Harrison Corbett

300355955

ESCI 202

Whanganui Basin Report

Abstract

The Whanganui Basin is a unique place to study geology. It is a basin where

sedimentation and subsidence are near equal. This gives the complete strata over an

extensive period of time while showing glacial-eustatic changes. Our goal was to determine

the different environments in which the strata at Ototoka Beach were deposited. By careful

identification and analysis of the section, a stratigraphic model was computed. By taking

fossil samples and dating them through various techniques, the ages of the strata were

determined. After all analysing had been completed, we were able to conclude that the

deposition of strata at Ototoka Beach were depositied during continual glacial-eustatic

changes.

Introduction

The Whanganui Basin, located in the southwest of the North Island of New Zealand,

is a back-arc basin in relation to the Pacific Plate-Australian Plate boundary that subsided

during the mid-Pleistocene. Sediment deposition has happened at the same rate as

subsidence, causing the deposition of approximately 4 kilometres of primarily shelf and

shallow-water sediment. It contains one of the most complete stratigraphic records in the

entire world giving scientists the ability to determine past environments by using different

aging techniques. The eastern margin of the basin has been slightly uplifted along the plate

boundary in the east with the Rangatikei River cutting through the middle of it resulting in

exposures of 47 sedimentary cycles which are on a Milankovitch cycle time scale and are

evident of glacio-eustatic changes. The basin is located near one of New Zealand’s active

volcanic zones, meaning that pyroclastic sediments have been deposited in certain strata

which are good proxies for dating. Seven stratigraphic layers were measured and out of those

seven, three of them contained shell beds. By examining the fossils and dating them, it is

possible to correlate their absolute age with the time of deposition. From analysing the

mircofossils and the macrofossils from the strata in Ototoka beach we were able to infer the

depositional environment of the strata as well as the climactic conditions of the times during

deposition. By using this data and previous data from geologists we were able to find

absolute ages for the strata in Ototoka Beach.

Methods

There were two main focuses of methods used during the Whanganui exercise.

Different procedures were used for collecting both stratigraphic data and paleontological

data. For collecting the stratigraphic data, we first identified the seven strata of importance.

We described the sections in great detail. A pacing technique was used with trigonometry in

order to determine the heights of the different strata. One would walk 50m and count the

paces. By using trig, one can then infer the height of the strata by walking from a certain

point until the next occurrence of a certain strata. Then it is just a simple calculation to

determine the vertical characteristic. The following weeks were spent in lab creating a

detailed measured section of Ototoka beach. By using the stratigraphic data collected, one

was able to create a detailed stratigraphic record of the area before the tilting event took

place. Samples were also taken from both the Mangahou Siltstone and the Pukekiwi Shell

Stone. The fossils found in the samples were sorted and identified as their respective species.

The ages for the fossils were found in Beu and Maxwell (1990). After determining the ages,

age-range charts were created which helped determine the ages of the deposition for the beds

in which the fossils came from.

Samples taken from the Lower Okehu Siltstone were sorted and identified using

microscopes. The sample was poured onto an examination plate. From there, the microfossils

were identified and placed on a faunal slide. With the microfossils identified, another age-

range chart was created that helped further the accuracy of the depositional age of the Lower

Okehu. Lastly, using both first and last presence of certain foraminifera, coccoliths, and

molluscs that coexisted during the deposition of the strata, an age-depth curve was produced

that shows ages of the Whanganui strata. The stratigraphic column is the Y-axis and time is

the X-axis. By plotting a number of biostratigraphy datum (FADs and LADs of species) one

was able to create the age-depth curve and infer absolute ages of the strata using two pieces

of information: the paleo-magnetic anomaly data and the presence of species in the certain

strata.

Results

The palaeontology of the section can be used to infer the climactic conditions under

which the strata were deposited. The measured section, shows both the vertical profile of the

section before any tilting takes place and the order in which the different stratum were

deposited. Also, when co-ordinated with the age-depth curve, it can be used to determine the

absolute ages of the strata. It is another tool that can be used to further infer the depositional

environment.

The oldest formation is the Lower Maxwell, a grey siltstone with fine laminations. It

was deposited 1.78 Ma during an interval of polar reversal between Cobb Mountain and

Olduvai, as shown on the age-depth curve (all layers beneath the Butler’s Shell Conglomerate

underwent this polar reversal). The Lower Maxwell contains three lignite beds. The first

lignite bed shows no evidence of salt-marsh like conditions. The presence of the non-forest

pollen, Gleichenia sp, Dracophyllum sp, Leptospermum sp, Coprosma sp, Phormium sp, and

Typha sp. are indicative of a semi-swamp like depositional environment (Harris and Couper

1953). The second lignite bed has traces of typical coastal plants but not many. Thi is

indicative that it was deposited in a swampy environment as well. The last lignite layer also

shows traces of plants evident of a swampy-terrestrial depositional environment.

