New Zealand tephro-chronology as a tool in geomorphic history of the c. 140 ka Mamaku Ignimbrite Plateau and in relating oxygen isotope stages

Abstract

The Mamaku Plateau, located west of Rotorua and adjacent to the Taupo Volcanic Zone (TVZ), is largely formed from Mamaku Ignimbrite (erupted c. 140 ka) which is overlain by tephras varying in age from 1.8 to c. 130 ka. Interbedded tephras, paleosols, loesses, and correlation with the marine oxygen isotope record enable a detailed reconstruction of geomorphic events. The major acidic tephra markers at Mamaku also occur extensively in North Island in dominantly loess mantles of river and marine terraces and in andesitic tephra mantles of marine terraces. Accordingly, past climate and vegetation changes interpreted at Mamaku are linked with similar data elsewhere and have a wider interest in the context of oxygen isotope stages. Major erosion occurred soon after emplacement of the ignimbrite and during two intervals of cold climae: c. 68 to c. 56 ka and c. 23 to c. 15 ka (oxygen isotope stages 4 and 2, respectively). Cover beds, especially in the southern and central part of the Plateau, were stripped and the underlying ignimbrite exposed. Widespread erosion was triggered at the beginning of these cold intervals by the destruction of the vegetative cover. Stripping of the cover beds was more severe during isotope stage 4 than stage 2, suggesting either a more extreme climate, or a more fragile vegetation community during stage 4 than during stage 2. The erosion produced deep valleys with cuspoid features on south facing slopes. The cuspoid features are mainly attributed to the differential effects of cold subpolar air masses which are thought to have destroyed the vegetation cover on south facing slopes and triggered mass wasting or catastrophic slope failure. Near the crest of the Plateau erosion also produced very distinctive tor fields. The tors are remnants of welded ignimbrite exposed by the removal of surrounding softer, less welded ignimbrite. This study provides a model for understanding other volcanic landscapes of similar and older age in the Taupo Volcanic Zone.