ケニヤ, モンバサ地域のカルー系, Duruma 砂岩の堆積構造

概要

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The Karroo Beds (Duruma Sandstones) distributed in the coastal region of Kenya consist essentially of arkosic sandstones and grits. They show, as a whole, a monoclinal structure inclined gently to the east-southeast.The upper half of the Duruma Sandstones exposed along the Mombasa Road was examined for the purpose of paleocurrent analysis. The most ubiquitously occurring current indicators are cross laminae both of small scale and of large scale. Current directions shown by small scale cross laminae in a single bed have a rather high consistency ratio. Parting lineations are also common in flaggy micaceous sandstones. Oscillation ripples with little directional significance are abundant, whereas current ripples are rare in spite of common development of cross laminae which are the product of migration of current ripples. Current marks such as groove casts are rarely recognized on the lower surface of graded sandstone beds alternating with mudstone beds. Probably these flysch-like beds were laid down from small and local “turbidity currents” originated from the sudden release of dammed-up sediments which had been accumulated behind some barriers in the realm of bed load transportation.
日本アフリカ学会の論文