Paleoecology of a Late Eocene Mobile Rockground Biota from North Otago, New Zealand

A distinctive mobile pebble and cobble rockground biota of Late Eocene age is described from volcaniclastic sediments between Oamaru and Kakanui, North Otago, New Zealand. Thousands of subangular to subrounded basaltic pebbles and cobbles are encrusted with a diverse range of very well-preserved epibionts including crustose coralline algae, serpulids, bivalves, foraminifera, brachiopods, and more than 70 species of cyclostome and cheilostome bryozoans. The preservation of thin sediment layers beneath and between encrusting bryozoan colonies indicates the probable occurrence, during life, of agglutinating microbial mats. The abundance of subspherical rhodoliths, the diversity of epibionts, and their occurrence on all faces of the volcanic clasts reflect intermediate levels of overturning and rolling in a moderately current-swept channel adjacent to small volcanic islands and seamounts. The occurrence of large foraminifera (Asterocyclina), bryozoans (including two extant species), and brachiopods with warm-water affinities indicates subtropical sea temperatures and water depths of 25-50 m for this community, which represents one of the few described Cenozoic examples of a mobile rockground biota.