Geo Junkets: New Zealand, North Island (Part 3)
Jesse Garnett White (USA)
Kohioawa Beach and Matatā Escarpment, Putauaki Volcano and the Kawerau Geothermal Field
Kohioawa Beach and Matatā escarpment
Kohioawa Beach, an uninterrupted sweep of sandy beach, dunes and wetlands, is directly below the near vertical Matatā escarpment between the towns of Otamarakau and Matatā. The escarpment gradually gains elevation to its highest point behind Matatā. Infrequently cut by active and inactive canyons, flowing streams debouche across the beach into the Bay of Plenty. Atop the escarpment are rare, mature pohutukawa, puriri and manuka, and various types of scrub and grass. Part and parcel of the Whakatane Graben, the terraced escarpment is composed of Castlecliffian marine sediments, remnants of the Aranuian Interglacial period. On the western margin of the graben near Matatā, marine sandstone and siltstone outcrops contain bivalves, gastropods, crustaceans, sponge spicules and microfossils, overlain by tuffaceous sediments, ignimbrite gravels and conglomerate (Nairn and Beanland, 1989).
The coastal dune and wetland areas of the Rangitaiki Plains and Tarawera River Valley near Matatā exhibit Holocene backswamp and floodplain deposits, including levees and meander sediments associated the Awatarariki, Waimea and Waitepuru stream catchments. The catchments rise from sea level to 370m elevation and drain into the Bay of Plenty. The Awatarariki and Waitepuru wetlands were destroyed in 2005 by large, storm-induced debris flows and associated floodwaters. Waimea was largely unaffected, retaining the majority of its pre-storm event character. Matatā township was severely impacted by debris flows, with over a hundred homes and properties damaged or destroyed.
Prior to the storm event, soft volcanic and sedimentary deposits over resistant ignimbrite were largely vegetated by secondary and regenerating forestland and pastoral areas. Twenty percent greater than a potential hundred-year flood, the precipitation was intense and quickly saturated soils, setting debris flows loaded with rock, rubble, soil, trees, woody debris, and other vegetation in motion. The entrained debris scoured the stream channels, leaving behind boulders greater than 7m in circumference and fallen trees over 15m long (Davies, 2005).
The view of Tarawera River Valley, Rangitaiki Plain and Putauaki Volcano (Mount Edgecumbe) from State Highway 2 between Matatā and Whakatane are stellar on a clear day (Fig. 1). Located near the eastern boundary of the Taupo Volcanic Zone (TVZ) and south-eastern flank of the Whakatane Graben Putauaki rises 820m above the surrounding landscape.
It is a young, dormant, steep sided, vegetation covered, multiple vent andesite-dacite composite cone (Nairn 1995; Wilson et al, 1995), with its last known eruption occurring approximately 1,850 years ago. There are two craters on the summit, separated by a narrow ridge. A brackish lake fills the western crater, but the eastern crater is vegetated, deep and dry. Situated on the lower northern flanks are two other craters and a small incipient cone. Common volcanic lithologies include indurated dacite, scoriaceous and altered andesite, block and ash flow, and ignimbrites (Hewitt, 2007).
Merapi-type nuee-ardente block-and ash-flow deposits associated with dome collapse are confined by lava lobes and dominated by thick lenticular channel-confined units containing charred wood (Carroll et al, 1997; Nairn et al, 1995). Radiocarbon dating of charred wood and Holocene Waimahia Tephra stratigraphy date Putauaki Volcano at 3280 BCE (Carroll et al, 1997).
Kawerau Geothermal Field
Outside the boundary of the Okataina volcanic centre, near the northeast edge of the TVZ, the Whakatane graben is infilled with rhyolite, andesite lava, ignimbrite and recent sediments deposited over a downfaulted Mesozoic greywacke basement (Bignall and Milicich, 2012; Milicich and Bardsely, 2013; Nairn and Beanland, 1989). Supplied by basement faults and fractures associated with a series of strike-slip generated northeast trending half-grabens and faults cross-cut by northwest faults (TVZ and North Island Shear belt respectively), 300°C geothermal waters ascend into the overlying volcano-sedimentary sequence. The hot fluids spread laterally into permeable zones along sub-horizontal volcanic and sedimentary layers into the Kawerau field (Wilson et al, 1995).
The Kawerau Geothermal Field, northwest of Putauaki volcano and east of Kawerau township, covers an area of 22km2 (Horie et al, 2009; Milicich et al, 2016). Geothermal development initiated with drilling in 1951 to supply the Tasman pulp and paper mill with steam power. Natural thermal areas nearby were in a state of decline prior to development. Sludge from the mill and continued drawdown of the reservoir destroyed numerous natural surface thermal features, including seeps, steaming ground, fumerols, hydrothermal vents and hot springs. With heat flow of 100 to 150MWt (Bromley, 2002) and with a current output of about 121MWe, Kawerau is currently the second largest single geothermal field in New Zealand after Wairakei Geothermal Field (Horie et al, 2009).
