Fossils

Darkeys Spur’s rock record of sea level change

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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.

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Waipatiki Beach

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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. (for more geological and access information see also www.geotrips.org.nz/trip.html?id=24) A track leading south of the beach takes you to a good view of the cliffs. (Update: there have been very big big rockfalls in this area and it is likely to be safer to explore the north end of the  beach.) 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.

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Tyre Tubing the Mangahouanga Stream

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On our second day in the Maungataniwha Forest, some of us explored the lower Mangahouanga Stream using the well established kiwi river transport method of tyre tubes. This allowed us to visit parts of the river that would otherwise be very difficult to reach. Initial access to the river was via bush bashing through pine and then native forest, and down a steep climb to the water’s edge. In the second photo I am following Ben towards our next fossil hunting stop off. James Crampton gets speedy on one of the faster sections of the stream. Where possible we stopped to closely inspect each boulder for the tell tale signs of fossil bones, wood or shells. Here is an example of fossil reptile bone (centre of photo). Because of the hard surrounding rock, these bones are not removable except using painstaking laboratory methods over many months. Although we found several interesting fossils, we were surprised that they did not seem to be as abundant as they were in 2009. This will have been due to the higher river levels, and the random redistribution of boulders during occasional flood events. Pete Shaw, forestry conservation manager, about to launch down some rapids. Finally we arrived at the Rockhounds Huts, – built by Joan Wiffen and her team in the seventies as a base for their summer explorations of the Mangahouanga Stream.

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Reptile Fossils from an Unexplored Valley.

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The first barrier to accessing Mangahouanga Stream Three years ago I visited Mangahouanga Stream in Hawkes Bay  along with a group of GNS Scientists. This is the area where Joan Wiffen, New Zealand’s famous “Dragon Lady” found the only known bones of dinosaurs from New Zealand, as well as various marine reptiles, prior to her retirement from fieldwork about ten years ago. I described this visit in my blog at the time. What I didn’t mention in my blog was the existence of a remote valley that we believe might have only been visited once by a geologist prior to 2009.  The valley is a tributary of the Mangahouanga Stream. Pete Shaw with marine reptile bones from Wiffen Valley As well as being protected behind a large privately owned forestry block, this hidden valley is made particularly inaccessible by a high waterfall and densely forested ridge. Along with the forest manager Pete Shaw, I had managed to enter the valley via very steep and rugged bush in 2009. We spent several unforgettable hours travelling down the un-named stream (now called Wiffen Stream), finding several Cretaceous reptile bones in large concretions. The photo shows Pete in 2009 with the prize find of the day moments after he pulled it out of the stream. It is a cluster of several reptile vertebrae, subsequently identified as belonging to an elasmosaur. Although heavy, Pete managed to carry it out, whereas most of the fossils we found that day had to be left in place. Last week I returned to the area with a team of GNS palaeontologists along with Victoria University student Ben Hines. One of our aims was to explore the hidden Wiffen Valley to  have a closer look at its geology and fossils, This photo shows GNS palaeontologists James Crampton and John Simes in the upper section of Wiffen Stream. There were log jams, tree trunks and waterfalls to negotiate as we travelled down the stream. We took our time to throughly check out the boulders for fossils as we moved slowly along. The reptile bones are typically found in very hard concretions like this one. We were unable to identify this particular bone, or remove it from the concretion, so it was left in situ along with several others that we saw. In this photo  Marianna Terezow of GNS Science can be seen tackling the dense bush that must be traversed to access and return from the hidden valley.

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Moa Hunting

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Takaka limestone country Whilst on holiday in the Nelson area last week, I went for a look around the limestone plateau of Takaka Hill, not far from the huge natural shaft of Harwood’s Hole. I managed to persuade a couple of friends to come along for the adventure. The area is very rugged, covered with rock outcrops and tangled vegetation. There are many caves (see my earlier blog post from January 2010) and my particular interest was to look for small vertical shafts that might have acted as lethal traps to the moa that once roamed the area. The delights of moa hunting With some careful searching, it did not take long to find some cave entrances. Some of the shafts are very deep and obviously care is needed in this environment to avoid the fate of becoming entombed and fossilised just like the moa that we were hoping to discover. As you can see, some of these caves are very small. With a bit of wiggling and squirming, we were able to push down into them. Moa bones lie scattered at the bottom of a cave Sure enough, a couple of them contained parts of moa skeletons lying at the bottom. In this image you can see a variety of bones, including leg bones and a pelvis. The number of different bones that we saw in this cave indicated that at least three or four moa individuals had been caught there. Moa pelvis  This is a close up of the pelvic bones of a moa Moa bones in narrow fissure At the very bottom of this cave, there were more bones visible, but the fissure was too tight to get close to. We were very satisfied with our discoveries, and happy to leave the bones in place for future rediscovery and study.

