Post-1.8-ka Marginal Sedimentation in Lake Taupo, New Zealand: Effects of Wave Energy and Sediment Supply in a Rapidly Rising Lake

The eruption of Taupo caldera, New Zealand, c. 1.8 ka, climaxed in the emplacement of the Taupo ignimbrite, which dammed the previous outlet of the caldera lake and allowed rise of the lake behind the dam to c. 34 m above current lake level. After an estimated 20 yrs of lake refilling, the ignimbrite dam was breached, and the lake drained rapidly to a position 2-4 m above current level. From that level, the lake apparently dropped slowly to its present position. Post-1.8-ka lacustrine sediments are exposed at various levels between the highstand and the modern shoreline. The distribution of lacustrine facies at Taupo reflects the location of the mouths of rivers that drained uplands buried and/or denuded of vegetation by the erupted material, together with the position of depositional sites with respect to prevailing winds and energy systems. Four broad facies environments can be recognized, defined by combinations of high or low energy and high or low sediment influx. Sediments deposited in areas of high energy (i.e. strong wave action) and high sediment input are rich in vitric material, but contain abundant lithic material in those deposits interpreted as storm influenced. Deposits in low-energy areas that had high sediment input are dominantly vitric, with abundant shoreface and pumice-raft deposits. High-energy, low-sediment-input areas are rich in lithic detritus, apparently reworked from ignimbrites along the pre-eruptive lake shore during storms. Deposits that characterize low-energy, low-sediment-input environments are poor in lithic detritus, reflecting the inability of currents to transport material to these sites. Terraces prominent above the modern shoreline of Lake Taupo expose transgressive sequences that range from beach face deposits at the base to below-storm wave-base deposits at the top. Many terraces are capped by deposits of well-rounded, cobble- to boulder-size pumice clasts, which are interpreted as stranded deposits of previously floating clasts, formed during rapid lake-level fall. The Lake Taupo sedimentary record provides insights into sedimentary facies and styles that may be expected during lake filling that immediately follows caldera collapse. The inferred rapid filling rate of the lake, estimated to be 5-9 m yr-1 for the exposed section, together with the dominantly vitric nature of the source material, permitted preservation and recognition of deposits such as shoreface storm-wave deposits that are unusual in non-volcaniclastic lake settings.