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Mangroves are a most common shrubs or trees especially near the coastal waterways in the tropics and sub-tropics, but mangrove species become less frequent in southern parts of the mainland and are not part of the Tasmanian floral makeup. As a group they have an interesting name, arise from a mixture for plant families and because of the habitats in which they grow have several unique features. Following are illustrated features of mangroves with the majority of images from the Kimberley coast, Western Australia (W.A.), supported with others from the Northern Territory (N.T.), Queensland (QLD.), New South Wales (N.S.W.) and South Australia (S.A.).

The word mangrove

The interesting point about the name mangrove is that it refers to both individual species that live in a particular habitat as well as the communities they form. In broad terms there are 41 different mangrove plant species found in Australia and their shrub and tree forms are all limited to tidally influenced habitats with most of these being in the tropical and sub-tropical bays, coastal lakes and river estuaries. Occasionally readers may come across the word mangal, this is a less commonly used alternative name for a mangrove community.

Botanical descriptions such a eucalypt, wattle or saltbush, refer to groups of species in Australia in three particular plant families ,respectively, Myrtaceae with about 700 eucalypt species, Mimosaceae with 980 wattles species and Chenopodiaceae with 60 saltbush species. Australian mangroves however belong in 19 diverse families that have one or more species that has adapted to grow within the mangrove habitat. The Rhizophoraceae family is the largest and includes three genera, Rhizophora (stilt mangroves), Ceriops (yellow mangroves) and Bruguiera (orange mangroves), each being represented by several species. At the other extreme Osbornia octodonta (myrtle mangrove) and Excoecaria agallocha (milky mangroves) are represented by just a single genus and single species and belong respectively in families Myrtaceae and Euphorbiaceae

The value of mangroves

Given that mangroves occupy tidally influenced waters and are associated with estuary river flats and sheltered bays they may become less pleasing to look at as a community at low tide than when flooded. Such sites were in the recent past often considered to be smelly (explained later) unattractive lowlands.  Particularly in well populated Australian locations, such sites were often ’developed’ using landfill like sand and soil then used for ‘better’ waterside purposes such as parklands, housing estates with canals and factory building sites. In most cases planners and developers have seen the error their ways as the positive ecological values of the tidal flats and mangroves become widely known and accepted.

Ecologically, mangrove communities are extremely significant and a study in Darwin Harbour, cited by Norman Duke in Australia’s Mangroves, identified 9 species of bats, at least 11 other mammal species, 128 species of birds, and around 3000 species of invertebrates e.g. insects and spiders, all living as mangrove canopy dwellers. Aquatic fauna and organisms in the mud as sampled in the Darwin Harbour study found 36 species of crustaceans and 31 mollusc (shell fish) species associated with mangroves.

Crustaceans like crabs are most important since many they take large quantities of leaf material below the ground to be used as food.  And as most recreational and commercial fishers are aware many fish species in their juvenile stage live within the mangroves.

Molluscs and fish as a food source, found withing mangroves, have been utilised by Indigenous people. Species of the Teredo worm that live in dead mangrove stems and trunks are also sought for food and medicinal purposes. The wood of some mangroves was used for making spears and boomerangs and naturally straight buoyant trunks of Camptostemon schultzii (kapok mangrove) was favoured in the Kimberley to make raft like watercraft.

Some characteristics of mangrove habitats.

Saturated soils  Mangroves mostly live in soil that is regularly saturated. This means that all the pore space in the soil is filled with water and not the mixture of air and water as is the case for most other plant communities. With all the air replaced with water, anoxic (Oxygen free) conditions exist under-ground and mangroves have developed alternate appendages to overcome the problem of no soil Oxygen.  Possibly the most obvious of these adaptations are the long, looping aerial roots as seen in stilt mangroves species in the genus Rhizophora. The stilt roots have many lenticels over their surface allowing for gaseous exchange -oxygen in and water and carbon dioxide out. Lenticels are small pores, occurring as raised sections formed by cells with lots of intercellular space.

