GeoTrips

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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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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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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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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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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