Facies analysis of pyroclastic deposits within basaltic tuff‐rings of the Auckland volcanic field, New Zealand

Abstract

Three basaltic tuff‐rings (Pupuke, Motukorea, and Orakei) in the Auckland volcanic field, show a range of eruption mechanisms from dry magmatic to dry and wet phreatomagmatic. A complex mix of physical processes resulted in a variety of deposits within each tuff‐ring. Motukorea tuff‐ring shows a drying‐upward sequence from matrix‐supported facies at the base to clast‐supported facies in transitional and upper units. Pupuke tuff‐ring shows an unusual sequence for the Auckland volcanic field from clast‐supported facies at its base to matrix‐supported facies at the top. Orakei tuff‐ring deposits are dominantly matrix‐supported facies. Clast‐supported facies comprise black, incipiently vesicular lapilli and blocks with little ash, resulting from minor magmatic activity energised by steam. Clast‐supported facies are interpreted to have been deposited by fall or dry surges from explosions that involved a limited amount of water at the vent. Matrix‐supported facies are dominated by ash, poor sorting, rounded ash‐coated lapilli‐sized clasts and lithic lapilli in massive, planar‐bedded or dune‐bedded forms. Matrix‐supported facies are interpreted to have been deposited by relatively wet surges generated by explosions involving a high water component at the vent.

Keywords:

• Auckland volcanic field
• phreatomagmatic
• wet surge
• dry surge
• Strombolian
• tuff‐ring