男鹿半島南西部戸賀-門前間の地質

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Miocene volcanics and their pyroclastic rocks called the Kamo lavas and the Shinzan rhyolites after HUZIOKA (1959) are widely distributed in this district. The lowest member consists of porphyritic trachytic-andesites, probably correctable to MIYAGI (1956)'s Kuguriiwa lavas, and is conformably overlain by the "Kamo lavas". The Kamo lavas are divided into two members.the lower (basalts) and the upper (porphyritic two-pyroxene andesites). They are unconformably overlain by the Tateshima conglomerate (newly named by the authors) which is chiefly composed of subrounded gravels of porphyritic two-pyroxene andesites and altered basalts. The "Shinzan rhyolites" cover the above all members including the Kamo lavas usually with unconformity. The "Shinzan rhyolites" are subdivided into the first stage eruptive type and the second and third stage intrusive types. The well sorted and/or graded pyroclastic beds are found often intercalated with the Kamo lavas, the Tateshima conglomerate and the first stage rhyolites, suggesting that the volcanic .action in this district had happened in a fresh water environment. The presence of the Tateshima conglomerate may be considered to mean the following tectonic history: eruption of the Kamo lavas→fracturing followed by local tilting which is to be continued intermittently→formation of the local depression→sedimentation of the Tateshima conglomerate→eruption and intrusion of the Shinzan rhyolites (first, second and third stages). It is noted that there are shown general coincidence of structural direction between faults, dykes and flow structures of rhyolites (chiefly second stage), trending NNE〜NE-SSW〜SW. This may indicate the intimate relationship between eruption and intrusion of rhyolites and fracturing. As for the mode of occurrence and structural aspect of the intrusive type rhyolites, the second stage rhyolites may be dyke-form, whereas the third stage may be lava-dome form.
1972-09-25

男鹿半島南西部戸賀-門前間の地質

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