TVZ Geothermal Development
Near Lake Taupo, I followed some geothermal drilling-rig signs and arrows pointing the way. I’d hoped to find it and chat with some folks but was fenced out. Continuing on, I followed the above-ground piping system to a viewpoint near Alum Lake overlooking pipelines, traversing the valley towards Wairakei Geothermal Power Plant (Fig. 2). Pulling over to scan the scene, I listened to a fellow geoscientist give a lecture to a group of seniors about SO2 abatement.
Further down the road, I stopped at Wairakei Terraces thermal spa – the upper pool was the warmest. On the edge of the pool were dead bugs scorched by the hot water. Their carapaces were sticking to dry areas where tiny black ants collected them back to cracks in the rock amidst the steam. After a long soak, I went to Craters of the Moon (Fig. 3) then Huku Falls (Fig. 4).
Craters of the Moon is the result of geothermal drawdown followed by explosive eruptions and increasing subsurface steam and water temperatures. I walked around the drawdown collapse zone viewing fumaroles, dry and cracked mud pots (Figs. 5 and 6), and eruption craters stinking of H2S (Fig. 7).
At Huku Falls, a narrow gorge funnels electric- blue water towards the falls. They aren’t very high, but the amount of water going over the edge is impressive and the plunge pool is loud.
The TVZ and associated Taupo Fault Belt is rife with geothermal fields and calderas from Mount Ruapehu to Kawerau (Wilson et al, 1995). Active for the last two million years, the TVZ is the result of Quaternary back‐arc rifting and calc‐alkaline volcanism associated with oblique westward subduction of the Pacific plate beneath the Australian plate (Taylor et al, 2004;).
Continuing crustal extension (for 8 +/- 2 mm/yr) accommodates the rise of magma bodies creating positive conditions for a significant regional geothermal resource (Darby et al, 2000). Approximately 30km wide and 150km long, the TVZ total geothermal heat flow is nearly 4,500MW (Bibby et al, 1995). Fields within the region have heat flows ranging from 2 to 500+ MW(e) across west and east trends with nearly 90% geothermal fluid being meteoric origin (Stewart, 1978). Present volcanic activity is largely restricted to Ruapehu, Ngaruhoe, Tongariro, Putauaki, and Whale and White islands.
Wairakei–Tauhara Geothermal Area
Divided into two fields but connected in the subsurface, the Wairakei-Tauhara area is a geothermal wonderland of modern tourism, aquaculture, greenhouses, and industry working harmoniously. Located within the Waikato Region, east of Lake Taupo, the area is crossed by a conjugate fault system of NE-SW trending, high-angle normal faults resulting from crustal extension of Jurassic greywackes (Rowland and Sibson, 2001; 2004; Rosenberg et al, 2009; Bignall et al, 2010). This basement complex produced open fissures and the rise of hydrothermal fluids below a stratigraphic sequence of pyroclastic, volcaniclastic, and sedimentary rocks.
The Tauhara Geothermal Field, approximately 18km2 to 35km2 in area, contains a shallow geothermal aquifer with over 400 wells drilled to extract heat, steam and/or water. Similar to the Kawerau Geothermal Area, natural surface features have been impaired by long-term development. Electrical development potential is near 220MW, with over 250MW sustainable and a current capacity of 352MW.
The oldest geothermal field in New Zealand and largest generator of electricity, Wairakei, covers an area of 20km2 to 25km2 north of Lake Taupo. It is the world’s second geothermal power station, largest in New Zealand, and the first to use flash steam from geothermal water as an energy source. In over 60 years, more than 290 wells have been drilled in the field, reaching depths of over 3,000m, with laterals extending nearly 2,000m. Temperatures range from 260oC to 209oC, with recorded temperatures reaching 272oC. Drawdown at Wairakei is problematic and fluid temperatures continue to decline. Unfortunately, natural geyser activity has been irreparably damaged at Wairakei. Prior to development, pristine geological formations in the “Geyser Valley”, included geysers, alkaline hot springs and sinter terraces.
Tongaporutu Estuary, Mount Damper Falls, and Mount Taranaka
It was a long drive from Taupo to the black sand beaches of Mokua. Passing through Waireke, Whakamaru, Mangakino and Te Kuti, I stopped for a few nights in New Plymouth.
Coming down out of the forests, there was a roadside billboard that read, “The Sun is Responsible for Global Warming, NOT YOU! Do you understand?”. I thought of Rush Limbaugh and his nightmarish and outlandish non-scientific reasoning concerning anthropogenic global climate change. There are certainly sun cycles, but, as we all should be aware, denying anthropogenic ramp-up in global temperatures and associated variables is extremely dangerous.
The Tongaporutu Estuary
I travelled from Lake Taupo through second growth forests and clear cuts, past faulted sedimentary outcrops and roadcuts of the Mount Messenger Formation, and near black sand beaches surrounding multicoloured sea stacks. Views of the Tongaporutu Estuary (Fig. 8) and Mount Messenger cliffs and Patangata Island (Fig. 9) from Highway 3 are spectacular.
