Stratigraphy and nature of Taupo Pumice Formation

Abstract

Tephras erupted from Taupo Volcanic Centre, North Island, New Zealand, during the most recent eruptive event (c. 1820 years ago) are defined as 4 members of Taupo Pumice Formation. From youngest to oldest these are Taupo Ignimbrite, Taupo Lapilli, Rotongaio Ash, and Hatepe Tephra. All are airfall in origin except Taupo Ignimbrite which is the product of pyroclastic flows. Three stratigraphic units are here recognised within Taupo Ignimbrite. The Lower unit, restricted in occurrence mostly between Taupo and Turangi, is predominantly reddish brown in colour and has previously been included within Waitahanui Breccia. It is here recommended that use of the name Waitahanui Breccia be discontinued. The Middle unit is the most extensive and is discontinuously underlain by a “lithic lag layer”, rich in accessory lithics and crystals. The Upper unit is widely distributed and noted for the large pumice blocks found at the top of the ignimbrite, as well as interbedded fine ash airfall layers containing chalazoidites (accretionary lapilli). Each unit of Taupo Ignimbrite is highly variable in appearance and is described in terms of 3 lithofacies: “valley facies”, “fines-depleted facies” and “ignimbrite veneer facies”. Charcoal, from branches and logs, is common within the Middle unit. Radial orientation of these logs, as well as grainsize and distribution of individual members, indicates Taupo Pumice Formation was erupted from within the present Lake Taupo, at Horomatangi Reefs. The total volume of tephra ejected during the eruptive sequence is estimated at about 100 km³. Approximately 20 km³ constituted the airfall beds of Hatepe Tephra, Rotongaio Ash, and Taupo Lapilli. The total volume represents about 20 km³ of magma.