Sedimentary

Titahi Bay Geology

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Titahi Bay is a great place to visit if you are interested to see some of the geology near Wellington. There are a number of very interesting features to look at and explore. The first thing to check out is the coastal landforms caused by a combination of the atmosphere and  the sea, as well as the variable resistance of the rock, and a history of earthquakes (uplift). The first image is taken from the Pa site, a few hundred metres north of Titahi Bay beach. If you are a teacher, this is an excellent place to encourage your students to observe some of these natural features, such as sea caves, sea stacks, arches, marine terraces and wave-cut platforms. There is more information about how these features form on coastlines generally on the GNS Science websiteYou can also have a look at this GeoTrips page for specific information if you would like to visit this area. This sea cave marks the line of weakness of a fault. It is no longer at sea level, having been uplifted out of range of the water by earthquakes. It is also a useful way through the rocks between two small embayments. A striking feature of some of the rocks at Titahi Bay is this type of weathering out of the spaces between joints to form distinctive criss cross box structures Having looked at the erosion and weathering features along the coast, the next thing to do is have a look at the structures and the rocks themselves. A good place for this is just south of the Pa site, accessed down a short very steep track from Terrace Road. www.geotrips.org.nz/trip.html?id=69 In this photo you can see that the rocks are made up of alternating bands of massive sandstone, with in-between layers of dark mudstone. These rocks were formed from sands and muds eroded from the margin of Gondwanaland, long before New Zealand existed. The material flowed down into the deep sea and settled over wide areas. The coarser sediment, at the base of each of these submarine landslides, is represented by the sandstone, whilst the mudstone gradually settled on top.After deposition, the sediments were squeezed and deformed by the bulldozing effect of plate collision along the edge of Gondwanaland. You can see how the originally horizontal layers are now  almost vertical at Titahi Bay. Many faults are easy to spot, as they displace the clearly defined rock layers.As well as faults there are also folds in the rocks such as the anticline (upfold) shown here. An interesting challenge is to look for sedimentary features such as graded bedding or cross bedding, in order to tell the direction of younging of the steeply tilted rocks.  In this photo you can see some cross bedding, showing where the rock above my finger cuts across some fine layers that must have been layed down first. If you have time whilst at Titahi Bay, and if the tide is out, you should have a look at the tree stumps of the fossil forest which are sometimes exposed, usually at the south end of the beach. It seems almost unbelievable that these wooden stumps date from a time before the last ice age, about 100 000 years ago. The fossil forest does actually extend right along the beach, but is mostly covered with sand. On rare occasions, about once a decade, storms clear the sand away to expose much more of the forest than you can see here.Look carefully and you can see the growth lines of these ancient tree stumps. Check out the GeoTrip location here: www.geotrips.org.nz/trip.html?id=32https://youtu.be/A2Jed7P-pQ0

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A dynamic landscape in Hawkes Bay

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Last week I was in Hawkes Bay with geologist Kyle Bland, who led a field trip for teachers, students and parents of Crownthorpe School. Hawkes Bay geology is a story of uplift along fault lines, combined with rapid erosion and deposition by rivers flowing from the inland mountain ranges. This story is etched into the geomorphology of the landscape. The Mohaka fault last ruptured between AD 1600 and 1850, and forms an amazingly straight scar across the landscape. Like many faults in New Zealand, it is an oblique strike slip fault, including both sideways and vertical movement.  If you click on the image to enlarge it you can see how streams crossing the fault have been offset by sideways movement from the last rupture. Combined sedimentation, uplift and erosion have produced stepped terraces alongside the Ngaruroro river flowing from the Ruahine range out towards the coast. There are many fossils to be found in the sedimentary rocks that have been uplifted and exposed. Fossil hunting Hawkes Bay style involves using a digger to get access to your specimens! Ancient greywacke sediments are exposed in the Ruahine Range, having been uplifted by tectonic movements of the North Island fault system (Mohaka and Ruahine faults). These rocks were deposited in a trough at the edge of Gondwanaland, long before New Zealand ever existed. In the video below, Kyle gives us a Hawkes Bay case study of landscape evolution.

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Fossil Whale

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Last week I visited Palliser Bay in the Wairarapa. Along the coastline there are many exposures of mudstone from the Hurupi Formation, about 11 to 8 million years old. These mudstones contain abundant marine shell fossils, but are also known for occasional whale bones.  After some time searching, as luck would have it, I found a large piece of bone sticking out of the mud near the base of one of the cliffs. The bone was embedded in soft sediment and was easy to remove with a bit of digging. Nearby I found two other large pieces. Back at GNS, Craig Jones identified them as fragments of mandible (jawbone) from a large baleen whale species. Two of the pieces matched together to give a combined length of 75 cms. Initially we thought that these are part of the left mandible, whilst the other single piece is part of the right mandible.    John Simes is the manager of the fossil collection at GNS Science. He  helped me to give them an initial clean to remove some of the mud that coated the bones. Here you can see the typical mottled texture and brown colour of fossil bone. This is the largest  piece, half a metre long and about 25 cms across. There is an epifauna of bivalve and barnacle fossils attached to the bones. This tells us that they would have been lying in calm, relatively shallow water before they were buried by sediment. There are also several wood fragments in the surrounding clay, which suggests that the whale died not far from land. After many hours of cleaning, some interesting grooves appeared in the bones. These show where blood vessels were embedded alongside the bone. For an update on additional whale bone discoveries from this locality check out this blog post.

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Whakataki

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Whakataki is a spectacular spot on the Wairarapa coast just north of Castlepoint. It features as one of the localities on our GeoTrips websiteThe 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.

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NZ’s First Reptile Discoverer returns to Mangahouanga

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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.

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Cape Kidnappers

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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 caused 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 pale coloured 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. Cape Kidnappers features on our GeoTrips website where you can also find lots of other locations to explore geology and landforms: www.geotrips.org.nz/trip.html?id=182

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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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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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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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