Tuatara: Volume 30, Issue 1, December 1988
Geology and Revegetation of An 1855 Landslide, Ruamahanga River, Kopuaranga, Wairarapa
Geology and Revegetation of An 1855 Landslide, Ruamahanga River, Kopuaranga, Wairarapa
A combination of river undercutting and shaking during the 1855 earthquake caused a landslide of Pliocene mudstone and old river gravel/loess cover along the Ruamahanga River, Kopuaranga, northern Wairarapa. The landslide is roughly circular in plan and covers an area of about half a square kilometre. It consists of rotated blocks, swampy areas, hummocky topography and two lakes below a curved crown scarp situated some 100m above the Ruamahanga River. Except for one totara and some dead logs the original native forest was destroyed during the landslide. Growth ring ages indicate a minimum lag time of 30 years for the native forest trees that now colonise about one third of the landside. Breast-height circumferences of 132 trees range from 252cm to 40cm and indicate differences in the growth rate and colonisation history of the various species.
Introduction And Historic Aspects
The 1855 earthquake of January 23rd was the largest earthquake in New Zealand's written history with an isoseismal-estimated magnitude of about M8 (Eiby. 1965). The shock was felt over an area of some 579,000 square kilometres and permanent vertical movement raised about 7,400 square kilometres of the southern part of the North Island. The most visible and violent effects were in and around the Rimutaka Range (Fig. 1A), where about one third of the range was denuded by landslides and a maximum of 2.7m uplift took place at the coast (Lyell, 1868). Horizontal (around 12m dextral) and vertical (from 2 to 0.3m) displacement occurred along the Wairarapa Fault over a length of about 100km (Grapes and Wellman. unpublished data).
An account of the 1855 earthquake faulting along the Wairarapa Fault by Ongley (1943) described landslides, subsidence features, and rents in hilly topography near Palliser Bay, southern Wairarapa, and 79km north of Palliser Bay, a large landslide along the Ruamahanga River less than 1km west of the Wairarapa Fault (Figs. 1A: 2). The landslide was first described by Vennell (1891) who was in the Wairarapa during the earthquake, and relates that a “mountain near Masterton was literally rent in twain, and remains to be seen this day,” An account of the Wairarapa Fault by Iorns (1932) refers to the fact that the landslide blocked the Ruamahanga River and that when the river broke through again “Maoris living in pas lower down (the river) had to climb into trees to save their lives.”
A journey to the upper part of the Wairarapa Valley in November 1953 by William Mein Smith, chief surveyor of the Wellington Provincial Government, records the site of the landslide as a hill called Rerenga that “rises abruptly from the (Ruamahanga) river made of light blue clay”. The first recorded observation of the landslide was mae by J. Crawford, Provincial Government geologist, in early 1863 (Crawford, 1870) who attributed it to the 1855 event and described the landslide as “… a Tertiary hill having been split in two, and the western part having slipped down towards the river bed.” Crawford also mentions that “… on the Opaki Plain, and the adjoining hills, are very palpable marks of the earthquake of 1855 and perhaps of other shocks”.
Figs. 1A, B, C and D.
(A) Map showing the location of the 1855 landslide, Ruamahanga River, Kopuaranga, northern Wairarapa, and the trace of the 1855 earthquake rupture along the Wairarapa Fault. Shaded area of southern part of the Rimutaka Range represents the area of extensive landslide caused by the 1855 earthquake. Dotted area represents Pliocene sediments.
(B) Topgraphic and geomorphic map of the 1855 landslide area, 20 m contour interval, u = upthrown; d = downthrown side of the fault. Arrowed lines along fault indicate direction of horizontal movement. Ground slope between the fault strands is indicated by short arrowed lines.
(C) Geologic map of the 1855 landslide.
(D) Cross section of the 1855 landslide along line A-B in (C).
