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Table 9.14 Comparison of successive numerical age estimates of Periods in the International
Geologic Time Scale and their 95 %-confidence intervals
Holmes
1960
Base of period or epoch
GTS2004
2
˃
(2004) GTS2012
2
˃
(2012) Difference
Pleistocene
1
1.8
2.59
0.79
Pliocene
1.1
5.3
5.33
0
Miocene
25
23
23.03
0
Oligocene
40
33.9
0.1
33.9
0
Eocene
60
55.8
0.2
56
0.2
Paleocene
70
65.5
0.3
66
0.5
0.5
Cretaceous
135
145.5
4
145
0.8
0.5
Jurassic
180
199.6
0.6
201.3
0.2
1.7
Triassic
225
251
0.4
252.2
0.5
1.2
Permian
270
299
0.6
298.9
0.2
0.1
Carboniferous
350
359.2
2.5
358.9
0.4
0.3
Devonian
400
416
2.8
419.2
3.2
3.2
Silurian
440
443.7
1.5
443.8
1.5
0.1
2.9
Ordovician
500
488.3
1.7
485.4
1.9
Cambrian
600
542
1
541
1
1
Source: Agterberg (
2013
)
radioactive minerals of which he excluded just over half that were considered to be
problematic (
cf
. Jackson
2006
, p. 245). From the remaining eight analyses he
arrived at dates for the bases of the Silurian, Devonian and Carboniferous, setting
them at 430, 370 and 340 Ma, respectively. In 1960, after more age determinations
of rock samples had become available, Holmes published the ages shown in
Table
9.14
, where they are compared with ages for the same periods or epochs in
the 2004 and 2012 geologic time scales published by the International Commission
on Stratigraphy and Geologic TimeScale Foundation, respectively. All numerical
time scales involve the conversion of a relative geological time scale into a linear
time scale along which rock samples and chronostratigraphic divisions are mea-
sured in millions of years. Holmes based his relative time scale on age-thickness
interpolations. He was aware of limitations of this method, stating: “I am fully
aware that this method of interpolation has obvious weaknesses, but at least it
provides an objective standard, and so far as I know, no one has suggested a better
one” (Holmes
1960
, p. 184). As pointed out by Gradstein et al. (
2012
, p. 16),
Holmes's estimate for the base of the Cambrian is curiously close to modern
estimates (see Table
9.14
).
Subsequently, Walter Brian Harland spearheaded several broadly based projects
between 1960 and 1990 that resulted in two widely used international geologic time
scales (Harland et al.
1982
,
1990
) known as GTS82 and GTS90. Geochronologists
commonly report analytical precision of age determinations as a 95 % confidence
interval written as
2. Many dates of greatly variable
precision were used for constructing GTS82 and GTS90. After 1990, higher
precision dates with 2
2
˃
because
Φ
(0.975)
-values of 0.5 % or better have become available. Radio-
metric (e.g., Uranium-Lead,
˃
40
Ar/
39
Ar and Rhenium-Osmium) methods have
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