(Above) Two examples of arching prop roots of long-style stilt mangrove (Rhizophora stylosa).

Pneumatophores are another adaptation of the root system They too are covered with lenticels and grow up above the soil level that is exposed with dropping tides. These grow up from cable roots that run out horizontally from the base of the plant as seen with grey mangrove (Avicennia marina). In this species pneumatophores are pencil shaped and generally less than 30 cm tall. With apple mangrove (Sonneratia alba) pneumatophores are more robust and cone shaped.

(Above) Two examples of the sharp triangular pneumatophores of apple mangrove (Sonneratia alba).

Other adaptations are knee roots where the plant forms tight loops in roots the rise above the ground surface, while other species have lenticel covered buttress shape roots formed at the base of their stem or trunk.

Break down of organic matter  Bacteria are important organisms in the decomposition of soil organic matter such as chewed up fallen mangrove leaves that have been taken underground in mangrove systems by crabs. In the anoxic (oxygen depleted) environment of saturated soils, aerobic (oxygen requiring) bacteria cannot function and are replaced by species with an alternate form of cellular respiration.

In aerobic organisms (including ourselves) respiratory energy production during cellular processes relies on a food source like carbohydrates and Oxygen.  Following a complicated set of enzyme-mediated chemical steps energy from the breakdown of carbohydrate is released along with Carbon Dioxide and water. Hydrogen is produced during the chemical breakdown and the cellular enzymes combine the Hydrogen with Oxygen to form the water.

In a summary:      Carbohydrate + Oxygen            →         energy release + carbon dioxide + water

In anoxic conditions anaerobic bacteria take over the respiratory process to breakdown carbohydrates (from chewed up leaves) but they have enzymes that substitute Sulphur not Oxygen to combine with the Hydrogen. In this case, Hydrogen Sulphide, H₂S or rotten egg gas, rather than water H₂O, is produced.

In a summary:   Carbohydrate + Sulphur        →     energy release + carbon dioxide + hydrogen sulphide

When soil is disturbed in mangrove habitats, we often get a whiff of the Hydrogen Sulphide! Alternatively, as the tide rises, water entering the crab holes or cracks in the soil. may push out the smelly gas H₂S.

The influence of tides  Most Australian mangroves have a habitat that is flooded at least once daily and many parts of the coast, with semi-diurnal tides, are flooded twice a day. Interestingly the range between low to high tide levels varies from day to day and so we have spring and neap tides.

Spring tides are a response to full moon and new moons while neaps fall between these two lunar phases. In summary during there are two spring tides and two neap tides each 29.5 days.  During spring tides, as opposed to neap tides, the range between high and low tide increases. Shorter plants growing close to the water’s edge are therefore completely immersed in saline water by high tide on most days and it is the only the inner, landward margin of the mangal that is flooded during spring tides.

Whereas close to the open water plants may be flooded on more than 45 events per the 28 day lunar cycle, this reduces dramatically to less than 20 on the upland margin.  Plants in the intermediate area will be flooded by normal high tides at a frequency between these two extremes. 

In practical terms the soil surfaces in mid to lower tide are flushed with water during the ebb and flow of tides but soils above this on the landward upland margin of the mangal are only wet during the highest of the spring tide. Soils on these upland margins tend to become more saline and they accumulate salt following evaporation of the water once the tides have receded. In effect, the surface dries as the water is evaporated leaving the salt.

These differences in exposure to tidal flooding and salinity influence where particular mangrove species are able to grow so that plants of the same species are not randomly distributed but, in response to their habitat preferences, grow together. An aerial photo of a mangrove lined shore demonstrates this phenomenon with similar species forming bands of similar leaf texture, height and colour generally occurring parallel to the shoreline.

Salinity and position in the estuary  There is also a gradient in salinity distribution along an estuary and this too means that different associations of mangrove species occur along the length of an estuary.

During the dry season of northern Australia for example there is reduced inflow of fresh water to estuaries from creeks and rivers. A stand of mangroves at the head of an estuary is a long way from the coast and at this point there will be reduced exchange of tidal water. These factors combined with evaporation from the estuary leads to more saline conditions. Closer to the estuary mouth however tidal exchange is more complete so salinity here remains much as it is in the open sea.

