Saturday, 29 April 2017

The Tortured Geology of the Mackay Coast

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.
Just two years, it was discovered that the world’s longest continent chain of volcanoes starts at Cape Hillsborough on the Mackay Coast. The Cosgrove volcanic chain as it is now known runs through inland Central Queensland, then disappearing under a section of very thick crust, before re-emerging in Southern Victoria, approximately 2000 km away.

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
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. 

This is what the bomb would have looked on landing (Photo USGS)

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:

Saturday, 11 March 2017

River Monsters in Mackay?

Soft muddy banks are like a canvas that records the animals of the intertidal forest.  Incredibly delicate feeding patterns from fiddler crabs and fin prints from mudskippers can be perfectly preserved.  And so can the prints of much larger and more mysterious creatures.

There is no guide to animal tracks in the mangroves, and perhaps it is time to make one, but first I need to figure out what creatures are making the marks.

In the brackish headwaters of a small river near Mackay, I noticed that the tidal riverbanks were perforated with triangular pits that were the size of a man’s fist.  There were no footprints associated with the pits so this rules out birds and mud crabs.  The pits were too delicate for crocodiles.  That leaves only fish, rays and turtles.  The mud is not dug out; rather a face has been pushed into the mud judging by the raised lip of displaced mud around the pit.  A mouthful of mud seems to be gulped in.  Plugs of mud with mouth prints could be found scattered around beside the holes.  Some were ribbed, suggesting the creature’s mouth had a ribbed roof or that the mud was pushed out past grooves in the creature’s mouth.  Other plugs were curved and smooth.  The plugs of mud were as big as a cast from a human mouth would be.  

Each pit is about 10 cm long and are similar to coffee cup in volume

A 'mouth' print in the discarded plug of mud.

A smooth plug, which might represent the other lower surface of the plug.
Rarely some very unusual fin prints were found near the triangular pits. The tracks were about half a metre wide.  I suspect that these are from a different creature.

Mysterious track - sample 1 (click to enlarge)

Mysterious track - sample 2

The stream was too narrow to turn the boat and there was a good sized crocodile slide just metres away, so I did not retrieve samples.  The place was heavily pitted.  Fiddler crabs were the only obvious source of food on the riverbank.  There were present in extraordinary density and could barely be bothered to hide when I approached. 

Crocodile slide with polished belly patch and scratches from a largish croc.

The density of the pits in this are is similar to the density of fiddler crabs
Since then, I have started to see the triangular pits elsewhere.  On the tidal flats, there are also signs that fiddler crabs are on the menu.  Holes of a slightly different shape are present along with long cylindrical plugs of mud.

Round holes with fang grooves and long plugs of mud 

Whatever makes these strange marks is likely to be very hard to observe.  It is difficult to sneak up on animals in shallow waters, and the turbidity of the water makes using underwater cameras a challenge.  If you know which creature makes these tracks, please leave a comment.

Friday, 3 March 2017

Postcard from Clairview, Central Queensland

After traveling for hours through dry cattle country on Australia's national highway, suddenly encountering an uncanny light blue sea at Clairview makes a big impression. The township of Clairview is a string of houses along a beach without even a petrol station or a shop.  However, there is a caravan park where people can and should stop for a day and experience the late afternoon vista from the beer garden on the beach.

Beach at Clairview
Thirteen islands hover on the far horizon (click to enlarge)
The bright colour of the sea is from clay suspended in the water.  Huge tides slosh back and forth in the enormous and famously muddy Broad Sound which lies just to the south.  Mixing of these muddy waters with the clean blue Coral Sea creates the curious sea colour at Clairview.

Colour of the water at high tide in a mangrove creek
At low tide, sand flats stretch toward the horizon and hold the promise of interesting creatures to anyone brave enough to seek the low water mark. Large tides have a reputation for catching people unaware and sweeping them away. With that in mind, I thought I would take a look.

Stony shoals extent for hundreds of metres from tiny creek mouths
The seabed is like a wet desert. In places it has sand so fine that it flows around your feet like mud. Elsewhere, hard sand ripples form troughs that hold soupy liquid clay. Surprisingly most sandflat creatures like sand dollars and moon snails are not smothered by the sediment despite being immersed in it. Small sand crabs attack you in their crazy style of self-defense. Dugong feeding scars crisscross the sparse beds of seagrass. Sometimes there are turtle body prints in the mud.