The next layer was the Pukekiwi Shell Sand, medium grain sized, grey sandstone. It

had widespread shell beds. The species found in the sample, like Myadora striata, are

evident of a marine depositional environment. The Pukekiwi is from the Nukumaruan at 1.75

Ma.

The Middle Maxwell formation is a well-sorted, grey sandstone with interbedded

lenses and fine laminations. It is 1.74 Ma and was most likely deposited in a cooler, moister

environment than the Lower Maxwell (Harris and Couper 1953). The presence of Sedges as

well as wood fossils, different pollens, evidence of burrowing species, and a lignite layer near

the upper contact mean that this stratum was most likely deposited in a terrestrial

environment.

The Mangahou Siltstone is a very-fine grained deposit that is 1.72 Ma. It contains

crossbedding that occurs on a millimetre scale and has widespread shell beds throughout.

The shell beds are full of concave down, articulated and disarticulated shells, mostly

Austrovenus sturchburyi.

The Upper Maxwell formation is 1.71 Ma and is made up of fine-grained packed

deposit and has a mudstone lens. There is a 40cm long, horizontally orientated ancient wood

macrofossil and pumice from the Ototoka tephra as well as abundant amount of sedge and

other carbonaceous materials. Another dominant plant is Myriophyllum sp. which is an

aquatic plant meaning that the Upper Maxwell was most likely deposited in a wet, swampy

environment (Harris and Couper 1953).

The Butler’s Shell Conglomerate is the next strata in succession and is dated to 1.06

Ma. It is composed of gravel-size, well-sorted grains and small shell fragments on a

millimetre scale. Unlike the rest, the Butler’s Shell Conglomerate has a normal polarity.

The Lower Okehu Siltstone also has normal polarity and is composed of coarse grains

and some mollusc fossils dating to 1.03 Ma. After careful observation of the sample from

this section under a microscope, two more fossils were identified: Bollivina Vellai and Oolina

melo. The Elphidium biofacies was determined for this stratum due to the ratio of benthic to

planktonic microorganisms being about 10%. This biofacies is indicative of a shallow

deposition in a sheltered coast or an area of similar geography. After creating an age-range

chart of the microfossils an age of Castlecliffian could be inferred.

Discussion

The first stratum of interest, the Lower Maxwell, due to the presence of pollen-

containing lignite, leaves, and other carbonaceous materials, can be inferred that it had a

terrestrial depositional environment. After analysis of the palynology of the Lower Maxwell,

it was evident that the depositional environment changed from swamp-like to more of a bog.

This is clear from the species found in the upper portion of the layer which could have only

existed I shallow water, humid conditions (Harris and Couper 1953).

1.74 Ma. the Pukekiwi Shell Sand was deposited. From the vast amount of mollusc

fossils in the samples, it can be concluded that it was marine depositional environment. His

means that between the deposition of the Lower Maxwell (1.78 Ma) and the deposition of the

Pukekiwi, there was significant sea-level rise. The stratum has many mud laminations as

well as relatively articulated shells which can only settle in calm conditions so we can infer

that during the time of deposition for the Pukekiwi, environmental conditions were relatively

stable and calm like in an estuary.

The Middle Maxwell, which contains traces of wood, pollen, burrowing species, and a

lignite layer near the upper contact, was deposited in a terrestrial environment. This means

that there was a drop in sea-level between the deposition of the Pukekiwiw Shell Sand and

the Middle Maxwell. The contact between the layers is very sharp meaning the change in

sea-level must have been quick on a geologic timescale. The fine-grained sandstone,

evidence of migrating dunes, and planar crossbedding is indicative of a flooding river during

deposition. Sedges dominated the time of deposition and (Harris and Couper 1953) propose

that forest changes had occurred during this time. They state that the depositional

environment was rather cool and moist.

The Mangahou Siltstone, dated at 1.72 Ma, has extensive shell beds full of

Austrovenus sturchburyi indicating another glacial/interglacial cycle where sea-level again

rose. The shells were concave down and both articulated and disarticulated and the fact that

the Managhou is comoised of fine-grained siltstone with small crossbedding festures means

that the conditions were again relatively calm. A. sturchburyi were able to live in a range of

ses conditions but most shells would eventually be deposited in relatively shallow bays (Beu

and Raine 2009).

