Thursday, 13 July 2017

Salt Pans of Wunjunga

Bowen famously has a salt mine at the entrance to the town. Tourist information used to impel people not to be put off by the ugliness of the salt flats and to continue through to the town. Yet in their own way hypersaline environments are beautiful and interesting. The only place I have found a publicly accessible salt pan which dries to a salt crust is at Wunjunga, which is just south of the Burdekin and on the Bruce Highway.  Driving into Wunjunga during the wet is like driving through a lake.

In the wet season, the salt pan is a shallow lake
Mosquito larvae are present in numbers so don't go there at night if you value your blood

A freshwater fish (spangled perch) swimming on its side in shallow water in an attempt to find freshwater
During the wet season when rains are heavy, the salt flats on the inland side of the salt pan complex drain slowly and become temporary freshwater wetlands.

Bird life is intense, so many birds and so many species
Some birds are timid but others will tolerate a careful approach
Closer to the coast tidal influence dominates and signs of freshwater influence are lost. Just before the beach is a shallow basin that is a saltwater lake during the wet season (shown above), and a bare salt pan with a small hypersaline lake in the dry. Large salt crystals form patterns in this lake.

The shallow lake almost completely dries

The bottom of the lake is encrusted with large cubic salt crystals

Delicate petals of salt spread over the surface
On the bare dry silty surface exposed by the drying lake there are unexpected signs of life. Every solid object is encrusted in barnacles, all dead as the lake fills seasonally and not with tidal cycles. A non-tidal saline lake with barnacles is not a common thing. The only other occurrence that I could find was the Salton Sea, a lake in California that is 70 m below sea level. The beach there is actually composed of barnacle shells. At Wunjunga, meat ants range across the bare flats and probably consume the dead barnacles, leaving empty shells.

Barnacles encrust every surface
A large green tiger beetle was running around in a completely bare expanse. Tiger beetles are predators and I assume they specialise in feeding on insects that land on the salt pans. When I tried to catch the beetle, it ran so erratically that it was almost impossible to catch. Tiger beetles can fly as well as wasps but their running skills are even better.

At the edges of the bare flats, succulent Tecticornia shrubs resemble staghorn coral. Their flowers are almost microscopic yellow specks that poke out between the stem segments. After colouring up, the stems shrivel, releasing segments that contain seed.

Tecticornia going purple

A diversity of succulents is present
Small banks of raised ground lie within salt pan and support an array of grasses and succulents. These areas are important to nesting birds and with their light silty soils are rather delicate places. Tread carefully in these places and look for animal tracks.  For such a harsh environment, there is plenty to find if you take the time.

Wednesday, 21 June 2017

Striped Possums in the Mangroves

Near Cairns, most of the mangrove swamps appear to have a good population of striped possums. The possums leave distinctive marks on dead trees, when they chew holes into the timber to get at the beetle grubs inside. Sometimes the possums leave great pits in the sides of trees. Mangrove timber is very hard and I often cannot even mark the sides of the pits with my fingernails. It must take the possums a great deal of effort to get at the beetle larvae or witchety grubs (moth larvae).

Striped possum feeding marks
Dead mangrove at end of boardwalk with possum damage
In June 2017, I was able to make a video of a striped possum (Dactylopsila trivirgata)  breaking open the bark of a mangrove tree on the Cairns Airport Mangrove Boardwalk.

During the day, the mangrove forest is revealed to be a dense, 20 m tall stilt mangrove forest. This forest has no connection to any terrestrial areas, not even stranded beach ridges, so the possum must live in the mangroves entirely. This makes me wonder how it obtains freshwater and enough variety of food to survive. The possum I observed appeared to be fairly small for a striped possum and was so hungry that it completely ignored me even though I was standing about 4 m away with a bright light.

