Fossil tree - Inverloch, Victoria

Cliff panorama showing a fossilised tree stump in-situ in the center left.

Close up of the tree showing bedding of sands above and below it. All beds tilt approximately 10 degrees to the East.

Zoom in of the stump showing the roots still twining through the now lithified sediments. Floods can do some amazing things and it's pretty awesome that this tree has been preserved like this!

Conglomerate core - Sydney Basin

Conglomerates from the Sydney Basin featuring cherts and jasper clasts. Core samples provided by J.Driscol of Monash University.

Fossil leaf - Sydney Basin sediments

 

A fossil leaf in a core section (rare) from the Sydney Basin. Section provided by J.Driscol of Monash University.

Fossil leaves - Illawarra coal measure, New South Wales

Both of these images are leaf fossils from the Illawarra coal measures in New South Wales. They have been dated to the Permian! They were provided by Dr C. Mays as teaching samples for a class I assisted teaching at Monash University and I couldn't resist snapping a few photos. They're lovely examples of preservation.

Places I've been - Dights Falls, Melbourne

Looking south at Dights Falls, Yarra Bend Park, Melbourne. Photo by Ant.

Here you can see the completed Dights falls (the fish weir is under the POV). Repairs and maintenance have been ongoing for sometime now. To the right are the apartment blocks of Abbotsford and in the foreground it is possible to make out the basalt blocks used to construct previous versions of the falls.

In the early 19th century, this was the location of a water powered flour mill. The falls were important as they marked the boundary between brine and fresh water on the Yarra River. Downstream of the location was an industrial zone and upstream was suitable for residences and farming (see Collingwood Childrens Farm).

Geologically speaking the bank the photograph is taken from is composed of basalts from the Newer Volcanics while on the far side of the river are Silurian turbiditites. The turbidites are deep ocean sediments where muds are generally deposited. Occasionally a significant storm or landslide event will result in the deposition of a layer of sands. The interbedded sands and muds of the turbidites make up much of geology east of the City of Melbourne and are interestingly tilted and folded.

In the image above, the sediments strike approximately North-South and dip around 45 degrees to the East. There is a locally thick bed of sandstones (approximately 1m thick) in this deposit which is responsible for the development and location of Dights Falls.

Geology around town: Flinders St Station, Victoria, Australia

 

Flinders Street Station, with an iconic Melbourne tram.

During a recent trip to the city of Melbourne I spent some time admiring the granites at Flinders Street Station. The granites most likely originate from the Harcourt Granite, located near Castlemaine, Victoria. They are a lovely creamy colour, course grained and feature a number of xenoliths and enclaves.

 

A xenolith, hornfels (with remnant bedding?)

 

An enclave featuring large K-feldspars located under the ticker sellers window.

 

Under the clocks to the right this xenolith is halfway up a pillar.

Places I've been: Organ pipes National Park, Victoria, Australia

 

Looking east at the leading edge of the organ pipes structure.

The second stop on our Igneous Geology field trip is the Organ Pipes National Park. As this was part of our first date, I've already posted about this location so won't go into much more detail. I will however show to pictures of new things I got to see this time:

 

Basaltic columns near the edge of Jacksons Creek just past the Tesselated Pavement. Hexagonal columns are created by the contraction of the cooling lava cracking at approximately 120 degrees. These cracks propagate downward as the body of lava cools creating columns.

 

The contact between the basalt and the underlying sediments. The white line is the metamorphised sedimentary rock, and is about 5mm thick.

 

Just below the contact, something I've never noticed before!

 

A conglomerate containing rounded quartz pebbles (1cm) and large angular sandstone cobbles (up to 10cm). This indicates the rock is not the Ordovician sediments but reworked into terrestrial sediments from the Eocene.

Places I've been: You Yangs granite, Victoria, Australia

It's that time of year again - yes the end of May. This comes with the inevitable two or three weekends of teaching the 1st year geology igneous field trip. Our first stop is the You Yangs granite. One of the biggest, and closest examples of a granite to CBD Melbourne (WGS84 37° 57′ 0″ S, 144° 25′ 48″ E), it towers over the basaltic plain to the west of the city. On a clear day the views are lovely, and you can even just see the Menzies building towering over Clayton in the far distance.