Adaptations to growing in saline water  Some mangrove species have the ability to resist uptake of particular minerals (mainly sodium chloride) by their roots while others have developed glands that excrete salt crystals onto their leaf surfaces, some of which are likely to be the washed off during high tides and rain. Salts may also be removed during leaf fall and the shedding of bark.

There is the only one deciduous mangrove and it grows in the mid tidal position of river estuaries across northern Australia. This species the cedar mangrove (Xylocarpus moluccensis), changes leaf colour from green to yellow, red and orange and losses leaves during the dry season. There are suggestions that this function may be another adaptive mangrove strategy to remove accumulated salt.

Dispersal of species following seed setting. There are several mechanisms used by species to distribute offspring and one involves the production of a buoyant, woody round fruit that eventually break up to release the seeds after they have fallen -both Australian Xylocarpus species have these rounded buoyant fruits that split into four parts releasing seeds.  Alternatively, many mangroves are viviparous where seeds germinate and partly develop while still attached to the parent plant. The seedling so produced is called a propagule and the stage of embryonic development reached before release depends on the species. The yellow mangroves Ceriops, produce seedlings with slender embryonic stems called the hypocotyl that is about 15 cm long in C. australis, 27 cm for C. decandra and is ribbed and up to 35 cm long in C. tagal.  In the closely related Rhizophora species the hypocotyl is also buoyant but is 65cm long in R. stylosa (long style stilt mangrove) and around 80 cm in R. mucronata (upriver stilt mangrove).

These propagules are buoyant and rely on the tidal movement to disperse once they fall from the parent plant. After varying periods in the water they may be stranded and the partly developed roots on the tip of the hypocotyl may anchor the seedling in what may or may not be a suitable habitat for the development of an adult plant.

Distribution of Species in Australia.  Whereas all mangrove species and most of their varieties grow in tropical waters, some like Rhizophora mucronate grow in the wet topic estuaries toward the tip of Cape York Peninsula. One like Avicennia integra, the stilt grey mangrove, only grows in estuaries of the Northern Territory with its definite wet and dry seasons. This area like the Kimberley and other parts of the Queensland coast have a monsoonal climate characterised by a wet season during the hotter time of the year and a dry season in the cool months.

In the arid tropics of the Western Australia such as the coastline of the Great Sandy Desert, there are up to nine species but further south the number of species is reduced with only one in the temperate south, Avicennia marina (grey and white mangroves).  A similar pattern of distribution is found in eastern mainland Australia with a diminution of species southward. In some bays, inlets and gulfs of Victoria and in South Australia one of the varieties of A. marina is the only mangrove species represented.

Many Kimberley travellers particularly those visiting by sea, are generally fortunate enough to travel inland along estuaries like that of the Berkeley, King George, Hunter, Sale or the Prince Regent rivers. This article presents two contrasting river estuaries, one where the now flooded valley has been controlled by many angled joints as seen along the King George River, the other the Prince Regent River where the estuary follows a single straight joint. The King George River ends in Koolama Bay and the Prince Regent River terminates at Saint George Basin. Features of both the bay and the basin are considered in a separate Background Briefing article.

During their formation the course of the river valley is determined by many factors as run-off rainwater follows the effect of gravity heading downstream toward the sea. Water may run over rock surfaces that vary in their resistance to the flow such as a ‘tough’ quartzite or a ‘soft’ siltstone. The resultant depth of the are rills, gullies or valleys formed in less resistant rock is likely to be deeper than that which is resistant to weathering.

As modern day travellers we are unable to follow these early stages of river valley formation in the Kimberley but two of the Kimberley estuaries, the King George and the Prince Regent rivers provide superb examples where after many, many million years the pattern of the rock jointing has had a major influence on the shape of the resulting valleys and the estuaries they now fill in their tidal section.

A primer – rock joints, weathering and erosion, land uplift and sea levels.

