Red Bluff, Duck Point, Yanakie Isthmus

Welcome to Red Bluff at Duck Point on the Yanakie Isthmus. This place has great scenery, (which a regolith geoscientist appreciates, naturally), but more importantly it represents the lowest part of the immediate Wilsons Promontory-Yanakie Isthmus stratigraphy. The lowest part of the stratigraphy we visit during this trip is the Cambrian greenstones at Waratah Bay, as you may have already read. At Red Bluff we see mangrove swamps skirting the edges of Corner Inlet, with a few outcrops of granite saprolite and weathered sedimentary rock poking out through the tidal flats in the distance. Red Bluff itself is just visible in the left-hand side of the image.

Walk up to Red Bluff and... parts of it are red! The red colour comes from secondary iron- (Fe-) oxyhydroxides, most notably hematite (a-Fe₂O₃) and goethite (a-FeO(OH)), which are two commonly occurring Fe-bearing regolith minerals. The presence of Fe-oxyhydroxides tells us that the rocks have been exposed to chemical weathering at the Earth's surface for some time. We can therefore use the generic term "saprolith" (weathered rock) to describe them, unless we look closer to discover whether the rocks are "saprock" (less than 20% of weatherable minerals are actually weathered, breaking with a hammer blow) or "saprolite" (more than 20% of weatherable minerals are actually weathered, breaking with a boot kick).

How do the secondary Fe-bearing minerals get there? Chemical weathering due to the attack of oxygen and weak acids or bases in water forces Fe-bearing minerals to chemically dissolve and precipitate Fe-oxyhydroxides. Alternatively, Fe-bearing groundwaters moving through the rocks precipitate Fe-oxyhydroxides as they evaporate.

The remainder of the saprolith at Red Bluff is white, or is mottled with alternating red and white colours.

Chemical weathering alters many of the primary (original) minerals of a rock, converting them to secondary minerals that are more suited to surface temperature and pressure conditions. Weathering can be regarded as a chemical overprint on surface and near-surface rocks, often (but not always) occurring some time after the rocks formed.

Looking west towards Red Bluff, note the mangrove swamp growing right up to the cliff base. There is only a narrow beach covered in coarse quartz sand which slopes gently down to the high tide mark, at which point the mangrove swamp takes over. Also note the linear outcrop jutting out towards us.

The rocks at Red Bluff are Early Devonian turbidites (sandstones and mudstones) intruded by Middle to Late Devonian granites of the Wilsons Promontory Batholith. They are slightly to moderately weathered, so in regolith terms can be collectively called saprolite. In this image we see mottled Early Devonian turbidite saprolite with a small blob of granite saprolite (centre) where the granite intruded the original turbidite. Notice the differences in texture between the two rock types. The turbidite saprolite is pervasively ferruginised (iron-enriched) with poorly-developed Fe-oxyhydroxide mottles in the rock mass but has strongly ferruginised joint planes. The granite saprolite is pervasively ferruginised, but the mottles are much smaller and have a "leopard-spot" texture.

As a generalisation, without delving too deeply into the chemistry of regolith processes, mottling occurs where oxidising and reducing (redox) conditions alternate or coexist in the regolith. It is common to see well-developed mottles where rocks or soils are strongly jointed or where there are preferential fluid flow pathways in regolith, for instance, voids left behind by tree roots, insect or worm burrows or soil fractures (ped structures).

Iron is highly soluble under reducing conditions, where it exists in the Fe²⁺ state, but is highly insoluble under oxidising conditions where it exists in the Fe³⁺ state. Where these conditions alternate or coexist, Fe in the regolith is repeatedly dissolved and precipitated, eventually building up complex red and white mottles like those we see here.

In this image we again see mottling, but this time in close-up and developed in granite saprolite. The granite is fractured and has sub-parallel, near-vertical quartz veins running through it. Mottles are developed as "leopard spot" textures. There are also prominent sub-horizontal Fe-enriched bands in this outcrop, visible in the photo. These are a regolith feature known as "liessegang banding", or fake bedding. These are a feature formed by fluctuations in the groundwater table where Fe-oxyhydroxides precipitated at the water table surface, creating the sub-horizontal bands. Do this enough times, and the liessegang bands appear as fake beds. Liessegang bands can also appear as coloured rings in heavily jointed rock masses.