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Te Mata Peak

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Last week, following my visit to Castlepoint I also headed further north to Hawkes Bay. State Highway 2 runs parallel to the central North Island mountain ranges, which had just received fresh snow from a recent southerly blast, to provide a classic New Zealand pastoral scene… Te Mata Peak near Havelock North is a popular spot for runners, hikers and paragliders. On a clear day there are  spectacular views across the landscape from the coast all the way to the volcanic peaks of Ruapehu in the centre of the North Island. The buttresses of Te Mata Peak are made of Awapapa Limestone. This formation, which is about three and a half million years old, is also found to underly other nearby coastal hills in the Hawkes Bay area. It was formed along a string of offshore shallow water shoals and tidal banks. At that time the coastline was about 40 kilometres to the west, along the present edge of the central mountain ranges. In between, the sediments of the same age are mudstones that represent much deeper water than the Awapapa limestone. Armed with my Kiwi Fossil Hunter’s Guide, I located a way to climb down on the eastern side of the peak to have a close up look at the cliff section just north of a radio mast, a few hundred metres down from the summit car park.  Although there isn’t a wide variety of fossils in these rocks, there were some vary well preserved specimens such as these examples of a scallop called Phialopecten marwicki, as well as barnacles, oysters, brachiopods (lamp shells) and coral-like bryozoans. In places, thinly bedded layers of shell fragments show that water currents were strong, indicating shallow water conditions when the limestone was deposited. Careful research by scientists has found that the alternating bands of hard, strongly cemented grey limestone and softer, orange sandy layers represent cycles of sea level change during the Pliocene Epoch. The harder layers formed because at shallower depths there were more water currents, which allowed more calcium carbonate rich water to flow through the sediments. This would have been during the ice ages, when huge amounts of sea water were locked up in polar ice caps, thus lowering the sea level. The warm period (interglacial) deposits have less carbonate cement to strengthen them and are therefore etched out more easily by erosion. These deeper water sediments are now underneath overhangs of the harder layers. The example here had clusters of large oyster shells scattered within it.

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Castlepoint and the Kiwi Fossil Hunter’s Handbook

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James Crampton and Marianna Terezow’s The Kiwi Fossil Hunter’s Handbook won the LIANZA Elsie Locke Award in the  non fiction category. With James away from town, Marianna attended the awards ceremony last night to accept the prize. Here you can see clearly that Marianna was one of the people behind the book! Armed with the fossil hunter’s guide, I recently went to visit a couple of the localities described within it. The first was Castlepoint, a popular spot on the Wairarapa coast of the North Island. It is a dramatic rocky promontary, enclosing a lagoon and extending out to sea. Just to the south is a steep track leading up to the top of high cliffs overlooking the bay. The spectacular coastline is a favourite spot for fishing and hiking as well as exploring for fossils. The Castlepoint Reef is jam packed with fossil fragments, indicating that it was created under the sea and has since been uplifted. The fossils have been dated at about 2.4 million years old. Because of fault lines on either side of the reef, nearby older and softer strata have been uplifted even more, and then eroded away, leaving the hard limestone of the reef to stand proud of the surroundings. The seaward side of the reef is a dramatic cliff, that would be easy to fall off if you weren’t careful. From studying structures such as slumped (buckled) beds and mixed rock fragments within the reef, geologists think that it represents the debris that accumulated in a steep sided canyon. Material occasionally avalanched down into the depths from above, mixing up fossil fragments and rocks into the sedimentary sequence. Here you can see an example of these disturbed beds, with a scattering of white fossil shells included. In this photo you can see that much of the reef is made up of densely packed shell fragments ( in this case clams and barnacles) that are piled up on top of each other. This univalve is similar to some that are found today on many New Zealand beaches. Both cold water and warm water fossil shell species can be found in the Castlepoint reef . It is thought that at least two ice age cycles are represented in the rocks, each lasting about 40 000 years. The fossils assemblages at Castlepoint show a mixture of deeper water species that would have lived several tens of metres below the surface in the bottom of the canyon, and shallower types that were washed down from the sea floor alongside. These brachiopods called Neothyris campbellica are about 5 cms long. They were filter feeders that attached themselves to the sea floor with a stalk. An early morning view of Castlepoint lighthouse with the Castle in the distance across Deliverance Cove.

