SHIBAOKA & BENNETT have developed a method for estimating the maturation level of organic materials in a sediment, where the lag of increase in maturity behind increase in geotemperature accompanied by subsidence of a sediment is taken into consideration. They have also presented a diagram which correlates vitrinite reflectance with geotemperature and time. The diagram, however, cannot be used if the burial history of a sediment is not reconstructed in detail, as in other nomograms ever published.
In this paper, the burial history of a sediment is assumed to be represented by a parabolic curve, and the increasing process of maturity shown by SHIBAOKA & BENNETT is methodologically employed. Within the limitations, a mathematical method for estimating the maximum paleo-geotemperature by which an already uplifted sediment had been affected is newly described on the basis of a set of data, i.e., burial time and reflectance of vitrinitic phytoclasts included dispersedly in surface rock-samples. As a result, the following statements can be made:
1) The increase of maturity of organic materials during the uplifting process of a sediment is very small as compared with that during the subsiding one. In case that a sediment had been affected by Tmax at the middle period of the whole burial time, organic materials in it had already gained the maturity of ca. 90% to the final one there. In that case, Tmax can be approximated from the measured vitrinite reflectance (R0) by NEWTON'S method on the basis of the following formula; log (0.90*R0)=0.0045*Tn+0.149*log tn-0.87, where Tn and tn are the formulae containing Tnax only as a variable, respectively. For example, in case that the burial time of a sediment is 20m.y., they are given by Tn=0.10+0.995Tmax, tn=1+2p+23*p+26*p+210*p+......+245*p, where p is presented by p=(20-Tmax)/500.
2) The calculated values of Tmax are substantially independent of the period when a sediment had been affected by Tmax if both the burial time and R0 are the same.
It is ascertained that Tmax calculated by this method on the basis of actually measured reflectance of vitrinitic phytoclasts occurred in the Lower Miocene sediments distributed in the western part of Tokyo is harmonized with paleo-geotemperature estimated from natural zeolite occurred in the same sediments.