The ‘Kimberley Landscapes’ post introduces many of the names and process mentioned in this article. Readers may find it a useful supplement to other articles about the Kimberley coast.
This post could also be read in association with that on the ‘Horizontal Waterfalls’.
In these articles I generally refrain from describing the magnificence of the local landforms but give a broad outline of the formation of the landscapes and a little of their natural history in the hope that these explanations may remove some of the bewilderment about how such natural, often complex wonders originated.
Talbot Creek
This creek, like the Horizontal Waterfall complex, joins Talbot Bay at its south eastern end.
The earth-moving forces that caused the steep folding of the rock formations at the Horizontal Waterfalls certainly folded the rock strata we see along the creek, but to a lesser extent (compare right and left of Figure 1). Over time the same weathering and erosive mechanisms have reduced many of the high points produced by folding to ‘stumps’ (as shown in Fig1 by the C shape) and these forces have acted to remove the less-resistant Elgee Siltstone and the Carson Volcanics formations with any un-weathered part mostly below the water. These mechanisms have involved flowing creeks that have cut down and opened-up gaps in the two of the more resistant sandstone rock ridges. Compared to the Horizontal Waterfall complex the two gaps along Talbot Creek are wider, and although the water rushes through these in response to tidal changes they do not offer enough resistance for the formation of horizontal waterfalls.
Figure 1. A diagrammatic section through the southern end of Talbot Bay to illustrate the relationship between the Horizontal Waterfalls (r. h.), Slug Island (middle) and along Talbot Creek (l. h.). Details of the effect of folding, weathering and erosion of the various rock formations, and the position of sea level following the last ice age is also shown.
Like the remnants on the walls of the embayments in the Horizontal Waterfalls complex, there is also plenty of evidence to show that particular rock types originally filled the spaces now filled with the Talbot Creek channel or tidally flooded lowlands covered with mangroves. The most prominent remnant is a narrow ridge (Fig.1 #15; Fig.2 #14) of Elgee Siltstone formation rock. This ridge is a formed from a bed of resistant conglomerate (Fig. 3) and not the soft siltstone beds typical of the Elgee Siltstone formation. There are also traces of this formation remaining on the tide washed wall (Fig. 2 #4). This sample (Fig. 4) also shows the wave like traces of micro folding compared to the large-scale folding that produced the surrounding ridges (Fig. 5).
Fig. 2. Showing details of the landforms and features along the estuary of Talbot(Cyclone) Creek
Fig. 3 Elgee Siltstone conglomerate found along the walls of the narrow ridge in Talbot Creek at low tide
Figure 4 Micro folding in the Elgee Sitstone found along Talbot Creek..
A cruise out of Talbot Bay and along its estuarine creek takes a circuitous route through the first gap (Fig.2 #3) which has been cut down through Pentecost Sandstone . This leads into and embayment once filled with the easily weathered strata of Elgee Siltstone and towered over by the high ridges of Pentecost Sandstone on both its northwest and southwest (Fig. 5). These are fairly shallow waters with a well developed mangrove community to its southeast. The creek channel follows a straight 3.5 km path to the southeast between a thin ridge of conglomerate dominated Elgee Siltstone and a rocky sloping ridge face of Warton Sandstone that dips down under the channel (Fig 2, #14 and #5). At the end of the straight is another gap (#15) cut through the Warton Sandstone with the creek channel flowing through this into a space once filled with Carson Volcanics basalt.
igure 5. Looking across the embayment originally occupied by Elgee Siltstone toward the gap in the ridge formed of Pentecost Sandstone. The mid background feature is one of the residual sandstone ‘stumps’.
Like the second embayment of the Horizontal Waterfalls complex, remnants of Carson Volcanics basalt many be spotted above the mangroves on the lower slopes of the Warton Sandstone. Further along this mangrove filled emabayment formed by Talbot Creek there are dark mounds of large black boulders on the surrounding low hills. These are not formed from Carson Volcanics rock but remnants of Hart Dolerite that once intruded the Volcanics (Fig. 4; Fig. 2 #8). Much of this lowland space is flooded during high tides, it is intersected by creek channels, has some salt flats at its margins and supports several mangrove species.