This granite is moderately weathered so is classed as a saprolite. In this case, the weatherable minerals are the ferromagnesian minerals (like biotite, muscovite and hornblende) and the aluminosilicates (like the feldspars, plagioclase and orthoclase). The other main constituent of granite, quartz, remains largely unweathered. In this rock the ferromagnesian minerals have mostly weathered out, contributing to the Fe-oxyhydroxide staining in the rock. The feldspars are starting to weather and have developed clayey rims that are easily scratched out, seen as white dots in the picture.

In this image we're looking south from Red Bluff towards Duck Point and Wilsons Promontory. The rocks here are saprolite developed on Devonian sediments showing pervasive mottling, ferruginised joint planes and differential weathering in the lower cliff face. Differential weathering occurs where soft parts of the rock get weathered away, but the hard parts remain. Some of the lower cliff face also has well-developed boxwork or "honeycomb" weathering (see below), another form of differential weathering.

How does differential weathering occur? Iron-oxyhydroxides like hematite and goethite precipitate in mottles or along joints. These secondary minerals are quite insoluble at the surface and armour the rock against weathering and erosion, whilst the surrounding rock that does not have Fe-oxyhydroxides is weathered and eroded in preference.

This view looks southeast towards Corner Inlet. This small rocky prominence juts out from the tip of Red Bluff. Notice the mangroves growing quite close to it. Why should this be here, when the remainder of the cliff face is some distance away (to the right of the photo)? What conditions caused this rocky remnant to survive?

Here's a close-up view of the rocky prominence. Notice how dark the rock looks? Is it the same rock as in the cliff face?

Indeed it is. This is a saprolite of heavily ferruginised (Fe-oxyhydroxide-cemented) Devonian sedimentary rock. The dark colour, like dried blood, is typical when rocks are ferruginised by hematite; that's how hematite got its name

The rock is heavily ferruginised. Can you think of a good Fe source locally?

In this example the rock shows some quite nicely developed boxwork or "honeycomb" weathering. The deep red-black material in the previous image is shown to actually be a surface feature. The saprolite underneath the surface is honey-coloured, indicating that it is only weakly to moderately ferruginised.

What is a good Fe source in the local area?

So, what is a good Fe source in the local area? This image is of the local mangrove swamp and high-tide line at Red Bluff. The tide line here is marked by dead seagrass that has been washed up from shallow marine flats in Corner Inlet. Below this there is a thin layer of white sand. About 5-10 cm below this is black, smelly mud, which mangrove swamps are renowned for. Why is the mud black and smelly? Because it's anoxic (it has no free oxygen), it's full of anaerobic bacteria, it's full of organic carbon, it releases H₂S (di-hydrogen sulfide) gas and it's also full of pyrite.

The soils in mangrove swamps are coastal Potential Acid Sulfate Soils (PASS). The soils are reducing (anoxic, without oxygen) and contain abundant bacterially-reduced pyrite (FeS₂). Should there be a geologically-sudden water level change (say, from uplift caused by an earthquake, the onset of a glacial period or, in an inland setting, a bad drought) and the water table lowers, soils can rapidly oxidise and become Actual Acid Sulfate Soils (AASS). What happens then? These black, stinking anoxic muds will suddenly turn acidic as pyrite is oxidised, liberating sulphuric acid, soluble Fe and S, in the following 3-step reaction:

1. FeS₂ + 7/2O₂ + H₂O = Fe²⁺ + 2SO₄^2- + 2H⁺

2. Fe²⁺ + 1/4O₂ + 3/2H₂O = FeOOH (precipitate - goethite) + 2H⁺

3. FeS₂ + 15/4O₂ + 7/2H₂O = Fe(OH)₃ (precipitate - Fe³⁺ hydroxide) + 2SO₄^2- + 4H⁺

Reaction 1 dissolves solid pyrite. Reactions 2 and 3 create solid products. Note that hydrogen cations are liberated in each reaction. Hydrogen cations combine with sulfate molecules to form sulfuric acid. Iron-oxyhydroxides are also precipitated in these reactions.

Each tonne of pyrite consumed in this reaction releases 1.6 tonnes of sulfuric acid.

Australia's coastline alone is estimated to contain 1 billion tonnes of PASS.

Australia also has similar amounts of inland PASS and AASS.

So, just below the surface of the mud, these reactions are occurring, liberating small amounts of sulfuric acid, which is diluted and washed away by seawater. Small amounts of Fe²⁺ are liberated and rapidly precipitate to form Fe-oxyhydroxides. This often occurs in the capillary zone of the water table (in this case around the tidal zone) resulting in rocks near the surface of the mud being coated in Fe-oxyhydroxides. An intermediate form of Fe-oxyhydroxide is ferrihydrite (see below).

