SURFICIAL BOTTOM SEDIMENTS OF LAKE TAUPO, NEW-ZEALAND - TEXTURE, COMPOSITION, PROVENANCE, AND SEDIMENTATION-RATES

Lake Taupo, the most voluminous lake in Australasia (c. 60 km(3)), is a large (623 km(2)), deep (max. depth 185 m) lake in central North Isl and, New Zealand. It infills a fault-controlled volcanotectonic depres sion and the contained caldera of Taupo volcano. A strong interrelatio nship exists between modern lake depositional environments and surfici al bottom-sediment textures: lake shelf deposits (0-10 m depth) are sa nd-gravel mixtures, wave rippled in exposed settings, and variably mud dy under weed beds in sheltered regions; lake slope deposits (10-50 m) are massive silty sands and sandy silts, sometimes gravelly; lake ris e deposits (50-100 m) are sandy silts and silts; lake basinal deposits (> 100 m) are mainly silts; and sublacustrine talus aprons at the foo t of sheer cliffs, and sublacustrine hills, host mixed gravel-sand-mud deposits. The terrigenous fraction of these sediments includes rock f ragments, volcanic glass, quartz, plagioclase feldspar, and clay miner als (illite, kaolinite, and smectite). Most terrigenous material is de rived from the greywacke- and andesite-dominated catchments south of t he lake, but also from pumice breccia deposits in the northeast. Silts and clays are segregated from gravels and sands at the shoreline, and carried offshore by mainly north- to northeastward-directed surface o r interflow currents, to settle progressively in basinal regions of th e lake. A minor fine sand fraction in basinal sediments is wind-borne andesitic ash. The intermittent dispersal of terrigenous sediment occu rs against a background of slow pelagic settling (mean 0.22 mm/yr) of diatom tests, which are least diluted by terrigenous input in central and northern parts of the lake. Acoustic reflection profiles and sedim ent cores show an extensive, shallow but irregular, sub-bottom reflect or horizon atop sedimentary pumice deposits that are inferred to have been rapidly emplaced during and soon after the c. 1850 year old Taupo eruption. The overlying lake muds range in thickness from > 2.5 m in southern basinal regions to < 0.5 m in central and northern parts of t he lake, corresponding to mean basinal sedimentation rates of > 1.4 mm /yr in the south, where the highest inputs of river-borne sediment occ ur, to < 0.3 mm/yr in distal central lake regions, where diatomaceous silt predominates. Spatial distribution patterns of sediment texture a nd composition, together with depositional rates, form the basis of a conceptual model of sedimentation for Lake Taupo.