Proxy Evidence of Relative Sea Level Change, Indicators and Implications, Raglan (Whaingaroa) Harbour, New Zealand
NZGS Progress Report 20/10/2023
Background
The June 2023 NZ Geomechanics News contained an article outlining the justification and objectives behind this MPhil research program. This article provides a current snapshot of the research undertaken to date.
The MPhil enrolment was motivated by observations made during my 20 years as a consulting engineering geologist in the Raglan region and during preliminary reconnaissance of the harbour in 2022. Low-lying terraces containing bivalve shellbeds were apparent at multiple locations along the 120 km foreshore.
The research priorities for determining sea level change include geological mapping, fossil dating, and accurate elevation measurements. In the absence of fossil dating, it cannot be established if these terraces were deposited due to a historic sea level high, vertical land movement, or both.
Following the start of this research in April 2023, further reconnaissance and detailed observations were completed at select sites throughout the harbour. Around 30% of the harbour area is tidal, necessitating multiple fieldtrips within 4-hr windows of opportunity, weather permitting. These were conducted in a 5-m shallow-draft canoe, driven by paddle.
Reconnaissance Findings
During reconnaissance, marine terraces were observed at 0.5 – 1 m elevation above the modern beach strandline at multiple locations. These are indicative of vertical land movement or sea level fall. How to establish and validate meaningful metrics on this displacement?
While these features could be explained by a recognised late Holocene sea level high, the freshness of the macrofossils suggests that these are too young to be of this era. Other geological evidence around the harbour, such as fresh fracture zones in the sea cliffs and the presence or absence of terraces within specific areas, indicates that the harbour is being subject to recent differential tectonic strain.
The bulk of published literature on marine terraces of the western Waikato reports uplift of the coastline, dating back millennia. This research was mostly based on proxies other than datable macro or micro fossils, which are rare (Pillans,1990; Gibb, 1986). The study objectives of these researchers were primarily the correlation of raised terraces with eustatic sea level curves, dating back beyond the Holocene. These terraces are significantly older and more elevated than the shellbeds of interest in this MPhil research, which are mostly < 1 m above the modern beach strandlines.
Foraminifera (forams) have a limited durability in even slightly acidic groundwater (Bruce Hayward, pers. comm.; Purna, et al., 2023; Murray and Alve, 1999). Consequently, their presence can substantiate that a shellbed sedimentary host-bed is relatively young and that it has been deposited by natural (non-anthropogenic) processes.
Consequently, the shellbeds’ associated sediments were checked for benthic foram concentrations. Greater than 90% of the intact bivalve samples retrieved for dating contained sediment infill which was checked for the inclusion of forams. Foram concentrations associated with the dating samples ranged from 0.5 – 15 % (by number of total grains) in the sieved diameter range 212 – 355 μm.
Dating
50% of the NZGS grant is being assigned to the cost of 14C dating. This is being matched 1:1 by a private funding source. Greater than 80 % of the bivalve dating samples consisted of 1 valve from intact (joined) mature cockles. 17 samples have been sent to the University of Waikato 14C dating lab. Of the 8 results that have returned, 6 are within an age of < 120 years BP,1 is dated at 620 years BP and 1 at 6000 Ka BP. Validation of these dates are being further established through the dating of support samples for each site, most of which are the second valve from prior-dated intact cockles. Depending on further finance a further 3 samples from each location will be added, resulting in a total of 25 14C dates.
In the interests of establishing maximum relative sea level within an established time-period a sample from a speleothem flowstone has been sent to the University of Melbourne for U-Th dating. The sample was located at 0.4 m elevation above the modern hightide erosion notch (Figure 1). The result is pending. Nearby flowstones at lower elevations display the effect of marine immersion.
Figure 1: Speleothem flowstone at 0.4 m above the modern mean high tide notch within Raglan Limestone
Establishing Elevations
Given that the degree of uplift is expected to be < 1 m, accurate elevation measurements of each site are required. These were established using RTK GNSS equipment with an accuracy of +/- 50 mm.
Establishing the degree of uplift requires comparing the elevation of a raised bed with its existing foreshore equivalent. The upper-most fossils in each bed were used as a consistent elevation measurement datum point. Whereas a raised strand (wrack) line’s equivalent in current time is above the high tide mark, a residual shellbed’s equivalent in current time is the elevation of living bivalves situated below mid-tide level.