Topography and Geology
Geomorphic and geologic maps and a cross section of the landslide are shown in Figs. 1B, C and D. The landslide is roughly circular with an area of approximately half a square kilometre. It consists of a number of rotated blocks, swampy depressions, arcuate scarps, hummocky ground (debris flows and mudflows), and two lakes to the west of a single primary failure plane that forms a curved crown scarp 100m above the present day flood plain of the Ruamahanaga River (Fig. 1B; Fig. 2). The primary failure plan is in blue-grey marine mudstone with thin sandstone layers of Late Miocene to early Pliocene age (Late Tongaporutuan-Opoitian) that are capped by 1-4m of river gravels of the Porewan aggradation surface (80-60 kyrs) overlain by about 0.8m of loess (Fig. 1C). The rotated slump blocks of mudstone all have a mean slope angle for their west-facing sides of about 30° and a little less for the east-facing slopes. Swampy areas and the two lakes have formed on the eastern side of the rotated blocks (Fig. 1B and C). The backward (east-facing) tilted slopes of the rotational blocks are covered by a mixture of gravels and loess that is partly weathered to a yellow-grey earth about 0.7m thick with weakly developed A (10 YR 6/4 dry) and B (10 YR 7/4 dry) horizons. The west-facing slopes are composed of a soil (about 0.3m thick), again with weakly developed A (2.4 Y 4.2 dry) and B (5 Y6/1 dry) horizons that overlies mudstone. The thicker soil developed on loess is the same as that exposed along the top of the crown scarp and contrasts with the shallow soil developed on mudstone in the 135 years since the landslide.
Fig. 2. Aerial photo showing site of 1855 landslide, Ruamahanga River. The trace of the Wairarapa Fault is indicated by the white arrow. (Reproduced with the permission of the Surveyor General. Department of Survey and Land Information).
Cores taken from 5 living trees (rimu, totara, white pine, beech and rewarewa) give growth ring ages of between about 90 and 100 years (Table 1). These ages indicate a lag-time for the beginning of reforestation of the landslide of between 30 and 40 years. Presumably, the first plans to have colonised the landslide would be such species as Coriaria (tutu), Cassinia and kanuka (e.g. Druce, 1957; Wassilieff, 1986). Growth ring ages of two felled kanuka are 82 and 88+ years suggesting that they are post-burning, second generation specimens. The kanuka are growing on the area of loess and boulders and are mainly situated at the edge of the forest trees and/or in canopy gaps. The average time lag for Melicytus ramiflorus (mahoe) colonisation of avalanche boulder terraces developed on one of the 1855 landslides from the western slope of the Orongorongo Valley, Rimutaka Range, and where there has been no subsequent burning, was estimated at 20 years by Robbins (1958).
Table 2 gives breast-height circumferences for 132 trees that now colonise the landslide and presumably began growing since 30 years after the event. The smaller trunk circumferences of typical understory species such as pigeonwood, red matipo, and lophomyrtus, imply that they began growing after the forest trees had become established. The smallest circumference (and presumably the youngest) kanukas are growing near the forest tree-open grass boundary.
|Species||Circumference (cm)||Tree Ring Count (years)|
|Dacrydiumcupressinum Lamb. (Rimu)||175||96±5|
|Podocarpus totara G. Benn. ex D. Don (Totara)||167||98±8|
|Dacrycarpus dacrydioides (A. Rich) Laubenf. (White Pine)||164||84+ (no centre)|
|Nothofagus fusca (Hook.f.) Oerst. (Red Beech)||208||89±3|
|Knightia excelsa R.Br. (Rewarewa)||186||99±2|
|Kunzea (Leptospermum) ericoides (A. Rich) (J. Thompson (Kanuka)||65||82±2|
|Nothofagus fusca (Hook.f) Oerst. (Red Beech) 293, 212, 208||N = 3||251±42|
|Elaeocarpus dentatus J.R. et G. Forst (Hinau)||N = 1||214|
