著者

北里 洋

出版者

東北大学

雑誌

東北大學理學部地質學古生物學教室研究邦文報告 (ISSN:00824658)

巻号頁・発行日

vol.75, pp.17-49, 1975-03-31

被引用文献数

19

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The present study is undertaken to clarify the age of deposition and extent of geographic distribution of the younger Cenozoic strata (Fig. 2) developed in the Oga Peninsula, Akita Prefecture (Fig. 1). Detailed tephrochronological investigation of the sedimentary rocks was done to trace the geographic distribution of the strata. A total of thirty key beds were traced over the Oga Peninsula area and a few of them were traced further to the Akita oil fields lying southeast of the Oga Peninsula (Fig. 9). Lithologic descriptions and type localities of the key tuff beds are given in Table 1. Magnetostratigraphy, microbiostratigraphy and radiometric dating assumed as the principal means to establish the geochronology of the strata. However, these methods were not accompanied by each other in most of previous works. Accordingly, an attempt was made to apply these methods to the same sections simultaneously and to synthesize their results for comparison. Two routes, the Oga Peninsula main route and the Sarukawa route, were selected in the Oga Peninsula for study. Along these routes, collected were 149 samples for paleomagnetic measurement, 30 samples for microfossil study and 4 samples for radiometric dating. The sampling localities and stratigraphic position in columnar sections are shown in Figs. 4 and 5. Magnetostratigraphy : -In the laboratory, samples were demagnetized in the alternation field of 150 Oersted to remove the unstable secondary components and then the samples were demagnetized in 200℃ thermal field. These samples were measured at every 45° around three axes of the sample with the automatic parastatic-type magnetometer. All numerical values obtained from the measurement were processed by an electric computer, NEAC 2200, Model 500, at the Computer Center of the Tohoku University. The result of measurements are shown in Fig. 6-a, b. The paleomagnetic studies indicate that the lowermost Kitaura, upper half of the middle Kitaura and upper Kitaura to Shibikawa formations are normaly magnetized and the uppermost Funakawa and lower to lower half of the middle Kitaura are reversely magnetized. The rest of the Funakawa Formation is interpreted as being remagnetized and unreliable for magnetostratigraphy. In order to magnetostratigraphic classification, these magnetic reversal patterns are named as AKOG-A, AKOG-B, AKOG-C, AKOG-D, AKOG-E at the Oga Peninsula main route, while AKSK-B, AKSK-C, AKSK-D, AKSK-E, AKSK-F at the Sarukawa route in descending order. Microbiostratigraphy : -Samples for microfossil study were used for planktonic foraminifera and Radiolaria. A part of samples used for paleomagnetic measurement were examined in calcareous nannoplankton and diatom. The results are shown in Figs. 7-a, b. The faunal and floral assemblages are generally monotonous in composition, as same as the present-day biota living in the North Pacific. Among them, species were found to be significant for biostratigraphical correlation, though they occurred sporadically both in the northern and southern sections : they are, planktonic foraminifera : -Globoquadrina asanoi, Globoquadrina kagaensis, Globoquadrina himiensis ; calcareous nannoplankton : -Gephyrocapsa spp. ; Radiolaria : - Eucyrtidium matuyamai, Ommatartus antepenultimus, Ommatartus penultimus, Ommatartus tetratharamus ; diatom : -Actinocyclus oculatus, Pseudoeunotia doliolus and Rhizosolenia praebergonii. Correlation : -Correlation of the magneto-zones with the Cenozoic magnetic time scale of marine sediments (Opdyke, 1972) were accomplished by correlating the associated micro-biostratigraphic events to those recognized already in the paleomagnetic polarity sequence of deep-sea cores (Fig. 8). From the interval of AKSK-F, occurence of Actinocyclus oculatus and Globoquadrina asanoi were recorded. Because of the first appearance of Actinocyclus oculatus and the last appearance of Globoquadrina asanoi, AKSK-F is correlated to the Matuyama reversed Epoch below the "Olduvai" event. Globoquadrina asanoi, Globoquadrina kagaensis, Globoquadrina himiensis, Actinocyclus oculatus and Rhizosolenia praebergonii were found in the intervals of AKOG-E, AKSK-F. The first appearance of Actinocyclus oculatus and Globoquadrina himiensis and the last appearance of Globoquadrina asanoi, Globoquadrina kagaensis, GloBoquadrina himiensis and Rhizosolenia praebergonii indicate that AKOG-E and AKSK-E are correlated to the "Olduvai" event. Within the interval of AKOG-D, AKSK-D, Gephyrocapsa spp., Eucyrtidium matuyamai, Pseudoeunotia doliolus and Actinocyclus oculatus were recognized. The first appearance of Gephyrocapsa spp., Eucyrtidium matuyamai and Pseudoeunotia doliolus and the last appearance of Actinocyclus oculatus and Eucyrtidium matuyamai assure that AKOG-D, AKSK-D can be correlated to the Matuyama reversed Epoch between "Olduvai" event and Jaramillo event. Correlation of the rest of magneto-zones are reserved because of poor occurrence in microfossils. Radiometric dating : -Dating was carried out by K-Ar method at the laboratory of Prof. Y. Uyeda of the Tohoku University. Measured minerals were biotite. The results of measurement are shown in Table 2. A distinct discrepancy is noticed between the radiometric age obtained and the results of magnetostratigraphic and microbiostratigraphic correlation.