Late Quaternary (post 28,000 year B.P.) tephrostratigraphy of northeast and central Taranaki, New Zealand

Abstract

Mt. Egmont/Taranaki in western North Island, New Zealand, has been inactive during historic time and thus its pattern of eruptive history must be reconstructed by inference from the stratigraphy and chronology of associated volcanic and pyroclastic deposits. The most complete record of eruptive activity from Egmont Volcano is found on the surrounding ring plain rather than on the volcanic cone, where surficial deposits are readily removed by erosion or deeply buried by the products of more recent eruptions. In this study, a comprehensive post‐28 ka record of the volcano's eruptive history is presented, and the relationship of andesitic tephra beds to andic soil material, Egmont‐sourced volcaniclastic detritus, and two silicic tephra beds from Taupo Volcano are discussed, along with implications for inter‐regional correlation. The post‐28 ka tephra succession is recorded in sixteen andesitic tephra formations. These formations and their approximate ages are as follows: Manganui tephra (4 beds; c. 3.1 ka), Inglewood Tephra (2 beds; c. 3.6 ka), Korito Tephra (2 beds; c. 4.1 ka), Mangatoki Tephra (2 beds; c. 4.4 ka), Tariki Tephra (6 beds; c. 4.6–4.7 ka), Waipuku Tephra (1 bed; c. 5.2 ka), Kaponga Tephra (10 beds; c. 8.0–10.0 ka), Konini Tephra (2 beds; c. 10.1 ka), Mahoe Tephra (4 beds; c. 11.0–11.4 ka), Kaihouri tephra (8 beds; c. 12.9–18.8 ka), Paetahi Tephra (6 beds; c. 19.4–20.2 ka), Poto Tephra (15 beds; c. 20.9–22.7 ka), Tuikonga Tephra (4 beds; c. 23.4–24.0 ka), Koru Tephra (2 beds; c. 24.8–25.2 ka), Pukeiti Tephra (1 bed; c. 26.2 ka) and Waitepuke Tephra (3 beds; c. 27.5–28.0 ka). Many of these tephra beds are sufficiently distinct and widespread enough to permit the dating and correlation of ring‐plain forming volcaniclastic deposits with which they are interbedded. The post‐28 ka tephra succession can also be subdivided into three broad sequences on the basis of major variations in the morphological and mineralogical characteristics of inter‐bedded andic soil material. At least 76 tephra events from Egmont Volcano with volumes exceeding 107 m3 have been recorded since c. 28 ka with an average eruptive periodicity of one in every c. 330 years. This frequency is considered minimal since more tephras of lesser magnitude may have been erupted but are only represented on the ring plain as intermittent accretion of fine‐grained ash that has rapidly weathered to andic soil material.