Scott L. Nichol

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.

New model of reef-island evolution: Maldives, Indian Ocean

A new model of reef-island evolution, based on detailed morphostratigraphic analysis and radiometric dating of three islands in South Maalhosmadulu Atoll, Maldives, is presented. Islands initially formed on a foundation of lagoonal sediments between 5500 and 4500 yr B.P. when the reef surface was as much as 2.5 m below modern sea level. Islands accumulated rapidly during the following 1500 yr, effectively reaching their current dimensions by 4000 yr B.P. Since then the high circum-island peripheral ridge has been subject to seasonal and longer-term shoreline changes, while the outer reef has grown upward, reducing the energy window and confining the islands. This new model has far-reaching implications for island stability during a period of global warming and raised sea level, which will partially reactivate the energy window, although it is not expected to inhibit upward reef growth or compromise island stability.

Geological effects of tsunami on mid-ocean atoll islands: The Maldives before and after the Sumatran tsunami

Low-lying coral islands are fragile landforms susceptible to long-term sea-level rise and extreme events, such as hurricanes and tsunamis. The Sumatran earthquake of 26 December 2004 generated waves that reached the Maldives 2500 km away. Observations of the effects of the tsunami are presented here, based on pre- and post-tsunami topographic and planform surveys of 13 uninhabited Maldivian islands. The surveys showed there was no substantial island erosion and no significant reduction in island area. Rather, the tsunami accentuated predictable seasonal oscillations in shoreline change, including localized retreat of exposed island scarps by up to 6 m, deposition of cuspate spits to leeward, and vertical island building through overwash deposition of sand sheets up to 0.3 m thick, covering up to 17% of island area. These results have implications for island stability indicating that low-lying reef islands are physically robust and the geological signature of tsunamis on atoll island development is minor.

Sheet-gravel evidence for a late Holocene tsunami run-up on beach dunes, Great Barrier Island, New Zealand

Abstract A semi-continuous sheet of granule to cobble-size clasts forms a distinctive deposit on sand dunes located on a coastal barrier in Whangapoua Bay, Great Barrier Island, New Zealand. The gravel sheet extends from the toe of the foredune to 14.3 m above mean sea level and 200 m landward from the beach. Clasts are rounded to sub-rounded and comprise lithologies consistent with local bedrock. Terrestrial sources for the gravel are considered highly unlikely due to the isolation of the dunes from hillslopes and streams. The only source for the clasts is the nearshore to inner shelf of Whangapoua Bay, where gravel sediments have been previously documented. The mechanism for transport of the gravel is unlikely to be storm surge due to the elevation of the deposit; maximum-recorded storm surge on this coast is 0.8 m above mean high water spring tide. Aeolian processes are also discounted due to the size of clasts and the elevation at which they occur. Tsunami is therefore considered the most probable mechanism for gravel transport. Minimum run-up height of the tsunami was 14.3 m, based on maximum elevation of gravel deposits. Optical ages on dune sands beneath and covering the gravel allow age bracketing to 0–4.7 ka. Within this time frame, numerous documented regional seismic and volcanic events could have generated the tsunami, notably submarine volcanism along the southern Kermadec arc to the east-southeast of Great Barrier Island where large magnitude events are documented for the late Holocene. Radiocarbon ages on shell from Maori middens that appear to have been reworked by tsunami run-up constrain the age of this event to post ca. 1400 AD. Regardless of the precise age of this event, the well-preserved nature of the Whangapoua gravel deposit provides for an improved understanding of the high degree of spatial variability in tsunami run-up.

Holocene reef growth in the Maldives: Evidence of a mid-Holocene sea-level highstand in the central Indian Ocean

Radiometrically calibrated ages from three reef cores are used to develop a Holocene reef growth chronostratigraphy and sea-level history in the Maldives, central Indian Ocean. Last interglacial reef (U-series age 122 ± 7 ka) was encountered at 14.1 m below mean sea level. An age of ca. 8100 calibrated (cal) yr B.P. immediately overlying this Pleistocene surface records the initiation of Holocene reef growth. Massive in situ corals occur throughout the cores and the consistency of the three age-depth plots indicate that the reef grew steadily between 8100 and 6500 cal yr B.P., and at a decreasing rate for the next 2 k.y. The position of modern sea level was first achieved ca. 4500 cal yr B.P. and sea level reached at least 0.50 ± 1 m higher from 4000 to 2100 cal yr B.P. before falling to present level. Emergent fossil microatolls provide evidence of this higher sea level. Results are significant to two long-standing issues relating to Maldivian sealevel history. First, the ambiguity of a late Holocene highstand has been resolved with clear evidence of its existence reported here. Second, the uncertainty of the regional pattern of sea-level change in the central Indian Ocean has been clarified, the Maldivian results broadly agreeing with island records in the eastern, rather than western Indian Ocean. Our results provide the first field evidence confirming geophysical model projections of a highstand 4–2 k.y. ago in the central Indian Ocean, though the observed level (+0.50 ± 0.1 m) is lower than that projected.