By definition, joints are rock fractures where either side of the fracture has not been substantially displaced laterally (to the left or right) or vertically (upwards or downwards). If displacement were to occur the fracture is referred to as a fault.

Running through most of the Kimberley rock surfaces are deep joints a feature that is not immediately evident to the average observer walking across a landscape. However, when identified by field geologists and mapped from broadscale aerial images taken from aircraft or satellites, we see joints that are quite numerous and follow a pattern. Although the joints may vary in length, they are mostly straight and over the Kimberley are oriented in either SE (southeast) to NW (northwest) or a SW (southwest) to NE (northeast) direction. It is believed that the alignment of the fractures in the rocks initially resulted from tremors emanating from earth moving tectonic forces that were building mountains along the east-south-east and the south-south-western edges of the Kimberley block 1,000 million years ago.

Under the influence of gravity runoff water flows downwards and this may be interrupted when it enters a waterbody such as a lake and ultimately stops at the level of the sea. During the flow water may be channelled into joint fractures and low points where frictional forces of the water tend to wear way the rock particles so making the joint wider and deeper. Any previously weathered rock sediments carried by the water increases friction thus accelerating weathering and forming deep channels. Should any crossing, intersecting joint offer a ‘better’ downhill direction the flow will match the new direction. As a result, water flow may follow joints that take a zig-zag route so determining the channel direction of the resulting creeks and rivers on their way to the sea.

Uplift of the Kimberley has occurred in the past on a couple of occasions many 100s of million years ago. A result of these massive uplifts the rivers too were uplifted and had further to drop to reach sea level. In terms of their land forming processes, rivers have rejuvenated powers of weathering and erosion so uplift is likely to result in the deepening of river channels. Cutting down to the base level by rivers was therefore determined by the sea level. With low sea levels in the past, river channels near the sea were much deeper. When sea levels rose again these valleys were flooded.

Most of the weathering and erosion events to produce today’s landforms were stimulated by an uplift just 20 million years ago.

Globally sea levels are intimately connected with temperature averages across the Earth. During cold conditions the volume of water in oceans and seas shrinks and in extremely cold water ice sheets and glaciers form so the liquid water is tied up as a solid thus reducing the level of the oceans and seas. Over time such glacial/ice age conditions may alternate with warmer interglacial) times when water expands, and glaciers and ice sheets melt. Phases of glacial-interglacial events have occurred multiple times during the relatively recent geological Pleistocene epoch that lasted over two million years

With very few mainland glaciers and being distant from the Antarctic, Australia contributed little to the freezing and thawing of water but by connected to the sea was still influenced.

The last ice age in this sequence was the most intense and is referred to by earth scientists as the Last Glacial Maximum (LGM). During the LGM Oceans around Australia saw water receded by a depth of 120m compared to today’s average high tide mark. Around the country this exposed the continental shelf which, for parts of the Kimberley, meant that the rivers valleys extended over the exposed continental shelf for up to 320km to deposit water in the Timor Sea. By way of contrast the narrow shelf as found along most of the NSW coastline had only few 100m exposed.

Timewise, the LGM occurred about 20,000 ya (years ago) and the interglacial melt began about 17,000 ya. As sea level rose the river valleys were flooded until the temperatures and the melt stabilised around 6,000ya. Existing valleys penetrated inland to varying distances so their lower section became tidally flooded and referred to as estuaries.

It is interesting to contemplate that Aboriginal ancestors lived through the colder conditions of the LGM and experienced the exposed shelf, and then the loss of land as the seas rose.

See ‘Background Briefing -overview of Australia’s Kimberley’ for further details.

King George River estuary the twin falls

King George River enters to Timor Sea after discharging into Koolama Bay in the north-eastern corner of the Kimberley. In the SW corner of the bay this river estuary remained hidden from the sailing explorers including Nicolas Baudin in 1803 and Phillip Parker King 1820.

It was not until 1911 that the upper part of the King George River was discover discovered and named by Europeans with an accurate prediction by the discoverer, C. Price Conigrave, of where the river would terminate i.e. Koolama Bay, then known as Rulhieres Bay.

