Middle–Upper Pleistocene tephras in the Papua New Guinea highlands

Abstract

Eighteen Middle–Upper Pleistocene (ca 200 ka) tephra units in the highlands of Papua New Guinea (PNG) are mapped and described. The study area is about 24 000 km2, extending from Tari in the west to Kainantu and the Kassam Pass in the east. It is estimated that at least 75 000 km2 of highland PNG received ≥50 cm tephra from various highland sources, and the total volume would have been well over 300 km3. Total tephra thicknesses near sources are >20 m, with thicknesses of individual units near source ranging from 1–2 to >4 m. Several units were deposited with coarse ash and lapilli basal layers that have been weathered to fine ash and clay sizes. Hagen, Giluwe and Yelia volcanoes are the main sources, with smaller centres near Giluwe and north of Hagen also producing mappable tephra units. One unit (Birip) was erupted about 40 ka, but most of the tephra units are 200 ka and older. Volumes of tephra suggest that many of the eruptions had volcanic explosivity indices of 4–6. On stable bedrock, tephras are present on slopes up to 35°, demonstrating the high stability of the tephra materials. Gaps in the tephra cover coincide with bedrock that weathers to unstable regolith where tephras did not accumulate; the tephra cover thus provides an indicator of bedrock stability. Mapped tephra units provide the potential for correlation of other Quaternary materials and can be used as widespread marker beds. The tephras are also important as the parent material for the dominant agricultural soils in the highlands.