The influence of limited accommodation space on the stratigraphy of an incised-valley succession: Weiti River estuary, New Zealand

Abstract The influence of limited accommodation space on estuarine stratigraphy was investigated using vibrocores from the Weiti River estuary; a narrow, fault-control estuary of the North Island, New Zealand. The Weiti incised-valley fill comprises eight lithofacies arranged into two distinct depocentres, separated by the main estuary channel. The seaward depocentre is occupied by a marine-influenced sand accumulation consisting of transgressive to highstand open-bay deposits overlain by regressive tidal flat and shelly beach-ridge deposits. The landward depocentre is dominated by a tidal-fluvial transgressive to highstand bay-head delta complex that includes point-bar, channel-lag and saltmarsh deposits. Radiocarbon ages for in situ shell and organics are used to establish the chronology of valley filling. Holocene marine deposition commenced late in the transgression (ca. 8 ka BP) and was dominated by an open-bay depositional environment in the seaward depocentre and by a tidal point-bar within the landward depocentre. At the time of maximum transgression (ca. 6.5 ka BP) most of the valley fill was in place. Highstand deposition has been characterised by formation of seven shelly beach ridges and vertical accretion of tidal flats in the seaward depocentre, and pointbar accretion to supratidal levels in the landward depocentre. The organization of transgressive and highstand deposits in the Weiti River estuary has been strongly influenced by limited accommodation space to the extent that, (i) marine flooding of the seaward depocentre was delayed due to a fluvial terrace at the present-day estuary mouth, thereby preventing formation of a continuous transgressive bay-head delta, (ii) the low-wave environment at the sheltered estuary mouth was suitable for barrier (beach ridge) construction only when accretion had bought the valley floor to within wave base during stillstand, and (iii) the narrow valley has promoted channelization of tidal and river flow which, coupled with the lack of a barrier, prevented a central basin environment forming and caused the highstand bay-head delta to develop in a restricted area. Despite these limitations on morphological and stratigraphical development, the Weiti River estuary has formed an overall transgressive- to regressive-depositional succession that is broadly consistent with “idealized” models of incised-valley stratigraphy.

Signatures of natural catastrophic events and anthropogenic impact in an estuarine environment, New Zealand

The sedimentary record of known natural catastrophic events and anthropogenic activity in an estuarine environment is assessed using sedimentological, chemical and geochronological techniques. Shallow cores collected from intertidal and salt marsh sediments in Ahuriri Estuary, Hawkes Bay, reveal a variety of signatures of natural and human disturbance. Evidence for the 1931 Hawkes Bay earthquake, which resulted in an uplift of one to two metres in the Napier area, is given by a change from silt- to sand-dominated sediment in the lower estuary, which is consistent with a shift toward higher energy depositional conditions following uplift. However, based on physical properties of sediments, the 1931 uplift event does not appear to have caused major changes in depositional conditions in the upper estuary. Similarly, no changes were recorded at one site in the lower estuary, which seems to represent an area of low energy depositional environment. Although the 1960 Chilean tsunami resulted in structural damage in Napier, it did not produce any recognisable sedimentological and geochemical signature in the sedimentary record, suggesting that the study sites were possibly beyond the limit of sedimentation of the tsunami. Post-European settlement impact is mainly restricted to the lower estuary, where increased concentrations of Zn, Cr, Pb and Cu are attributed to industrial discharges. Evidence of agricultural runoff is shown by an increase in Cu concentrations within a fine-grained depositional environment that is distal from industrial sources in the town of Napier. Chemical data (Cl and S) suggest a change in the depositional environment in the upper estuary due to increased freshwater influx and/or decrease in seawater influence. Dating by 210Pb suggests that this occurred around the middle part of the 19th century, and might be attributed to river flooding in the region at that time.

