Overseas, relict deep-seated coastal landslides have attracted significant analysis because of potential hazards caused by sea level rise and climate change. Despite the rapid growth in the Auckland region, regional coastal landslide hazards are poorly understood. We investigated the geomorphology and material properties of Ohuka coastal landslide on the southwestern coastline of the Auckland region, to better understand the failure mechanism. The site is formed on weak Neogene sediments of varying lithological properties, with very low angles of dip, exposed on both the seaward (west) and landward (east) flanks of Ohuka Hill. Displacement appears to have been rotational, along a low-angled basal shear surface, the Koheroa Clay Seam. The surface geomorphology consists of discontinuous uphill-facing scarplets indictive of rotational slump blocks, along the basal shear surface. However, the preservation of slump block surface topography is hindered due to the lack of a thick, strong caprock, and high annual rainfall (1400 mm), and erosion. The proposed failure mechanism and general geometry is consistent with correlations between graben width and depth to failure plane reported from bedding-controlled coastal landslides formed on low dip angles in southern England. This provides important context, given the paucity of New Zealand bedding-controlled coastal landslide case studies.
Abstract Landslides are widespread geomorphological features on the North Island of New Zealand, where they represent one of the primary landscape-forming processes. This study focuses on the steepland terrain surrounding Gisborne, a city located on the east coast of the North Island, at the Hikurangi subduction margin. This terrain consists of young, weak, sedimentary rocks and soils; the most common modes of slope failures are soil creep, slides and flows in shallow, clay-rich soil and regolith, triggered by heavy rainfall. Based on observational data from Sentinel-1 imagery, this study leverages results from interferometric synthetic aperture radar (InSAR) processing to reveal the distribution of deformation across Gisborne’s steepland periphery from January 2016 to December 2021. Velocities in the line of sight were obtained from the stack of interferograms and projected along the direction of maximum slope, to extract the true displacement on the slopes. The ascending and descending data sets were combined to reveal the vertical and horizontal components of the deformation. The results were combined with a regional LiDAR data set, aerial imagery and field observations to delineate areas of slope deformation. Finally, slope deformation time series data was compared with rainfall records to identify seasonal changes, due to shrinking and swelling of expansive soils. Although the InSAR displacement data contains some noise, results could be used to identify 132 unstable slopes within the study area, caused by soil creep and earthflows. Also, the shrink-swell of expansive soils causes a seasonal pattern of displacements, which varied by 10–20 mm/year between Austral winter and summer, strongly correlated to rainfall.
Abstract: Research initiated in 1909 by G. Leslie Adkin (1888–1964) suggested Park Valley in the Tararua Range was glaciated during the Late Quaternary, based on the ‘U‐shaped’ cross‐profiles in the uppermost parts of several valleys. Findings were published, but were not met with universal acceptance. Adkin's work remained the only glacial research undertaken on the North Island's axial ranges until the latter part of the 20th century. Adkin holds a special position in New Zealand, because although he worked full‐time as a farmer he published nearly 40 articles in scientific journals on topics as varied as Māori archaeology and geomorphology.
Abstract Recent geomorphologie and sedimentologic investigations in Park Valley, in the central Tararua Range, have identified several landforms of glacial erosion and deposition, including cirque basins, a U‐shaped glacial valley, and a lateral moraine ridge. The presence of Kawakawa tephra (Aokautere Ash) within loess c. 50 cm beneath the surface of the moraine has indicated that the moraine was formed prior to 27 ka, suggesting late MIS 3 glaciation in this sector of New Zealand. However, recently published 10Be cos‐mogenic dates from glacially‐scoured bedrock and an erratic block on the surface of the moraine indicate that glaciation was much later, the corollary being that the Kawakawa tephra has been re‐distributed post‐eruption, perhaps from the ridgelines surrounding the moraine. Here, two optically‐stimulated luminescence ages on the loess are reported that suggests most of the moraine was formed during late MIS 3 or early MIS 2, and that a readvance at c. 17.7–18.7 ka extended as far as the moraine. Keywords: Tararua RangeOSL datingglaciationMIS Notes School of People, Environment and Planning, Private Bag 11222, Massey University, Palmerston North 4442, New Zealand, m.s.brook@masseyac.nz
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