ESTIMATION OF THE MESOZOIC GEOLOGICAL TIME-SCALE

Geological time scales are constructed by combining the stratigraphic record with radiometric dates and their standard deviations. The strat igraphic record encompasses litho-, bio-, chrono-, and magnetostratigr aphy. The statistical methods employed should embody concepts and data available for the systems considered. Recently, in order to estimate the ages of 31 Mesozoic stage boundaries, use was made of a database w ith chronostratigraphic classifications for 340 dates, biostratigraphi c data including ammonite subzones, and information on seafloor spread ing. This paper is primarily concerned with the propagation of errors through the successive steps of the data analysis. The following stepw ise approach was taken for combining the different types of data: (1) maximum likelihood estimation with windows set around prior stage boun dary estimates, (2) averaging of estimates with variable precision inc luding magnetochronologic data, and (3) calibration by means of cubic smoothing splines assuming equal duration of ammonite subzones. The en d product is a time-scale in which the stage boundary ages are accompa nied by approximate 95 per cent confidence intervals.