Thrust Creek

BY JULIAN THOMSON (GEOLOGIST)
Accessibility: EASY
A wider view of the outcrop with the two rock types visible. J.Thomson / GNS Science
The Wairarapa Fault ruptured in 1855 during the most powerful earthquake to strike New Zealand since the arrival of Europeans. At its southern end, near Palliser Bay, this fault divides. At Thrust Creek an exposure of one branch (the Wharekauhau Thrust) is visible. This is an important site in the history of the science of geology!
Then entrance to Thrust Creek, with gravels in the cliff on the right. J.Thomson / GNS Science
A thrust (or reverse) fault is where the rock on one side is pushed up and over the rock on the other side of the fault because the crust is being squeezed or compressed.
At Thrust Creek, the rocks to the west of the fault is greywacke - the ancient rock that underlies the Wellington region as well as the Rimutakas and other North Island ranges. On the east side of the fault are much younger gravels (about 50 000 years old). The west side has been thrust up over the east side and in so doing has pushed flat lying beach gravels into the vertical.
The Wharekauhau Thrust is a branch that splays out at the southern end of the Wairarapa Fault in the Palliser Bay area. It continues southwards through the sea floor towards Cook Strait. The fault displaced the sea floor and generated a 10 metre high tsunami that accompanied the 1855 earthquake that is estimated to have exceeded magnitude 8.2.
Thrust Creek is of great historic and philosophical significance, not just in New Zealand, but globally.
The eminent 19th century British geologist Sir Charles Lyell reported on the 1855 Wairarapa Earthquake, publishing in the Proceedings of the Royal Society of London in 1858.
In so doing, he demonstrated for the first time in the scientific literature, that there is a direct relationship between fault movements and earthquakes.Lyell never visited New Zealand, but he interviewed three people who had experienced the 1855 earthquake and who observed and recorded changes in the environment. Those were Edward Roberts, Walter Mantell and Frederick Weld. All three held significant roles in colonial New Zealand society.
Older greywacke rocks have been pushed over much younger gravels. J.Thomson / GNS Science
When you approach the small valley of Thrust Creek from the beach, you will pass a cliff made up of horizontally bedded gravels on the right hand side. Carry on up and you will find a rough farm track leading northwards up the hillside on the right (east) side. The fault is exposed in the bank on the right of this track, a short way up.
Because the exposure cuts through the fault lengthwise, it may take a while to work out the layout of the rocks. The older greywacke is recognised as a fairly uniform, fine grained and darker coloured rock, while the gravels contain an abundance of pebbles of different sizes. You can see how the gravel beds that are exposed just beneath the fault plane (the contact between the greywacke and the gravels), have been tilted up vertically from their original horizontal orientation. This is due to the drag effect of the greywacke being shoved over them when the fault moved during earthquakes.
Can you work out the approximate angle of the fault from the horizontal? It may help to try to get a side-on view of the fault plane if you can.
Take the opportunity to explore the area around Thrust Creek. The Wharekauhau Fault extends beneath the beach (where it is hidden by recent gravels) and into the sea bed, alongside the coast southwards towards Turakirae Head.
Directions/Advisory

From Featherston, drive south along Featherston Creek Road, and continue where it becomes Western Lake Road, all the way to Ocean Beach. This is about 42 km and takes about 40 mins. Follow along Ocean Beach (drive if you are happy with using the gravel road) or walk for 2kms, past a stream that you will cross after about 1 km.

Accessibility: EASY

Gravelly terrain, but of no great difficulty

Features
Sedimentary Active Fault Rock Deformation
Geological Age
The greywacke rocks are Late Jurassic in age, 145 to 155 million years old, whilst the gravels are much younger, about 50 000 years old. The fault is thought to have been active for at least two million years.
Zealandia Evolution Sequence
Pākihi Supergoup: 5 million years ago – present