DISSECTION OF AN IGNIMBRITE PLATEAU: AFTERMATH OF THE 64 KA ROTOITI ERUPTION, BAY OF PLENTY, NEW ZEALAND

Following an explosive eruption, the duration and extent of the sedimentary response, i.e. the erosion of pyroclastic material, the perturbation of drainage networks and their return to equilibrium conditions, and changes in the rate and style of sedimentation and deposition, are functions of a number of factors. These include: (a) the volumes of material generated; (b) the eruptive style and dispersive power, which influence its type and distribution; (c) the pre-existing geomorphology of the impacted area; and (d) the climate at any point in time and space, which controls eustatic base-level and influences both the rate of erosion and the subsequent regrowth of vegetation, which in turn acts to stabilise pyroclastic deposits and inhibit erosion.

The Taupo Volcanic Zone in the central North Island of New Zealand is an ideal area in which to study such processes due to the youthfulness and degree of preservation of the products of a number of ignimbrite-emplacing eruptions. Eruption of the ~50 cubic km Rotoiti Breccia (RB) at the climax of the 64 ka Rotoehu eruption from the Okataina caldera buried an area of c. 850 square km beneath up to 100 m of unwelded pyroclastic flow units, damming the outlet to intracaldera Lake Rotorua where extensive highstand shoreline terraces developed as a result. The RB forms a broad, gently sloping fan that was deposited over a dissected plateau composed of older welded units including the Mamaku and Matahina ignimbrites. The distribution of valleys in this buried plateau largely controlled the development of drainages on the Rotoiti Breccia. Medial areas of the RB are characterized by abundant secondary phreatic explosion craters and their deposits buried beneath several metres of debris- and hyperconcentrated flow units. Sea-level was ~ 50 m lower than at present at the time of the eruption, so that fluvial deposits comprising braided stream sediments are graded to this base and lie buried beneath the modern coastal plain. Erosion and reworking was essentially completed under the cool interstadial conditions of Oxygen Isotope Stage 3: landscape stabilization occurred prior to eruption of the Mangaone Subgroup Plinian fall units between 43 and 31 ka which drape both the Rotoiti Breccia and its reworked units.