Figure 6. Mounds of Hart Dolerite along Talbot Creek that once intruded the Carson Volcanics
Much of the hilly country around the estuary supports savannah woodlands dominated by eucalypts, particularly northern woolybutt (Eucalyptus miniata) with an understory of grasses and patches of wattle. Some of the high slopes (background Fig. 5) with massive sandstone exposures may carry a small, powdery white, smooth barked Eucalyptus rupestris.This small tree has with no common name but a species name meaning ‘rock loving’ (Fig. 7) . On the ridge above the mico-folding (Fig.2 # 4) are a number of the native northern pines, Callitris intratropica, growing on the ridge (Fig. 8). As species, and as the name suggests these pines are distributed across tropical nothern Australia. Another less common small tree, a Kurrajong, which flowers toward the end of the dry season when it is deciduous. The bright red flowers of this Kurrajong (Brachichiton viscidulus) may be seen on a few trees at the nothwestern end of the Eleee Siltstone ridge (Fig. 2 #14). This species has no common name and is often distinguished by it sticky fruits (Fig. 9).
Figure 7. Eucalyptus rupestris in its rocky habitat.
Figure 8. Mid-left -native northern pine growing on the ridge.
Figure 9. Red flowering Kurrajong on the end of the Elgee Siltstone ridge.
Notes for travellers in Western Australian Kimberley on landscapes near
Talbot Bay.
The ‘Kimberley Landscapes’ post introduces many of the names and process mentioned in this article. Readers may find it a useful supplement to other articles about the Kimberley coast.
This post could also be read in association with that on the ‘Talbot Creek’.
In these articles I generally refrain from describing the magnificence of the local landforms but give a broad outline of the formation of the landscapes and a little of their natural history in the hope that these explanations may remove some of the bewilderment about how such natural, often complex wonders originated.
The Horizontal Waterfalls
Overview
The Horizontal Waterfalls (HW) are found in the south western corner of Talbot Bay a deep bay located on the north side Yampi Peninsula east of Koolan Island. Rather than horizontal rock formations this corner of the Kimberley is made up of well folded formations and in the vicinity of the HW folds were so steep to be nearly perpendicular (Fig. 1). This meant that the local rock formations including the Wunaamin Miliwundi Sandstone, Carson Volcanics, Warton Sandstone, Elgee Siltstone and Pentecost Sandstone that were originally horizontal and over 4 km deep were folded, in parallel, to stand vertically.
Figure 1. View through the first gap in the ridge and into the first embayment during a turbulent outflowing tide. On the other side of the embayment is the second gap in a ridge and is the entrance into another (smaller) embayment. The ridge in the distance, McClarty Ranges is about 3km away and has no gap.
What happened to make today’s phenomenon?
Today the set of landforms at the HW (Fig. 2) are made up of three ridges (#3, #5 and &7), two with gaps (#10 and #12) through them and two embayments (#11 and #13). The embayment are flooded valleys between the three ridges. Tidal water from Talbot Bay (#1) may flood in through the outer gap to fill the first embayment and as it rises forces water through the second gap to fill the inner embayment. As the tide falls, the flow is reversed and the level of water in the embayment drops again. In brief it is the narrow gaps and the high tidal ranges in Talbot Bay that creates the waterfall-like flow.
Figure 2 Map of the landforms at the southeast end of Talbot Bay including those of the Horizontal Waterfalls and Talbot Creek. (NB #7 McClarty Ranges is formed from Wunaamin Milliwundi Sandstone, the new name for that originally named King Leopold Sandstone.)
The high hills and deep valleys resulting from folding through past orogenic (mountain building) activity would then have been subjected to weathering and erosion (Fig.3 #1). Once the tops of the high points were removed each of the five formations would have been exposed end-on. As illustrated on the right hand side of Fig.3, the three sandstone formations (#9) would be least affected and weathered at a relatively slow rate to remain as ridges. However, the Carson Volcanics (#3) is less resistant due to its basaltic composition and has been chemically weathered. The Elgee Siltstone (#5), because of its physical weakness, would have been easily removed by running water. Most of this weathering and erosion occurred during times of lower sea level and the base the Volcanics and Siltstone is currently well below the low tide mark (#10 and #11), hence they are continually flooded and form today’s embayments.