The next layer to be deposited, the Upper Maxwell formation, contains a 40cm long

horizontal, wooden fossil as well as pumice from the Ototoka tephra, abundant amounts of

sedge, and other carbonaceous materials, which leads to the inference of a terrestrial

depositinal environment. Another change in the glacial/interglacial cycle. The Upper

Maxwell is composed of fine-grained siltstone with mudstone lenses. This means that the

environment must have been flooded or be a swamp which goes with (Harris and Couper

1953) who stated the carbonaceous material must have accumulated in a swamp. They also

stated that from the deposition of the lignite layer, an inference could be made that the

environment was wet and warm, although cooler than present day.

Again, a change in sea-level took place and in turn creating the Butler’s Shell

Conglomerate. It is composed of coarse, well-sorted, gravel-size sediments. The stratum

contains a vast amount of shells proving the marine depositional environment. The rounded

clasts, evidence of dune migration, and the coarse, well-sorted sediment is all evident of a

high-energy depositional environment.

The last stratum of interest, the Lower Okehu Siltstone was deposited in a marine

environment but at a time of sea-level fall. The grain size increases im coarseness as the

layer gets younger which is evident of a higher-energy, falling sea-level condition. The

presence of molluscs and foraminifera again suggest a marine depositional environment.

Conclusion

After careful analysis of the Ototoka strata in Whanganui, it was clear to see and

understand the glacial/interglacial cycle and how it affected deposition environments for

different strata. The first stratum of interest, the Lower Maxwell, was deposited in a

terrestrial environment. This is evident from the lignite layers that contain certain pollen

species and leaves as well as other carbonaceous materials. The sea-level then rose and the

Pukekiwi Shell Sand was deposited. A marine stratum evident from the abundance of

mollusc fossils and fine grained sediment. Sea-level fell again and the Middle Maxwell was

deposited in a terrestrial environment. There was presence of wood, lignite, and certain

pollens evident of a terrestrial environment that was cooler and wetter than the Lower

Maxwell. The Mangahou Siltstone was the deposited in a marine environment after sea-level

rose again. This is clear from the presence of Austrovenus sturchburyi which is indicative of

a calm marine environment. Again, sea-level fell and the Upper Maxwell was deposited in a

terrestrial environment. It was deposited in a swampy, wet environment evident from the

certain pollens found, the lignite layer, and the abundance of sedge. Another cycle took place

and sea-level rose deposited the Butler’s Shell Conglomerate. The marine depositional

environment is clear from examining the coarse, well-sorted, gravel-sized sediments which

means dposition in a marine, high-energy environment. Furthur sea-level fall occurred and

the Lower Okehu Siltstone was deposited. The presence of molluscs, and foraminifera

suggest a marine depositional environment. However, evidence in the strata would show

constant changing between terrestrial and marine environments. Above the Lower Okehu

Siltstone is the Okehu Shell grit and the Upper Okehu Siltstone respectively. Although not

analysed in this project, further examination would most likely show more glacial-eustatic

changes. In conclusion, the major stratigraphic facies characterised within the Wanganui

Basin were deposited in coastal plain, shoreface and shelf marine environments during the

late rise, highstand and early falling part of each glacio-eustatic cycle (Beu and Edwards

1983).

References

Beu, A.G. and Maxwell, P.A. 1990. Cenozoic Mollusca of New Zealand, New

Zealand Geological Survey Palaeontological Bulletin 58.

Beu, A.G. and Raine, J.I. 2009. Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990). GNS Science miscellaneous series no. 27.

Beu, A.G. and Edwards, A.R. 1983. New Zealand Pleistocene and late Pliocene glacio-eustatic cycles.

Appendices

Figure 1: Ototoka mouth, Whanganui

Figure 2: Macrofossil ages

Amalda mucronata

Zethalia zelandica

Aeneator marshalli

Austrovenus sturchburyi

Paphies porrecta

patro undatus

Tawera spissa

Dasinia subrosa

Notocallista multistriata

Myadarastriata

Recent Hawera Castlecliffian Nukumaruan

Mangapanian Waipipian Opoitian Kapitan Tongaporutuan Waiauan Lillburnian Clifdenian Altonian Otaian Waitakian Duntroonian

Pukukiwi Sand Shell fossils (vertical), Sample Age (horizontal)

Mangahou Siltstone Sand fossils (vertical), Sample Age (horizontal)

Figure 3: Microfossil ages

Eggerellabradyi

Lagenaadvena

Neoglobo.pachyderma

Globigerinabulloides

Bolivinaparri

Bolivinitapliozea

Gavelinopsishamatus

Discorbinellacomplanata

Nonionoidesturgida

Nonionellinaflemingi

RecentHaweraCastle.Nuku.Manga.WaipipianOpoitianKapitanTonga.WaiauanLillburnianClifdenianAltonianOtaianWaitakianDuntroo.Whainga.Runangan

Age of Sample: Castlecliffian-Hawera