CONSTRUCTING AN ERUPTIVE HISTORY: AN ANALYSIS OF THE TEKOA TUFF IN ISLAND HILLS, NEW ZEALAND

The Tekoa Tuff is a Miocene-aged, volcaniclastic deposit located in the Island Hills area of the Tekoa Range in North Canterbury, New Zealand. The tuff and surrounding deposits have been previously studied, but not in depth. This study constructed eruptive and depositional histories through physical observations from the field, stratigraphic methods, and grain size analysis. Additionally, well-studied volcanics near Oamaru, New Zealand provide data used for comparison. At Island Hills, distinct layers were identified within the Tuff at two different sites. Grain sizes were separated using a wet sieve after dissolving the carbonate cement of each sample. Variables considered include the color, size, and shape of the grains, the components of each layer, and the weight percentages of calcite cement in each layer. Preliminary data suggests that the eruptive history has been divided into four distinct episodes based on shape and color similarities and the presence of lithics. Preliminary analysis of the calcite percentages shows a near 30% increase in calcite content in the uppermost layers of the Tekoa Tuff. This indicates the occurrence of a significant change in the depositional environment. Research to further develop the eruptive history will continue through fall 2011.