This post didn’t require driving anywhere to collect our field data. We just looked out the window of our hotel room. Kangaroo Point is a famous rock-climbing spot, where climbers come to hone their skills; the city even put in powerful spotlights for night climbing. It’s also a fascinating rock exposure, revealed in a meander of the Brisbane River (Fig. 1).

 Fig. 1

Figure 1 is the attention getter because it is so massive (60 feet high), but it isn’t completely uniform. Towards the left of the photo collage the rock is more massive and the cliff higher, although it is also set back a little with buildings along the river. It abruptly drops. The slope is less to the right. This may mean nothing, but I also noted that there is more visible structure in the exposure to the right. I had to get closer.

Our investigation began where the hill began to rise, about 200 yards to the right of Fig. 1. Here we find a possibly thin bedded, highly weathered grayish rock (Fig. 2), that does not support a ledge.

 Fig. 2

The square block in the center of the photo is ~12 inches across. There are no sedimentary structures and no visible grains. However, it is highly weathered and no fresh surfaces were available for examination. The character of the exposed rock changes within 100 yards to a nodular form (Fig. 3).

 Fig. 3

The angular faces are rounded here, some into nodular forms. Without bedding it is impossible to be certain, but I get the feeling that these “beds” are approximately horizontal. There is no evidence of original texture. A few hundred yards further and the rock was forming ledges (Fig. 4).

 Fig. 4

At this time I’m certain that I’m looking at a Paleozoic metamorphosed sandstone because we’ve seen several examples on the previous day. So I keep looking for evidence of depositional texture.

By the time I made my way to the halfway point of Fig. 1, the only visible texture was fractures and differential weathering (Fig. 5).

Figure 5

 A   B

Figure 5A is dominated by vertical fractures delineating blocks of solid rock where’s Fig. 5B is dominated by what appear to be vertical bedding, weathered to a rounded appearance. I was convinced by what I’d already seen that these were not images of vertical beds, but rather fractures that weathered at different rates due to differences in mineralogy and water infiltration.

This interpretation is supported by examining the exposure another hundred yards further (estimate only), as seen in Fig. 6, which shows low-angle joints, angling down to the right.

 Fig. 6

I can’t make sense of this exposure, which should have given me the answer to what type of rock I was examining. Unfortunately, I have a terrible memory. Thus, I kept looking for bedding features and found a continuous, undulating fissile layer that could pass for a layer of fine-grained sediment (Fig. 7).

Figure 7

 A  B

 C

These three photos were taken within 12 feet. Figure 7A suggests crossbedding of an ephemeral nature, like during a flood, some mud that was buried quickly and forgotten. The layering thinned but was continuous with Fig. 7B, which suggests a pinching and swelling process (i.e., nodule formation). This hypothesis was consistent with the multiple, fissile layers seen in Fig. 7C.

The deposition of a single layer of mud in a nearshore bar (marine environment of massive sandstone) or point bar (fluvial depositional environment) is nil, so that should have been the end of it. Sufficient pressure and heat to destroy primary fabric would have recrystallized any clays that somehow found their way into these sediments. I had originally thought this was a sequence of metasedimentary rocks, but this hypothesis is contradicted by the layers seen in Fig. 7 and the lack of visible grains anywhere. This is a very fine-grained rock.

Of course, in the field, I hadn’t thought of all of these factors yet, so I found another piece of inconsistent evidence (Fig. 8).

 Fig. 8

A feature like the juxtaposition of unaltered, leucocratic rock and highly weathered, dark rock in this image is not associated with sedimentary rocks. This is the kind of relationship associated with hydrothermal processes in isolated pockets, like along fracture zones. There is no evidence of quartz injection into veins in these rocks, as we saw in metasediments from a previous post. There is no sign of magmatism or the proximity of a major fault, which would produce a range of fine-grained minerals as products of hydrothermal alteration.

We can never rule out slippage along fractures (see Figs. 5 and 6), lubricated by surface water, however, so Figs. 7 and 8 are probably the result of weathering along fracture zones.

The conclusion of this analysis of the field data should have leapt to mind immediately because I have seen rocks just like these many times. These are not sedimentary rocks and they never were. They are not metamorphic. They are not plutonic. We’ve run out of rock types, but one.

I admit that I cheated.

Kangaroo Point is famous and even has a Wikipedia page. The Brisbane Tuff was deposited during the Triassic Period (~230 MYA). The rock doesn’t show fine lamination or phenocrysts like we saw in much younger rhyolite (23-16 MYA) because it was created when melting-hot pieces of ash fell to the earth and formed what is called a welded tuff, not quite volcanic glass.

