Geological time scale

Abstract

The principal aim of the science of geochronology is to present the history of the Earth as a dated sequence of events of known duration. The only available matrix of evidence is that contained within the rocks of the Earth, supplemented to a minor extent by meteorites and by rocks retrieved from the Moon. The primary requirement of geochronology is the erection of a measured geological time scale. By a painstaking study of the mutual relationships of the rocks of the Earth's crust and their included trace of the evolution of life, geologists over the last 200 years have slowly constructed a more-or-less detailed relative time scale. Precise physical measurements from which the ages of specific rocks and minerals can be estimated only became possible with the discovery of radioactive decay. The first estimates based upon physical measurements were undertaken in 1907. Great advances have been made in the last 15 years and the present state of geochronometry is summarized in this paper. There is a brief discussion of the assumptions, accuracy, precision and geological ' errors inherent in physical geochronometry. The five major physical methods of rock and mineral age determination currently in use are described. The evidence from which an estimate of 4600 x lo6 yr for the age of the Earth has been derived is reviewed and the history of the Earth is divided into a number of 'major cycles' (early Precambrian, 4600-3500 x lo6 yr; mid Precambrian, 3500-2400 x 106 yr; late Precambrian, 2400-1550 x lo6 yr and 1550-850 x 106 yr; Varisco-Caledonian, 850-250 x lo6 yr; and Alpine, 250 x lo6 yr to sometime in the future). Correlation of Precambrian rocks and the Precambrian time scale is almost entirely dependent upon physical age measurements. Our knowledge of the geochronology of the last 600 x lo6 yr of the Earth's history is based upon a careful combination of the evidence from biostratigraphy and physical geochronometry. This latter, or Phanerozoic, portion of the geological time scale is discussed in detail. I t is concluded that geochronology as a science is still in its infancy. A vast amount of age measurement work will have to be done before its principal objectives can be achieved.

Impacts and tectonism in Earth and Moon history of the past 3800 million years

“…These episodes are listed in Table I, where three are designated by the names of their Canadian Shield prototypes following Fitch et al (1974). The six episodes were global (or at least broadly regional) in character, being now recognized on all or most of the major stable cratons.…”

K-Ar age of the East Peripheral kimberlite at De Beers Mine, Kimberley, R.S.A.

“…Three dating samples were prepared: the first was a total rock sample of the relatively fresh phlogopite-peridotite matrix, excluding, as far as was possible, any of the numerous exotic xenoliths carried within this matrix; the second was a concentrate of fine-grained phlogopite flakes from the matrix, and the third was a concentrate of coarse-grained phlogopite from one of the more prominent inclusions. All three dating samples were analysed by the argon-40/argon-39 age spectrum method using the techniques described by Fitch, Miller & Mitchell (1969); Fitch & Miller (1973);Fitch, Forster & Miller (1974) and Frost, Fitch & Miller (1981). A summary of the relevant analytical results is given in Table 1 and the characteristic feature of the three age spectra (in each a major plateau sector) is illustrated in Figure 1.…”

“…121-41;Fitch, 1972;Dalrymple, Gromme & White, 1975). In certain rare, favourable circumstances, argon-40/argon-39 age spectrum analysis can be used to help resolve these discrepantly too high apparent ages (Fitch & Miller, 1971, 1973Fitch, Forster & Miller, 1974;Lanphere & Dalrymple, 1976;Frost, Fitch & Miller, 1981); in the more commonly encountered, less favourable circumstances, it is at least possible for the severe disturbance of the argon-40/argon-39 age spectra to be used to demonstrate the presence and extent of an extraneous argon problem. Conversely, the absence of disturbance in age spectra, along with concordance between individual age spectra and conventional K-Ar 'spot' dates, can be used to provide confidence in the geological accuracy of any particular K-Ar date (Fitch, Hooker & Miller, 1978) and is equally important.…”

Some conventional potassium–argon and 40Ar/39Ar age studies of glauconite

“…According to the geological time scale compiled by Fitch, Forster & Miller (1974), the junction between Arenig and Tremadoc is 3= 510 Ma. The Phanerozoic Time-scale (Harland, Smith & Wilcock, 1964) Under the microscope samples A 1, A2, A3 and B displayed very similar morphological characteristics.…”

“…The arbitrary mean of 471 Ma is about 8 % lower than the time-scale value after Fitch et al (1974). Junction between Arenig/Tremadoc 3s 510 Ma (Fitch, Forster & Miller, 1974). Constants used: A p = 4.72 x 10" 10 y-\ A, = 0.584x 10" 10 y" 1 and "K/K = 0.0119 at.…”