Depositional environment of algal balls in the Ryukyu Group, Ryukyu Islands, southwestern Japan.

Fossil algal balls are abundant in limestones of the Pleistocene Ryukua Group. 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. 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.-Authors