In the eyes of the Traditional owners the lower reaches of this river formed the border between the country of the Miwa speaking people to the west and Kwini country to the east. Since Native Title Land ownership was finally determined in 2013 the area is under the control of the Balanggarra Land Corporation, for the Balanggarra Traditional owners.

Naming some local features

Koolama Bay was named after the ship Koolama was beached here during World War Two. Prior to this it had unofficially been called Rulhieres Bay after the rocky Cape Rulhieres on the north-eastern mouth of the bay and then Calamity Bay. The Cape had been named in honour the French administrator and diplomat Claude Carloman de Rulhière (1735-1791) by Nicolas Baudin (Captain of Geographe) or Louis de Freycinet (master of La Casuarina) when they sailed past in June 1803.

King George River was named by Charles Price Conigrave in honour of King George V. George V succeeded his father Edward VII in 1910.

Features of the River and Estuary

In contrast to the rest of the Kimberley coast, where tidal ranges are meso- (in the 2 to 4m), macrotidal (4 to 8 m) or even greater, this small section of the coast including Koolama Bay and the estuary is subjected to mostly microtidal conditions with just over two metres difference between the highest and lowest tides.

The low tidal range for this estuary means there is not a massive twice-daily in and outflow of tidal waters and these conditions have allowed the development of islands of sediment along the estuary that are populated by mangroves and shallower water populated with sea grasses. Under the current conditions, with the sand spit build-up near the estuary mouth makes this gap quite narrow and from a distance, difficult to distinguish from a 2.8 km long mangrove lined sandy beach to its east and the cliff on the other side

The rest of the large bay is surrounded by cliffs formed from Warton Sandstone one of the typically ‘tough’ Kimberley sandstones. This same rock lines either side to the 13 km long estuary as it zig-zags inland to terminate in twin waterfalls carrying King George River freshwater off the Karunjie Plateau.

The zig-zag channel of the river has followed the pattern of joints in the sandstone and these are aligned in either a SE to NW or SW to NE direction. Opening-up of the joints to form the river channel took place many millions of years ago as the river slowly weathered the sandstone. This has been supported by much wetter conditions in the past. At this time the current Koolama Bay is likely to have been much further out to sea and the bay itself may have been non-existent or quite a different shape without a perimeter of cliffs.

Although the Warton Sandstones were formed over 1,800 mya (million year ago) specialist earth scientists (geomorphologists) suggest that the land-forming processes on the Kimberley coast have produced most of today’s major landforms in the last 20 mya. During this period one could imagine the King George River initially discharging directly into the sea and that the earliest waterfall developed once the land had been uplifted. It is the processes of weathering (wearing away) and erosion (removal of weathered products), brought about by moving water, that have enabled the formation of a deeper cliff-sided channel and the progress of the waterfall inland which has formed this estuary.

In traditional Lalai stories these majestic falls were created by a male and female rainbow serpents, Wungkurr, who travelled from their distant country to the west, the Sir George Moore Islands. As they travelled inland the river was created.

Today the estuary is over 13 km long and ends in a pair of waterfalls pouring over 8Om cliffs. This long drop particularly during the wet, creates two deep plunge pools beneath falling water. On one occasion echo sounding measurement found the river to be 62 m deep below the eastern most fall. As the river flow decreases following the wet, flow in the west fall creases but that on the east continues though reduced to trickle at the end of the dry. Elsewhere the estuary channel depth varies considerably with most water depths less than 10 m with very moderate tidal effects.

The cliffs both around the Bay and lining the estuary expose the very blocky nature of the weathering Warton Sandstone. The blocks vary greatly in size but tend to have relatively straight margins and in places look as if they were engineered and positioned by modern cranes or past giants! The horizontal lines of the blocks follow the margins of successive layers of sediment from when the sandstone was being deposited. The vertical block surfaces are those of joints created well after the sandstone was rock had consolidated and after earth tremors had passed through the area.

