Hump Ridge Sandstone (Fiordland) EarthCache
Hump Ridge Sandstone (Fiordland)
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There are amazing views from the top of the Hump! You’re standing on top of the Hump Ridge Formation
(30 to 37 million years old) beside a beautiful alpine tarn, admiring spectacular sandstone tors.
Looking west over Lake Poteriteri is deepest darkest Fiordland, with some of the oldest rocks in New Zealand – granite and gneiss of the Western Province rocks, 320 to 380 million years old, formed back in the Tuhua Orogeny.
Looking east across Te Waewae Bay is the Longwood range, made of Median Batholith granites, 150 to 250 million years old.
Here in between are the relatively young Waiau Group sedimentary rocks, 2.5 to 40 million years old (with even more recent Ice Age gravels on top in some areas).
But, are these tors you’re looking at really sandstone? Or are they perhaps mudstone/siltstone, conglomerate or breccia?
For this earthcache, visit two waypoints on the Hump track. Describe and identify the rocks you see.
You may log this earthcache as a find as soon as you have emailed your answers to the cache owner. However, if your answers are insufficient, we will ask you to provide further information. If this happens you must change your find log to a note until satisfactory answers are received. Photos are very welcome, but please avoid or photo-shop out any spoilers!
Waypoint 1: This waypoint is right beside the alpine tor loop boardwalk.
You will see a plaque in memory of a local helicopter pilot.
Q1: What is the picture on this plaque, and what is its connection to Hump Ridge geology?
Immediately to the right of the plaque are several rocks lying on the ground, without lichen or weathering. Examine these rocks.
Q2: Describe the grain size and shape. From this information, would you describe this rock as mudstone/siltstone, sandstone, conglomerate or breccia?
Q3 Sandstones usually form under the sea. Can you see any other feature in these rocks which makes you think they were formed under the sea? If so, describe what you see.
Waypoint 2: A rock tor on the ridge; the track goes under an overhang on the west of the tor. Stand on the track on the downhill side of the tor and look up at the rock.
Q4: Give a general description of the layers of rock you see, including approximate thickness and grain size in each layer. From this information, would you describe this rock as mudstone/siltstone, sandstone, conglomerate or breccia?
Here's a guide to sedimentary rock names:
On the Earth’s surface, hills and mountains are always being eroded away. Rivers carry mud, silt, sand and gravel to the sea where they get buried, cemented and recrystallised. These layers of sediment harden to become sedimentary rocks – mudstone, siltstone, sandstone and breccias (or conglomerate if the gravel pebbles are rounded instead of angular)
Mudstone: grain size less than 0.005 mm
Siltstone: grain size 0.005 to 0.1 mm
Sandstone: grain size 0.1 to 2 mm
Conglomerate and breccia: grain size 2 mm to 1 m (Conglomerate has rounded stones; breccia has angular stones.)
From: “A Guide to the Geology of the Riverton- Aparima District” by Nic Mortimer, GNS Science
Now you've decided whether the Hump is sandstone or something else, maybe you're curious how it got here...?
How NZ got the Hump: blame it on the Moonlight?
Our micro-continent Zealandia separated from Gondwana 85 million years ago. Sea floor spreading in the Tasman moved us away from Australia, until movement ceased about 55 million years ago.
About 45 million years ago, Gondwana’s breakup was completed when Australia moved north and separated from Antarctica. A new spreading plate boundary developed in the Southern Ocean and propagated northeast into Zealandia. (Now the Macquarie Fault Zone between the Australian and Pacific plates.)
As Zealandia was wrenched apart from the south, faults formed. The most important was the Moonlight Fault Zone. This fault zone is still a major feature in Southwest New Zealand today - part of it is the Hauroko Fault along the base of the Fiordland mountains, through Lake Poteriteri and Lake Hauroko.
The Waiau Basin forms, sinks underwater and fills with sediment
Fault-bounded basins developed in the Waiau and Te Anau regions, on either side of the Moonlight Fault. As Zealandia sank, these basins were covered by the ocean. By 35 million years ago, deep layers of sediment had collected in the basins, up to 8 km thick in parts of the Waiau Basin. Out in the deeper sea sandstones formed. Where the sea was shallow, limestones formed from the shells of dead sea creatures (this is how the Te Anau and Clifden limestones formed).
New Zealand as it may have looked 25 million years ago. Most of the land has subsided below the sea - note that the fault-bounded Waiau Basin and Te Anau Basin are underwater.
The Sediments are folded into a Hump and uplifted
As the Alpine Fault began to develop about 20 million year ago, plate movements changed again. Instead of being stretched apart, the Pacific and Australian plates began to push into each other. This squeezed the Waiau Basin sedimentary rocks from the side and crumpled the strata into folds. The Hump is actually a huge upward fold of sandstone layers, called the Hump Ridge Anticline.
