Sedimentology of Lake Taupo, Central North Island, New Zealand

Lake Taupo, with an area of 616 km², is the largest of several lakes of volcano-tectonic origin in the Central Volcanic Region, North Island, New Zealand. In the past the lake has been affected by catastrophic volcanic events, the last being the Taupo Pumice eruptions of about 131 A.O. Since then the catchment has restabilised and sedimentation in the lake has diminished. A number of depositional environments in Lake Taupo are characterised on the basis of surficial sediment texture and depositional processes. These environments include: shelf (sands, gravels, sand-gravel mixtures); slope (silty sands, sandy silts); rise (sandy silts, silts); basin (silts); sub-lacustrine talus slope and sub-lacustrine hill (gravel-sand-mud mixtures). Close to existing or pre-existing river mouths, and on sub-lacustrine talus slopes, gravels typically include one or more types of catchment-derived rock fragments (andesite, rhyolite, ignimbrite, greywacke). In other parts of the lake, gravels are almost entirely pumice fragments derived from pumice tephras and breccias within the catchment. Shelf and slope sands consist of rock fragments, volcanic glass, quartz and plagioclase feldspar. Slope and rise sediments may also contain sand-sized benthic and aufwuch diatom tests (e.g. Synedra sp., Nitzschia sp., SurireZZa sp.). Basinal sediments include a minor,very fine-fine sand fraction of andesitic Ngauruhoe Ash. Silt in basinal sediments comprises planktic diatom tests (Melosira sp. » Cyclotella sp. ≃Epithemia sp.) and terrigenous grains (volcanic glass, rock fragments, quartz, plagioclase feldspar). Subordinate clay-sized material is dominated by diatomaceous and authigenic silica, detrital illite and kaolinite, and montmorillonite of probable hydrothermal origin. The distribution and provenance of sediment components in the lake indicate that terrigenous material is derived mainly from southern catchment regions (greywacke and andesite lithologies) and to a lesser extent from the northeastern catchment (rhyolitic volcanics). Gravels and sands are deposited in shallow water where they may be re-worked by coastal processes; suspended silts and clays are transported offshore by surface or inter-flow currents, and deposited in basinal regions of the lake. An extensive sub-bottom surface of sedimentary pumice, deposited in Lake Taupo during and soon after the Taupo Pumice eruptions, has been identified from sediment cores and seismic profiles of the lake bottom. Mud-layer thicknesses overlying this pumice surface indicate average basinal sedimentation rates of less than 30 mm/yr for central and northern lake regions, to more than 140 mm/yr in southern 1ake regions. Areal distribution patterns of sediment texture and composition, together with depositional rates, form the basis for a proposed model of sedimentation in Lake Taupo