Whakatane, a rocky mélange

 

Pied Shag drying its wings on a convenient rock. Whakatane Heads.
Source: Author (2017).

 

Aerial view of Whakatane Heads, 1944, showing outcrop of hard rocks subject to slower rates of erosion than surrounding sandy coastline. Source: LINZ/www.retrolens.nz (2017).

 

Detail of above photo, showing shore platform formed by wave erosion of outcrop. In the top right hand are visible near vertical sedimentary beds showing deformation. Source: LINZ/www.retrolens.nz (2017).

Statue of Wairaka at Whakatane Heads, atop the rock Turuturu-Roimata. Source: Author (2017).

While the Eastern Bay of Plenty is renowned for it's long sandy beaches and shallow harbours and sand-bars, Whakatane is unique for the rocky headland forming one side of the Whakatane river-mouth. Here can be seen rocky cliff faces and outcrops, forming a majestic backdrop to the statue of Wairaka, one of the voyagers from Hawaiki. The rock Turuturu-Roimata forms a pedestal for Wairaka, while smaller rocks make favored resting and drying spots for the pied-gannets. If you look closely at the outcrops that may appear solid as rock, you will find they are in fact brittle, sheared, and deformed, telling a story of their long journey from a distant time and place, and the forces they have endured on the way. 

Typical outcrop of mélange. Whakatane Heads. At the top of the outcrop (approx 3m. high) can be seen deformed and fractured beds of mudstone alternating with sandstone, formed by deposits from underwater landslides settling on an ancient ancient seafloor. Source: Author (2017).

Although you may have heard of a mélange, meaning a mixture or variety of food ingredients, less will have heard of a mélange made of rock.  Just like the culinary term, a geological mélange refers to a mixture, in this case a mixture of rocks that you would not typically find next to each other in an outcrop. The presence of a mélange demonstrates immense forces at play to bring these rocks together into the solid rocky mélange we find at the surface.

To get a sense of the nature and chaotic structure of the Whakatane Mélange, one of the best places to view it is in the rocky cliff faces and shore platforms at the river mouth at low tide. Keep your eyes open for deformed and “out of place” rocks that appear quite different from those surrounding them. In the cliff faces and on the shore platform you may find different types of rock broken, deformed, and fractured into pieces centimetres to meters across, forming a chaotic mix of rocky fragments suspended in siltstone. Take care though, because a close examination of the cliff will reveal the brittle and sheared nature of this rock. One of the main components of the Wakatane mélange is the metamorphic sandstone greywacke. While this rock in its undisturbed state may be hard, shearing and fracturing through tectonic processes can make it brittle and loose.This was demonstrated when a landslide triggered by weeks of heavy rainfall in 2011 exposed the deformed, sheared, and fractured nature of this rock formation forming the cliffs behind houses on Muriwai Drive. Check out the video below for footage of the landslide that buried a digger!

Freshly exposed face of rock uncovered by landslide in 2011. The layered beds of sandstone and mudstone would have originally been laid down horizontally on the seafloor, here on the left of the outcrop they have been folded, are dipping near 45 deg. on the right of the outcrop, and in the centre of the outcrop can be seen a massive greywacke sandstone breaking up the strata. Source: Author (2017).

 

The same forces at work as in photo above on a smaller scale. Boulder of coarse light grey sandstone embedded in dark grey mudstone forming the wave-cut shore platform at the Whakatane Heads. Source: Author (2017).

 

What this mélange tells us is that a large region of rocks in the distant past has been subject to huge forces crushing, shearing, and mixing rocks to produce this unique mixture. However, this process did not take place here at the Eastern Bay of Plenty coast, but far away, over 140 million years ago. Picture a huge landmass larger than the size of Australia and Antarctica combined. At that time, the rocks we find exposed at Whakatane, were on the edge of the ancient landmass called Gondwana. Heat, pressure, and fluids all conspired together to form a plastic-like mixture, as rock continued to be plastered to the edge of the continent. In time this mixture cooled, forming a mixture of rigid rock types.

Inevitably, the process of plate tectonics wrenched this slab of rocks away from Gondwana, and rifted them across the growing Tasman Ocean seafloor till they came to form the solid foundation of New Zealand. The collective term “basement rocks” identifies rocks of this age in Aotearoa, that started their journey on the edge of an ancient continent and 140 million years later form the geological foundations of our country from North Cape to Bluff.   

Map showing extent of greywacke basement rocks, extending in a belt from Whakatane, forming the axial ranges of the North Island, and the Southern Alps of the South Island. Source: University of Otago http://www.otago.ac.nz/geology/research/environmental-geology/metals-in-the-nz-environment/intro-metals-in-nz.html (2017).

  

One of the main components of the Wakatane mélange is the metamorphic sandstone greywacke. While these rocks may have started out hard, they can become highly fractured and loose over time. Light coloured veins of crystals through the rock form as it is subjected to heat and pressure through burial, causing crystals to precipitate in fractures and joints in the rock. Source: Author (2017).