Flynn's Beach

Welcome to Flynn's Beach, Phillip Island. This site contains materials from near the top of the South Gippsland stratigraphy and helps fill in some of the data missing from other sites visited during this field trip.

The outcrops we will visit at this site occur along the beach cliff to the north of the walking track. Here we're looking north to the headland. Notice the well-formed shore platform on the headland. This is composed of slightly weathered Tertiary basalt of the Flinders Subprovince of the "Older Volcanics", at Phillip Island ranging in age from about 40 to 48 million years old. We'll examine the contents of the cliff face in the photos below.

Almost the first thing you see when you walk out onto Flynn's Beach from the car park is this red-brown stain coming from small creek to the left. The creek comes out of the sand dunes behind the beach and quickly drains away into the sand. What's the stain?

Regolith materials behind the beach are a mixture of well-sorted quartz sand dunes and organic carbon-rich peaty swamp deposits in the dune swales (the depressions between dunes). From our previous experience with the mangrove swamp deposits at Red Bluff we know that they contain PASS (Potential Acid Sulfate Soil) materials, and have an abundance of pyrite. In this example, the swamp is now being drained by the small creek and contains AASS (Actual Acid Sulfate Soil) materials. The red-brown stain is composed of Fe-oxyhydroxide precipitates including ferrihydrite (approximate composition 5Fe₂O₃.9H₂O) and goethite (a-FeO(OH)) created during the pyrite oxidation process.

The oily stain on this little pool of water is not actually oil, it's ferrihydrite (approximate composition 5Fe₂O₃.9H₂O). This is a highly hydrated regolith mineral that, when it dehydrates under the heat of the sun, will convert to goethite (a-FeO(OH)). Ferrihydrite is very common around any swampy deposits near rivers, creeks, dams and coasts and indicates the nearby presence of PASS and AASS materials.

Walk further north along Flynn's beach and you'll come to the headland. The cliff face of the headland (seen here looking southwest back towards the walking track and the creek) is composed of highly weathered basalt saprolite. Note the prominent talus slopes at the bottom of the cliff. These materials are very loose and crumbly and are easily dislodged to form the talus - why? Notice also the red patches and white veins. What does this remind you of? Do you remember seeing these sorts of features at Red Bluff?

What's talus? It is fragments of rock and soil material that accumulates by gravity at the foot of cliffs or steep slopes. Scree is another name for it.

Here's the cliff face again, this time looking east, square on. Talus, overlying aeolian sand, covers the bottom half of the exposure. The remainder of the cliff face consists of moderately- to highly-weathered, mottled basalt saprolite. The mottles appear vein-like over much of the exposure, but there's small patches of more normal vertical mottling in the centre and right hand side of the cliff face. Why are there different mottling styles (veins and vertical mottles)? Probably because the vein-like mottles are formed by water penetrating large cooling fractures in the basalt mass around large blocks of basalt. The vertical mottles are probably the result of water penetrating finer fractures within basalt blocks and maybe fine holes after tree roots, termites, ants and worms.

Why is the basalt saprolite so crumbly, resulting in lots of talus? Let's consider the weathering products of some common rocks...

Fresh granite contains:

• Quartz SiO₂,
• Feldspar: Na- and Ca-bearing plagioclase NaAlSi₃O₈-CaAl₂Si₂O₈ and K-bearing orthoclase KAlSi₃O₈;
  
• Muscovite K₂Al₆Si₆O₂₀.OH₄;
  
• Ferromagnesian minerals orthopyroxene (Mg,Fe)SiO₃, biotite K₂(Mg,Fe)_6-4(Fe,Al,Ti)_0-2Si_6-5Al_2-3O₂₀.(OH)₄ and/or hornblende (Na,K)_0-1Ca₂(Mg,Fe,Al)₅Si_6-7Al_2-1O₂₂.(OH)₂; and,
  
• Minor minerals like zircon ZrSiO₄ and tourmaline Na(Mg,Fe,Mn,Li,Al)₃Al₆Si₆O₁₈(BO₃)₃(OH)₄.

What are the weathering products of this? The quartz is relatively stable so will remain as quartz, zircon and tourmaline will do the same, but the feldspars and ferromagnesian minerals will weather to clays (hydrated alumino-silicates). Metallic cations (K, Mg, Ca, Na) and anions (Cl, F, etc.) are normally flushed away in groundwater but Fe may remain as hematite or goethite, staining the weathered rock.

Fresh basalt contains:

• Volcanic glass;
• Olivine (Mg,Fe)₂SiO₄;
  
• Plagioclase NaAlSi₃O₈-CaAl₂Si₂O₈;
  
• Orthopyroxene (Mg,Fe)SiO₃ and/or clinopyroxene (Ca,Fe,Mg)Si₂O₆; and,
  
• Minor minerals like apatite Ca₅(PO₄)₃(OH,F,Cl), biotite K₂(Mg,Fe)_6-4(Fe,Al,Ti)_0-2Si_6-5Al_2-3O₂₀.(OH)₄ or phlogopite K₂(Mg,Fe)₆Si₆Al₂O₂₀.(OH)₄ and magnetite Fe₂O₃ and/or another spinel.