From around 15 million years ago, from Fiordland southwards along the Alpine Fault, the Australian plate dipped beneath (was subducted under) the Pacific plate. The plates kept being shoved into each other and therefore Fiordland was gradually uplifted.
As the Hump was pushed up towards sea level 8 million years ago, bivalves, brachiopods and barnacles lived in the shallow seas. This shellbed eventually formed limestone – the Port Craig Formation (5.28 to 8 million yeas old). These are the sculptured shelves of rock you can see along the beaches at Blowholes Beach and Mussel Bay (fossil shells- hence the name!).
The Hump gets a Horst
Stress caused faulting along each side of the Hump – the West Hump and East Hump faults.
The whole block of rocks between these faults has been pushed up: the top ridge of the Hump is a horst.
As you walk the Hump track you will easily recognise where the East Hump fault runs. It’s that very steep bit of the track between the Water Bridge and Stag Point, where you are holding onto roots to haul yourself up!
The Hump gets steps: Waitutu Marine Terraces
Subduction-related uplift of Fiordland is still going on – Fiordland is still rising today, at the rate of 0.5 mm per year (that’s half a metre in a thousand years).
During the Ice Ages (from about 1.8 million years ago) this continual uplift of Fiordland created a famous series of 13 marine terraces on the Hump Ridge and continuing west around the coast to Puysegur Point.
A beach would form in the usual way, the sea eroding a beach terrace over thousands of years. Then, during a colder interval, a lot of the world’s water became frozen in icefields and glaciers so the sea level dropped. Fiordland would be slowly but inexorably pushed up… and when the climate warmed and the sea level rose again, that older beach terrace had been lifted tens of metres higher and out of reach. So, the sea would erode a new beach terrace…
As the sea level rose and fell during the Ice ages, at least 13 marine terraces formed and were preserved. The oldest terrace is over half a million years old – you are standing on it at Waypoint One. The youngest terrace is about 3,000 years old. The second-youngest terrace, about 80,000 years old, is the wide flat platform where you walk along the Port Craig tramtrack.
The classic side view of the Hump shows a series of “steps” which are the raised marine terraces.
See a map of the marine terraces on the last page of C M Ward’s 1988 paper: Marine terraces of the Waitutu district .
And yes, during the Ice Ages it was cold enough to have small glaciers on top of the Hump. Okaka Hut is sited within a classic semicircular glacial cirque basin.
Sketch map of the main geological features in southwest NZ, showing the Waiau Basin (in yellow) and the Te Anau Basin on either side of the Moonlight Fault Zone (in blue). The Moonlight Fault Zone runs from the offshore Solander Basin south of NZ, right up to join the Alpine Fault at the Turnbull River.
Background Information:
Department of Conservation Hump Track information: visit link
Note that the Hump track is public with unrestricted access, although camping is not permitted within 500 m of the track or huts. Walking the loop clockwise is recommended - the climb is more measured, and there are innumerable boardwalk steps on the Luncheon Rock to Edwin Burn section which cause knee problems going downward.
Geological History of NZ visit link
Te Ara: Breakup of Gondwana visit link
A quick video version!
NCEA Level 2 Geological History of NZ (Wayback Machine link) https://web.archive.org/web/20160321052756/https://ncealevel2sci.wikispaces.com/Geology
This has lost the images, but will tell you about the Tuhua Orogeny, Rangitata Orogeny, Kaikoura Orogeny...
Otago University GEOL 252 2013. Geology of the South Island. From the Wayback Machine visit link
Excellent overview of tectonic processes, with maps showing the changes in NZ over time and a timeline of tectonic events.
Fiordland geology:
GNS NZ geology webmap visit link
The essential NZ geology reference map. See lognote below with tips on how to use it.
Geology of the. Fiordland Area. I. M. Turnbull. A. H. Allibone. R. Jongens, GNS Science.
Go to www.gns.cri.nz/content/download/5417/29723/file/Fiordland_text.pdf or find pdf download here
The basic reference for Fiordland geology.
C. M. Ward (1988) Marine terraces of the Waitutu district and their relation to the late Cenozoic tectonics of the southern Fiordland region, New Zealand, Journal of the Royal Society of New Zealand, 18:1, 1-28, visit link
See the map of marine terraces and glaciation at the end of this paper.
R.J. Norris, R.M. Carter (1982) Fault-bounded blocks and their role in localising sedimentation and deformation adjacent to the alpine fault, southern New Zealand Tectonophysics 87: Issues 1–4, Pages 11–23 visit link
Beggs, J M & Ghisetti, F C. (2006). Transpressional structures in Western Southland basins. New Zealand Petroleum Conference Proceedings. visit link
General Interest:
Viaducts Against the Sky: The story of Port Craig. W. Bird, 1998
IPENZ - Port Craig Viaducts visit link
NZ Historic Places Trust Register - Port Craig Sawmill and Settlement visit link
Additional Hints
(No hints available.)