McCraw, J.D.Briggs, Roger M.Lowe, David J.2026-05-042026-05-041987https://hdl.handle.net/10289/18250Aspects of late Quaternary tephrostratigraphy and tephrochronology of the Waikato and other regions in northern North Island, New Zealand, are investigated using distal tephras contained in lake sediments and peats. Such environments are advantageous for tephra studies, particularly in distal localities where tephra identifications have previously been difficult, because the tephras are preserved as discrete layers relatively little affected by postdepositional reworking or weathering processes. Tephras can thus be correlated with confidence much further from source and mapped with a degree of thickness resolution around 10-100 times better than previously possible. Sediment cores from lakes and bogs enable the stratigraphic, chronologic, and compositional relationships of tephras erupted from different sources to be elucidated, and a record of explosive volcanism to be documented. Forty-one distal tephras of rhyolitic or andesitic composition have been deposited in lakes in the Waikato area over the past c.17 000 years. The tephras originated from six volcanic centres and are identified using their mineralogy (particularly ferromagnesian silicate assemblages) and glass and mineral composition (determined by electron microprobe) in combination with stratigraphic position and ¹⁴C chronology. The correlated tephras associated with each source (approximate ages in years B.P., old T½ basis) are: Taupo: Taupo (1800), Mapara (2200), Whakaipo (2800), Hinemaiaia (4500), Opepe (8900): Okataina: Whakatane (4800), Mamaku (7000), Rotoma (8500), Waiohau (12 200), Rotorua (13 300), Rerewhakaaitu (14 700), Okareka (17 000): Maroa: Puketarata (14 000): Mayor Island: Tuhua (6200). uncorrelated (14 500); Tongariro: Mangamate (Te Rato Lapilli?) (9950), Okupata (8 informal units Oa-1 to Oa-8: 10 100, 10 500, 10 800, 11 050, 11 200, 11 700, 12 100, 12 700), uncorrelated (13 100). Rotoaira (13 700); Egmont: 15 informal (uncorrelated) units Eg-1 to Eg-15 (2500, 3700, 3750, 4100, 4400, 5250, 5850, 5900, 9300, 9600, 10 100, 11 050, 14 500, 15 000, 15 500). Eleven Holocene tephras, mostly rhyolitic, are identified in bogs in Urewera National Park (Kaipo Lagoon, Te Rangaakapua): Kaharoa (700 years B.P.), Taupo, Mapara, Waimihia (3200 years B.P.), Hinemaiaia, Whakatane, Rotoma, Opepe, Poronui (9900 years B.P.), Karapiti (10 100 years B.P.), and Okupata (10 300 years B.P.). Distal rhyolitic tephras identified in Auckland (Lake Waiatarua) include Tuhua, Mamaku(?), Rotoma, and Opepe(?); in Northland (Lake Omapere), Mamaku and Rotoehu (c.50 000 years B.P.) tephras are identified. New methods for detecting thin tephra layers in organic sediments are investigated: x-radiography of unopened, small diameter sediment cores, rapid x-ray fluorescence of peat samples, and the application of subsurface interface radar (SIR) to tephra-bearing lake sediments and peats. SIR is fast and precise with depth penetration up to 10 m, and could be very effective in mapping shallow tephra layers in various environments. Controls on the rates of weathering and the genesis of clay minerals in tephras of acid to intermediate composition are reviewed. A study of controls on weathering and clay mineral genesis in subaerial late Quaternary tephra deposits and soils in the Waikato region is greatly facilitated by the stratigraphic, compositional, and pattern of distribution of the unweathered counterpart tephras preserved in adjacent lakes and bogs. The study demonstrates that rates of weathering and clay mineral transformations in tephra materials are strongly influenced by composition and environmental factors, especially microenvironmental factors, rather than a tephra age-dependent factor. The dated tephras in c.17 000 year-old lakes in the Waikato region provide time-stratigraphic markers for multidisciplinary palaeoenvironmental studies, many of which are currently in progress, on the lake sediments. The developmental history of Lake Maratoto, near Hamilton. is investigated using tephrochronology and ¹⁴C dating, and enables some aspects of postglacial climatic change to be inferred.enAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.Thesis