James Goff

Sedimentary differences between the 2002 Easter storm and the 15th-century Okoropunga tsunami, southeastern North Island, New Zealand

Internationally, a key problem in reconstructing chronologies of coastal hazards is the ability to distinguish between storm and tsunami deposits. This situation has been exacerbated by the low number of known locations where both types of deposit occur along the same stretch of coastline. The sedimentological characteristics of a tsunami and a storm deposit laid down on the same stretch of coastline on the southeast coast of the North Island, New Zealand, are distinctly different. The 15th-century tsunami was probably caused by fault rupture in the Cook Strait region, whereas the Easter 2002 storm was generated by a meteorological depression centred some 900 km to the southeast. The differences include areal extent, thickness, and grain-size characteristics. The tsunami deposit thins abruptly at the margins, fines inland, is more poorly sorted, has entrained rip-up clasts, and has an erosional lower contact, often with a buried soil. The storm deposit has a highly variable grain-size distribution with a marked coarsening at its landward extent, is better sorted, coarser, and has a sharp, non-erosional lower contact associated with buried vegetation and soil. The coarser grain size is probably the result of differences in sampling regime as opposed to wave energy. The storm deposit extends about 40 m inland as opposed to about 200 m for the tsunami. Variations in the preservation of evidence are a reflection of the age of deposition. Records of tsunamis and storms in New Zealand indicate that there are probably several coastal sites where both types of deposit can be compared.

Palaeotsunami deposits: a New Zealand perspective

Abstract Over the past few years, geological investigations of coastal sediments in New Zealand have provided evidence of past tsunami. The identification of palaeotsunami deposits is helping to increase our knowledge about the sources, magnitude and frequency of these events. This paper briefly outlines both the key diagnostic characteristics used in New Zealand to identify palaeotsunami deposits and the emerging findings related to recent research.

On the Interpretation of Subglacial Till Fabric

ABSTRACT The modal distribution of stone long-axis fabrics and their respective eigenvalues can be used to infer the genesis of subglacial till. In this paper we offer a two-axis diagram that compares fabric modality to fabric isotropy (S3/S1) and addresses the problem of eigenvectors falling between the modes of some well-developed till fabrics with low eigenvalues. Our simple five-fold scheme of modality categories includes: (1) unimodal clusters, (2) spread unimodal, (3) bimodal clusters, (4) spread bimodal, and (5) polymodal to girdle-like fabrics, and requires the analyst to study equal-area, lower-hemisphere (Schmidt) plots of the fabric data. After assigning the fabric to a morality category, isotropy is calculated and both results are plotted on the graph, whic helps to separate two main fields of subglacial till: (1) lodgement and subglacial meltout tills, and (2) deformation fill. On the basis of selected published fabrics from tills at modern glaciers, as well as our own Pleistocene till data, lodgement and subglacial meltout tills tend to have unimodal or bimodal fabrics. In contrast, deformation tills and tills that experienced multiple processes tend to have polymodal to girdle-like fabrics. Some overlap occurs between fields be. cause of the complex nature of till formation (i.e., because pure end-member till facies are rare and most tills are hybrids). We strongly recommend that Schmidt plots be visually analyzed and used in conjunction with eigenvalues when studying till. However, fabric data alone is not enough. Multiple criteria including structural, lithologic, and stone morphologic data from the till must also be considered before drawing conclusions on till genesis. Furthermore, if eigenvectors fall between fabric modes, then they cannot be used to indicate former ice movement directions. Finally, our new modality-isotropy diagram may have wider applications.

A tsunami (ca. 6300 years BP) and other Holocene environmental changes, northern Hawke's Bay, New Zealand

Abstract Sediment cores collected in a coastal lagoon a few kilometres east of Wairoa, northern Hawkes Bay, New Zealand, were examined using sedimentological, geochemical, palynological and micropaleontological analyses. A distinct short-lived catastrophic saltwater inundation (CSI) about 6300 years BP and possibly other minor marine incursions are preserved in the coastal estuarine to lagoonal freshwater sedimentary sequences, which have been deposited in the last 6500 years. The CSI is characterised by a gravel unit that thins landward and decreases in particle size to sand, within a sequence consisting mainly of brackish estuarine muds. Diatom assemblages indicate a marked change from the shallow brackish estuarine muds to marine gravels and sands to brackish estuarine muds. The marine influence in the gravel and sand is also shown by the presence of marine dinoflagellates and a peak in Na/Rb. Sedimentological, chemical and paleontological (in particular diatoms) evidence indicates it is a CSI. We conclude that this was a tsunami and propose the most likely propagating mechanisms. Marine influence decreases upcore and totally freshwater conditions are evident in the upper section of the cores. The geochemistry of the sediments mainly reflects the change in stratigraphy, with distinct signatures for tephra (Na, Fe, Cr), organic-rich and peat units (As, Br) and the coarse gravel-sand CSI unit (Na/Rb, Cr, Fe), but it is also indicative of changes in depositional environment. The change in chemistry (Na/Rb) in the CSI event is indicative of a saltwater influence, whereas a marked change in S content suggests a sudden change from brackish to freshwater conditions shortly after 4800 years BP. Another peak in S and Br content about 3200 years BP may indicate another temporary change to brackish conditions.

