Stratigraphy, age, and correlation of voluminous debris‐avalanche events from an ancestral Egmont Volcano: Implications for coastal plain construction and regional hazard assessment

Abstract

Two previously unrecognised debris‐avalanche deposits have been identified on the eastern flanks of Egmont Volcano beneath a thick mantle of tephric and andic soil material that has mostly subdued their topographic expression. The Ngaere Formation is a c. 23 ¹⁴C ka large volume (>5.85 km³) debris‐avalanche deposit that is widely distributed over 320–500 km² of the north‐east, south‐east, and south portions of the Egmont ring plain. The second deposit, Okawa Formation, is a c. 105 ka large volume (>3.62 km³) debris‐avalanche deposit that has been mapped over a minimum area of 255 km² in northern and north‐eastern Taranaki. Both debris‐avalanche formations contain axial facies with hummocks composed mainly of block‐supported brecciated andesitic debris. A less conspicuous marginal facies, texturally resembling a mudflow, is more extensive. A third debris‐avalanche deposit (Motunui Formation) is extensively preserved along the north Taranaki coast where it is truncated by a c. 127 ka wave cut surface (NT2) and closely overlies a c. 210 ka wave cut surface (NT3). The source of this debris‐avalanche deposit is unknown. Side‐scan sonar and shallow seismic profiling have been useful in accurately delineating the distribution of combined Okawa and Motunui debris‐avalanche deposits in the offshore environment but cannot distinguish between the two deposits or enable onshore spatial and volumetric estimates for each unit to be revised. However, the widespread occurrence of debris‐avalanche rock material offshore does emphasise the importance of this lag material altering the orientation of the coast influencing both wave climate and rates of coastal erosion. Similarly, the extensive onshore occurrence of debris‐avalanche rock material appears to be a significant factor in widening of the north Taranaki coastal plain and preservation of the NT2 and NT3 uplifted marine terrace surfaces. Initiation of collapse by magmatically‐induced seismicity is apparently common at many stratovolcanoes. Emplacement of Ngaere Formation was immediately preceded by a magmatic fall unit and is directly overlain by a closely spaced sequence of 13 fall units. In contrast, there is no evidence to indicate that an eruptive event triggered or immediately followed the Okawa debris‐avalanche event, but seismically induced gravitational sliding cannot be discounted. Egmont Volcano has repetitively collapsed over its c. 127 ka history and has generated at least five voluminous landscape‐forming debris‐avalanche deposits. Probabilistically‐based return times are calculated at c. 1967 ¹⁴C yr for volumes ≥0.15 km³ and c. 21 000 ¹⁴C yr for volumes ≥7.5 km³. Despite lower return times in comparison to tephra emission, Egmont Volcano is an inherently unstable cone because it comprises interbedded lavas and unconsolidated volcaniclastic deposits with a high slope angle overlying a faulted basement of Tertiary sediments. Should eruptive activity recommence and coincide with significant upper cone dilation, then the likelihood of a gravitational cone collapse is expected to increase although critical thresholds remain to be modelled. Fortunately, the Taranaki Regional Volcanic Contingency Plan is based on pre‐emptive evacuation which is intended to minimise loss of life in advance of an eruptive and/or cone collapse event occurring.