Whakataki

Whakataki is a spectacular spot on the Wairarapa coast just north of Castlepoint. The shoreline is a large area of tilted rock strata that have been eroded into a broad, flat platform extending for hundreds of metres along the coast. The rock layers are alternating sandstone and mudstone layers that stand out as distinct lines.

It is believed that they formed as cyclically repeated turbulent flows of sand and mud that avalanched down and over the sides of an underwater channel about 500 to 1000 metres below the ocean surface. These sorts of deposits are know as turbidites. Here they are of early Miocene age (roughly 20 million years old)

As the sediment laden water surged across the sea floor it laid down a deposit of sand and mud with several distinct layers. The base layer typically has very flat laminations, followed by a more convoluted and rippled layer above it. Above that the particles get finer as the remaining cloud of mud slowly settled on top of the coarser sandy layers below.

It is interesting to look at the different structures and imagine how they formed in the dark depths of the sea so long ago. Here I am pointing at some climbing ripples in the upper sandstone layer, above a more regularly laminated base layer of the flow. They show that the current was moving from the left (south).

Exploring the area shows up many interesting geological features.

Here you can see that the beds are not only tilted up, but they have been dislocated by faults.

 

In this image you can see joints cutting across the beds at a right angle. They develop as the pressure on the sequence decreases due to erosion of overlying material. You can see how the spacing between the joints is wider for the thicker beds, and closer together on the thinner ones.

 

The rock layers are of interest to geologists because similar thin bedded fine grained deep sea sediments are often found to be important reservoirs for hydrocarbons which penetrate into the tiny pore spaces between the individual grains of sand. 

By studying these beds where they are exposed at ground level, we can gain important information about similar but more inaccessible  sequences deep below the surface that may actually contain trapped oil or gas.

During our visit, Garth Archibald was making a laser scan of the surface of the shore platform. This will be translated into a 3 dimensional computer image of the platform which can then be used for detailed analysis of the different layers in the sequence.

Garth has used his laser scanner in a wide variety of settings, including a number of Christchurch cliffs that were seriously shattered by recent earthquakes, as you can see in this video.

NZ's First Reptile Discoverer returns to Mangahouanga

In 1958, Petroleum Geologist Don Haw was mapping the rocks in the Mohaka river catchment of Western Hawkes Bay. The project was part of a wide ranging exercise to evaluate the hydrocarbon potential of the East Coast basin at that time for BP, Shell and Todd.  His discovery of reptile bones in the Cretaceous sediments was recorded on Company maps which subsequently caught the eye of Joan Wiffen in the early seventies. She ventured into the region to take a closer look. Remarkably this led to her eventual unearthing of New Zealand's first dinosaur fossils, as well as many other new species of exciting Cretaceous reptiles. For her significant effort Joan became known as the “Dinosaur Lady”.  For his essential initial work, Don was awarded the Wellman Prize in 2001.

On March 24th 2012, 54 years after his initial explorations,  Don returned to Mangahouanga along with the teachers and school children who were participants of our GNS Science "Dinosaurs and Disasters Geocamp". This was a historic day as it was his first return to the valley in all that time. In the photo, Don (centre) is with Robyn Adams, one of Joan Wiffen's long term fossil hunting assistants who still leads trips into the valley.

In the following transcript, Don describes his experiences from all that time ago:

 

"We were mapping outcropping sediments in the Upper Mohaka river tributaries, observing for the first time, what might be there. Nobody had really mapped that steep isolated terrain before. We were keen to find what was present between the greywacke basement rocks and the overlapping Upper Tertiary sandstone section. Perhaps nothing - we just didn't know - maybe the Upper Tertiary rested directly on basement.   Was there any Cretaceous section exposed?  This was so important to the assessment of the hydrocarbon prospectivity of the region."  

