ON THE AGE OF EVENTS OF UNKNOWN AGE - APP LICATION TO THE GEOMAGNETICPOLARITY TIME-SCALE

Events of unknown ages have by definition no known age. However, our k nowledge on these unknown ages is not completely non existent, since t hese events can be inserted in an ordered set of events of known ages (dated by radioisotopic methods for example) and events of unknown age s. Our knowledge on such ordered sequences can be formalized by using the noninformative (uniform) prior probability density distributions f or the ages of events of unknown ages and Bayes's theorem to introduce the time-order relationship condition. We deduce that the conditional density probability distributions of the ages of events of unknown ag es are given by various forms of Euler's beta distribution. These dist ributions allow us to estimate the probability for an event of unknown age to occur at a given time interval. We use this method to build ap propriate probabilistic images which allow us to express our actual kn owledge/ignorance on the ages of reversals of the Earth's magnetic fie ld from the Middle Miocene to the Upper Pliocene. These images take in to account the uncertainties which arise from the non-regularity of oc eanic spreading rates and the uncertainties in absolute calibration po ints. The ambiguity which exists in the polarity state of the geomagne tic field at a given time can be estimated using the Entropy function.