Sticky, stinking, rich, black mud. A mangrove's favorite habitat. Note the prolific aerial roots that gather oxygen from the air rather than the water. If you look around this site you can find small pools of water that have an oily-looking film over them. This is not oil, but rather another mineral product of the pyrite oxidation process called ferrihydrite (approximate composition 5Fe₂O₃.9H₂O), which dehydrates to form hematite very easily. You can see pictures of ferrihydrite at Flynn's Beach.

The modern mangrove swamps are generating small amounts of acid and Fe-oxyhydroxides. However, we know that these conditions also existed in this location sometime in the Tertiary, after the gravelly Haunted Hills Formation was deposited. The Haunted Hills Formation is shown on the Warragul 1:250,000 geological sheet as "Tph".

In this image we see rafts of silcreted Haunted Hills Formation gravel at the preset coastline, with black mud and mangroves overlapping.

Here we see platy rafts of groundwater silcrete of the Haunted Hills Formation gravels.

What's a silcrete? Why, it's a duricrust! What's a duricrust? It's a secondarily cemented regolith material - a durable crust. The cement can be silica (silcrete), calcite (calcrete), dolomite (dolocrete), alumina (alcrete or bauxite), iron oxyhydroxides (ferricrete), magnesia (magnecrete), manganese oxyhydroxides (manganocrete), salt (salcrete)... the list goes on. The materials being cemented are usually resistate (resistant to chemical weathering) minerals like quartz, zircon, anatase or other rocks containg these minerals. Silcretes are silica-rich duricrusts containing at least 95-98% SiO₂. The remainder can be resistate minerals like zircon (ZrO₂) and anatase (TiO₂), with maybe a little hematite.

How are silcretes formed? Well, to form a silcrete you need to strip a piece of regolith all of its other weatherable minerals like clays, except the resistates. How can this be accomplished? It's best done using powerful acid, which dissolves the clays and flushes metallic cations (like K and Al) away in groundwater. Silica is left behind as a kind of hydrous gel which may move slightly but mostly remains in situ. Where's the best place to find powerful acid in the regolith? In AASS environments! The Al and Fe cations generated by this reaction will precipitate as hydrous flocs somewhere downstream of where the silica gels are precipitating.

We now believe that most silcretes formed in palaeo-AASS environments, either near a coastline or inland around swamps. Silcrete is one of the more common duricrusts encountered around Australia, so at some time in the past, a lot of Australia was affected by AASS.

You can see silica gels, alumina gels and Fe-oxyhydroxide flocs in modern AASS environments like coastal swamps and deltas (for instance, the Burdekin River Delta) and inland settings like the Western Australian Wheat Belt.

Close-up of silcreted Haunted Hills Formation gravel. Note the subangular quartz fragments indicating that the fragments have not been transported very far. What's a good quartz source close to Red Bluff? Why, the Wilsons Promontory Batholith of course! The Haunted Hills Formation gravels, including this silcreted version, are the remains of chemically weathered and physically transported granites from Wilsons Promontory.

Learn more about Red Bluff, rock weathering, mottling, coastal and inland acid sulfate soils and silcrete by reading:

W.D. Birch ed. 2003. Geology of Victoria. Geological society of Australia special publication No. 23. 842 p.
J.G. Douglas 1979. Explanatory notes on the Warragul 1: 250,000 geological map. Geological Survey of Victoria, 27 p.
R.A Eggleton ed. 2001. The Regolith Glossary. CRC LEME, 144 p.
S. Lamontagne, W.S. Hicks, R.W. Ritzpatrick and S. Rogers 2004. Survey and description of sulfidic materials in wetlands of the Lower River Murray Floodplains: Implications for floodplain salinity management. CRC LEME Open File Report 165. [4.21MB]
R.H. Merry and R.W. Fitzpatrick 2005. 2005. An evaluation of the soils of Tilley Swamp and Morella Basin, SA. CRC LEME Open File Report 195. [2.02MB]
Sammut, J., White, I., Melvilles, M.D. 1996. Acidification of an estuarine tributary in eastern Australia due to drainage of acid sulphate soils, Marine and Freshwater Research 47, 669-684
G. Taylor and R.A. Eggleton 2001. Regolith Geology and Geomorphology. John Wiley & Sons, Ltd. Chichester, 375 p.

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