1. VIZUALIZING EARTH HISTORY
By Loren E. Babcock
Chapter 3
Geologic Time

2. Compare relative age dating and numerical age dating.
Geologic Time Scale
Compare relative age dating and numerical age dating.
Relative age dating – (Older vs. younger)
Relative age dating is a technique of establishing a
chronology of events arranged in sequential order.
Relative dating puts events in sequence using Nicholas Steno’s
principles of superposition, original horizontality, and original
lateral continuity; applying Charles Lyell’s principles of
cross-cutting relationships and included fragments; and
also applying William Smith’s principle of biotic succession.

3. Compare relative age dating and numerical age dating.
Geologic Time Scale
Compare relative age dating and numerical age dating.
Numerical age dating — Numerical age dating is a technique
of establishing when events occurred according to how
much time has elapsed since their occurrence.
Numerical age will involve in calculating many thousands,
millions, or billions of years ago an event occurred. Numerical
age dating techniques include counting tree rings,
counting layers of ice drilled from glaciers,
and counting sediment layers in ancient lakes.

4. Understand the various techniques of stratigraphic correlation.
Geologic Time Scale
Understand the various techniques of stratigraphic correlation.
Stratigraphy — Stratigraphy is the study of layered rocks,
including their compositions, origins, geometric
relationships, and ages.
Correlation — Correlation is defined as matching of strata
from one location to another.
The goal of studying stratigraphy and correlation is to try
to match rock units from one place to another to determine
their environment of formation and deposition.

5. Understand the various techniques of stratigraphic correlation.
Geologic Time Scale
Understand the various techniques of stratigraphic correlation.

6. Understand the various techniques of stratigraphic correlation.
Geologic Time Scale
Understand the various techniques of stratigraphic correlation.

7. Understand the various techniques of stratigraphic correlation.
Geologic Time Scale
Understand the various techniques of stratigraphic correlation.

8. Understand the various techniques of stratigraphic correlation.
Geologic Time Scale
Understand the various techniques of stratigraphic correlation.

9. Understand the various techniques of stratigraphic correlation.
Geologic Time Scale
Understand the various techniques of stratigraphic correlation.
Correlating Rock Units
Rock units can be correlated, using relative age dating techniques,
on the basis of their lithology (lithostratigraphy) or on the basis of
criteria that are proxies for geologic time (chronostratigraphy).
Fossils, variations in chemical isotopes, variations in physical
properties (such as radioactivity levels), and sequences of
unconformity-bounded strata are some of the variables that
provide an understanding of the relative timing of events in
geologic history, and which can be used to correlate strata
from one place to another.

10. Geologic Time Scale LITHOSTRATIGRAPHY
Lithostratigraphy involves the correlation of rocks on the basis of
lithology, or rock type.
The lithology of a rock usually involves some combination of the rock
composition, grain or crystal size, and color.
Lithostratigraphic units can be correlated according if they are
composed of sandstone, shale, conglomerate, limestone, or granite,
for instance, but commonly additional qualifiers are needed to
help distinguish one unit from another.
Color of a rock is often used as a guide to correlation.

11. Geologic Time Scale LITHOSTRATIGRAPHY
Lithostratigraphy — Stratigraphic correlation on the basis of rock type.
Formation — Fundamental unit of lithostratigraphy; it has a
definable top and bottom, and is mappable across
geographic space.

12. Geologic Time Scale BIOSTRATIGRAPHY
Biostratigraphy — Zoning of stratigraphic layers, and
arrangement of those layers according to relative time of
deposition, using the ranges of fossils.
Zone (biozone) — A stratigraphic interval defined by its fossil
content, and usually given the name of a characteristic fossil
present in that interval.

13. Geologic Time Scale BIOSTRATIGRAPHY

14. Geologic Time Scale CHEMOSTRATIGRAPHY
Chemostratigraphy — Correlation of strata using ratios of chemical isotopes.
Isotopic excursion — A positive or negative shift in the isotopic ratio of an element as recorded through a succession of stratigraphic layers.

15. SEQUENCE STRATIGRAPHY
Geologic Time Scale
SEQUENCE STRATIGRAPHY
Unconformity - A surface of erosion or non-deposition.
Sequence stratigraphy - Correlation of strata on the basis
of erosion surfaces that separate packages of sediments
or sedimentary rocks called depositional sequences (sequences).

16. SEQUENCE STRATIGRAPHY
Geologic Time Scale
SEQUENCE STRATIGRAPHY
Unconformities fall into three basic kinds:
nonconformities,
angular unconformities, and
disconformities

17. SEQUENCE STRATIGRAPHY
Geologic Time Scale
SEQUENCE STRATIGRAPHY
Distinguish the three types of unconformities.
Nonconformities occur where sedimentary strata have been deposited
over top of crystalline (igneous or metamorphic) rock.
Angular unconformities occur when horizontally parallel strata are
deposited on tilted and eroded layers that may be
either vertical or at angle to the overlying horizontal layers.
Disconformities are unconformities between parallel layers
of sedimentary rocks which represents a period of
erosion or non-deposition.

18. Geologic Time Scale SEQUENCE STRATIGRAPHY

19. SEQUENCE STRATIGRAPHY
Geologic Time Scale
SEQUENCE STRATIGRAPHY
Understand how events in Earth history are dated.
Eustatic (sea level) — Global sea level.
Sequence (depositional sequence) — A relatively
conformable package of sedimentary strata that are
bounded below and above by unconformities or their
equivalent conformities.

