| Abstract |
Following the debris flows at Matata, eastern Bay of Plenty, on May 18, 2005, detailed field investigations of the geology, freshly exposed in the scoured channels of the Awatarariki and Waitepuru streams, were performed. Within a sequence of marine and terrestrial sediments, previously thought to be as old as early Pleistocene, the Rangitawa tephra (correlated with the ~320-340 ka Whakamaru group ignimbrites) has been recognised, resting on a palaeosol above a sequence of shallow marine, beach and fluvial sediments. The tephra is overlain by several packages of cross-stratified fluvial sediments dominated by coarse Whakamaru-derived pumice clasts, then by a mixture of terrestrial sediments and two packages of shallow marine sediments recording incursions of the sea across a gently sloping alluvial plain. The sequence is capped by the ~280 ka Matahina ignimbrite, which rests on thin terrestrial sediments above the upper of the two marine incursion sequences. The Rangitawa tephra and Matahina ignimbrite are key chronostratigraphic markers that can be used to predict the stratigraphy above and below their correlatives which are widely distributed beneath the Kawerau Geothermal Field. Stratigraphic work at Matata serves to provide models for the geometry, age and origins of rock units important in determining the permeability structure of the Kawerau geothermal field, and in providing a framework for development of structural controls on growth of the Whakatane graben. |