Mangroves beside Jack Barnes Memorial Boardwalk
This tall stilt mangrove stand is about 250 m wide and 750 m long 
This particular trip into the mangroves taught me quite a lot. Firstly, there are very few flying insects in the mangroves at night (other than bloodsuckers). There also seemed be very few spider webs. Bright eyes revealed occasional wolf spiders on tree trucks. Wolf spiders jump on prey, rather than using webs. Only one sleeping bird was observed and one large Papuan Frogmouth.

Podargus papuensis
Papuana frogmouths are large and have red eyes
It is counter intuitive for a place with such exuberant vegetation to be so devoid of wildlife. However, on reflection, very few insects can eat mangrove foliage and even then, they usually only take a few bites, so it makes sense that the terrestrial food pyramid is virtually absent. Mangroves have a detrital food web that is based on plant material that has died and been reprocessed by bacteria and fungus into a less toxic form. Normally mangrove leaves which have fallen partially decompose and are then consumed by crabs and shrimps. 

Perisesarma messa feeding on fallen mangrove leaves
In a way, the striped possum is also part of a detrital food web. It feeds on beetle larvae that burrow into dead timber and spread the spores of fungus, which grows and provides food for the beetle larvae. When watching the video of the striped possum, it almost looks like the possum is either drinking or feeding on the fungus lining of the beetle tunnels in addition to feeding on the beetle larvae. Perhaps someone should research this someday.

Striped possum feeding marks
Marks created by striped possum in video

Saturday, 6 May 2017

Most Beautiful Fish in the Mangroves

Sometimes floating mangrove leaves are really fish in disguise. Batfish take pretending to be mangrove leaves to an extreme. This post shows a Round Batfish (Platax orbicularis) that I found on the Cairns waterfront, in North Queensland, Australia.

A round batfish beside a floating mangrove leaf 
During April, the surface waters of harbour are alive the juvenile fish and mangrove-leaf-like batfish are suddenly present. They lie on their sides and almost drift passively with the current, making only slow motion movements to capture small food items.

The fish's face would often come out of the water when it was feeding

The fish would only be vertical when turning

Mangrove leaf mimick
So great was the fish's faith in its disguise, that I could almost poke a camera in its face.
In the few hundred metres of the Cairns waterfront, perhaps half a dozen are present. Naturally, I would prefer to be searching for fish in pristine mangrove wilderness, but my experience is that the marina is by far the best place to see fish that mimic mangrove material. Mimics seem to be more abundant in the outer estuary which has large tidal flows. Mangrove flotsom mimics are difficult to find in the long mangrove creeks where the same water moves back and forth within the creek instead of being flushed and replaced with new water on each tide. As the marina is at the mouth of the harbour and the floating concrete fingers trap floating objects, ideal conditions for observing mimics are present. Still, it is no coral reef and the pursuit of mimics needs a long attention span and time to waste. Many species of juvenile fish are only present for a few weeks of the year as they grow quickly and move to new habitats

Batfish begin life in the mangroves but live on coral reefs when they are mature. Where they come from before they become 'mangrove leaves' is not clear. I think that they first appear as the small dark fish that lurk in the shadows under the floating fingers of the marina. The dark young batfish swim vertically and are hard to photograph from above. Perhaps they mimic dark mangrove detritus, which sometimes swirls around in eddies below the surface. Fins turning orange is the main clue that these are juvenile batfish.

In Cairns Harbour, there are two species of batfish that pretend to be mangrove leaves, the Longfin Batfish (Platax tiera) and the Round Batfish (Platax orbicularis). There are also a several other fish that pretend to be bits of floating vegetation including sea grass, green leaves or pieces of bark. Perhaps, there are about 20 vegetation mimicking fish species in total.

Long-finned bat fish swimming around floating concrete fingers

Platax tiera juveniles

Below are some (suitably licensed) photos from the net that show how the batfish change after they leave the mangrove environment. Sometimes the adults return to the mangroves and can be seen swimming around the mangroves by snorkelers.

Longfin batfish in transition to adult form

Platax tiera
Mature longfin batfish

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:

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.  They 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.