Biostratigraphy type relative dating technique
A second problem is that to draw a signiﬁcant stratigraphic boundary at an unconformity or at some other signiﬁcant stratigraphic change is to imply the hypothesis that the change or break has a signiﬁcance relative to the stratigraphic classiﬁcation, that is, that unconformities have precise temporal signiﬁcance.This was speciﬁcally hypothesized by Chamberlin who was one of many individuals who generated ideas about a supposed “pulse of the earth”.A new unit could be erected, but it seemed likely that with additional detailed work around the world many such chronostratigraphic problems would arise, and at some point it might be deemed desirable to stabilize the suite of chronostratigraphic units.For this reason, the development of some standardized procedure seemed to be desirable.
The amount by which the sediment bed of interest is younger than the lowest ash bed is then equal to its stratigraphic height above the lowest ash bed divided by the rate of sedimentation, thereby yielding an “absolute” age, in years, for that bed.
This procedure is typical of the methods used to provide the relative biostratigraphic age scale with a quantitative basis.
The method is, of course, not that simple, because sedimentation rates tend not to be constant, and most stratigraphic successions contain numerous sedimentary breaks that result in underestimation of sedimentation rates.
Timescales for the Cenozoic and the Jurassic and Cretaceous are particularly noteworthy for their comprehensive data syntheses, although all have now been superseded.
More recent detailed summation and reconciliation of the global data provided a comprehensive treatment of the subject.
This raised the question of how to classify the rocks that formed during the interval represented by the unconformity.