Titahi Bay North, bedrock structures

BY JULIAN THOMSON (GEOLOGIST)
Accessibility: MODERATE
Folded Bedrock at Titahi Bay, Photo J.Thomson @ GNS.Science
Faults, folds and sedimentary structures can be seen in the local greywacke bedrock.
Bedrock at Titahi Bay, Photo J.Thomson @ GNS.Science
This coastal area shows ancient basement rocks (older than 200 million years). They were formed in the ocean off the coast of Gondwana, before New Zealand ever existed. The lighter coloured sandstone layers are coarser grained than the darker, more fractured siltstone layers. These alternating layers are referred to as turbidites and are the product of marine avalanches (turbidity currents) bringing sediment into the deep ocean down a continental slope. Each turbidite consists of a lower sandstone layer, fining upwards into siltstone (argillite).
After millions of years of burial, compression, tectonic movement, uplift and erosion, the rocks are again exposed. Originally they were horizontal, now they are tilted vertically, highly deformed and dislocated, with many folds and faults visible. Faulting often weakens the rocks and makes them more susceptible to erosion hence you will often see a fault associated with a gully or cave where the crushed rock has been eroded out.
Faulted Bedrock at Titahi Bay, Photo J.Thomson @ GNS.Science
Notice the sedimentary layers that look like stripes of rock, tilted up vertically.
There are many faults cutting across the shore. They can be spotted by looking for places where the sedimentary layers have been offset (pushed sideways) along the fault plane. Some of the faults branch and some of them cut through the sedimentary layers at different angles. What is the biggest offset that you can find?
Have a look also for folds in the rock. There are some good examples here.
A close-up look at the rock layers may reveal some sedimentary structures (look for variations in textures, shapes and colours) such as very small scale cross-bedding and graded bedding. These tell us about the processes of sedimentation, such as the direction and strength of the turbidity currents.
There are also good examples of box weathering in places.
Directions/Advisory

Drive to Titahi Bay and continue until you reach the ramp at the right (north) end of Titahi Bay beach. Turn right here onto Richard St, then first left, then right. There is a parking area on the left at the start of Terrace Road.

These cliffs are unstable. Avoid standing under them unnecessarily or scrambling over them. Sturdy footwear advisable.

Accessibility: MODERATE

The area of this GeoTrip is a short (5 minute) walk from the parking area. Walk along Terrace Road for 100 metres to the top of the cliff and find a very steep track down to the shore and head to the south (left) for about 100 metres.

Features
Sedimentary Rock Deformation
Geological Age
Late Triassic, over 200 000 million years old. These rocks are part of the Gondwana history of Zealandia.
Zealandia Evolution Sequence
Eastern Province (Mesozoic growth): 300 – 110 million years ago