The You Yangs are a small granite body, more of a stock than a batholith. The granite is rich in quartz, oligoclase and orthoclase with minor mafic minerals such as biotite and hornblende. The You Yangs are porphyritic with large K-feldspar (orthoclase) crystals up to 7cm long and a course grained groundmass. The granite also contains numerous xenoliths and enclaves.

 

A granitic tor located behind the picnic ground bathrooms.

 

An enclave in the tor - note the concentration of dark minerals and the presence of large K-feldspar crystals suggesting a similar chemistry to the granite. This is a large example of an enclave and spans 25cm. Some of the enclaves I've seen in this granite are 60cm wide.

 

A close up of the side of the granite

 

Closer still - a xenolith is bisected by two aplite veins. Xenoliths are pieces of country rock that have been surrounded by the magma during intrusion. They are then metamorphised by the heat produced by the cooling granite. The xenoliths seen in the You Yangs are typically hornfels, formed from metamorphising the Ordovician siltstones and sandstones (marine). Aplite is a quartz rich vein with small amounts of feldspar. It is thought to be the residual melt that solidifies in the cracks formed in the cooling granite.

 

Pseudofossils - 12 Apostles, Victoria

I spotted some beautiful faux fossils in the sandy-limestone used to make the underpass at the 12 Apostles while I was there with the JMSS geology class a month ago. These dendritic patterns are formed by the deposition of minerals by hydrothermal fluids. They are often mistaken for plant fossils by the geologically-uneducated. Still, they're quite pretty.

 

Sandy-limestone? in the underpass at the 12 Apostles, Victoria.

Historical places I've been: Tower Hill Maar, Victoria

 

It's been a while since I've done one of these. This is an image of Jim Driscoll and I at the Tower Hill Maar in Western Victoria, 2013. He's into sedimentology and geothermal research and is just about to finish his PhD at Monash University. For the last couple of years we've assisted with the JMSS geology field trip and this is one of the locations we take the students.
This is a nested maar system is the result of a sequence phreatomagmatic eruptions (note the huge number of layers behind us) resulting in a complex structure seen today. Multiple scoria cones are surrounded by a crater lake (which is probably formed by multiple vents). The countryside around Tower Hill is covered by these ash and scoria layers which all slope gently away from the site of the volcano.The volcano also ejected larger bombs, the impact of one can be seen over my right shoulder. The shape of these craters can help determine the trajectory of the bomb and allows volcanologists to determine the sequence in which the vents were actives.

Historical places I've been: Eldee Creek structure

 

Eldee Creek Station is about an hour and a bit northwest of Broken Hill. Each year in winter, Monash University takes it's 3rd year field geology class up there for a two-three week mapping course. I've participated once and have taught it twice. Ever time I go, I make a new map of the structure, and each time it's different. The images above and below were taken from Robert Weinburg's Eldee Project page on the Monash University website.

 

The core of the Eldee structure is pegmatite which is enclosed by pelitic gneiss, meta-sediments and felsic gneiss with the occasional amphibolite pod to make things interesting. There are beautiful folds, some nice clear faults and goats a plenty in this field area. I quite enjoy quietly tramping across the hills listening to the skitter of pelite shingles as they skid away from me.

Yellow - Amphibolite, , Green - Biotite Gneiss, Blue - Muscovite Schist, Pink - Felsic Gneiss, Red - Pegmatite.

This is a map from 2010 I made of the northern part of the structure while supervising small groups of students. As a result I take no structural measurements and often walk the same patch three or four times while having to skip other areas. The area is a lot more complicated than shown here and later maps that I've made show complicated F2 folds in the eastern half instead of those large pegmatite blobs. As you can see, the pegmatite gives the main radiometric response.

The rocks we map (felsic gneiss).

