Island Times Blog

Geology in the Field: A New Zealand Experience, Pt. 2

March 30th, 2015 by Gus Meloy |

This blog series details the travels and experiences of SeaTrek’s Gus Meloy, a geologist going through the pinnacle of his education. Six weeks of intense field and course work are described while providing insight about the country of New Zealand. This is a unique perspective he intends to share through a series of entries.

Week 2: A Week in Westport

Now that our first section of the trip was complete, we turned south and drove along the West Coast of New Zealand. There were lots of vineyards, forests, and pastures and among other things, hitchhikers.  Hitch hiking is a very common practice in New Zealand. In the U.S., hitch-hiking is seen as dangerous, whether you are the one picking up a stranger or the one catching a ride, and for good reason. Fortunately, the culture of New Zealand is different and you can feel safe standing by the road and holding your thumb out. The VI’s are similar to New Zealand in this way. On our way to Westport we actually picked up two hitch-hikers who were on vacation, exploring New Zealand. One was from Spain and the other was from Israel.  They climbed into Nick’s van, and we continued on until we reached West Port and the west coast.

We arrived at the Canterbury University Field Research Station around 5 p.m. We had enough time to unload our groceries and get settled in before joining Nick in one of the classrooms for our introduction to the local geology. The research station had two buildings, both with dorm style rooms. The building we stayed in had an enormous kitchen with industrial sized pots and pans that we would use later. There was a washer and dryer that we could use to do our laundry. Something that was new to me was what is called a drying room. It is often very wet and rainy along the west coast of New Zealand and therefore if you are doing research in the area you will want to be able to dry out your boots and gear every day when you come home. This drying room was full of heaters and dehumidifiers that hastened the process. A drying room in New Zealand is what we call the lifeline at Seatrek!. Hooks and shelves lined the walls to maximize useful space. The classroom was like a small lecture hall with a white board on the back wall and many chairs stacked to accommodate a class of fifty if necessary. The ceiling had cool support feature that allowed the building to sway a little safely in the event of an earthquake. This type of feature is necessary if you wish to live in New Zealand and keep your home as safe as possible. There are old inactive faults throughout the entirety of the South Island that cannot be mapped because they are so numerous. Any one of these old faults can reactivate and create an earthquake, making all of the South Island susceptible to earthquake hazards.

Once we were settled into our rooms and the first load of laundry was going, we gathered in the classroom with our notebooks. The six of us were eager to listen to Nick’s spiel about the West Coast geology. Karoly was in the kitchen busy making some fantastic meal. Nick explained that the west coast of New Zealand is partially a metamorphic core complex trending along the coast line. Conceptually this shows how far geologists have come in their thinking with structural geology. This is a geologic feature found in areas where extension is occurring, and the plate is thinning. The American West Coast is where metamorphic core complexes were first characterized. Imagine a rectangular block is representing your tectonic plate. In order to stretch it you have to break it or create faults, and then you have to slide along these faults. These are called normal faults and they are one way that you can accommodate for the extension of the crust. The other way, which is the way that produces metamorphic core complexes, is to have one dominant detachment fault that curves and goes to great depths. Other related faults will occur in parallel with the detachment fault and curve with the detachment fault eventually joining together at depth. The movement or extension occurs when the detachment fault releases and “detaches”, sliding along the curve as if you were to pull the carpet out from under a string of dominoes set to fall. A roll back effect is created, giving a sawtooth appearance to the old units, which their valleys will fill with younger material. The middle of the metamorphic core complex becomes thin from the stretching and is now deformed from the heat below more so than before. Recrystallization occurs along the “carapace” that rises up due to low density from the increased temperature. Igneous intrusions are also possible as the crust is thin and can be melted easily. Over the next few days we would visit different parts of this core complex and at the end of the week we would have to tell the story based off of the locations we visited and where those places fit into the story. Structural data would be taken in order to try to characterize the shear in the area if any could be found as well for a one day mini-mapping project. Once Nick’s talk had concluded, we went to the kitchen where Karoly had prepared for us a seafood alfredo pasta with Hungarian mashed potatoes. It was a welcome feast and some of the richest mashed potatoes I’ve ever eaten. Real cream was involved so you knew it was good.

For this portion of our field study, we would visit various sites that told part of the story of the metamorphic core complex. Additionally, we did go to a few sites that had more recent significance and spoke to the quick changing environment that is New Zealand. Along a road cut while driving the west coast we stopped to look at an outcrop. The unit was younger than the core complex and was all sedimentary rock. It represented throughout time what the environment was like when that material was deposited. What made this stop most interesting were these black bands that kept showing up. Upon closer inspection the black material was this lightweight substance that broke into blocks; unmistakably coal. Our professor Karoly explained that the coal we were looking at was not of industrial grade but one that they could, in fact shovel into the back of their pick-up truck and use in their own furnace. The difference between the coal we were looking at and high grade coal, such as what is found 20 miles further inland, is what is called the volatile content. Volatiles are particles within the coal that are not carbon but cause the coal to burn much hotter. The presence of coal represents that when this material was deposited, it was mostly likely a swamp or an estuary, what we know in the BVI’s as salt ponds and mangrove forests. The environment had to be anoxic in order to preserve organic material and not have it react and breakdown. Additionally, the conditions after burial had to be hot enough to generate the coal but not burn it off secondarily.