The Prince Regent River and Estuary

The Prince Regent River has been known to Europeans since its discovery by Phillip Parker King in October 1820.  He had sailed into St George Basin an inner basin of Brunswick Bay. King continued his explorations by travelling from the Basin in a whale boat up this newly discovered Prince Regent River on an incoming tide. His ship Mermaid remained anchored in the Basin.

This basin and river area is the homeland of the Adbalandi clan who were within the territory covered by the Worroran language and its people. This local country was traditionally referred to as Malandum. Their land was mainly on the southern side of the river with country to the north bordered by people who spoke the Wunambal language. Many of the Lalai (Dreamtime) stories of creation relating to river and that of St George Basin, the river and King Cascade, are told in both languages.

Prince Regent River begins its +100 km course originating near the foot slopes of and Mt Agnes (735 m above sea level). It follow a zig-zag joint path from the mountain slopes and then follows a relatively straight NW-SE joint line. But for one small section on the upper straight where the channel deviates a little then runs for 70 km to St George Basin. For at least half of this distance, the section that was explored by King, the estuary is influenced by the oceanic semidiurnal tidal pattern of two highs and two lows each 24 hours and 50 minutes. Still on the river and returning to the Mermaid, King decided that rather than rowing against the tidal current they were to sleep in a small river island -here he estimated that  the tide had risen by 30 feet (between 9 and 10 metres) during the night between the low and high.

An aerial view of the river illustrates the checkerboard nature of the rock joints and shows that where the main channel is joined by a tributary it tends to enter at right angles having cut its channel along a NE-SW joint. One of the larger sized navigable tributaries, referred to a Camp Creek, runs its first 3 km to the SW along a straight channel.

A most historic and interesting feature about 32 km upstream from the Basin is a 200m wide cascade below Cascade Creek and it falls into a large pool that also enters the main channel at right angles. As this creek is spring fed means that it tends to have some flow of fresh water even during the dry season and is not wholly dependant on the wet season flows. The cascade is rock stepped and drops about 45 m to the tidal pool which during very low tides, exposes large expanses of its muddy base.

The rock type here and for the majority of the Prince Regent River course is Wunaamin Mulliwundi Sandstone (previously known as King Leopold Sandstone). This is a relatively resistant rock type and most of the river and estuary lies between rocky resistant sandstone cliffs. The valley between the parallel cliffs is about 200 m wide but narrows further upstream. Downstream the valley has been widened in a few locations due to the presence of a less resistant rock. At these points there is evidence of the rock, Hart Dolerite.

About 1,760 mya molten Dolerite lava the was forced between the sandstone strata and cooled to fill joints and form horizontal sheets called sills. Dolerite, unlike the resistant sandstone is easily weathered when exposed to air and water. As the river channel has deepened it cut across a sill embedded in the sandstone. Once exposed it weathers and undermines the sandstone as weathering products are washed away. With no support the sandstone falls and the so gradually widening the channel at this point. The Dolerite is dark in colour and when weathered forms reddish soil. Weathering of large, above ground sills of Dolerite forms rounded hills unlike the sheer rocky landforms of sandstone.

Further reading:

See ‘Background Briefing -features of Koolama Bay and Saint George Basin’ for the changed geology and landforms once the Prince Regent River estuary reaches St George Basin.

In these articles I generally refrain from describing the magnificence and uniqueness of the local landforms but give a broad outline of the formation of the landscapes they are part of, and a little of their history in the hope that these explanations may remove some of the bewilderment about how such natural, often complex wonders originated

 The Kimberley is a large regional area of north west Western Australia and fronts the Indian Ocean and the Timor Sea to its west and north, with the Great Sandy Desert to its south and the Northern Territory to the east. Being in tropics means the Kimberley is warm to hot throughout the year with a climatic pattern described as monsoonal with hot and wet summers and cooler dry winters

Two major rivers, the Ord and the Fitzroy arise in the south east Kimberley around Mount Wells (983m) the regions highest point. The Ord tracks east then north via Lake Argyle to Cambridge Gulf north of Wyndham and the Fitzroy flows to the diagonally opposite south eastern corner of the Kimberley and after being joined by several tributaries discharges in King Sound northeast of Derby. There are many other rivers flowing off mounts and ranges on the central Kimberley Plateau and like the Ord and Fitzroy are seasonal, responding the to the wet season for their major flows usually between October to April, with greatly reduced flow volumes during the long, intervening dry season.

