Many stony beaches near Mackay have an exceptional diversity of rocks. Bedrock exposures also reveal a complex mixture of rock types. How did the regional geology become such a complex mess?
|Rocky beach on the 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 a dozen different rocks types and these rocks can be interbedded of mixed up in complex ways.
|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.
|Red Cliff Island near Seaforth|
The Mackay Coast and nearby Whitsunday Coast are the final small remnants of one of the greatest volcanic episodes known. The Whitsundays are the remains of the largest slicic igneous province on earth. Rather than issuing floods of basalt, these volcanoes 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 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.
|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 so high up into the sky 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.
|Funnel Mountain (344 m, viewed from Taffy Island) is formed of volcanic deposits have been protected from erosion by a hard sandstone cap.|
|Metasediment cliffs on Outer Red Cliff Island|
The volcanism 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. The other characteristic of the Mackay Coast is how hard igneous rock has been cracked in every direction by the stresses, similar to how ice is cracked by a moving glacier. Weathering along these cracks may be the reason behind the jagged boulder beaches we see today. Rifting also led to the creation of some of the granitic mountains islands in the region.
Unfortunately most of the Mackay Coast’s dramatic volcanic history has eroded away and only traces remain. Cape Hillsborough is the only coastal volcano remaining and even then what remains is only a small remnant of one side of the volcano. The crater is gone. 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 near shore 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 near shore islands.
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 to clays and softer
rocks formed from volcanic ash. A single 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. Here is the geology of Taffy Island in photos.
|Fine-grained hard rocks with a dull red cortex protect the eastern end of the island.|
|Running through this area is a dull grey stripe of 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 welded by heat.|
|A single and quite large basalt bomb was present on the shoreline of the island. Behind the bomb is the soft rock derived from ash that makes up most of the island.|
|The western end of the island has cliffs of unconsolidated clay and stone|
|The top of the island has windswept grassland and horizontal rainforest|
I am not a geologist and am investigating the regional geology to understand the distribution of fauna and flora in the landscape, which is my profession. Background information used to prepare this post has been drawn from a number of sources including:
- ‘Rocks and Landscapes of the National Parks of Central Queensland’ by Warwick Willmott (book);
- Volcanic origin of Whitsundays;
- Zircons blasted from Queensland to Western Australia;
- Rocks and Landscape of Bruce Highway between Rockhampton and Mackay;
- Cosgrove Volcanic Chain; and
- Volume of material ejected by Whitsunday Volcanic Province.