Depositional Environment of Algal Balls in the Ryukyu Group, Ryukyu Islands, Southwestern Japan

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Abstract
Fossil algal balls are abundant in limestones of the Pleistocene Ryukyu Group in the Ryukyu Islands, southwestern Japan. Their concentration forms a characteristic lithofacies, the algal ball limestone, which is an important component of the Ryukyu Group. Textural and paleontological examination indicate that these algal balls originated in a moat or lagoon, growing while they rolled about on the floor of a sandy channel, and finally accumulated on a gentle fore-reef slope. In places, algal balls are found in the deeper water facies of the Ryukyu Group into which some of them were brought by slumping or gravity flows to be included in biogenic detrital limestone accumulated on the fore-reef slope, together with those which originated in deeper water. The present paper exclusively deals with the algal balls of the algal ball limestones.

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