This post is going to summarize everything I’ve learned about Australia from all my field trips so far. The first section, presents our last field site in the Australian Capital Territory, however, before summarizing the geology of the ACT, Queensland, Victoria, and Tasmania.

Tertiary Sediments

This is the last post from the ACT. Site 31 (see Fig. 1 for location) is located in the Botanical Garden, where a gully was excavated to create a rainforest. Despite the low rainfall in the area, it worked and a path leads through what appears to be an erosional gully from Queensland.

 Fig. 1

The fanglomerates exposed by the excavation haven’t been dated but are believed to be 2-3 million years old. This is too old for carbon-14 and too young for other radiometric techniques. Apparently, they don’t contain sufficient fossils to date either. These rounded cobbles of all sizes (Fig. 2) were deposited on an alluvial fan (hence the name) near steep mountains.

Fig. 2

 A   B

 C

The cobbles range in size from ~one foot in diameter to less than an inch. Such a disparity is size results from short transport distance from the source, but the rounded shape of many of the cobbles suggests that transport in steep and rapidly flowing streams before deposition.

ACT Summary

Most of the rocks we’ve seen in the ACT were deposited/intruded/extruded between 485 and 400 million years ago. The oldest rocks were originally deposited in an ocean trench near rapidly rising land with volcanoes erupting periodically, someplace like Japan today. Sedimentation accompanied by granite plutons pushing into older rocks as well as explosive volcanism continued throughout this interval. Sometime after ~420 Ma, many of the older rocks were buried and subsequently folded during a major compressional tectonic event. All of this occurred within a relatively small area, but over an immense time interval.

Whatever event created the mountains that must have existed ended and erosion began. Erosion of this immense mountain range continued for almost 400 million years, at which time the remnant mountains were still large enough to have active alluvial fans between 2-3 Ma.

Events in Queensland

We can fill in some of the gaps from the ACT by reviewing our trip to Brisbane.

First, Sand and mud were collecting in a shallow marine environment between 383-323 Ma, less than 600 miles NE of Canberra. These sediments were deeply buried, deformed, and slightly metamorphosed sometime after deposition. Volcanism was occurring as well.

Between 237 and 200 Ma, sand and mud were being deposited in a delta environment with enough organic matter collecting to form commercially viable coal beds.  During this same period, massive layers (>300 feet) of rhyolite were ejected from a shallow magma chamber, forming one of the largest calderas in the world. Erosion ensued until 23-16 Ma, when basalt flowed over the Brisbane area. (Note the change from felsic to basic volcanism during this geologically brief time span.)

Changes in sea level are currently leaving a record in beach sediments that may someday become rocks.

Events in Tasmania

Sedimentary rocks were collecting in Tasmania (~600 miles to the south) between 1600 and 540 Ma. Volcanism was also occurring during this interval, recording a mountain building event not that different from that which occurred later. In fact, the dates are close enough (540 Ma here versus 485 Ma in Canberra) to justify calling this a continuous orogeny. Slightly later (500-470 Ma), a coastline was present in central Tasmania, and intermediate-to-felsic volcanic rocks were produced when a large granitic pluton was emplaced into older sedimentary rocks. More granite was emplaced between 420-360 Ma.

To get an idea of what might have been happening over such a long time (~1600-350 Ma), imagine all the islands east of Australia (preferably viewed on a globe) colliding with it, being swallowed beneath the much more massive land mass. Finally, picture Australia colliding with South America about 350 million years ago. It was that massive an event. Faults and folds are everywhere.

Between 252-201 Ma, Tasmania was collecting rocks in lakes and rivers, i.e., terrestrial sediments. There was also some alkali volcanism occurring. This period didn’t last long, however, being followed (201 and 145 Ma) by the emplacement of a huge basic intrusion close to the surface, between layers of sedimentary rock (aka laccolith).

Basalts flows spilled out of multiple volcanos around 60 Ma. Gravels and sandy sediment collected after ~2 Ma and the current surface is eroding as the land rebounds from removal of miles of sediments.

