Stratigraphy, age, composition, and correlation of late Quaternary tephras interbedded with organic sediments in Waikato lakes, North Island, New Zealand

Abstract

Cores from 14 peaty lakes in the central Waikato region, northern North Island, contain a sequence of 41 well-preserved, mainly macroscopic, occasionally bedded, ash and lapilli layers ranging in thickness from c. 2 to 110 mm and interbedded with fine-grained organic lake sediment. The layers, whose field and compositional properties are described in detail, are distal airfall tephras that were erupted between c. 17 000 and c. 1800 ¹⁴C years ago from six rhyolitic and andesitic volcanic centres located c. 70–200 km from the Waikato sites: Taupo (5 tephras), Okataina (7), Maroa (1) (rhyolitic); Mayor Island (2) (peralkaline); Tongariro (11), and Egmont (15) (andesitic). These sources were distinguished using the tephras' mineralogical assemblages and glass and mineral compositions (determined by electron microprobe). The tephras were correlated with named proximal eruptives using their stratigraphic relationships and radiometric ages (based on multiple ¹⁴C dates on enclosing lake sediment) in combination with the mineralogical and chemical criteria. The correlated tephras associated with each source (listed youngest to oldest) are: Taupo—Taupo, Mapara, Whakaipo, Hinemaiaia, Opepe; Okataina—Whakatane, Mamaku, Rotoma, Waiohau, Rotorua, Rerewhakaaitu, Okareka; Maroa—Puketarata; Mayor Island—Tuhua, uncorrelated; Tongariro—Mangamate (?Te Rato Lapilli), Okupata (8 informal units Oa-1 to Oa-8), uncorrelated, Rotoaira; and Egmont—15 informal units Eg-1 to Eg-15. In total, the post c. 15000 year lacustrine tephras range in thickness from c. 20 cm (possibly equivalent to c. 37 cm on dry land because of compaction or dissemination in the lakes) in the north of the study area, to c. 25 cm (c. 47 cm on dry land) in the south, to c. 42 cm (c. 78 cm on dry land) in the east. Of these total thicknesses, rhyolitic tephras make up c. 65–95%; andesitic tephras are more common in the south (c. 30–35%), decreasing markedly to the north (c. 10–15%) and east (c. 5%). These different proportions of andesitic to rhyolitic material may have influenced weathering and argillisation in the tephra-derived soils in the Waikato area. Provisional isopach maps for 11 tephras (Whakaipo, Eg-2, Tuhua, Mamaku, Rotoma, Opepe, Mangamate, Waiohau, Oa-8, Rotorua, and Rerewhakaaitu) are presented. The thickness resolution of these is generally one to two orders of magnitude greater than in most previous isopach maps in New Zealand, and attests to the value of using lake sediments for tephra mapping. Most of the tephras found in the Waikato lakes should persist well beyond the Waikato area. Extrapolated plots of isopach thickness against distance from isopach centre suggest that c. 1 mm isopachs should occur c. 200–300 km from source. Many of the tephras were probably emplaced by powerful eruptions (possibly including directed blasts), or were dispersed by strong winds, or both. Co-ignimbrite ash may have contributed to some of the resulting lacustrine tephra deposits. The 41 tephras identified in the Waikato lake cores record, on average, an eruptive event every c. 400 years (rhyolitic, 1 per c. 1100 years; andesitic, 1 per c. 650 years). The tephras, particularly Taupo, Tuhua, Mamaku, Opepe, Mangamate, Waiohau, Rotorua, Rerewhakaaitu, and Okareka, are useful as time-stratigraphic markers in paleoenvironmental studies on the lakes and catchments.