Geologic Time: Concepts and Principles - The Changing Earth: Exploring Geology and Evolution - page 461

◗ Figure 17.26 Determining Absolute Dates for Sedimentary
Rocks The absolute ages of sedimentary rocks can be determined
by dating associated igneous rocks. In a and b , sedimentary rocks
are bracketed by rock bodies for which absolute ages have been
determined.

⎧
⎩

150 to
180 MYA

Nonconformity

Batholith
180 MYA

150 MYA

a

Lava flow (600 MYA)

> 600 to < 675
MYA

⎩

Ash fall (675 MYA)

⎨
⎩

> 675 - 750
MYA

Sedimentary rocks
regionally
metamorphosed
(750 MYA)

b

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 lithostrati-
graphic boundaries (

Mission Canyon Formation

Figure 17.28). The fundamental bio-
stratigraphic unit is the biozone . Several types of biozones
are recognized, one of which, the concurrent range zone , was
discussed in the section on correlation (Figure 17.16).
The category of units expressing or related to geologic
time includes time-stratigraphic units (also known as
chronostratigraphic units) and time units (Table 17.2).
Time-stratigraphic units consist of rocks deposited during
a particular interval of geologic time. The system , the basic
time-stratigraphic unit, is based on a stratotype consisting of
rocks in an area where the system was fi rst described. Systems
are recognized beyond their stratotype area by their fossil
content.
Time units are simply designations for certain parts of
geologic time. The basic time unit is the period ; however, two
or more periods may be designated as an era , and two or more
eras constitute an eon . Periods also consist of shorter designa-
tions such as epoch and age . The time units known as period,
epoch, and age correspond to the time-stratigraphic units
known as system, series, and stage, respectively (Table 17.2).

◗

Lodgepole Formation

◗ Figure 17.27 Lithostratigraphic Units The Madison Group in

Montana consists of two formations, the Lodgepole Formation
and the overlying Mission Canyon Formation. The Mission Canyon
Formation is the rock unit exposed on the skyline. The underlying
Lodgepole Formation are the rocks that are mostly covered by
vegetation on the slopes below.

TABLE 17.2 Classifi cation of Stratigraphic Units

UNITS DEFINED BY CONTENT UNITS EXPRESSING OR RELATED TO GEOLOGIC TIME

Lithostratigraphic Units Biostratigraphic Units Time-Stratigraphic Units Time Units

Supergroup Biozones Eonothem — — — — — — — — — Eon

Group Erathem — — — — — — — — — — Era

Formation System — — — — — — — — — — — Period

Member Series — — — — — — — — — — — — Epoch

Bed Stage — — — — — — — — — — — — — Age