Physical volcanology of Miocene basaltic pyroclastic deposits at Pigeon Bay: Remnants of flank scoria cones of Akaroa volcano, Banks Peninsula, New Zealand

Abstract

Coastal exposures of four lithified pyroclastic units at Pigeon Bay on Banks Peninsula in Canterbury, New Zealand, record flank explosive activity at the Miocene Akaroa volcano. These eroded units are the surface expression of lateral dike injections of hawaiite magma from a central conduit beneath the Akaroa lava shield. The form of the deposits was severely modified by erosion before burial within the volcanic pile and during exhumation. Four lithofacies are proposed, based on the size and shape of clasts and inferred degree of welding: (1) ash‐rich deposits; (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 grainsize of the beds enable us to infer that the deposits are the eroded remnants of Hawaiian and Strombolian cones. Within the deposits, variations in clast shape and size reflect subtle variations in the processes of fragmentation, in turn controlled by the size and rise rate of bursting gas bubbles. Different degrees of welding reflect local variation in accumulation rates and clast temperature on deposition (in turn partially dependent on residence time in the vent, distance from vent to point of deposition, and clast size).

Keywords:

• Banks Peninsula
• Akaroa volcano
• hawaiite
• scoria cone
• Strombolian
• welded tuff