Geology Reference
In-Depth Information
Table 14.1—Cont'd
Method
Age range (years)
Basis of method
Materials needed
Tephrochronology
0-10,000,000
+
Recognition of individual tephra by
their unique properties, and the
correlation of these to a dated
chronology
Pyroclastic rocks
Palaeontology
Evolution of
microtine rodents
8,000-8,000,000
Progressive evolution of microtine
rodents
Terrestrial animal remains
Marine
zoogeography
30,000-300,000
Climatically induced
zoogeographical range shifts
of marine invertebrates
Marine fossiliferous deposits
Climatic
correlations
a
1,000-500,000
Correlation of landforms and
deposits to global climate
changes of known age
Most sedimentary materials and
landforms
Notes:
a
Experimental method
b
Depends on nuclide used (beryllium-10, aluminium-26, chlorine-36, helium-3, carbon-14)
c
Depends on series (uranium-234-uranium-230, uranium-235-protactinium-231)
d
Depends on material used (zircon and glass, apatite)
Source:
Adapted from Sowers
et al.
(2000, 567)
about rates.
Calibrated-age
methods may provide
approximate numerical ages. Some of these methods
are refined and enable age categories to be assigned to
deposits by measuring changes since deposition in such
environmental factors as soil genesis or rock weathering
(see McCarroll 1991).
Relative-age methods
furnish
an age sequence, simply putting events in the correct
order. They assemble the 'pages of Earth history' in a
numerical sequence. The Rosetta stone of relative-age
methods is the principle of stratigraphic superposition.
This states that, in undeformed sedimentary sequences,
the lower strata are older than the upper strata. Some
kind of marker must be used to match stratigraphic
sequences from different places. Traditionally, fossils
have been employed for this purpose. Distinctive fos-
sils or fossil assemblages can be correlated between
regions by identifying strata that were laid down
contemporaneously. This was how such celebrated
geologists as William ('Strata') Smith (1769-1839)
first erected the stratigraphic column. Although this
technique was remarkably successful in establishing the
broad development of Phanerozoic sedimentary rocks,
and rested on the sound principle of superposition,
it is beset by problems (see Vita-Finzi 1973, 5-15). It is
best-used in partnership with numerical-age methods.
Used conjointly, relative-age methods and numerical-
age methods have helped to establish and calibrate
the geological timetable (see Appendix).
Correlated-
age methods
do not directly measure age, but suggest
ages by showing an equivalence to independently dated
deposits and events.
Dating techniques may be grouped under six
headings: sidereal, isotopic, radiogenic, chemical and
biological, geomorphic, and correlation (Colman and
Pierce 2000). As a rule, sidereal, isotopic, and radio-
genic methods give numerical ages, chemical and
biological and geomorphic methods give calibrated or
relative ages, and correlation methods give correlated
ages. However, some methods defy such ready clas-
sification. For instance, measurements of amino-acid
racemization may yield results as relative age, calibrated
age, correlated age, or numerical age, depending on
the extent to which calibration and control of environ-
mental variables constrain the reaction rates. Another
complication is that, although isotopic and radiogenic
methods normally produce numerical ages, some of