Another curious spot that we explored was the pancake rocks of Punakaiki. This was a limestone formation along the water’s edge that had been wave cut and channelized in some areas to make arches and caves that filled with ocean surge, creating spray as the tide came in. The limestone is very unique because it has a feature that is not observed anywhere else called stylobedding. Stylobedding is responsible for the appearance of the rock, which looks like a stack of flapjacks, hence the name pancake rocks. Limestone is supposed to be made of calcium carbonate, CaCO3, but can contain imperfections. These imperfections in the crystal lattice can migrate under pressure through a process called grain boundary diffusion. This has taken place on a large scale and all of the imperfections align in a plane. The rock is weaker in this plane and therefore, preferentially weather’s along the plane of imperfection. This creates the pancake effect. Normally the environment is disturbed in some way or doesn’t persist long enough to generate this effect. The Maori people considered the place to be sacred and have a god that protects the area.

We stopped for lunch one day in a cave at the mouth of the Fox River. The cave was very puzzling to us and we spent a couple of hours trying to figure out what was going on. The rock was obviously sedimentary because there where rocks within the whole rock that were cemented together. The rounding, sorting, grain size, and clast type all varied throughout the cave. After thinking all through lunch what could be going on, enjoying my increasingly elaborate sandwich, Karoly and Nick explained that what we are looking at was the result of a lot of fill generated from the detachment fault slipping and extension occurring. Along the fault you will create relief and this creates potential for landslides and erosion to fill the fault. After burial and compaction you are left with a breccia. This represented one of the bookends of the metamorphic core complex and we now knew how to orient the structure for our individual projects.

South of the metamorphic complex, we did an afternoon mapping project along Three Mile Beach. The beach represented an offshore sedimentary package that would represent a continental shelf setting. This can be determined by the sequence of sediments found in the rock. The sequence is referred to as the Bouma Sequence. We saw a repetition of this as we walked along the beach, which was to be expected. After a little further distance, however, the sequence was still there only in reverse. This meant that we must have walked across the fold axis, and we were now on the other side of either a U-shaped syncline, or an A-shaped anticline. Within the group we had debate over whether or not there were two folds present at Three Mille Beach. The scale with which we were working with made it difficult to completely conclude what was going on but the general consensus is that there was one fold with perhaps some parasitic folding as well. We managed to leave just as the rain started to come.

The remaining key locations for the metamorphic core complex were the Meybille Mylonites and the Cape Foulwind Granite. The mylonites were found along a beach that required it to be low tide in order to access it. Sometimes we had to go through the bush to get to the next beach and we were introduced to some of the most inhospitable vegetation New Zealand had to offer. I wore shorts that day and came away with considerably less leg hair than when I started. Once on the other side of the “jungle”, we encountered these huge blocks of rock on the beach. The rocks were metamorphic, meaning that the grains had been recrystallized. In metamorphic rocks, you can often observe the direction of strain and stretching by looking at features in the rock that used to be round. A previous existing round clast when pushed from one direction on the top and another on the bottom will start to smear and roll at the same time. Grains like this are called shear sense indicators because it can tell you which way movement was occurring.

As we left that beach, we picked mussels that were attached to the rocks. Karoly would cook them up for dinner and we’d be eating them about three hours after picking them. The Cape Foulwind Granite represented some of the igneous intrusions that can occur as the crust is thinned and magma rises up. This granite was unique in that it had phenocrysts or extremely large clasts of feldspars. This would have required fluid to be present at the time of emplacement. The cape is called Cape Foulwind because of the seal colony that resides there. As it turns out, seals smell really bad and a lot of seals smell even worse, so the cartographer felt it necessary to point out. The seals were fun to watch playing in the surf, but their stench was not welcome. When we returned to the field station we started our individual projects.

After lots of coffee, book reading, reviewing notes, study breaks, discussions, and six hours I came away with a stereonet, a cross section, and a poster illustrating the geologic history of the west coast. The stereonet is a way of compiling your structural data that includes the orientation of the rocks of interest, in such a way that you can start to make assumptions and formulate an idea of what is going on with the big picture. A cross-section is a diagram that predicts how the different units of rock will match up together at depth. This takes into consideration folding, faulting, and stratigraphic relations. Two of the other students finished around the same time as me, and we decided to walk to the beach and stick our feet in the ocean. In a short time we had been to about 9 different stops along a 150 km stretch of coast line and came away with a very good understanding of what had taken place. New Zealand was starting to come into focus and we felt a sense of belonging. The trip had just started to pick up the pace and we would be doing our big mapping project next. I had been looking forward to the mapping portion of the trip because it’s where you can really show your strengths as a geologist. In the next blog I will go into the mapping project in detail.

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