"It was high summer, February 1958 I think, and we were scrambling up this really difficult stream bed, huge boulders, and totally bush covered. We recognised we were stepping on boulders and outcrops of massive concretionary sandstones which we had not seen before. These appeared to be of marine origin, and had fine shell debris in them which was triggering off alert signals to me – There might be other important fossils here!  We should look carefully! I was with field assistant Ken Fink Jensen to whom I owe much for his support and encouragement in those days, Together we began to examine some odd protuberances on the surface of certain boulders, which I quickly recognised, because of their shape and texture, had to be organic and which were almost certainly bone remains from some marine creature.  I think my initial reaction was that they were fish remains. The rock was hard, very hard, and we extracted several and brought them back to Gisborne."

"They were sent off to Jack Marwick, a retired NZGS chief palaeotologist,  who identified them as reptilean bones. Eventually they were recognised to be Mosasaur fossils, a type of  marine Plesiosaur.  It was a first for New Zealand."  

"This region became the hunting ground of Mrs Joan Wiffen who followed up our fossil discovery, with many years of hard work there, excavating numerous other finds from the same stream bed.  She, with her husband and family team, found many new fossils, some really exciting, including some terrestrial dinosaur remains which must have been washed into those early primeval seas. It has now become one of the most prolific fossil sites in New Zealand."


The final image shows a mosasaur skull that was found by Joan and her team and is now kept at GNS Science in Lower Hutt.

Cape Kidnappers

Cape Kidnappers and the Clifton Cliffs make for a spectacular geological site in Hawkes Bay. The cliffs extend for several kilometres southwards from Clifton, on the coast near Hastings. They  are very high and consist of quite loose rocks, so it is important not to go too close where possible. It is also important to start your visit on a falling tide which will give enough time for a return trip without being cut off by high water.

 At the start, near Clifton, the cliffs are made up of thick river gravels, with thin layers of white pumice (volcanic ash) and occasional dark layers of plant material.  Initially the beds are about 300 000 years old. Because they are dipping gently down to the north, you will pass further and further down the sequence as you walk along the beach to the south.and east. Here you can see the fluted erosion of the unconsolidated gravels by rainwater.



In this photo, a layer of light coloured volcanic ash separates overlying river gravels from marine mudstones below. Just above the ash is a very thin dark organic layer with plant remains in it.

There are many ash layers in the sequence. They have been erupted from the Taupo Volcanic Zone in  the Central North Island, at least 150 kms away. The thickness of the layers even at this distance, testifies to the magnitude and violence of these past rhyolitic eruptions. In this photo you can also see how a fault has dislocated the beds by several metres.


Further along the beach, towards Black Reef, there is a distinct change in the bedding, seen in this image about half way up the cliff. The lower gently dipping beds have been eroded flat with much younger beds deposited on top of them. This unconformity represents a time gap of about two and a half million years. The lower unit is three and a half million years old - the upper one starts at about 1 million.

An exciting find on our visit was this fossil whalebone. It extended through the boulder for about one metre.







Out on the reef itself were some well preserved shell fossils as well as another orange coloured whalebone fossil slowly being eroded away.

Last but not least I should mention the gannets, for which Cape Kidnappers is most famous. The young birds here will take their first flight soon, and without looking back or touching down will travel all the way to Australia.

Lake Tutira - tectonic uplift, ice ages, landslides and cyclones

Lake Tutira is a scenic spot on the route between Napier and Wairoa in northern Hawkes Bay. It is in a very rural setting, surrounded by steep hillsides and farmland.

The landscape around the lake contains several powerful geological stories.

The first is that the hills themselves, made up of rocks that are about 1.8 million years old, reach a height of up to 800 metres above sea level. Before being uplifted and exposed by erosion, the rocks may have been buried to depths of 500 to 1000 metres. This means that they have been rising at an average rate of about a metre every 1000 years.