This outing was a wild goose chase in that I was “expecting” to find sedimentary rocks from the Paleozoic. It was only field work, which contradicted my expectations, and the hard work of previous geologists, that revealed the depth of my ignorance. This should have been a no-brainer and here’s why:

• Figure 1 shows no overwhelming structure. Nothing shouts out, “Sedimentary” or “Metamorphic” or “Plutonic.”
• Figures 2 and 3 are ambiguous; such close-up analysis of an unknown stratigraphic unit is problematic, jumping the gun so to speak. These could be older rocks on which the tuff was deposited. They do resemble some of the poor exposures from our trip to the caldera.
• Figures 4 through 6 should have sealed it because they “suggest” bedding going every way – coincidentally, just like the fracture patterns.
• Figures 7 and 8 nailed the coffin shut because the geological requirements to create such juxtapositions have never been reported (to my knowledge).

The one legitimate error I would have made is the age. Without knowledge of stratigraphic relationships between this unit and those above and beneath it (structurally and chronologically), I probably would have guessed it was age equivalent to the rhyolite we saw at Purling Brook Falls.

The bottom line is that this area received ~150 feet of ash over an unknown period of time about 200 MYA, but not from the caldera we visited before. The source of this volcanic rock is buried beneath the ash it produced. It was a tumultuous time.

After driving through the mountains and seeing 400 million years of the history of Queensland in the last post, we finished the day on the beach in a tourist area known as The Gold Coast (Fig. 1).

 Fig. 1

As can be seen in Fig. 1, this is a wide beach, called dissipative. We saw a similar beach form on a low-energy beach in Tasmania, but that was not in equilibrium with wave conditions. This beach has the appropriate morphology for the unrelenting surf impacting the fine sand (Fig. 2).

 Fig. 2

Breakers on such beaches are 6-9 feet in height and construct several bars parallel to the beach. The small sand grains are easily moved and result in a relatively flat beach profile.

We noted the evidence for storms in Tasmania and potential evidence of the beaches not recovering. That is a problem here as well. Figure 2 was taken from the top of a continuous dune that faced the beach. It was about ~10 feet high in this location but varied along the beach. For example, less than 1/4 mile along the beach, it was only 6 feet (Fig. 3).

 Fig. 3

Note the erosion at the toe of the dune and the exposed roots of the small tree. Thick grasses are partially armoring the dune face but there are still signs of permanent sand loss. How severe depends on when the damage occurred. The vegetation advancing towards the beach suggests that it has been a couple of years at least. If so, this beach is eroding because sand blown off the swash zone should heal any damage within a year.

Some locations seemed to be recovering whereas others reveal scarps (Fig. 4).

 Fig. 4

Note the scarp in the foreground of Fig. 4 and the lower slope in the middle part of the photo. This apparent recovery is probably due to slumping from higher up the slope, as indicated by erosion runnels perpendicular to the beach. Further along the beach, in an undeveloped area where human impacts are minimum, the dune is high but shows evidence of multiple erosion events (Fig. 5).

 Fig. 5

Not the flatter area halfway up the dune (right side of photo), covered with grass. This location appears to have recorded a large event long before a more recent one. There is no appreciable recovery from either.

It would seem that beautiful, sandy beaches fed by multiple rivers carrying sediment from eroding mountains isn’t enough to maintain equilibrium with rising sea level. The world’s coast lines are sediment starved and cannot fight against existing sea level, much less further increases.

This is the photo I couldn’t take.

As part of an ongoing series, we have been exploring the geology of Victoria in several posts, and recently spent some time on describing the geological history of Tasmania. This post explores the geology of Queensland, focusing on the southeast part of the state, near the city of Brisbane (pin marker in Fig. 1).

 Fig. 1

Eastern Australia’s geology is dominated by collision and orogeny from the late Proterozoic through the Paleozoic, followed by extension tectonics and volcanism during the Mesozoic. We’ve seen this from our travels through Tasmania and  Victoria. So we won’t be surprised to see similar rocks in Queensland. Our first day’s travel was from the Brisbane Airport to Springbrook National Park, ending up at the Gold Coast (Fig. 2).

 Fig.2

We didn’t stop to look at the rocks around Brisbane, so this post begins in Area 1 (see Fig. 2). We’ll get to Brisbane next time. The black rectangle outlines the approximate area contained in Fig. 3, a geologic map from the Rock-D app.

