PHYSICAL VOLCANOLOGY OF MIOCENE BASALTIC PYROCLASTIC DEPOSITS AT PIGEON BAY - REMNANTS OF FLANK SCORIA CONES OF AKAROA VOLCANO, BANKS PENINSULA, NEW-ZEALAND

Coastal exposures of four lithified pyroclastic units at Pigeon Bay on Banks Peninsula in Canterbury, New Zealand, record flank explosive ac tivity at the Miocene Akaroa volcano. These eroded units are the surfa ce expression of lateral dike injections of hawaiite magma from a cent ral conduit beneath the Akaroa lava shield. The form of the deposits w as severely modified by erosion before burial within the volcanic pile and during exhumation. Four lithofacies are proposed, based on the si ze and shape of clasts and inferred degree of welding: (1) ash-rich de posits; (2) non-flattened scoria deposits; (3) mixed scoria deposits; and (4) densely welded scoria deposits. Spatial relationships between lithofacies, steep primary dips, and rapid changes in thickness and gr ainsize of the beds enable us to infer that the deposits are the erode d remnants of Hawaiian and Strombolian cones. Within the deposits, var iations in clast shape and size reflect subtle variations in the proce sses of fragmentation, in turn controlled by the size and rise rate of bursting gas bubbles. Different degrees of welding reflect local vari ation in accumulation rates and clast temperature on deposition (in tu rn partially dependent on residence time in the vent, distance from ve nt to point of deposition, and clast size).