Historic places I've been - Lake Purdiguluc, Colac, Victoria

 

Geophysics survey, Lake Purdigulac, 2009. -38.260268, 143.499328

Purdigulac is a nested maar complex of multipe vents and some scoria cones. It lies south of the Red Rocks complex and right in the middle of the Eastern Volcanics. It was a pain to survey as the lake occasionally fills with water and the concentration of salts creates meter deep cracks in the sediments at the bottom of the craters during dry times. Flicity (volcanologist), Tegan (volcanolo-physicist) and I (geo-physicist) quite enjoyed our time exploring this volcano.

 An image of the Red Rocks complex taken nabbed from the AltonaBeauty blog. Purdigulac is part of the Red Rocks Complex which contains at least 9 craters and a number of scoria cones. Work by Teagan Blakie and Flicity Piganis has identified that Purdigulac is one of the most complex craters in this complex with at least 9 eruptive points and shallow diatreme structures with multiple coalescing vents.

Field trips - Billa Kalina in the Womera Protected Area

 Sunset in the South Australian desert during winter - cold but beautiful.

 
The Woomera Prohibited Area! 

I have recently completed a short field trip to the Womera Prohibited Area. This zone covers an area greater than England in the South Australia arid area. Still occasionally used for jet / missile (and things) testing, much paperwork is required to gain access. The company I currently work for has a couple of tenements in this area so my boss decided we should go up there and oggle the rocks. Given this area is in the Gawler Craton there weren't many rocks to look at, but we made the most of the ones we could find.

 A normal fault in some Bulldog Shale. The resistant layers are reinforced by gypsum and have been displaced by approximately 15 meters.

There were three of those pod like structures separating two different mudstones / weathered shales.  Below the main outcrop, slightly sandy and more resistant layers can be seen (near write-in-the-rain fieldbook)

In some sections of the pod, cross beds can be seen.

The edges of the pods show the bedding of the mudstone wrapping seamlessly around them. This + xbeds suggests to me that these are paleo-channels. The Bulldog shale is thought to have formed in shallow marine conditions during oceanic transgression.

Not on our patch, but we visited some Adelaidian sediments anyway. This hills were mostly composed of quartzites and conglomerates. Note the 6m high sand dunes in the middle distance and the lack of topography so typical of this area.

 Getting around is SO much easier when you have a Gator.

Historic places I've been - Mt Tamborine, Queensland

 

Mount Tamborine is the remnants of the edge of a shield volcano. In 2012 I found this more resistant bit, or possible dyke on the side of the road and had to stop and pose with it. I know nothing of the local geology except Brisbane area is one giant volcano (possibly hotspot related) and as a result they appear to have ignimbrite as their paving stone of choice (Melbourne uses Basalt - what does your city use?). It's on the side of a road around -27.909855, 153.181835.

Further down the road at Ceder Creek Falls, -27.894321, 153.185086. It's hard to take a selfie AND capture what looks like a contact between lava flow and ignimbrite (on the right)

 

Large clast in the ignimbrite with bracelet for scale - no compass, no geopick, no handlense and no scale bar, what sort of holidaying geologist am I?

 

Late stage faulting with quartz/calcite? infill - where's your hardness kit when you need it?

And finally what we came here for - Ceder Creek Falls looking down at the main pond. This is the best shot - I got everything else came out blurred. I suspect this'd be a lovely place to swim in summer.

Historical - Places I've been, Forest Creek, Chewton, Victoria.

Forest Creek, Chewton, Victoria. -37.079266, 144.260201. The outcrop in question is located in the northwest of this image, in the creek bed.

Geological map of the area in question. The outcrop is either Castlemainian Zone 1 or Chewtonian Zone 1+2 (Castlemain Chewton Goldfield map, C.E. Willman 1994)

 

 Partial outcrop facing South. This is the western most fold of a three fold section. The northern outcrop is pretty cool too but I don't have any large scale shots.

 

The outline of the structure not shown in the photograph. Boxes show the location of the closeups that follow. Eastern most, Big then Central.

 Two ideas for this set of crenulations: 

Option 1 - Not a F2 syncline, a F1 recumbent fold in the limb of the anticline.

Option two - The limb photographed has been progressively tightened by continued orogeny. 

S1 cleavage is rotated as western limb is tightened, later overprinted by upright cleavage (S1.5) due to ongoing compression..