Near the Tongaporutu Estuary mouth, a fence line was adorned with hundreds of single flip-flops nailed to the pickets. In the parking lot, I talked with couple guys from Australia who’d just returned from the sea stacks I wanted to visit. Mount Messenger coastal cliff outcrops and sea stacks at and near the estuary expose excellent stratigraphic sections, some of which are accessible only at low tide.
Indigenous coastal forest along coastlines in New Zealand are rare. Along the Tongaporutu river and estuary, these coastal forests reside on uplifted marine terraces of Miocene Mount Messenger Formation and Holocene slumps. The formation is composed of lower continental slope basin floor fans and channel levee complexes, deposited in the Taranaki Basin intracontinental foredeep. Outcrops of the Mount Messenger exhibit lithofacies associations including:
- Massive sandstone and thin bedded mudstone gravity-flow deposits;
- Heterolithic interbedded thin bedded sandstone and mudstone high-density current deposits;
- Finely laminated thick bedded sandstone turbidity current deposits; and
- Deformed thick-bedded slump deposits (Helle, 2003).
Skirting the escarpment, I viewed twisted, folded and overturned beds of deformed slump deposits exhibiting syn-sedimentary faults (Fig. 10).
Further down the escarpment and at Patangata Island, caverns have eroded out from sub-vertical fracture sets (Fig. 11) associated with the Patangata Island Fracture Zone.
Standing on rippled estuarine sand dissected by gastropod trackways, while looking up the near vertical cliff face, the danger of debris fall was clear. As if to ram home the point, there was a decaying sheep at the base nearly devoid of flesh, covered in flies and maggots. However, wandering around and literally through Patangata Island the sedimentary structures are fantastic (Figs. 12 to 14).
Meandering down the coastline, weaving my way in and around sea stacks, viewing outcrops and the “sisters”, I was eventually cut-off by rising tidewater (Fig. 15).
I was hot, sweaty, and a bit dehydrated when I walked over to a massive slump associated with the 14 November 2016, 7.8 magnitude earthquake (Figs. 16 and 17).
The water was pouring out of a crack, clean and clear, from a fresh water spring behind a grouping of dead trees. Hot, dizzy and sweating, I stripped down to my tubby-skivvy, hippo-skin and showered in the cold water. It felt so good to cool down as I took large gulps of the best water I have ever put in my mouth. The earthquake and subsequent aftershocks caused significant damage to Elephant Rock. A famous landmark for tourists and locals alike, it “lost its trunk”.
On the way back to the vehicle, I noticed oblique, vertically bedded sandstones (Fig. 18), channel deposits (Fig. 19) and sandstone float containing rafted mud chips.
Mount Damper Falls
Leaving the estuary and river, I passed orchid greenhouses, fruit and vegetable stands, flax and grass farms, and native plant nurseries along the highway. I then drove up, down and all around on mountain roads on my way to New Plymouth. I became side-tracked on back roads in search of Mount Damper Falls, but decided I wasn’t up for the hike. From there, I wandered into an incredible experience where I found a tower of bee boxes on the side of a gravel mountain road (Fig. 20).
What an adventure to walk into a few hundred thousand bees, take a good listen and look around. The buzz was incredible. I could feel them landing all over me and flying all around. It was awesome watching them organising in flight towards the box openings and their octagonal patterns above me. They checked out my nostrils, as I decided to take the experience to a whole other level by sitting down between the boxes. I slowly sat Indian style on the grass and closed my eyes. The intensity of the sound increased arriving in circular waves. Bees all over me and crawling near my ear canals and nostrils. I lightly crinkled my upper lip and huffed out my nose to get them away. It was so fun I almost started laughing but I didn’t want to let any of them into my mouth. I’m not sure how long I sat there, but it couldn’t have been more than ten minutes. Slowly getting up, I walked back to the car brushing the beautiful, light-yellow, and fuzzy little bees off of me.
Further up the dirt road, I saw deforestation on an epic scale. I wasn’t prepared for the sight, after such a fun and natural experience. The mountains were devoid of trees, with the exception of massive Kauri skeletons under which were hundreds of grazing sheep (Fig. 21).
Literally sick to my stomach, I drove back past the bees to the near-ghost town of Ohura. It must have been quite the logging town back in the day but all I noticed were elderly zombies wandering around. I was completely turned around, way off track and going the opposite direction of New Plymouth. I didn’t realize how far off I was until I came out of the forest at a high-point near Taumarunui, where I could see Mount Ruapehu (Fig. 22) and Mount Ngauruhoe (Fig 23).
I eventually made it to back to Highway 3 near the Shell Taranaki Limited tank farm. Along the highway, the views of the uplifted White Cliffs and towering Mt. Taranaka (Fig. 24) were stunning.
And so, I arrived in New Plymouth and Sunflower Lodge late in the afternoon and hit the sack.
|The parts in this series comprise:|
|Geo Junkets: New Zealand, North Island (Part 1)|
|Geo Junkets: New Zealand, North Island (Part 2)|
|Geo Junkets: New Zealand, North Island (Part 3)|
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