Summary of Victoria

Melbourne was part of a massive delta system during the Ordovician period (~490-440 Ma), at the same time that so much was happening ~400 miles to the north, in the ACT. These rocks were later folded. Sedimentation shifted to sandy during the Silurian period (~440-416 Ma), probably due to a river changing course. Sometime after 440 Ma, regional compression changed from SE-NW to SW-NE, and increased in intensity.

During the Cretaceous period (145-65 Ma),  clastic marine sediments were collecting, as well as nonmarine. Sea level was oscillating. No radiometric dates are available.

The SE tip of Australia was covered by a massive volcanic field after 65 Ma. These flows reached the ocean and extended offshore. Some ash layer were created as well. Nonmarine sediments were also collecting during this period, but they weren’t buried deep enough to become hard.

Between 10 and 5 Ma, the Melbourne area changed from a shallow marine/deltaic environment to an arid land, with episodic sedimentation in streams. Basalt flows continued to pour onto the land, until 700 thousand years ago. During the last million years or so, the climate has dried and most rivers don’t reach the coast, leaving relict sands scattered over older rocks exposed at the coast. Cliffs are common features, partly because of sea level change, partly because of isostatic rebound.

That’s it so far. I’ll update the summary as the field data accumulate.

As I’ve been discussing in the last few posts, the rocks of the ACT record a mountain-building episode between about 485 and 400 million years ago. This post is going to examine some volcanic rocks extruded towards the end of this period.

We drove to the top of Mount Ainsley, Site 16 from the regional guide (See an earlier post for details), from where we looked down on Canberra (Fig. 1), standing on top of a stack of mostly volcanic rocks extruded between 433 and 427 Ma, towards the end of the orogeny. The top of the mountain is approximately 800 feet above the base, which also reveals outcrops of the Mount Ainsley Volcanics formation.  That is a thick pile of volcanic material deposited over a 6 million-year interval.

Fig. 1

 A   B

However, as we’ve learned before, geological time is long. For example, (using SI units for convenience), the mean extrusion rate is 250 m/6 million years, or 40 cm/1000 years. The number is simply too small to comprehend as an annual rate.  This is a good rate, however, because it suggests that on average about 40 cm (16 inches) was extruded from a nearby volcano every 1000 years. So it wasn’t as if hell was raining down daily. Let’s see how our estimate compares to the rocks.

We’ll start at the highest point on Mount Ainsley. It’s typical for volcanic rocks to look like they have bedding because they are often produced in episodes and thus form layers, but they aren’t deposited by water so they don’t have well developed layers. That’s what we see in Fig. 2

Fig. 2

 A  B

 C  D

Figure 2A shows a well-preserved face, and this irregular bedding as well as some fractures are visible.  A close-up (Fig. 2B) shows a gray rock, varying from lighter to darker shades. There is some irregularity in the coloration on this relatively fresh surface and xenoliths (pieces of rock) and phenocrysts (minerals) are visible as lighter color blebs and very small particles in panel B. The variability of the deposit is obvious in Fig. 2C. The left side of the steps is thinly bedded whereas thick layers are visible to the right.  In fact, Fig. 2D (taken on the other side of the knob that defines the highest point of Mount Ainsley), shows massive, rounded appearance.

The only way to identify individual volcanic deposits is if enough time passes between them for the surface to weather. That isn’t obvious here and there was no mention of it in the guidebook. Thus, it’s possible that Fig. 2 represents a single episode, or many scattered over decades or centuries.

Massive layers are exposed in road cuts as we head down the mountain (Fig. 3), through the thick section of volcanics.

Fig. 3

 A  B

Figure 3A is featureless but panel B reveals a different texture in the middle of the photo, as if the material was fractured before it cooled, giving it an irregular appearance.