Figure 3. A diagrammatic section through the southern end of Talbot Bay to illustrate the Horizontal Waterfalls, Slug Island and along Talbot Creek to illustrate the effect of folding, weathering and erosion and the rise of sea levels following the last ice age
The two gaps (Fig. 1) are likely to have formed as creek water escaping from the higher landforms cut down through joints in the Warton Sandstone and the Pentecost Sandstone. These creeks would also have carried away any products of weathering. The inner embayment deepened because of the rapid chemical weathering of Carson Volcanics and it is possible today to find remnants of basalt on the sides of this embayment (Fig.2 #6). The outer embayment represents the space where Elgee Siltstone once stood but because of its susceptibility to physical weathering by running water it too has been lowered well below current sea level. Like the Carson Volcanics remnants of the siltstone also remain on the embayment walls (Fig.4 ; Fig. 2 #4). The outer ridge is formed from Pentecost Sandstone, the middle ridge from Warton Sandstone and the inner wall, without a gap is formed from Wunaamin Miliwundi Sandstone.
Figure 4. Remnants of Elgee Siltstone in the first embayment near the second gap of the Horizontal Waterfalls. An indication of the tidal range is revealed by the dark surface of the siltstone (due to Cyanobacteria growth) that is regularly covered with tidal water.
The section of the coast outside Talbot Bay is subject to maco-tidal conditions (greater and a 4 m range) and as the tidal influence moves deeper into the Bay it tends to intensify. As the high tide water builds the narrow gaps resist the flow the water, it builds up on the outer side and has to run ‘down hill’ into the first embayment (Fig. 5). This creates one horizontal waterfall that is repeated at the second gap. These flows are all reversed with the outgoing tide and between these tides there are short periods of slack water.
Figure 5. Looking along the northwest side of the first embayment, Horizontal Waterfalls.
Boating operators taking passengers through the Horizontal Waterfalls are required to observe safe practice given the ferocity with which the water moves especially during Spring Tide events of the lunar month as occurs over the full moon and new moon periods.
Sea levels reached their current level some 6,000 year ago and prior to this the tidal flow would have progressively moved further out of Talbot Bay and eventually at the peak of the last Ice Age, 18,000 to 20,000 years ago, seas were some 120 m lower than today. Given the very wide continental shelf in this part of Australia the shoreline then would have been 100 km seaward. At this stage we could imagine that Poulton Creek (Fig. 2 #16) which runs into the inner embayment would have flowed as a stream out through the two gaps, possibly fallen over a waterfall into Talbot Bay where it joined Talbot Creek out of the bay and across the continental shelf to the sea.
We must conclude that in Geological Time this is a very recent event and although the current conditions may be adding to the weathering and erosion, these were not what led to the initial development of this somewhat unique set of landforms.
Figure 6. An inflatable boat about to ‘climb’ the horizontal waterfall out of the second embayment through the narrower (and more dangerous) second gap during a building high tide..
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
The Kimberley (Adapted from kimberleysociety.org)
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.
The Kimberley Basin formations and Hart Dolerite intrusions making up the Kimberley Series
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.
(above) Cliff of Warton Sandstone (Berkeley River) & (below) Hart Dolerite intruded under hills of Wunaamin Miliwundi Sandstone (Prince Frederick Harbour.
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.
Folded and overfolded strata of Pentecost Sandstone (near Nares Point Yampi Peninsula)
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.
Today we can see evidence of these compressive forces on an originally horizontally layered series. Compression produced hills and valleys but all the high tops have been weathered and eroded away leaving only their stumps and associated landforms.
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.
Sites for Kimberly coast I have been fortunate to visit over the past 24 years. Most cruises with Coral Expeditions including a selection of 12 to 15 sites during a 10 day cruise.
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).