Physically the Kimberley is composed of a number of regions but the largest and most prominent is the Plateau that occupies its main northern and western section and forms the majority of the Kimberley’s coastline. This region lacks any large towns but has several smaller ones with many Indigenous centres and outposts. Other than those connected by sealed roads (the “black-top”) most of the smaller communities remain somewhat inaccessible by road during the wet season.

Makeup of the Kimberley Plateau

The rock of Kimberley Plateau began its history over 1,800 million years ago (Ma) when sediments from surrounding mountains were deposited in a large, shallow marine basin with dimensions of at least 450 km north to south and over 400 km east to west. Rivers carrying sandy sediments would have fanned out over the basin and deposited their load forming what is todays Wunaamin Miliwundi Sandstone (formerly referred to as the King Leopold Sandstone) that was built up to depths of more than 800 m in some locations. Following this event that lasted for several million years volcanic lava was extruded over the surface of the sandstone.  On cooling this mainly basalt rock varied in depth from 60m deep to over a kilometre. This rock is referred to as the Carson Volcanics. The eruptions were not from typical volcanic cones, but from surface fissures and the lava flowed and filled in hollows and valleys where it became the deepest and remained shallower over higher surfaces of the Sandstone.

Following another pause over many millions of years new mountains had arisen and the basin had continued to slowly sink. The new lot of sediments formed another sandstone to be spread over the Carson Volcanics.  This sandstone is referred to as the Warton Sandstones and dating of Zircon crystals from within the sandstone suggest a maximum age for this formation of 1,786 Ma. Two later formation-forming episodes led to the deposition of Elgee Siltstone and over these thick beds of the Pentecost Sandstone.

Two other rock types found in the Kimberly Basin group are those of the Yampi Formation and Hart Dolerite. Unlike the earlier formations Yampi Formation has a distribution limited to the south west of the basin and it this sedimentary formation formed after the Pentecost Sandstone that has become economically important. One of its main rock components is hematite, the iron ore mineral and there has been large scale mining of this on the Yampi Peninsula since 1936. The complex of formations plus the Hart Dolerite is referred to as the Kimberley Series.

Each of the titles allocated to these Formations has its own interesting history and the Pentecost Sandstone for example was named after John Pentecost, the geologist member of a survey team led in 1882 by Michael Durack. The Yampi Formation simply takes its name of the location where the formation is found and the state’s naming authorities have no idea of the origin of the name Mount Hart.  The site where the type specimen of Hart Dolerite was collected was Hart Range shown on Alexander Forrest’s chart after he had traversed this area in 1879.

Hart dolerite has a similar composition to the Carson Volcanics basalt but given that dolerite did not reach the surface, it cooled more slowly underground and so developed larger crystals in its makeup. Dolerite is an intrusive rock where the magma, pushed up with great pressure was forced up through joints in the rock and intruded horizontally between the beds of the sedimentary rock where it cooled to form sills.  Most of Hart Dolerite is found within the Wunaamin Miliwundi Sandstone and up into the Warton Sandstone. Because of its mineral composition and resultant dark colour, it is hard to distinguish Hart Dolerite when found associated with the Carson Volcanics.  Dating of the Dolerite indicates an age of formation around 1795 Ma.

The Effects of Earth Movements.

Most of the Kimberley Basin formations were laid down horizontally and have remained that way even during the Basin’s uplifts to become a plateau.  However, the landforms of the Yampi Peninsula in the Plateau’s south western corner, and areas south of this, including the Wunaamin Miliwundi Ranges (originally referred to as the King Leopold Ranges) were initially formed by mountain forming forces called orogenesis. In the south west Kimberley orogenic events were compressive acting mainly form the pushing forces from the south and these occurred about 1000 Ma and again between 670 to 510 Ma ago. These huge forces produced folded mountainous landscapes with many high tops and low valleys and in some cases pushed the more pliable folds over one another and caused faulting in brittle rocks.