I haven’t mentioned what kind of volcanic rock this is. First, it’s gray color is intermediate between darker volcanics (e.g., andesite) and rhyolite, which is very light gray. The guidebook identifies this as dacite, which is in fact intermediate between the two. Dacite consists primarily of quartz (gray material in photos) and plagioclase feldspar (white material). Some of the 800 feet of volcanics was probably extruded as a thick substance because magma with lots of quartz tends to be thicker than low-silica melts. However, these magmas also tend to be explosive (remember Mt. St. Helens), trapped gasses being released catastrophically.

At the bottom of Mount Ainsley, we went to Site 26, where blocks of agglomerate are exposed in a small area (Fig. 4).

 Fig. 4

These boulders are in place, being the tips of a larger outcrop covered by the thin soil. A piece broken off by a previous visitor was waiting for me (Fig. 5A). I didn’t have to touch it. Note that we are now at the bottom of the mountain, somewhere near the bottom of the pile of volcanic material. These rocks are certainly thousands, if not millions, of years older than those in Fig. 2.

Fig. 5

 A  B

The sample in Fig. 5A is 18 inches long. The lighter areas are either pieces of country rock, or feldspar (which crystallizes at a higher temperature than quartz). Figure 5B is close enough to see the wide range in the size and shape of the phenocrysts (the image is ~5 inches across).

I’m going to take a moment and look at this rock (not the same sample) even closer (Fig. 6).

Fig. 6

 A  B

 C

Figure 6A is about 3 inches across, and individual phenocrysts can be identified, including both white (feldspar) and black (unknown) minerals. Zooming in on the photo, the crystallization of the phenocrysts is clear, and even the dark gray groundmass takes on an irregular shape, no longer appearing uniform.

Figure 6B is at even higher magnification. It shows the edge of one of the white phenocrysts against the matrix. The contact is not as sharp as one would expect if it was already solidified and then ejected along with the fine particles within which it was trapped. This magma blasted out of the volcano so hot that the material raining down on the land fused together, while retaining individual mineral characteristics. It wasn’t molten. This kind of structure is unique and well known because we can study ignimbrite in the modern world.

I threw Fig. 6C in for the hell of it. This is at the highest magnification my phone could achieve. Zoom in on this photo and you’ll see the assortment of crystal shapes that existed in the magma chamber when the phenocryst (angular, light-colored shape) composed of individual crystals was ejected. Note the sharp contact in the lower-left corner of the photo, so different from Fig. 6B.

Neither the magma chamber nor the external environment was uniform. Imagine a pot of boiling oil, bubbling, spilling out of the pan and making a mess. That what an eruption from this kind of volcano is like.

I’ll wrap up this series of posts next time.

Introduction

I’m taking a break to examine the rocks exposed by the recent excavation of State Circle, which obviously hadn’t occurred when the documentation I used for our geological investigation of the ACT was written.  Apparently, this site has become a part of the teaching of earth sciences from elementary school through a graduate program, and I can understand why. I briefly introduced these rocks in the last post. I promised to investigate them further.

The length of the road cut is more than 300 yards, but it’s never more than twenty feet high. It was so wide and narrow that creating a panorama was impossible, because of bridge supports, etc. I did my best (Fig 1).

 Fig. 1

This is only the western half of the exposure.  It is a self-contained sedimentology laboratory, from rocks deposited more than 400 million years ago. I am unwilling to spend the time required to treat this exposure with the care it deserves, so I’m going to hit on a couple of sedimentological processes and try not to go too far out on a limb, but I’m certain I will fail. This is a blog, not a stratigraphy course. This blog is about thinking, not being told what to think, so I’ll be brief and rely on photographic evidence to make my points.

Depositional Environment

I mentioned the evidence for this exposure representing a nearshore shoal, submarine bar, or barrier island in my last post. Geologists are pretty familiar with the structures of nearshore marine sand bodies and we didn’t see anything surprising here. For example, the steep crossbedding observed to the right of Fig. 1 (landward) suggests that the silt/fine sand part of this unit (Fig. 2) was deposited in a sediment-dominated environment.

