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Welcome to San Remo. Here we look at another part of the landscape evolution story of southeastern Gippsland and Philip Island by examining the coastal platform and cliffs on San Remo Peninsula. This image, taken looking south from on top of the cliff, shows the western part of the Sand Remo Peninsula looking towards Cape Woolamai on the eastern tip of Phillip Island, separated by The Narrows, a narrow waterway. |
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San Remo is famous for its small fishing fleet and fish and chips on the way to the Phillip Island Moto Grand Prix, but it's also great for its exposures of Cretaceous terrestrial volcanoclastic sediments of Strzelecki Group. In this image we see gently north-northwesterly (NNW) dipping Cretaceous sediments with obvious bedding. These dip shallowly northwest towards the Bass Fault, which has had a major affect on the local geology by drag-folding the Cretaceous rocks and overlying rocks. The sediments are assigned a Lower Cretaceous age, so could be anywhere from 141 to 97 million years old (without further research). Can you find out? |
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The Cretaceous sediments are grey-coloured when unweathered, but display a range of yellow and red hues where exposed to the air, from hematite and goethite stains introduced by groundwater and modern weathering processes. This image shows a great example of weakly-developed boxwork (or "honeycomb") weathering, which leaves little pits or caverns in the rock surface, and is quite common in coastal areas. In this case, the raised portions are weakly cemented by silica dissolved from the rest of the rock, forming slightly more resistant "boxes". The material in the boxes is relatively less resistant to weathering and is plucked out by salt weathering to be washed away by seawater, rainfall and also fretted by windblown sand. You can learn more about salt weathering at Squeaky Beach. |
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Here's another example of boxwork weathering, this time with the boxes composed of ferruginised sediment, most likely goethite, given the yellow colour of the rock. Again, the "boxes" are produced where goethite cement holds the sediment together, leaving the less well-cemented sediment to be weathered out by salt weathering, seawater and rainfall and fretting by windblown sand. This is very similar to the boxwork weathering seen at Red Bluff. Boxworks are also very common in gossans above orebodies, so it always pays to take notice when you see this style of weathering - you might have found the next Broken Hill or Mount Isa! What's a gossan? It's the weathered surface expression of an orebody. |
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In this image we see the next part of the stratigraphy of Southeast Gippsland that's important to our landscape evolution story. Look closely at the picture. Do you see red material on the right, with a sloping band of white material over the top, then a thick sequence of grey-brown material overlying everything? This is an unconformity between the Cretaceous terrestrial volcanoclastic sediments of the Strzelecki Group and the overlying Eocene to Oligocene (Tertiary) basalts of what used to be known as the Older Volcanics. In other parts of Phillip Island we can see basalts directly overlying Devonian Woolamai Granite (for instance, at Pyramid Rock), but here we see that the basalts overlie Cretaceous rocks. The white material between the basalts and the underlying Cretaceous rocks is alluvial sediment, part of a drainage system that eroded valleys into the top of the Cretaceous rocks and partially eroded them. This sediment has then become trapped by basalt lavas that flowed down the valley commencing in the Eocene, 50-odd million years ago. |
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In the previous image, and in this one, things are not straight and level. Did you notice that the white sediments above the unconformity in the previous image were tilted? What's going on in this picture? This image shows weathered basalt columns in the top centre. How do basalt columns form? They're a cooling feature - as molten lava cools and solidifies it shrinks slightly, causing polygonal cracks to form within the rock body. The cracks propagate from the cooling surface (above, below and from the sides of the lava flow), forming columns as the cracks meet. These then form ideal conduits for water and oxygen to pass through, accelerating the weathering of the basalt rock. Normally, basalt columns are vertical, so why are these tilted? So too, why are the white sediments in the previous image tilted? It's probably because they are close to the Bass Fault, and neotectonic (recent tectonic) activity on the fault has tilted these sediments away from their original near-horizontal positions. |
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Here's a close-up of the cliff face showing the pervasively weathered basalt. Notice the angular boxwork and the rounded gray blobs surrounded by darker material? What you're seeing here are remnant basalt corestones (the rounded blobs), surrounded by a skins of a combination of iron and manganese oxyhydroxides, and remnant cooling cracks now filled with secondary minerals. How much of the original basalt mineralogy remains here? Probably not much - actually, I'd say none. It appears to have all been replaced by secondary minerals, the products of weathering. The angular cracks are filled with a combination of clays, calcium-magnesium carbonates and zeolites, giving them their white-buff colour. The corestones are composed mostly of clay replacing the original mineralogy. You can learn more about basalt weathering at Flynn's Beach. |
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Finally, Here's a wide view of the shore platform and cliff face at San Remo, showing the tilted basalts and tilted intra-basaltic weathering profiles present at this location. It serves to show that some things are not as they initially seem, and that old landscapes may be modified by later processes including burial and exhumation, but also by tectonic activity. |
Learn more about San Remo and its weathering by reading: W.D. Birch ed. 2003. Geology of Victoria. Geological Society of Australia special publication No. 23. 842 p. |
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