Dugong feeding trails
Marine turtle body print
Portunus pelagicus
Baby Sand Crab on the defensive
The tide does not rush in so badly and anyone who can walk would be safe although it does gurgle as it flows over the rippled sand.

On the calmest of days the incoming tide stirs the sediments into milk
Near the beach is a sculptured layer of rock. All around the region, this concrete-like layer of rock also appears in the beds of streams both freshwater and marine. Vast areas of land in Central and Southern Queensland apparently have this rocky hardpan lurking just below the soil surface. It is an ignored geological feature as it is neither mineral or soil. The sole scientific paper I could find struggled to explain how and why the rock formed. It appears to be a relict from a past climate and is composed of aluminium and silicate compounds of similar chemistry to clay.  It is solid, contains embedded stones and is neither laterite or silcrete.

hardpan with encrusting oysters
Relict hardpan sculptured by the sea

Clairview Creek, Central Queensland
Hardpan studded with stones in the bed of the mangrove creek
The presence of this sheet of rock just below the surface makes the bush on the coastal plain struggle to survive as the shallow soils flip from very wet to very dry with the seasons. It is perhaps one of the reasons why Central Queensland has so little agriculture and so few people.  At Clairview the rock forms reefs that are encrusted with oysters. The hollowed and twisted stone shapes are strangely photogenic.

Definitely wear shoes, as every stone has a forest of oysters. The oysters grow vertically on the stone surface and colonies are like a shag pile carpet. Perhaps this orientation frustrates the oyster drills that feed on these colonies as moving over the colony that presents only sharp edges must be hard.

A small variety of oyster that stands on end.
Scattered along the coast and sometimes well out on the sand flats are small stands of mangroves. Some open stands of large trees resemble parkland.

Flock Pigeon Island lies just off-shore

Larger mangroves provide some handy shade
Clairview is a place of sand and sky and a milky sea. It offers a picturesque desolation and serenity.

Wednesday, 11 January 2017

The Rakali, Australia's answer to an Otter

A Rakali is a large semi-aquatic rodent that is Australia’s closest equivalent to an otter.  They are also commonly known as water rats, however this name bothers me as the animal is clearly different from a rat.  Recently the Australian government decided that it was time to refer to native Australian animals using aboriginal names and Rakali is the name used for this animal by the people of the Murray-Darling river system.  Rakali are present across most of Australia, along both rivers and near the sea.
A Rakali (Photo: Mike Trennery,
The Rakali is a major predator of crabs, particularly the larger semi-terrestrial crabs such as the crabs that live in mangroves and ghost crabs from sandy beaches.   Seeing a Rakali is very difficult as they are nocturnal and usually, they are detected by footprints.  In mangrove swamps near Cairns, it is common for the mangrove forest floor to be almost covered with foot prints, such is the scale of their activity.  I suspect that their presence is one of the reasons why mangrove crabs are mainly diurnal.  In turn Rakali might fall prey to large owls and pythons which also visit mangrove swamps.

Front feet leave star-shaped prints and back feet, long prints with webbed toes
Rakali also forage along beaches and I often see their footprints in freshly reworked sand of creek mouths.  Recently I found Rakali track on the beach at Slade Point near Mackay and decided to follow them.  The beach is almost a surf beach and is exposed to strong winds and high wind waves.  At both ends of the beach there is a rocky headland and the Rakali tracks ran along the high tide line from one headland to the other.  As the tide peaked just after dark, the Rakali must have traversed the beach whilst there were still traces of sunlight in the sky.  It is an audacious move for such a small animal to travel a 850 m distance completely exposed.  On the way across the beach, the Rakali caught and ate a ghost crab.

Remains of the ghost crab
Rakali reach approximately 1.3 kg in weight with a body length of nearly 40 cm so they are much larger than rats.   They are known to forage almost 3 km a night, however I don’t know if anyone has previously recorded them crossing of 850 m km of exposed beach in a few minutes. 

Dangerous, rogue waves often wash 10 m higher up than normal waves
Photo of Lamberts Lookout in moonlight at 8:30 pm
Lamberts Beach, which the Rakali crossed
Rakali often drown in crab and yabbi pots, particularly in freshwater.  In South Australia, net crayfish pots have been banned due to the toll they take on turtles, Rakali and Platypus.  If you want to use yabbi traps, please check on the internet how to avoid wildlife kills. Rakali may not swim as often in estuarine systems as they would be prone to attack by large fish and crocodiles.  Tides also expose their prey, so they may not have need to dive.