Coastal dunes in Westland, New Zealand, provide a record of paleoseismic activity on the Alpine fault

Regional episodes of coastal progradation and dune formation in South Westland, New Zealand, have quickly followed all known Alpine fault earthquakes since A.D. 1200. This reflects rapid transport of large postseismic sediment pulses from mountain catchments to the coast and accumulation of this material as a dune ridge. This study provides the first demonstration of this link for multiple events over a region. The dune sequences also provide evidence of another previously unrecognized regional aggradation event, which may be earthquake-related, and which occurred just 50 yr after a large Alpine fault earthquake. Coastal dunes have great potential for paleoseismic application because the spatial separation of earthquake-induced sediment pulses on a prograding coast allows identification of events closely spaced in time. Coastal dune systems have the potential to improve paleoseismic understanding over the Holocene for many plate boundary faults near coastal areas.

Seismic driving of nationwide changes in geomorphology and prehistoric settlement: a 15th Century New Zealand example

Abstract We propose a model for the seismic driving of environmental changes and illustrate it using a case study from New Zealand dated to the 15th Century AD. A “seismic staircase” shows a chronological progression of environmental outcomes that includes tsunami, rock avalanches, vegetation disturbance, rapid coastal dune building, river aggradation, and abandonment of prehistoric coastal settlements. The 15th Century event appears to be unique in the past Millenium and is most notable for its period of rapid coastal dune building. The coincidence of a catastrophic El Nino episode in the mid 15th Century AD may have served to move sediments rapidly through the terrestrial system thus delimiting clearly separate geomorphological after-effects. We contend that seismic driving is a major factor in determining paleoenvironmental change in tectonically active areas during the Holocene and provides the palaeoenvironmental context within which smaller scale events operate.

Māori environmental knowledge and natural hazards in Aotearoa‐New Zealand

Abstract Based on a long and close association with the land and its resources, Māori have developed a detailed knowledge of local natural hazards. This includes oral histories and traditions that record past catastrophic hazard events, place names that designate areas that are high hazard risk, and environmental indicators that inform about the safety and viability of activities linked to changes in the environment. Māori Environmental Knowledge is a valuable and neglected area of information on natural hazards and provides a unique source of expertise that can contribute to contemporary natural hazards management and mitigation in New Zealand.

A chronology of natural and anthropogenic influences on coastal sedimentation, New Zealand

14C and 137Cs chronologies of sediment accumulation were obtained from five sediment cores taken from Wellington Harbour, New Zealand. A 10,000-year chronology records the Holocene transgression, European colonisation, and variations in the general sediment accumulation rate caused by earthquake uplift and anthropogenic activity. Rates vary from a high of ~ 60 to a low of 0.1 mm a−1. In general, rates increased at the beginning of the Holocene marine transgression, but by ~ 5000 yr BP they reached a stable level. Harbour-wide, these rates remained stable until the second half of the 19th Century when deforestation by European settlers caused order of magnitude increases in sediment accumulation. In the past 40–80 years rates have increased again as a result of urban growth and river channel management, although the effects are less pervasive. Harbour-wide influences can be placed in two categories, natural and anthropogenic, the latter being recent contributions to a sedimentary regime dominated by the Holocene marine transgression. Sediment accumulation rates indicate that two major earthquake uplift events had only a local effect on harbour sediments. Anthropogenic influences are considered to be more significant sedimentologically than earthquake activity.

Possible tsunami deposits from the 1855 earthquake, North Island, New Zealand

Abstract A series of three fining-upward sequences from deposits in the Okourewa Stream bank on the south coast of the North Island, New Zealand, investigated by grain-size, diatom, radiocarbon, geochemical and macrofaunal analyses have been tentatively interpreted as the products of a tsunami. The proposed event consisted of three separate waves (the second being the largest) generated by a surface rupture of a local fault. Changes in diatom assemblages and the presence of marine shells, pumice, and beach pebbles may represent a tsunami advancing inshore over beach, freshwater channel, and coastal wetland environments. Deposition occurred between ad 500 and 1890. The event in question may have currently the ad 1855 rupture of the West Wairarapa fault.

Coastal dune ridge systems as chronological markers of palaeoseismic activity: a 650-yr record from southwest New Zealand

A study of two shore-parallel dune ridge sequences in southwest New Zealand shows that tectonic activity has been the primary controlling influence in their formation since at least AD 1450. The timing of dune-building episodes at the mouths of the Haast and Okuru Rivers was determined using the ages of colonizing trees. Episodic dune formation was indicated by clear discontinuities in tree ages, with distinct cohorts having colonized successive, newly formed, dune ridge-swale units. At both sites, four dune ridge-swale units have formed since AD 1450, with each unit closely postdating an Alpine fault rupture (c. AD 1460, c. AD 1615, AD 1717, AD 1826). Colonizing cohorts of trees started growing within 20-46 years after an earthquake at both sites, and all known major regional earthquakes have resulted in a dunebuilding episode. No other dunes are present at either site. Progradational coastal dune systems have potential as a tool for palaeoseismic studies. In regions with high background levels of sediment delivery to limit erosion/burial of dunes and with little coseismic subsidence, dune systems may preserve a spatially discrete record of major earthquake-induced sedimentation events over the Holocene. Earthquakes are a key driver of palaeoenvironmental change and coastal plain development in this tectonically active region.