ERUPTIVE HISTORY OF AN EOCENE SURTSEYAN VOLCANO, MOERAKI PENINSULA, SOUTH ISLAND, NEW ZEALAND

A model for the explosive formation of a shallow submarine basalt volcano on the East Otago shelf is presented. Surtseyan lapilli tuff deposits with maximum stratigraphic thicknesses of at least 175 m record the existence of a volcano that built into storm wave base. Data from two stratigraphic columns show that the volcano was formed by at least two phases of eruption, separated by a quiescent period during which cross-laminated volcaniclastic sandstones derived from the volcano were deposited. With the exception of these sandstone beds, the entire volcano is composed of lapilli tuff. Units below the sandstones are generally massively bedded, while units above are usually well-bedded and often alternate between coarser and finer grained lenticular bedsets. A diverse range of large clast types, including fragments of schist-derived xenoliths, dike fragments, and irregularly shaped basalt clasts exhibiting possible fluidal deformation, is present throughout the lapilli tuff at Moeraki. Examination of these rocks and detailed petrographic study of the lapilli show that a variety of eruptive and fragmentation processes produced the lapilli tuff. Included in these processes was the explosive phreatomagmatic interaction of magma and particle-seawater-sediment slurries in the vent. It is proposed that the volcano was formed through the explosive eruption of lapilli tuff at depths no greater than 400 m below sea level. Deposition of lapilli tuff, both through fall deposits and eruption fed density currents, built a volcanic pile at least 100m above the sea floor. Eruption then ceased for a period during which erosion and deposition of volcaniclastic sandstone occurred. Renewed eruption of lapilli tuff resulted in a volcanic cone that rose above storm wave base and may have emerged. As with Surtla and Kavachi, it is likely that erosion to storm wave base occurred rapidly following the cessation of eruption.