When you look at this steep hillside, you can see lines of cliffs running almost horizontally across the slopes. These bluffs are made of relatively hard limestone, with softer muds and sandstones hidden beneath the grassy slopes separating the cliffs.The top limestone band that you can see correlates with the one on the top of Waipatiki Beach that I showed in a recent post.

These cliff lines therefore represent the cycles of global change that repeated every 40000 years. The hard limestones were deposited as sea levels were slowly rising, while ice caps melted at the end of each glacial period, as shown also in my previous posts from Waipatiki and Darkeys Spur.

The next landscape feature of interest is this area of grassy hummocks just beyond the pine plantation. These are the debris pile from a massive landslide that slid down from the nearby hills about 7200 years ago. It blocked the stream that flowed down the valley, thus forming the present day Lake Tutira.  Similar huge rock slides occurred in other parts of the region at the same time. Scientists believe that they may have all been triggered by a single massive earthquake.

One of our activities on the recent 'Dinosaurs and Disasters Geocamp" with Hawkes Bay schools was to drill a sediment core from Lake Tutira. Kyle and Richard used a PVC drainpipe which they pushed into a shallow part of the lake bed.

Although only about half a metre in length of core was extracted, you can clearly see a number of layers. The top of the core is to the left of the photo, with several organic rich layers visible. The lower half consists of varying amounts of pumice that will have been washed into the lake from where they accumulated after the Taupo eruption 1800 years ago.





The lakefloor sediment has in fact been studied in detail by researchers from GNS Science and other institutions . In 2003 a drill rig was set up that retrieved a 27 metre core right through to the base of the sediments. It revealed a detailed history of the environment around Lake Tutira over the last 7200 years:

• Almost 1400 storms were intense enough to leave their traces in the form of layers of mud washed down from the surrounding hills. Periods when storms were more common started abruptly and could last for several decades.
• Volcanic eruptions from the Taupo Volcanic Zone (including the well known 'Taupo Eruption' of 1800 years ago)  have left layers of ash that can be dated.
• Changes in land use from native forest to pasture due to human occupation, have increased the sedimentation rates tenfold..

During Cyclone Bola which passed over Hawkes Bay in 1988, over 750 mm of rain fell over four days.  A huge number of mudslides came off the hillsides over the whole region.

In this photo, Richard Levy and I have exposed a buried soil layer next to Lake Tutira. It is beneath about metre of pale brown 'Cyclone Bola Mud'  (top half of image). The dark soil layer below contained branches of wood. Further down there was another pale coloured mud layer from an earlier rainstorm.

Darkeys Spur's rock record of sea level change

Along with Waipatiki, another excellent Hawkes Bay locality for looking at rocks laid down over a cycle of sea level change is Darkeys Spur, about twenty minutes by car west of Lake Tutira. The road is very narrow and care should be exercised to park safely and watch out for vehicles. Fortunately the road is not a busy one. The road winds up a hill and the rocks are well exposed in the cutting on the uphill side.




Kyle Bland of GNS Science has mapped the geology of Hawkes Bay in detail. He showed me what there is to see at Darkeys Spur, and also led our recent Geocamp visit there.

As at Waipatiki Beach (see previous blog post) the deepest water deposits are grey muds with occasional oyster shells.

Not far up the sequence, the colour changes, the particle size increases and a wide variety of fossils starts to appear in rocks which now indicate decreasing water depth.

In this close up image, you can also see that the shallower water sediments are cross bedded, indicating strong water currents. The bivalve shells are also facing concave side down which is what happens to them when washed along by moving water. This indicates very shallow water just below or within wave depth.

A little further up the road, the rock type (lithology) changes to gravels, indicating a beach environment as sea levels decreased further. In this image you can see the marked transition between the nearshore sediments and these gravels.

Finally, river gravels can be found mixed in with the beach gravels. They can be distinguished because the stones on a pebble beach tend to be flat whereas in a river bed they are more rounded or blocky shaped, such as the ones that Kyle is showing here..

