Saturday, 29 April 2017

The Tortured Geology of the Mackay Coast

Many stony beaches near Mackay have an exceptional diversity of rocks. Even bedrock exposures reveal a complex series of rock types. How did the regional geology become such a complex mixture? 

Varied geology of Mackay Region
Diversity of rocks on shore of Taffy Island (Click to enlarge)
Firstly, the geology in other parts of Queensland is usually not like this. In Far North Queensland, the grand landscapes have large areas of uniform rock types such as basalt, granite and metamorphic rocks formed from hardened deep sea sediments. Stony beaches usually have rocks of similar geology.  There is nothing like the confusion of rocks seen on the Mackay Coast, where there can be several wild swings in geology within a space of a few metres. A single small island or bay may have more than half a dozen different rocks types and these rocks can be interbedded or mixed up in complex ways.

Breccia on beach at Cape Hillsborough
Breccia and basalt at Cape Hillsborough
Despite the crazy geology, the coastal landscape of the Mackay Coast has an almost painfully low topography and the geological wonders are pocket-sized rather than awe-inspiring tourism grade. The places shown in this post are mostly accessible by car and the remainder are close enough to reach in a small boat on a good day.  There are also a few islands that can be walked to on a low tide if you take care not be stranded by the huge tides near Mackay.

Vocanic ash features in Mackay region, near Whitsunday Islands
Red Cliff Island near Seaforth
The Mackay Coast and nearby Whitsunday Coast were shaped by the largest slicic igneous province on earth. Rather than issuing floods of basalt, the volcanoes of this igneous province were explosive and produced mainly volcanic ash. The Whitsunday Islands are mostly formed from water-laid deposits of volcanic ash which were compressed into stone. Time has filled the submerged calderas of the volcanoes that produced the Whitsundays with sediment and has hidden them from view. The positions of the calderas are inferred by changes in rock types on islands and the mainland.

Geology of Cape Hillborough, hardened volcanic ash island
A hollow island formed from fused volcanic ejecta
Recent scientific papers reveal the power of volcanic blasts from this province.  Some were so powerful, that sand-sized zircons were blasted up so high that they came down in Western Australia. In total, the volume of material discharged is estimated to have been approximately 1.4-2.5 million cubic kilometres. That would be enough material to cover all of present day Australia to a depth of more than 300 m. 

Geological feature in Central Queensland
Funnel Mountain (344 m, viewed from Taffy Island) is formed of volcanic deposits have been protected from erosion by a hard sandstone cap.
Just two years, it was discovered that the world’s longest continent chain of volcanoes starts at Cape Hillsborough (active ~32 million years ago) on the Mackay Coast. The Cosgrove volcanic chain as it is now known runs through inland Central Queensland, then disappears under a section of very thick crust, before re-emerging in Southern Victoria, approximately 2000 km away.  Volcanic chains are the result of a continent drifting across a plume of magma, known as a hot spot.

Geological tourist attraction
Metasediment cliffs on Outer Red Cliff Island
The volcanic episode that created the Whitsundays was associated with the separation of Australia from Antarctica and the relatively unknown other continent in our region, Zealandia. In addition to volcanism, the Mackay Coast has had intense cycles of rifting (stretching) and compression. Compression has tilted many of the originally low angled volcanic ash deposits to much steeper angles.  This tilting can be seen in the cliffs of Outer Red Cliff Island.   In other areas, solid masses of igneous rock have been cracked in every direction by the stresses.  I imagine that the process was similar to how ice is cracked by a moving glacier. Weathering along these irregular cracks may be the reason behind the jagged boulder beaches we see on the Mackay Coast today.

Rifting led to the creation of the granitic islands off the coast. Magma filled expanding rifts deep within the earth's crust and then cooled into granite. Subsequence cycles of compression resulted in faulting which uplifted and exposed the granite.  Where the surface sank due to faulting, basins were formed that accumulated sediment. New sedimentary rocks were formed including sandstone. Some of these rocks have also been uplifted.

Granite island near Mackay, Central Queensland
Foreground: Flat Top Island showing granite outcrop

Geological history of Central Queensland
Wedding Cake Rock (uplifted sandstone), Newry Islands
Unfortunately most of the Mackay Coast’s dramatic volcanic history has eroded away and only traces remain. Cape Hillsborough is part of the side of the only remaining coastal volcano. The crater has been lost to erosion and subsidence. The next nearest volcano is the Nebo Volcano which is 70 km west of the town of Sarina. There are however many volcanic features on the Mackay Coast that are associated with side vents and dykes. Most of the features of Mackay Coast and nearshore islands, including Taffy Island, seem to be a product of dykes.  Dykes occur where magma has forced its way into a crack in the overlying bedrock or compressed sediment. Heat from the cooling magma cooks the surrounding rock into a harder stone that can resist erosion and it is these rocks that comprise many of the nearshore islands.

Taffy Island
Taffy Island is located near Freshwater Point, Sarina. It is one of the few islands that I have been to rather that a specially selected example. At the eastern end of the 300 m long island is a dyke and the rocks in the contact zone have been cooked to produce much harder rocks. Moving away from the contact zone, the ground reverts softer rocks formed from volcanic ash. Volcanic ash breaks down to clay and much of the volcanic ash has turned into clay with the passage of time.  A single basalt rock, which might have been a basalt bomb lies on the beach in the middle of the island. As the nearest volcanoes are approximately 80 km away, the bomb possibly flew this distance to reach its current position.  Strong trade winds and high waves continue to shape the island today. Here is the geology of Taffy Island in photos.

Metamorphosed fine-grained hard rocks with a dull red surface protect the eastern end of the island.

Running through this area is a dull grey stripe of igneous rock, which is the dyke that many have metamorphosed the surrounding softer rock.

In areas that received less heat, the volcanic ash was only slightly hardened.

A brightly coloured rocky islet beside Taffy Island seems to be created from ash consolidated by heat. 

Basalt bomb
A single and quite large possible basalt bomb was present on the shoreline of the island.  Behind the basalt is the soft rock derived from ash that makes up most of the island. 

This is what a fresh basalt bomb looks like (Photo USGS)

The western end of the island has cliffs of unconsolidated clay and stone

Critically endangered littoral rainforest, Central Queensland.
Windswept grassland and horizontal metre-high rainforest attest to the strength of the winds
I am investigating the regional geology to understand the distribution of fauna and flora in the landscape, which is my profession.  This post has been prepared to provide an entry point to other resources which provide more detailed and peer reviewed information.  Background information used to prepare this post has been drawn from a number of sources including:

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