 Fig. 3

Location 1 is at the upper-left (NW) corner of Fig. 3. It was a rainy day, we were on a narrow road with few opportunities to pull over, and there was heavy Sunday traffic with many people from Brisbane heading to the country for the day, despite the miserable weather. Nevertheless, we got some photos of exposures of sedimentary rocks. Note that the rocks in Fig. 4 are wet, so color isn’t as useful because hues change in subtle ways. Rocks are like that.

Figure 4

 A  B

The geologic map (Fig. 3) indicates complex lithology in Area 1 (see Fig. 2 for location), including thick-bedded sandstone with finer grained sediments and high organic content (Fig. 4A). Road cuts also revealed interlayered thin beds of sandstone and shale (Fig. 4B). Between 237 and 201 MYA, this area was receiving mixed sand and mud while accumulating enough organic matter to create coal beds.

Still within Area 1, we move into a volcanic zone (yellow regions in Fig. 3) of much younger age (23 – 16 MYA) that includes basalt flows and rhyolite (Fig. 5).

Figure 5

 A

 B  C

Some of these volcanic beds are quite thick, forming bluffs up to thirty feet or more in height (Fig. 5A). At this location, a few miles east of Fig. 4, a waterfall had formed. These rocks also displayed incipient columnar jointing (Fig. 5B) similar to that seen in a previous post. Where eroded, these rocks formed blocks that formed steep slopes and gathered in the bottom of canyons (Fig. 5C).

The next observation area (Area 2 from Fig. 2) took us back to the Late Devonian to Mississippian (383-323 MYA), where metamorphosed clastic sediments with some volcanics (Fig. 6) are exposed. This is distributed as a broad swath running from north to south on the entire eastern half of Fig. 3.

Figure 6

 A  B

These beds were tilted and there was evidence of faulting throughout the area. Figure 6A shows thicker beds of sandstone whereas Fig. 6B is dominated by fine-grained sediment. These photos were taken within a mile of each other. This is a complex sequence of metamorphosed rocks that doesn’t form cliffs so good exposures were hard to find. However, I did manage to examine some of the sandstones (Fig. 7).

 A  B

 C  D

There is no evidence of sedimentary texture (Fig. 7A) and examining the photo up close reveals that it looks more like an igneous rock than sedimentary. Recrystallization during cementation reveals quartz filling fractures (Fig. 7B) and partial melting (Fig. 7C). The rocks reveal fissile texture (Fig. 7D) similar to a schist.

We travelled through these rocks until reaching Area 3 (see Fig. 2 for location) and Purling Brook Falls (Fig. 8), a 300 foot drop over a rhyolite cliff.

 Fig. 8

These rocks are more of the Early Miocene (23-16 MYA) rocks we saw before (yellow in Fig. 3). The stream is retreating along the edge of the 300-foot-thick sequence, which quickly erodes so that there is no canyon downstream. Close observation shows fine flow and depositional features (Fig. 9A) and phenocrysts (the white grains in Fig. 9B).

Figure 9

 A  B

Let’s summarize what happened in SE Queensland over the last four-hundred-million years. We rambled across time during our drive, so we’ll put the rocks in chronological order.

The oldest rocks we saw were Devonian to Mississippian (383-323 MYA) sediments and volcanics (Figs. 6 and 7). At that time this was a shallow sea with volcanism occurring intermittently. Australia was a peninsula attached to Gondwana with SE Queensland an open-ocean margin. During the subsequent assembly of Pangea, more mountain building occurred, until the early Mesozoic. These oldest rocks were buried and metamorphosed, almost to the point of becoming high-grade metamorphic rocks.

During the early Mesozoic (237 and 201 MYA), a nearshore environment obtained, with the collection of sandy and muddy sediments, along with the accumulation of organic matter to later form coal beds (Fig.4). These rocks also probably contained some basalt but we couldn’t identify it from poor exposures and bad weather conditions. This was when Pangea was breaking up.

A global system of mountains eroded after this as the modern ocean basins formed. Here in Queensland, the next rocks we find are volcanics from Miocene (23-16 MYA). These are well preserved (Figs. 5, 8 and 9). They were erupted from a caldera that we reached at the southerly end of our trip today (Fig. 10). This was a period of volcanism throughout SE Australia, as we saw in a previous post.

 Fig. 10

I couldn’t take the photo myself because “Best of All Lookouts” is at 3800 feet and we were in the clouds. The floor of the caldera is ~500 feet, so it was a serious drop. There was a volcano in the center, as shown in Fig. 11.

 Fig. 11

Figure 11 shows about half the original caldera, the eastern margin having been eroded more, leaving a few mountains and the central volcano.

That does it for today. Considering the crummy weather, we were pretty successful.