Sequence stratigraphy of a coastal-plain incised valley estuary, Lake Calcasieu, Louisiana

ABSTRACT The stratigraphic profile along a 100 km section of the Lake Calcasieu incised-valley fill was reconstructed from 87 bridge borehole records at four sites along the tidal reaches of the Calcasieu River, an additional 18 boreholes aligned along the chenier plain near the estuary mouth, and shallow seismic data collected along 160 km of track line within the Lake Calcasieu central basin. Valley incision occurred during the d18O stage 2 sea-level lowstand, when the paleo-Calcasieu River flowed across the inner continental shelf and incised into older fluvial terraces of the Prairie Formation. The valley floor is 57 m deep beneath the modern chenier plain and rises to -30 m along the upper reaches of the modern estuary. Sea.level rise commenced about 20 ka and has continued episodically throughout the Holocene. Shoreline transgression has proceeded at varying rates in response to sea-level rise, and the incised valley is now occupied by a wave-dominated estuary, fringed by the chenier plain of western Louisiana. Lowstand deposits are poorly preserved within the Calcasieu incised valley. Deposits constituting the transgressive systems tract (TST) lie directly above the lower sequence boundary at the top of the Prairie Formation. The TST shows greatest facies diversity and clearest separation of bounding surfaces toward the present seaward end of Lake Calcasieu. Here, the TST comprises three parasequences that include fluvial, bayhead-delta, and central.basin (estuarine) deposits. Multiple flooding surfaces within the TST relate to the episodic style of sea-level rise. Highstand infilling of the valley has been characterized by nearshore marine deposition and chenier-plain progradation. Along the lower reaches of the modern Calcasieu River, the valley fill comprises transgressive central-basin nd landward-stepping bayhead-delta facies, overlain by the highstand bayhead-delta complex. This study documents the most recent phase of deposition within the compound fill of a coastal-plain incised valley and demonstrates the potential importance of fluvial controls on facies within incised-valley systems.

Predictive modelling of seabed sediment parameters using multibeam acoustic data: a case study on the Carnarvon Shelf, Western Australia

Seabed sediment textural parameters such as mud, sand and gravel content can be useful surrogates for predicting patterns of benthic biodiversity. Multibeam sonar mapping can provide near-complete spatial coverage of high-resolution bathymetry and backscatter data that are useful in predicting sediment parameters. Multibeam acoustic data collected across a ∼1000 km2 area of the Carnarvon Shelf, Western Australia, were used in a predictive modelling approach to map eight seabed sediment parameters. Four machine learning models were used for the predictive modelling: boosted decision tree, random forest decision tree, support vector machine and generalised regression neural network. The results indicate overall satisfactory statistical performance, especially for %Mud, %Sand, Sorting, Skewness and Mean Grain Size. The study also demonstrates that predictive modelling using the combination of machine learning models has provided the ability to generate prediction uncertainty maps. However, the single models were shown to have overall better prediction performance than the combined models. Another important finding was that choosing an appropriate set of explanatory variables, through a manual feature selection process, was a critical step for optimising model performance. In addition, machine learning models were able to identify important explanatory variables, which are useful in identifying underlying environmental processes and checking predictions against the existing knowledge of the study area. The sediment prediction maps obtained in this study provide reliable coverage of key physical variables that will be incorporated into the analysis of covariance of physical and biological data for this area.

The palynology and sedimentology of a coastal swamp at Awana, Great Barrier Island, New Zealand, from c. 7000 yr B.P. to present

Pollen and sediment analysis of two Holocene cores from Awana, Great Barrier Island, shows that at 7000 calibrated yr B P the local swamp was an estuanne salt marsh dominated by Restionaceae By c 6000 yr B P the water table was lower, and a fresh water swamp (Gleichenta‐Leptospermum) had replaced the salt marsh Regional conifer‐hardwood forest c 7000 yr B P was initially co‐dominated by Libocedrus and Dacrydmm cupressinum Libocedrus declined from c 6000 yr B P During the period c 6000‐c 2500 yr B P, relatively stable environmental conditions ensued with little change in local or regional vegetation Around 2500 yr B P, the swamp surface became drier and was invaded by Dacrycarpus and Laureha swamp forest This forest was subsequently repeatedly disturbed (not by fire), indicating climatic change to drier and windier conditions Ascanna lucida was periodically a major component of swamp forest Disturbance is also recorded in the clastic (mineral) sediments, where beds of sand within finer‐grained sediment and...