As an aside, the white sediment is very fine grained, so I tasted it. It is not sand, not even fine sand. I didn’t grind it between my teeth (the test for silt), but it didn’t have the greasy taste of clays, although it was a little salty tasting.

 Fig. 2

Thick sets of cross-bedded silt can only occur with weak waves and high input from a river transport system, from a source distant enough to reduce quartz and feldspar particles from granites (sand sized) to silt. A wild guess would be 50 miles from the source. These are relatively hard minerals. Even correcting for any structural deformation, it is obvious that the silt beds in Fig. 2 are close to their original orientation because the adjacent intercalated silt/sand/mud is sub-horizontal.

It isn’t so obvious when we look at the landward side of this sand feature, where post-depostional deformation has occurred (Fig. 3).

 Fig. 3

The failed seaward margin (in the distance) of the sand body can be seen at the left margin of the photo. We have seen that the sediment layers are nearly horizontal there. That isn’t the case on the landward side of the feature, where we see draped intercalated silt/mud dipping at 30 degrees. I don’t want to say too much yet because I’ll address this in the next section, but we need to remember that the landward side of the sand body would have been shielded from wave action; thus, the sediments would not have been as winnowed or dewatered. When an infrequent storm event occurred, therefore, the effects would have been erratic. That is what we see in Fig. 3.

The only other issue to raise, but only briefly, is the sudden change from uniform silt to mixed sand/silt/mud. This is only conjecture, but I’d be willing to propose that transitions are not easily recognized in the stratigraphic record. I’m going out on a limb…maybe I’ll wait.

I should spend a few words on the mixed silt and mud layers, which are obviously important. This area (god only knows how many millennia later) became a tidal flat, as indicated by the mixed sand and mud above the silts we’ve been discussing (Fig. 4).

  Fig. 4

The depositional environment here in central Canberra didn’t change in a heartbeat. Figure 4 is about 10 feet  in height. This looks like a pretty stable environment, from the rhythmic layering of clay and sand/silty sediment. How stable was it?

Figure 4 reveals a stable depositional environment that created at least 3.3 m (I’m using SI for convenience) of sand and mud. Using a conservative depositional rate (for a tectonically active region) of 3 mm/year, we can look at the sediment in Fig. 4 from a temporal perspective: Yoko is standing in front of about 1000 years of sediment. My jaw drops just like yours (should).  I had expected it to be at least a million years; there is a caveat, however. Tectonic processes have to make accommodation space (as it’s called in sedimentology) for all that sediment. We can skip to the chase and say that in the Canberra basin, the accommodation space was available because tectonic plate movements (vertical as well as horizontal) made it happen.

Syndepositional Deformation

Anyone who has ever walked along the seashore understands syndepositional deformation. How in the world were the footprints of dinosaurs, birds, even people, preserved in ancient sediments? To be clear, very few events are preserved. When I go to the beach and stand there, letting the waves dig a hole around my feet, my pleasant experience will not be shared with future generations. Very little is preserved in the constantly moving surface world, kept in flux by wind and water. I can’t answer my own question (it’s rhetorical), but here are a couple of examples that might make sense.

I used this picture (Fig. 5) to suggest that mud is stronger than sand and would form boudins but, to be honest I don’t think I have to prove anything. Just look at the record that was in front of my eyes (Fig. 4).

 Fig. 5

The slip-face and resulting Shear Zone led to failure of the seaward-facing (to the left) sand body, so the newly accumulating (over centuries) mud slipped down the slippery slope, forming boudins (indicated by ellipses with squiggling lines), and then everything stabilized. The reason the boundary between the sandy sediment and the mixed sediment is so distinct is the buffering effect of hydrodynamics.

There’s a saying, “It will all come out in the wash.”

That is what coastal hydrodynamics does. Here’s another example (Fig. 6), this time showing how wet sediments can do things impossible for rocks.