Other Proxies
By June 2023 it became apparent that all shellbeds fitting our criterial were confined to one area of the harbour. This correlated directly with the presence of shallow, wave-cut rock platforms within the tide-zone and at greater depths. These 2 proxies are absent in another specific part of the harbour, constituting 20% of its total area. This anomaly was the first indication that the harbour may be subject to differential vertical land movement and active faulting. Swales et al. (2005) established that while thick sediments overlay base-rock in the Waitetuna branch of the harbour, it was thin or absent in the Waingnaro branch where wave-swept hard rock platforms at shallow depths dominate.
Site Selection
Three sites have been selected for focused study. The first of these (Site A) has over 200 m of shellbed exposed in a beach erosion scarp, demarcated near its centre by an area of drained swamp (Figure 2). The shellbeds differ notably north and south of the drain. Aerial images show that north of the drain a terrace of ∼ 100 m length and ∼ 15 m width appears to have been uplifted by 0.3 – 0.5 m since 1949 (Figure 3).
Figure 2 (Site A): Fresh, intact cockles (A) @ 0.3 m above the modern strandline within a 200 m-length shell-rich beach erosion scarp (B)
Figure 3 (Site A): A young terrace absent in a 1949 aerial photograph
Dating and drilling is ongoing at this site to fully characterise the shellbeds, as the (unlikely) prospect of it being an accretionary deposit cannot yet be discounted.
Site B has 2 notable features: an elevated residual shellbed and its surrounding geology.
Raised shellbeds occur due to 1 of 3 possible depositional processes:
- Reworked residual
- Strandline (wrack and washup debris)
- Undisturbed residual
While the first two processes can be a result of progressive uplift, an undisturbed residual shellbed cannot. It must be uplifted in one motion to avoid being reworked by wave action. Its value is that we can establish an accurate metric on the degree of uplift by comparing its elevation with the modern living equivalent in the tide-zone. A genuine undisturbed residual shellbed has obvious characteristics (Figure 4). These include a confined vertical thickness, high concentrations of intact bivalves and the presence of forams: within both the host sediment and the shell infill.
Figure 4 (Site B): A residual shellbed dominated by intact bivalves @ 0.3 m above the modern strandline
The 14C age of 104 years BP has been established for 1 macrofossil sample at Site B. A second sample is currently being processed.Forams are present within the host sediment and the intact cockles’ infill. The surrounding geology displays relatively fresh fractures, rockslides, and evidence of active faulting.
The significance of Site C is due to its locality within the Raglan Township boundaries. It has a confined residual shellbed at the top of the host sediment unit (Figure 5). RTK GNSS measurements established a 0.9 m height-difference between the residual shellbed and its modern living equivalent in the tide zone. The Intact bivalves have a relatively high concentration of forams within their sediment infill. The age of samples from this site is pending. Their freshness suggests that it is likely to be < 2 ca.
Figure 5 (Site C): Residual shellbed comprised of 50% intact cockles @ 0.9 m above its offshore equivalent
Conclusion
14C dates of 8samples representing 3 sites on the Raglan Harbour foreshore appear to support geomorphological, geological, palaeontologic, and bathymetrical evidence that a major portion of Raglan Harbour has been subject to upward vertical land movement within the last few centuries. Results of dating from a further 9fossil samples are pending. The current interpretation may be amended following the receipt of additional dating results.
A comprehensive description of the stratigraphy, sedimentology, and evolution of the 3 study sites is in progress. Given the variation in apparent uplift, the prospect that there may be active faults in the harbour is being further investigated.
Should the site and laboratory evidence support the hypothesis that significant uplift has occurred within the last few centuries, this study could become a credible foundation on which to base further research from which to make long-term projections on the rate and influence of relative sea level change in the wider Raglan Harbour.
References
Gibb, J.G. 1986. A New Zealand regional Holocene eustatic sea-level curve and its application to determination of vertical tectonic movements. Royal Society of New Zealand, Bulletin 24, 377-395.
Murray, J. W., Alve, E. 1999. Natural dissolution of modern shallow water benthic foraminifera: taphonomic effects on the palaeoecological record. Paleogeography, Palaeoclimatology, Palaeoecology. Volume 146, issue 1-4. Pages 195-209
Pillans, B. 1990. Pleistocene marine terraces in New Zealand: A review. New Zealand Journal of Geology and Geophysics, 33 (2), 219-231.
Purna, S. P., Yulianto, E., Nugroho, S. H. 2023. Distribution patterns of foraminifera in paleotsunami. A review. Natural Hazards Research. Volime 3, Issue 1. Pages 1-13
Swales, A., Ovenden, R., Budd, R., Hawken, J., McGlone, M.S., Hermanspahn, N., Okey, M.J. 2005. Whaingaroa (Raglan) Harbour: Sedimentation and the effects of historical catchment landcover changes. Environment Waikato Technical Report. No 36