|Carpodetus serratus J.R. et G.Forst (Putapuaweta)||N = 1||214|
|Dacrydium cupressinum Lamb (Rimu)||N = 1||175|
|Myoporum laetum Forst. (Ngaio) 205, 187, 174, 158, 150, 136 (dead), 119, (258, 104)||N = 9||166±44|
|Alectryon excelsus Gaertn. (Titoki) 243, 204, 166 (dead), 104, 83, 77, 66||N = 9||161±77|
|Podocarpus totara G.Benn ex D.,Don (Totara) 329, 286, 270, 219, 203, 195, 191, 188, 182, 144, 140, 138, 111, 108, 99, 88, (88, 184), (107, 102), (78, 55)||N = 22||153±77|
|Prumnopitys taxifolia (D.,Don) Laubenf. (Matai)||N = 1||150|
|Melicytus ramiflorus J.R. and G.Forst. (Mahoe)||N = 1||146|
|Nestegis cunninghamii (Hook. f) L. Johnson (Black Maire) 150, 116||N = 2||133±17|
|Knightia excelsa R.Br. (Rewarewa) 150, 110||N = 2||130±10|
|Sophora microphylla Ait. (Kowhai) 173, 150, 145, 122, 106, 92, 91, 80, 64, 63||N = 10||119±43|
|Pseudopanax crassifolius C. Koch (Lancewood)||N = 1||99|
|Hoheria sexstylosa Col. (Ribbonwood)||N = 1||98|
|Dacrycarpus dacrydioides (A. Rich) Laubenf. (White Pine) 82, 74, 71||N = 3||76±5|
|Kunzea (Leptospermum) ericoides (A. Rich) J. Thompson (Kanuka) 108, 108, 97, 90, 88, 86, 85, 84, 83, 80, 79, 78, 73, 73, 73, 72, 71, 71, 68, 64, 62, 60, 54, 54, 52, 51, 51, 50, 49, 49, 48, 48, 45, 45, 29, 29, (88, 58), (83, 68), (66, 37, 47), (49, 39), (52, 42, 38, 28, 27), (96, 64)||N = 51||61±21|
|Hedycarya arborea J.R. and G. Forst. (Pigeonwood) 62, 44, (100, 100), (44, 43, 35)||N = 7||61±26|
|Myrsine australis A. Rich (Red Matipo) 73, 72, 57, 42, 33||N = 5||55±6|
|Lophomyrtus obcordata (Raoul) Burret. (No popular name) 48, 37, 34||N = 3||40±6|
Column A - Number of trees measured; Column B - Breast-height circumference (cm). Multiple trunks for single trees are enclosed in brackets and are averaged as if single trees. The measurements sample about one fifth of the total trees present and are considered to be representative. Nomenclature follows Connor and Edgar (1987).
Reference to the landslide site in 1853 by the Government Surveyor Smith when there was no landslide, and by the Government Geologist Crawford in 1863 when there had been a landslide, make it certain that the landslide took place sometime between November 1853 and February 1863, and almost certainly during the 1855 earthquake. The observatin of Smith in 1853, that the true left bank of the Ruamahanga River was a high cliff, indicates that the landslide probably resulted from a combination of the undercutting of this cliff and earthquake shaking. The similar angle of repose of the large rotated blocks within the landslide implies that all of the blocks slid at the same time and as a result of short-term failure along a single arcuate crown scarp.
It is assumed that the native forest cover mentioned by Smith in 1853 was destroyed during the landslide. Only one living totara and several dead trunks of page 83 the pre-landslide forest remain. Native forest tree ring counts give a maximum age of about 100 years indicating a minimum lag time of regeneration on the landslide of about 3 years. Younger (<90 year) ages for kanuka, usually an early colonising species, imply that they are a post-burning, second generation growth. Circumference measurements of 132 post-landslide trees vary from 252 cm to 40 cm and indicate differences in growth rate and colonisation history of the various species that now inhabit the landslide.
Many thanks are due to Professor Harold and Mrs Joan Wellman, Mrs Diane Kelly, Janet and Peter Kelly for a pleasant day's outing on the landslide site measuring the circunmferences and identifying the trees. Harold Wellman provided some critical comments on the manuscript and Dr Maggie Wassilieff provided valuable botanic advice. Dr Ross McQueen, Botany Department, Victoria University of Wellington, is thanked for his helpful suggestions and for supplying the tree borer, most of which now remains in the last tree to be cored.
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