extinction) of species allows biostratigraphic correlation to be
rapidly determined
o Species are not uniformly distributed over Earth’s surface because of
different environmental tolerances; eurytopic species are best for
biostratigraphic correlation
o Index fossils are species that are easily identified, widespread, and
abundant and thus especially useful for biostratigraphy
 Integrating Different Stratigraphic Datums
o The different types of datums (lithostratigraphic, biostratigraphic, etc.) are
routinely integrated into standardized chronostratigraphic frameworks
5.6 How Complete Is the Geologic Record
 Unconformities and Diastems
o The geologic record is not complete and does not record every event in
Earth history
o Much of the geologic record is not represented by sediment but by
surfaces of erosion or non-deposition called unconformities
o Angular unconformities, disconformities, and nonconformities are
different types
o The time represented by the missing sediment is called a hiatus
o Diastems are gaps in the stratigraphic record so short that are virtually
undetectable
 Sequence Stratigraphy
o Unconformities (especially disconformities) are the basis for the discipline
of sequence stratigraphy
o The depositional sequence is the basic unit of sequence stratigraphy; it
consists of rocks or sediments laid down relatively continuously
o A depositional sequence is bounded above and below by an unconformity;
each unconformity represents a sequence boundary
o These depositional sequences are normally first recognized on seismic
sections and later correlated to well logs and biostratigraphic extinctions
o Sea level changes (transgressions and regressions) play a role in the
development of depositional sequences and sequence boundaries
5.7 Why is Sea Level So Important?
 Sea level determines the broad patterns of depositional environments and facies,
which record the history of Earth and life in rocks and fossils
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o Without the geologic record a great deal of Earth’s history would be lost
and our understanding of the planet’s evolution more fragmentary
Sea level also reflects and interacts with tectonics and climate to shape the Earth
o The relative importance of these processes varies at different scales of
time
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