FACIES ANALYSIS OF PYROCLASTIC DEPOSITS WITHIN BASALTIC TUFF-RINGS OFTHE AUCKLAND VOLCANIC FIELD, NEW-ZEALAND

Three basaltic tuff-rings (Pupuke, Motukorea, and Orakei) in the Auckl and volcanic field, show a range of eruption mechanisms from dry magma tic to dry and wet phreatomagmatic. A complex mix of physical processe s resulted in a variety of deposits within each tuff-ring. Motukorea t uff-ring shows a drying-upward sequence from matrix-supported facies a t the base to clast-supported facies in transitional and upper units. Pupuke tuff-ring shows an unusual sequence for the Auckland volcanic f ield from clast-supported facies at its base to matrix-supported facie s at the top. Orakei tuff-ring deposits are dominantly matrix-supporte d facies. Clast-supported facies comprise black, incipiently vesicular lapilli and blocks with little ash, resulting from minor magmatic act ivity energised by steam. Clast-supported facies are interpreted to ha ve been deposited by fall or dry surges from explosions that involved a limited amount of water at the vent. Matrix-supported facies are dom inated by ash, poor sorting, rounded ash-coated lapillisized clasts an d lithic lapilli in massive, planar-bedded or dune-bedded forms. Matri x-supported facies are interpreted to have been deposited by relativel y wet surges generated by explosions involving a high water component at the vent.