20. OTHER CORRELATION TECHNIQUES
Geologic Time Scale
OTHER CORRELATION TECHNIQUES
Measurements of the geophysical properties
of rocks are also indicators of lithology.
Gamma-ray profile - Sedimentary rocks emit varying amounts of
gamma rays depending on the amount of radioactive uranium,
thorium, and potassium they contain.
Gamma-ray profile can be used to correlate strata exposed at
the surface, or ones penetrated by drilling into the
subsurface.

21. Geologic Time Scale

22. Geologic Time Scale

23. OTHER CORRELATION TECHNIQUES
Geologic Time Scale
OTHER CORRELATION TECHNIQUES
Polarity directions — Measuring polarity directions recorded
in rocks from the time the rocks were formed.
Earth’s polarity has switched many times during this planet’s
long history, and the sequence of normal and reversed
polarity episodes provides a good basis for the correlation of strata.

24. Geologic Time Scale OTHER CORRELATION TECHNIQUES

25. Understand the geologic time scale.
The geologic time scale is a chronology, or “calendar”
of Earth history.
Units of geologic time are not “natural” or followed a
preset plan (or “chapter outline”) that Earth adhered to.
Geologic time units have been defined and named
by geologists who recognized that Earth
has passed through a series of stages, each
having its own distinct characteristics.
The switchover from one stage to the next, or the
boundary point between two time units, must be
arbitrary but based on sound scientific reasoning.

26. Geologic time scale — A chronology of Earth history.

27. Geologic time scale — A chronology of Earth history.
GSSP - Acronym for Global Standard boundary
Section and Point, an internationally ratified point
in strata marking the boundary between two
time-rock (chronostratigraphic) units, and their
equivalent time units.
Chronostratigraphic unit - A time-rock
unit; the tangible representation of a
geologic time (geochronologic) unit.

28. Distinguish geologic time units from chronostratigraphic units.
Geologic Time Scale
Distinguish geologic time units from chronostratigraphic units.
CHRONOSTRATIGRAPHY
System - The fundamental unit of chronostratigraphy.
Period - The unit of geologic time equivalent to a system.
Eons, eras, periods, and epochs are formally defined
units of geologic time. They are abstract entities, just
like minutes and hours.
The geologic time scale shows abstract time
units in their correct order, and hierarchically arranged.

29. Geologic Time Scale CHRONOSTRATIGRAPHY
Correlation techniques used to establish chronostratigraphic
units must be time-parallel, meaning that they must define units
representing the same “instant” (in geological terms)
everywhere in the world.
Biostratigraphic units (biozones or zones) qualify as
time-parallel units because biological evolution and
extinction is a non-repeating process. Each species
has had only one time of appearance on Earth, and
each has gone extinct or eventually will do so only once.

30. Geologic Time Scale CHRONOSTRATIGRAPHY
Chemostratigraphic profiles record variations in chemical isotopic
ratios locked into sediments through geologic time. Details of the
sine-wave pattern of positive and negative excursions
provide a virtual “fingerprint” for time units.
Sequence-stratigraphic units, or sequences, are bounded by
disconformities of short duration (geologically speaking).
They are commonly the result of rapid global sea level changes,
which means disconformities of equivalent age appear in
stratigraphic sections around the world.

31. Geologic Time Scale CHRONOSTRATIGRAPHY
Lithostratigraphic units, especially formations, are, for the most
part, not time-parallel.
Formations usually represent a sedimentary facies, the type of
sediment deposited in one area (say a beach, a reef, a lake,
or a stream), and are not directly tied to time intervals.
In the world today, many different sedimentary
facies coexist in time, and this was also true
in the geologic past.

32. Geologic Time Scale GEOCHRONOLOGY
Parent - In a radioactive decay series, an unstable isotope that
decays, or transforms, into a daughter product.
Daughter - Isotope formed from the
radioactive decay of a parent isotope.
Geochronology refers to the dating of rocks according to
their numerical ages. Geochronologic dating involves
laboratory analyses of radiometric isotopes.
In radioactive decay, a radiogenic parent isotope
changes through one or more steps, each referred to
as a daughter product, until becoming a stable end product.

33. Geologic Time Scale GEOCHRONOLOGY
Radioactive decay occurs in a number of ways.
Three processes useful in geochronology are called
alpha decay, beta decay, and electron capture.
Alpha decay is a type of nuclear fission, in which the parent
splits into two daughter products. It tends to occur in
isotopes that have large atomic numbers.
Beta decay is of two forms. In neither one does the atomic
mass of a nucleus change. Emission of a beta particle
(an electron, β-), and the emission of an electron’s
antiparticle, which is called a positron (β+).

34. Geologic Time Scale GEOCHRONOLOGY
In electron capture, a parent nucleus captures one
of its own electrons and then emits a neutrino. The atomic
mass of a nucleus does not change. Potassium-40 (40K)
transforms to argon-40 (40Ar) by means of electron capture.
Half-life - the amount of time it takes for one-half of the
parent to decay to a daughter product is called a half-life.

35. Geologic Time Scale GEOCHRONOLOGY

36. Geologic Time Scale GEOCHRONOLOGY
After one half-life, 50% of the parent remains,
and 50% of the atoms have become a daughter
product. After two half-lives, 25% (or half of the
50% amount) of the parent remains, and 75% of
the atoms have become a daughter product.
After three half-lives, 12.5% (or half of the 25%)
of the parent remains, and 87.5% of
Further decay continues in the same way.