What are the weathering products of this? The weathering products of basalt are pretty much clay, along with some minor minerals like zeolites and phosphates, again with most metallic cations (Mg, Fe, Ca, Na, P) and anions (Cl, F, etc.) flushed away in groundwater and Fe remaining as hematite or goethite to give the red stain.

In both cases, the Si and Al cations will recombine with oxygen and hydroxyl (OH^-) ions to form clay in situ.

So why are the rocks so crumbly?

A close-up view of the moderately- to highly-weathered basalt from Flynn's Beach. Note the white-coloured amygdales. These are relict gas bubbles (vesicles) that are now filled with secondary weathering products including zeolite, carbonate and clay. The rock mass itself is stained red-brown signifying that is contains a reasonable proportion of hematite or goethite, but the mass is mostly clay. Remember - it only takes a little bit of hematite to make a dramatic colour change in any rock.

So why is the basalt saprolite so crumbly? It's because of the type of clay that it is composed of. Basalt weathering results in the formation of moderate to large amounts of smectite clays. Smectite is the generic name for a family of clays (formerly known as montmorillonite) that swell when they come in contact with water. Smectite clays form "self-mulching" soils. These shrink on drying and swell on wetting, churning the soil surface over and creating crumbly red-brown soils (that are great for growing potatoes, by the way). Smectite clays can also form in weathered granites too, providing that there is an adequate water supply, but usually in small amounts. Smectites have a high cation exchange capacity (CEC) and bond water molecules to Ca, K, Mg or Na in their interlayers, allowing them to swell when hydrated, and shrink when dehydrated.

The talus slopes are formed by the shrink-swell action of smectite clays in the basalt saprolite causing it to fall apart. This represents a considerable geohazard; it is often difficult to stabilize slopes composed of weathered basalt.

Walking back towards the car park, we come across this exposure. These materials sit above the weathered basalt in the stratigraphy (you see see weathered basalt at the base in the next two photos below).

If you take a closer look, you can see that the exposure has a range of materials. The most obvious feature is the well-developed vertical mottling, consisting of red-yellow and white-gray colours. Mottles are developed mostly in clays reminiscent of the weathered basaltic clays seen earlier and well-rounded, well-sorted, fine- to medium-grained quartz sand occurring in a wedge-shaped sub-horizontal lens, seen thinning towards the right-hand side of the photo, and in the base of the section.

How would you interpret the materials you see here. What kind of environment were they deposited in and what happened to them subsequently?

Think carefully about the kinds of materials and the present- and possible paleo-environments. The clays are friable and break into small aggregates similar to those developed on the weathered basalts we saw before. Given the proximity of this section to basalt saprolite outcrops nearby (and indeed at the base of this section) it is reasonable to conclude that the clays were derived from weathered basalts. The clays are friable because they contain a large amount of smectite - a common weathering product of basalt. The sands are well-rounded and well-sorted. What does this imply? That they're from a beach or near-shore marine environment, where wave action has knocked all the edges off the grains.

We believe that what we're seeing here are clayey swamp deposits, derived by weathering the local basalts, deposited in a lagoonal environment behind a beach.

Where's the beach? Well, if we look a little further down in the section in this photo and the next, we can see rounded basalt pebbles and cobbles deposited over the top of quartz sand and weathered basalt.

How would you interpret this section?

We believe that here we're seeing a basalt shore platform (now weathered) at the base, overlain by the remnants of a sandy beach facies, then cobble beach facies, then lagoonal swamp facies deposits. This kind of succession is possible where sea levels are falling or where the beach is prograding (moving outwards), leaving lagoons and swamps behind the beach dunes, which is more likely.

Close-up of the weathered basalt shore platform with overlying sandy beach, cobble beach and overlying heavily mottled swamp facies deposits.

Walk back a little further towards the car park and you'll see this exposure. This is the unconformity between the weathered basalts and the overlying swamp deposits and the modern dunes. Notice the dramatic colour change between the lower deposits and the modern dune sands. What does this say about the styles of weathering and the chemical reactions taking place (or that have taken place) in each deposit?

A close-up of the unconformity between the modern beach dunes and the older swamp deposits. Notice the carbon-rich horizon marking the unconformity, which is a peaty layer indicating that there was vegetation growing on this interface before it was covered by the modern dunes. Note also that the sand below the unconformity is bleached gray (it as been reduced), whereas the modern sands are yellowish, mostly likely indicating that they contain a small amount of Fe-oxyhydroxide (they are oxidised).

Learn more about Phillip Island (and processes occurring at Flynn's Beach in general) by reading:

W.D. Birch ed. 2003. Geology of Victoria. Geological Society of Australia special publication No. 23. 842 p.

Go back