Surficial bottom sediments of Lake Taupo, New Zealand: Texture, composition, provenance, and sedimentation rates

Abstract

Abstract Lake Taupo, the most voluminous lake in Australasia (c. 60 km3), is a large (623 km2), deep (max. depth 185 m) lake in central North Island, New Zealand. It infills a fault‐controlled volcanotectonic depression and the contained caldera of Taupo volcano. A strong interrelationship exists between modern lake depositional environments and surficial bottom‐sediment textures: lake shelf deposits (0–10 m depth) are sand‐gravel mixtures, wave rippled in exposed settings, and variably muddy under weed beds in sheltered regions; lake slope deposits (10–50 m) are massive silty sands and sandy silts, sometimes gravelly; lake rise deposits (50–100 m) are sandy silts and silts; lake basinal deposits (>100 m) are mainly silts; and sublacustrine talus aprons at the foot of sheer cliffs, and sublacustrine hills, host mixed gravel‐sand‐mud deposits. The terrigenous fraction of these sediments includes rock fragments, volcanic glass, quartz, plagioclase feldspar, and clay minerals (illite, kaolinite, and smectite). Most terrigenous material is derived from the greywacke‐ and andesite‐dominated catchments south of the 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 or interflow currents, to settle progressively in basinal regions of the lake. A minor fine sand fraction in basinal sediments is wind‐borne andesitic ash. The intermittent dispersal of terrigenous sediment occurs 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 sediment cores show an extensive, shallow but irregular, sub‐bottom reflector horizon atop sedimentary pumice deposits that are inferred to have been rapidly emplaced during and soon after the c. 1850 \ ear old Taupo eruption. The overlying lake muds range in tnickness from >2.5 m in southern basinal regions to <0.5 m in central and northern parts of the lake, corresponding to mean basinal sedimentation rates of >1.4 mm/yr in the south, where the highest inputs of river‐borne sediment occur, to <0.3 mm/yr in distal central lake regions, where diatomaceous silt predominates. Spatial distribution patterns of sediment texture and composition, together with depositional rates, form the basis of a conceptual model of sedimentation for Lake Taupo.