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.