The maximum paleotemperature or paleo-geothermal gradient is one of the important parameters for evaluating hydrocarbon potential in the sedimentary basins. Authigenic zeolites, found in Japanese Tertiary and Cretaceous oil/coal fields, show systematic distribution of a vertical zonal arrangement: the fresh glass zone, the clinoptilolite zone, the analcime zone, and the albite zone in descending order. The zeolites in sedimentary rocks are formed mainly through the function of temperature during the burial diagenesis. The temperatures of the first appearences of the minerals are particularly affected by heating time, geothermal gradient and the concentration of Na+ in pore water. The present paper studied the relationship between the present geothermal gradient and the appearance of zeolite observed in twenty wells in the Akita and Niigata oil fields of which the burial depth of hole rocks are geologically considered to be maximum at the present time. The temperatures (T, °C) and the depths of the first appearances (Z, m) can be expressed by functions of the geothermal gradient (G, °C/100m): Clinoptilo1ite: Tc=70.4-2.42G, Zc=5271/G-141, Analcime: Ta=145.6-10.6G, Za=15434/G-1501, Albite: Tab=191.1-11.3G, Zab=19103/G-1434. Thethicknesses of the clinoptilolite zone (ΔZc)and the analcime zone (ΔZa) are respectively given by ΔZc=10050/G-1317 and ΔZa=3669/G+67. The paleo-geothermal gradient and the maximum paleotemperature can be estimated from the above relations whenever the appearances of zeolites will be clear.
Biostratigraphic studies of both calcareous nannofossil and planktonic foraminifera were made on the Nanatani Formation exposed in the Imogawa section and the Nishikurosawa Formation in the Oga Peninsula, northeast Japan. The studies revealed the interrelation between calcareous nannofossil datum planes and planktonic foraminiferal ones shown by means of their occurences in ascending order as follows: Datum level a: Last appearance of Helicosphaera scissura (NN4/NN5 boundary), Datum level b: Abundant top of Discoaster def landrei (CN3/CN4) and first appearance of Orbulina suturalis (N8/N9), Datum level c: Abundant base of Reticulofenestra pseudoumbilica and first appearance of Globorotalia peripheroacuta and G. miozea (s. 1.) (N9/N10), Datum level d: Last appearance of Sphenolithus heteromorphus (CN4/CN5a or NN5/NN6). These fossil criteria indicate that the Nanatani Formation is CN3 to CN5a and N8 to N10. The Nishikurosawa Formation is the lower part of CN4 which corresponds to N9. Abrupt increment of Reticulofenestra pseudoumbilica in the late Nanatani stage implies the influence of cold water in early Middle Miocene to the area of Japanese islands which had been under the subtropical climate.