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Palliser Bay

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Palliser Bay is an isolated sweep of coastline about 2 hours drive from Wellington. It is separated from New Zealand’s busy capital city by the Rimutaka Range. Yesterday I visited the area with a group of Lower Hutt school children as part of their Year Seven geology camp. Fully armed with the “Kiwi Fossils Hunter’s Guide” as well as another excellent book by Lloyd Homer and Phil Moore that describes the geological features of the Wairarapa Coast called “Reading the Rocks“, we visited several great geology hotspots along the coastline. A striking feature that we noticed straight away was the flat topped escarpment that runs along much of the coast. This is a raised marine terrace that was at sea level about 80 000 years ago. It indicates that the whole area has been undergoing an enormous amount of uplift which continues to this day. First stop was Hurupi Stream. (This is described in detail in the “Kiwi Fossils Hunter’s Guide“). The soft mudstones at the sides of the stream were deposited under the sea in the Miocene Epoch (sometime between 11 and 7 million years ago) , when the Aorangi Range just to the North was an island, separated from other parts of the North Island by a shallow sea. We found quite a few marine molluscs that are very well preserved and easily spotted. Not far along the coast road are the Putangirua Pinnacles. These spectacular features have been eroded out of a thick sequence of conglomerate. Hard layers or large individual boulders within the conglomerate form a protective cap at the tip of each pinnacle. The ground is strewn with loose rubble – testament to the fact that the erosion here is still very active. This might not be the best place to visit in a rainstorm! A few kilometers along the coast road, there is a dramatic example of coastal erosion where a whole section of the original road itself has disappeared! We followed the coast past the small settlement of Ngawi, and a huge tilted slab of fossiliferous sandsone called Kupe’s Sail, to the Cape Palliser lighthouse. This is built on a cliff of volcanic rock that was erupted under the sea as pillow lavas about 100 million years ago. The long staircase up to the lighthouse leads up to a great viewpoint. This is the Southeastern tip of the North Island of New Zealand, with nothing but ocean between here and Antarctica or South America. Just a few kilometres out to sea is the Hikurangi Trench, the collision boundary between the Pacific and Australian tectonic plates. The connection between uplifted terraces, fossils, erosion, earthquakes and volcanoes gave us all something to think about to round off our geological excursion.

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Turning over an old leaf with the Fossil Hunter’s Handbook

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If you are reading this blog, you presumably like the idea of getting outside and appreciating the landscape and its underlying geological features. James Crampton and Marianna Terezow are paleontologists here at GNS Science. They have just published a great book for fossil enthusiasts called the Kiwi Fossil Hunter’s Handbook. It is full of interesting information and highlights a number of prime localities around the country for unearthing nice fossil specimens. After a conversation with James,, and armed with information from one of the chapters in the book, I recently visited a fossil locality near Murchison in the South Island. About 6.5 kms north of Longford on the main State Highway 6, there is a sign indicating Nuggety Creek Road. A few hundred metres along the track there are some crumbling cliffs by the roadside. This is a fantastic place to collect fossil leaves from the Miocene (about 16 – 13 million years old). They were deposited in a river valley that was surrounded by a rainforest. Boulders at the foot of the cliff are absolutely packed with leaves of different plants. There were large trees here as well as smaller plants such as ferns living beneath them. The rock is quite crumbly, so it can take a while to find a lump that is solid enough to stay in one piece. These fossils are evidence that the climate in New Zealand in Miocene times was very humid and warmer than at present. As you can see from the last photo, some of the fossil leaves are superficially very similar in appearence to modern leaves found at the same locality today.

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