 Fig. 6

Figure 6 is less than 10 inches across. The heavy black line tracks a single layer of sediment folded at a ridiculous angle. The ellipses, sometimes filled with squiggles, are boudin structures. The hand-drawn arrow shows the continuity of the “bedding.”

The basic process revealed in Fig. 6 scales up (Fig. 7).

 Fig. 7

Figure 7 is key to understanding syndepositional deformation. It doesn’t matter what caused the shear zone in the figure. The orange lines show layers of sediment remaining coherent while sliding downhill, until the rate of shear/strain was too great, when they became jumbled (scribbly orange lines at bottom-left of Fig. 7). The incompressible sands of the subjacent unit flowed smoothly (black lines), jamming  into the shear zone. Some turbulence is indicated at the top of the transition after the slump, but then sedimentation continues, only now dominated by mixed sand and mud (see Fig. 5 above).

Postdepositional Deformation

I haven’t covered a fraction of what this site offers. I am overwhelmed. Serious research  studies, integrating this exposure into the other stratigraphic and sedimentological data from the area, are probably underway. I’m going to limit my analysis to what I’ve seen with my own eyes. My interpretation is probably incorrect, but it isn’t because of negligence.

Recalling that these sediments were deposited between 443 and 427 Ma, it is surprising that they are not in worse condition than they are. For example, compare Fig. 3 to photos of the Canberra Formation and the Black Mountain Sandstone from my post on Siliciclastic Sedimentation. All of these rocks are about the same age and were deposited within a few miles of each other.

The rocks comprising the Black Mountain Sandstone are fractured, show signs of extensive cementation, and are tilted in different directions because of faulting. The Canberra Formation rocks are faulted and folded to the point of being overturned. It’s true that the time interval for deposition at these three locations was large (16 million years), which is a very long time. However, we know that regional deformation hadn’t ended because of folding of the younger (424 Ma) rocks at Deakin Anticline.

I would have thought these were Tertiary sediments if I hadn’t looked at a geological map (actually, I used the ROCKD app on my phone). This is an anomalous site.

Nevertheless, these rocks have been deformed.  There are high-angle normal faults in this sediment with no evidence of brittle fracture (Fig. 8), i.e., no crushed zones (breccia) or re-cementation (unique minerals and micro veins).

 Fig. 8

Examining Fig. 3 above, we see that the outcrop reveals a very low-angle anticline, which implies that these rocks were compressed (folded) and later went through an extensional stress regime, as clearly indicated by normal faults in Fig. 8. The thin, upper beds have been folded, however, and the faults also show some curvature.

The next example could be discussed as either syndepositional or post-depositional deformation (Fig. 9). It has some elements of both, but mostly I think this feature was created during shallow burial, no longer at the surface and thus post-depositional.

 Fig. 9

The white, horizontal line indicates what I think was originally a continuous sediment layer, so there’s about 3 feet of displacement along the normal faults indicated by dashed lines. The block arrows show relative direction of movement of the three blocks seen in the photo. The stress field is tensional, i.e., the sediments/rocks were being pulled from both ends, somewhere other than this outcrop. Note the upward curvature of the sub-horizontal line on the left side of the image. This sediment was not cemented when faulting occurred.

I admit this is pure conjecture but, the narrowness of the Horst (upward moving block) suggests that the two normal faults either converged less than 20 feet up the section, or these sediments were very close to the seafloor when this brittle deformation occurred. It isn’t too difficult to imagine faults splitting and joining because rocks are heterogeneous when buried and somewhat ductile, so maybe…

Spatial and Temporal Variability

I was unwilling to say too much earlier, but now is the time to speak openly. The sudden change from relatively pure very-fine sand and silt to intercalated silt/mud that is so obvious in this outcrop didn’t happen. The river and ocean system didn’t switch one day, and stop delivering/depositing massive volumes of silt. Mud didn’t occur magically as if a dump truck (of immense size) had unloaded into whatever primordial river ran through this area ~430 million years ago.

This is an example of how subtle changes in a complex system like the earth’s surface aren’t evident immediately. A system as complex as the lithosphere-atmosphere-hydrosphere adjusts to slight changes, until it doesn’t. Then — Voila — a suddenly very different situation. The childish interpretation, that the world is a very simple place and abrupt changes in the geological record, as we’ve shown in this post, are the result of intelligent-design — Gaia, God, a horned toad with superpowers, Aliens…whatever — fails to account for our nascent understanding of nonlinear systems.

I have a simpler explanation: Stratigraphy doesn’t immediately record transitions in the geological environment because fluid-dynamic-driven, processes like sedimentation, act as a buffer that smears transitions out, over thousands of years. The earth’s surface is three dimensional. The stratigraphic sections we’ve been examining make that obvious. Time is another element; but it isn’t directly correlated to the vertical dimension in stratigraphic sections of sedimentary and volcanic rocks (both deposited horizontally).

I will address the disparity in deformation in the last post of this series.

This is the third post from the ACT. For an overview take a look at the first post.

This time I’m going to visit several locations from the Canberra area that show the variability of sediments during the orogeny. Figure 1 gives some idea of the complex history sand and mud particles have after being deposited in the ocean. We’ll talk more about that in the next post, but for now we’ll compare the sediments.

 Fig. 1

The rocks in Fig. 1 are estimated to have been deposited between 443 and 427 million years ago (Ma). The white rock has a lot of features that we’ll look into next time. For now, notice that the overlying thin bedded light-and-dark layers appear to be in continuous contact with the lighter rock. Ignore the displacement along faults.

This sediment consists of uniformly fine-sand-to-silt particles. The steep cross-bedding (Fig. 2) suggests this was a nearshore marine environment with strong wave action, producing submarine bars and probably a steep delta-front winnowed by wave turbulence.

 Fig. 2

The cross-bedding suggests transport from the right side of the photo, so fine sand was coming from an easterly source (e.g., NE to SE), probably near a river mouth. Imagine the mouth of the Columbia River, draining the Cascade Mountains. A lot of sand is deposited in giant spits and sand bars. The cross-bed sets (Fig. 2) are ~3-4 feet thick and dipping seaward (to the left). Note the near-horizontal orientation of the conformable sediments overlaying this unit. This is very close to the deposition angle.

What about the overall shape of the sand body? At this outcrop, we are able to see what appears to be a cross-section of this submarine feature (Fig. 3).

 Fig. 3

The photo is taken looking offshore along the paleo-shoreline, so the contact between the sand body and the overlaying sand/mud sequence dips landward and shows some irregularities (note the tongue protruding upwards to the right of the motorcycle). It reaches its maximum thickness in Figs 1 and 2 (left of the biker in Fig. 3) and is truncated by a fault. It looks like a fairly large submarine bar , maybe 200 yards across. Could have even been a barrier island or shoal that was buried by mud.

There is one last feature to mention in passing about this outcrop. The contact between the fine sand body and the overlaying mixed sediment indicates deformation of the sediments when they were still sediments, i.e., before burial (Fig. 4).

Fig. 4

 A  B

Figure 4A shows a pinching to the right of Yoko, forming narrow necks separated by lozenge shaped dark sediment. This structure is called “boudin” because it looks like a sausage. In sediments, it occurs when mud is stretched overlaying sandy sediments, which are very weak in extension. This conjecture is consistent with extension indicated by normal faults seen throughout the exposure. Figure 4B shows filling of channels by the underlying coarse sediment, analogous to a river flood channel being filled in. Note that the sediments are dipping ~10 degrees to the right in panel B and take that into account.

A closeup view of Fig. 2 reveals extensive dragging of sediment layers along both sides of the faulted contact between the two units. This cannot happen with deeply buried and partially cemented rocks.

The exposure we’ve been discussing is located near Site 22 in Fig. 5. The next stop takes us to Black Mountain, Site 4 in Fig. 5, where we’ll examine some turbidites deposited during the same broad interval (443-427 Ma).

 Fig. 5

The Black Mountain Sandstone is called a flysch, part of a sequence grading from deep water turbidity current sediments to shallow water shale and sandstone. We aren’t going to be able to determine that much detail from our field work. The beds vary from nearly horizontal (Fig. 6) to dipping at more than 30 degrees (Fig. 7).

Fig. 6

 A  B

Figures 6A and B were taken at a single exposure along a road cut. The thin sand layer (white in both panels) is continuous between them. This layer is less than 1 foot thick. A massive sand layer is separated from it by layer of mud.

 Fig. 7

Figure 7 is how the rocks typically appeared. They are jointed and brittle and it is difficult to discern any sedimentary structures in them (Fig. 8).

Fig. 8

 A   B

A close-up examination of Fig. 8A suggests that there are some larger particles present, but I wouldn’t bet on it. Discerning fine sediment structures in fine-sand turbidites requires breaking a lot of hand samples off from the exposure, and I don’t do that. I have to live with what’s visible at the outcrop. There is a hint of slight cross-bedding, dipping to the left in the lower-right corner of panel A. Figure 8B gives the impression of layering that dips to the left as well (these samples were in place but the photos are not oriented). There is also evidence of angular fragments consistent with a turbidity current, which transports larger grains as bed load in a predominantly fine matrix.

We had the opportunity to take a few photos of a spectacular road cut of the Canberra formation (also 443-427 Ma) located on the eastern edge of Fig. 5. This formation consists of finer clastic sediments and some volcaniclastics. We couldn’t stop for a close examination because of heavy weekend traffic and no place to pullover, so we drove slowly where there was a passing lane and took a lot of photos. Figure 9 is representative of what we saw (after the fact).

Fig. 9

 A   B

These were two identifiable (to me) folds with thrust faults. The area labeled “Fault Zone” had no recognizable bedding and tended to be fragmented. Other photos revealed vertical to overturned beds or breccia zones with no discernible structure.

The final example of siliciclastic sediments from this orogeny is from Site 10 (Fig. 5), a fine-grained, volcaniclastic sandstone deposited around 424 Ma.

Fig. 10

 A  B

 C  D

Figure 10A gives more technical details than I can give, and it is a good example of how seriously Australian’s take geology. Anything accessible has a sign explaining its geological history. Panel B shows the anticline hinge that made the site famous. Figure 10C shows the fine lamination expected in a fine-sandstone, with indications of soft-sediment deformation as well (near the key fob). Panel D shows what’s left of the hinge point. Looking back at Fig. 10B, the hinge (greatest curvature) is to the left of Yoko.  That’s what is shown in panel D. In other words, this anticline was folding over like so many others (e.g., Fig 9) when the rocks were deeply buried and being compressed.  I walked all around the site but found no glaring evidence of the fault referred to in Fig. 10A.

That does it for this post. We’ve looked at sediments deposited over a 60 million-year interval during an orogeny, and how they’ve been deformed since. We’ll get back to that in a later post, when we put this road trip into the bigger picture.

For details about the big picture, take a look at the first post in this series.

This post goes back to the beginning of the mountain building event, about 485 Ma in the past. We took a drive to Lake George, about 40 km NE of Canberra, just over the border in the state of New South Wales. Lake George is a shallow depression with no outlets and very little inflow. Thus, it is often dry and water depths are on-average about 1 m (Fig. 1).

 Fig. 1

It was mostly dry when we visited. Figure 1 looks across the lake to the east from the west side (location shown by red circle in Fig.2). We’ll be circumnavigating the basin in this post.

 Fig. 2

Lake George has a long geological history, including uplift of the west side of the basin within the last tens of millions of years. This faulting has brought Adaminaby Group (485-443 Ma) sedimentary rocks to the surface after more than 300 feet of uplift. This is a turbidite sequence of sandstone, mudstone, shale, greywacke, chert; quartzite, phyllite, slate (Fig. 3).