Above the gravel deposits (seen in the lower cliff in the centre left of this photo),  there are lime rich sands (upper cliff), indicating that sea levels were  rising again. At the time when these deposits were laid down, these sea level cycles were about 40000 years long, related to the variation in the tilt of the earth's rotational axis.

Rock records that show cycles of sea level change are also found along the Wanganui coast for example at Ototoka Beach. They offer a globally significant geological insight into the way polar ice sheets have expanded and retreated during repeated ice ages.

Waipatiki Beach

Waipatiki Beach north of Napier is a great place for family holidays in the summer.  It is enclosed by cliffs at either end that happen to provide one of Hawkes Bay's many classic geological sites.

A track leading south of the beach takes you to a good view of the cliffs. You can see several colour changes in the rock strata from the base of the cliff to the top. These are due to the fact that the water depth in which the rocks were laid down changed through time. The blue grey band in the middle of the cliff is fine grained mud with a few oyster fossils, that was deposited offshore in about 50 to 80 metres of water depth.The more orange coloured rocks were laid down in shallower water, with beach sand and many fossils.

Because of erosion and rock falls, there are many boulders rich in fossils that have fallen down onto the beach below. This is where you can find lots of interesting specimens. In this photo, Richard Levy, a sedimentologist from GNS Science is looking at a slab full of bivalves and sand dollars. This is reminiscent of many modern New Zealand beach environments such as along the Kapiti Coast north of Wellington. 

At the top of these orange beds the fossils have been washed around and damaged by wave action, indicating a very shallow environment of deposition. 

A close look will show that the fossils here include very few actual shells. This is because many sea shells are made of aragonite, a form of calcium carbonate that differs in its structure from the other common alternative which is calcite. Aragonite tends to dissolve relatively easily during the rock forming process, and to re-precipitate as calcite in the matrix of the sediment. This makes these rocks very hard, but with many gaps where shells have disappeared, leaving only the internal casts.

In this photo you can see some trace fossils made by some sea animals burrowing into the sediment about two million years ago.    

So why do the rocks show this change from the grey muds, deposited in relatively deep water, to progressively shallower sandstone and limestone?  Either the land was going up or the sea level was going down, or perhaps both were happening at the same time.

The rocks around Hawkes Bay and other parts of New Zealand show clearly that the main cause was sea level change, which in turn was due to global ice age cycles which themselves were driven by changes in the earth's orbit around the sun (called Milankovitch Cycles). So if you ever go to Waipatiki for a holiday, you may like to look for some fossils and consider the relationship between Astronomy and the colours of the cliff.

Dinosaurs and Disasters Geocamp 2012

Over the last two weeks, GNS Science, with support from the Todd Foundation, The Royal Society and the National Aquarium in Napier, has been running a 'Dinosaurs and Disasters Geocamp' for a group of Hawkes Bay Intermediate level students and teachers.

The participants investigated many landforms and cliff sections around Hawkes Bay. GNS Science geologists Kyle Bland and Richard Levy also helped to organise and lead the Geocamp. In this photo they are encouraging the participants to look closely at a cliff section with fossils and structures above Lake Tutira that help explain the formation of the lake by a giant landslide.

One of the participants Michael Young is showing a section of sediment core that we drilled out of the bed of Lake Tutira using a length of drainpipe, and then wrapped in clingfilm for transport back to our base at the Napier Aquarium.

Kyle Bland is seen here, showing participants how sediment from Waipatiki Beach is washed and sieved so that it can be checked for microfossils.

The two week geology immersion experience was created to open the eyes of young people, their schools and the local community to the wonders of the natural environment in their local area. It culminated in a two day expo created and run by the students to show their discoveries to the public. In the photo Phoenix Hancox-Thompson is introducing visitors to some of the activities.

Here are a couple of videos that capture some of the activities and the enthusiasm of the participants. The first looks at some of the locations we visited:



The second is a chance to learn geology from some very bright young geoscientists: