The advent of 
Ar/
Ar ages and the implication from
cyclostratigraphy [e.g. Johnson, 1982; Hilgen and
Langereis, 1989; Shackleton et al., 1990] that the
conventional K-Ar ages for Plio-Pleistocene [ Mankinen and
Dalrymple, 1979] were too young led to a large number of
Ar/
Ar age determinations. The conventional K-Ar age for the
Brunhes/ Matuyama boundary (0.73 Ma) has been modified to 0.78 Ma
by a number of independent 
Ar/
Ar studies [ Izett and
Obradovich, 1991; Baksi et al., 1992; Spell and
McDougall, 1992; Tauxe et al., 1992; Hall and Farrell,
1993]. This age is consistent with the cyclostratigraphic
estimates [ Johnson, 1982; Shackleton et al., 1990]
giving strong support to the new chronology. The
astrochronological ages derived by Hilgen [1991a,b] have now
been supported by 
Ar/
Ar ages for the Jaramillo [ Glass
et al., 1991; Spell and McDougall, 1992; Tauxe et al.,
1992], Cobb Mountain [ Turrin et al., 1994], Reunion [
Baksi et al., 1993a], Olduvai [ Walter et al., 1991;
Baksi, 1994], Kaena and Mammoth [ Renne et al., 1993;
Walter, 1994, Walter and Aronson, 1993], and Gilbert [
McDougall et al., 1992, Baksi et al., 1993a].
The Cande and Kent [1992a] timescale utilized the following 9 absolute age tie points for the last 83 Ma. (1) Gauss/Matuyama boundary at 2.60 Ma from astrochronology [ Shackleton et al., 1990; Hilgen, 1991a,b]. (2) C5Bn at 14.8 Ma through the correlation of foraminiferal zonal boundary N9/N10 to the GPTS [ Miller et al., 1985] and the age estimates for this foraminiferal boundary from Japan at 14.5 Ma [ Ikebe et al., 1981] and from Martinique at 15.0 Ma [ Andreieff et al., 1976]. (3) Oligocene-Miocene boundary correlated to the middle part of C6Cn [ Berggren et al., 1985], chronogram ages for this stage boundary (23.8 Ma) from Harland et al. [1990]. (4) Eocene-Oligocene boundary correlated to chron C13r in the Apennines [ Nocchi et al., 1988], with absolute age estimate (33.7 Ma) from Odin et al. [1991]. (5) A date (46.8 Ma) for the upper part of C21n from Bryan and Duncan [1983] correlated to the GPTS by Berggren et al. [1983]. (6) An age of 55 Ma for the NP9/NP10 nannofossil boundary [ Swisher and Knox, 1991] correlated to the Paleocene/Eocene boundary [ Berggren et al., 1985]. (7) Cretaceous-Paleocene boundary at 66 Ma [ Harland et al., 1990]. (8) Campanian-Maastrichtian boundary at 74.5 Ma [ Obradovich and Cobban, 1975; Obradovich et al., 1986] based on the correlation of this stage boundary to the late part of C33n [ Alvarez et al., 1977]. (9) Campanian-Santonian boundary at 84 Ma [ Obradovich et al., 1986; Alvarez et al., 1977].
The Cande and Kent [1992a] timescale, based on the analysis of oceanic magnetic anomalies, is discrepant with the astrochronological estimates for the Plio-Pleistocene (Table 1). Wilson [1993] analysed the magnetic anomalies at various spreading centers and concluded that the anomaly spacing is, in fact, consistent with the astrochronological estimates. Cande and Kent [1992a] adopted the spacing of Plio-Pleistocene anomalies given by Klitgord et al. [1975], which appears to be the source of the error. In the revised version of their timescale, Cande and Kent [1994] adopted the astrochronological estimates [ Shackleton et al., 1990; Hilgen, 1991a,b] for all Plio-Pleistocene reversals, and the 65 Ma estimate (as opposed to 66 Ma) for the Cretaceous-Tertiary boundary [ Swisher et al., 1992, 1993]. Otherwise the absolute age tie points are as for Cande and Kent [1992a].
Since the publication of the Cande and Kent [1992a]
timescale, several Paleocene to Miocene 
Ar/
Ar age
determinations correlated to polarity chrons have become available.
For Miocene polarity chrons, Baksi et al. [1993b] and Baksi
[1993] [modified from Baksi and Farrar, 1990] determined
Ar/
Ar ages of 9.6 Ma and 16.32 Ma for the termination of C5n
and the onset of C5Bn.2n, respectively. McIntosh et al.
[1992] have determined the ages of polarity chrons close to the
Eocene-Oligocene boundary, and estimated an age of 33.4 Ma for this
stage boundary. In the Bighorn Basin (Wyoming), a tuff dated at
52.8 Ma lies at the older boundary of C24n.1 [ Tauxe et al.,
1994].
Obradovich [1993] has reviewed 
Ar/
Ar age control
on Late Cretaceous ammonite zones in the US Western Interior.
These zones can be correlated to the European ammonite zones to
provide the best available numerical age estimates of
Albian/Cenomanian and younger Late Cretaceous stage boundaries
(Table 2). One clear discrepancy concerns the
Campanian/Maastrictian boundary. This stage boundary is generally
considered to coincide with the top of the Globotruncana
calcarata foraminiferal Zone in Italian pelagic limestone
sections, and this zonal boundary lies in the upper part of C33n.
The Cande and Kent [1994] timescale therefore implies an age
>73.62 Ma (age of top of C33n) for the Campanian/Maastrichtian
boundary, whereas the definition of the stage boundary in the
Western Interior (at the base of the Baculites eliasi Zone)
yields an age of 71.3 Ma [see Obradovich, 1993] (Table 2).