Horizontal strata v’s folded strata.

Lower beds in horizontal strata (left side) are not weathered and eroded until those above have been removed. Not only does folding produce hills and valleys (middle image below) but, following weathering and erosion of the high points (right side), several of the ‘lower’ beds are also exposed to weathering and eroding agents.

Another forceful effect transmitted throughout the plateau crated jointing seen particularly in the sandstone formations. Joints ran through the mass of rock rupturing it vertically which combined with the horizontal bedding layers created blocks. The vibrating forces that created the joints is believed to have emanated from orogenesis in both the east and south of the plateau and have created a series of often deep seated joint lines that run from the southeast to the northwest and from the southwest to the northeast. Some joints run for many 100s of metres and a few extend for many 10s of kilometres.  

Geologists have also gathered evidence to illustrate that the plateau has had several uplifting episodes the most recent being in the order of 200 Ma and about 50 Ma ago. Each time there is an uplift, weathering and erosion by rivers is rejuvenated and it is these most recent uplifts that have been responsible for the development of many of the landforms we see today. 

In some locations the only formation remaining today is the lowest one, the Wunaamin Miliwundi Sandstone, clearly resulting from long periods of weathering and erosion and the removal of those formations originally above it.  However, there are locations visible around its seaward edges of the Kimberley Basin, where Pentecost Sandstone and Elgee Siltstone still remain as the upper predominant strata. In other parts of the Plateau geomorphologists who study these landforms note that these are several high spots assessed as being remnants of the Plateau from earlier times.

What weathers the fastest sandstone or basalt?

In general, the Kimberley sandstones are the most durable sedimentary rock with respect to weathering and the siltstone is most susceptible. We are likely therefore to find that most ridges, escarpments and cliffs are formed from sandstones, whereas valleys are more likely to occur where Elgee Siltstone once predominated. 

Contrary to common opinion, basalt and dolerite due to their mineral composition weather more rapidly than the sandstone under environmental conditions that have prevailed in the Kimberley. These two igneous rocks have minerals with a high proportion of iron and so chemically weather more rapidly than the somewhat inert sandstone, composed mostly of silicon dioxide.

Should either of these igneous rock types be exposed within or below a sandstone, their rapid weathering weakens the strata since it no longer offers support and the sandstone collapses and may be broken up by the fall.

The Plateau as seen from its coastline

My most recent Kimberley visits have been aboard Coral Expeditions vessels sailing between the Berkeley River and Broome. The cruise itineraries mostly involved experiences along the edge of the Plateau but during the section between Broome and south King Sound we pass Kimberley landscapes of the Dampier Peninsula that are not part of the Plateau.  This section also includes the offshore Lacepede Islands.

Along the edge of the Plateau cruises pass many sea cliffs and venture into estuaries and bays. Here it is possible to see the characteristics and relationships between each of these six Plateau formations and the Hart Dolerite especially at sites where they are washed clean and/or devoid of soil and vegetation. Such features include-

• Warton Sandstone cliff-lined estuaries of the Berkeley (see above) and King George rivers,
• visits to sites like Jar Island, Bigge Island and Swift Bay to see rock art from past indigenous clans painted mostly on Wunaamin Miliwundi Sandstone surfaces;
• mangrove lined Porosus Creek with its great biological diversity surrounded by Wunaamin Miliwundi Sandstone cliffs and sills of Hart Dolerite (see above);
• the wave cut underwater plateau of Montgomery Reef and its fascinating tidal patterns;
• the complex of colours, shapes rock types including the Horizontal Waterfalls in Talbot Bay and nearby, Nares Point in Yampi Peninsula (see above).

Read more in  ‘Australia’s Kimberley Coast’ by A W (Sandy) Scott.