Fig. 3

 A  B

These beds consist of alternating fine grained sandstone and siltstone, with shales containing sand lenses. They are dipping steeply to the NE, but this faulting is not associate with the Paleozoic orogeny discussed in these posts.

Examining the rocks up closer (Fig. 4) reveals fine textures supporting the proposed turbidite depositional environment.

Fig. 4

  A   B

 C

Figure 4A shows a bed ~6 inches thick, tilted 60 degrees to the right. Zooming in on it reveals thin cross-bedding intersecting joints. Figure 4B shows graded bedding, with slightly larger grains revealed in the white layers against the grey background of smaller grains to the right of the sample.  Panel C reveals sets of laminae at low angles, intersected by joints filled with cement.

We followed a narrow asphalt road, which became gravel, to the NE side of Lake George (see Fig. 2), crossing through volcaniclastic and siliciclastic rocks deposited towards the end of the orogeny (433-410 Ma). There were no outcrops of these sedimentary rocks, however, and only a few sandstone beds were visible in ditches. The dominant exposed rocks were outcrops of small boulders of a gray, fine-grained  igneous rocks on low hills (Fig. 5).

Fig. 5

  A  B

 C

Figure 5 shows small outcrops (< 2 feet in length) of in-place fine-grained rock with large quartz crystals along joint surfaces (Fig. 5A) and as veins within joints (Fig. 5B and C). This fine-grained rock is part of a granitic pluton about 420 Ma in age, towards the end of the orogeny. The fine-grained rock may be overlaying volcanics into which the granitic rock was intruded at this location, although no intrusive rock was visible.

We followed a track to get as close to Lake George as possible (Fig. 6), where a small hill could be seen in the distance with outcrops visible. Most of the larger hills were covered with soil. This is a heavily eroded granitic pluton.

Fig. 6

A few miles further east took us back into the oldest rocks again (~485 Ma), to the east of Lake George (Fig. 7A), where we found thin bedded siltstone and shale (Fig. 7B).

Fig. 7

  A   B

A short distance further finally revealed the granitic basement (Fig. 8).

Fig. 8

  A   B

 C  D

Figure 8A shows the largest outcrop we found. It is about 4 feet across. A closer look reveals a granitic texture, with alkali feldspar and quartz visible as the pink-to-white and gray crystals, respectively (Fig. 8B). An even closer look (Fig. 8C) verifies the overall impression we have from panel B. However, this is not a homogeneous granite as seen in panel D, which shows contamination by lithic fragments (angular grains) and evidence of strain (shear displacement) as the magma was cooling, producing weak lamination within the darker part of Fig. 8D.

This post has circumnavigated a shallow basin surrounded by rocks from representative intervals during the orogeny. The oldest sedimentary rocks (~485 Ma) contain turbidites deposited on steep continental shelves where the land is rising. Deposition of these proximal (deposited near rising land) turbidites and greywackes continued as explosive volcanism began to dominate nearby. The sediments are from the ocean whereas the volcanics (Ignimbrites and tuffs) were deposited on land. This continued until granites were emplaced by about 420 Ma.

Next time, we’ll closer at the sediments deposited during this long interval.

We are on a quick trip to Canberra the capital of Australia located in the Australian Capital Territory, which is analogous to Washington DC, but it’s called the ACT for short. It’s about halfway between Melbourne and Sydney, in the state of New South Wales. I’m doing the post a little different this time. This is the first post. I may update it to keep the information all together, or I may make a separate post. I don’t know yet.

We’re going to start in the present. Figure 1 shows what the Paleozoic orogenic belt we will be examining looks like today. The figure was taken from the Telstra Tower, sitting atop Black Mountain on the outskirts of Canberra, looking to the NNE. We’ll be heading that way in our next post.

 Fig. 1