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The Adelaide Superbasin

The terminology and nomenclature used in geology change constantly, just like in every other discipline. Thus, what was originally a geoscyncline, became a rift complex, and is now a superbasin. I guess that being bigger is better than being complicated. Adelaide is in the Wild West of Australia. Because of the name change, this post is about the Adelaide Superbasin (Fig. 1).

Extent map of the Adelaide Superbasin.svg
Figure 1. Outline of the Adelaide Superbasin with topography of the Flinders Range.

It isn’t possible to describe hundreds of stratigraphic formations in a blog post, so I’ll do as I always do and describe what I saw with my own eyes. I couldn’t find a geological map of South Australia, so I’ve compiled a schematic map of the the region (Fig. 2).

Figure 2. Schematic of the mountain range, path, and starting point discussed in this post.

The Starting Point (Fig. 2) was reached after crossing the Murray River at Murray Bridge(Fig. 3), a view not unlike entering the Rio Grande Valley at Las Cruces.

Figure 3. Approaching the Murray River, passing through Quaternary and Tertiary sediments.

The Murray River was the lifeline for the agricultural regions of South Australia until the1940s, when rail and trucks finally became more profitable. The river remains a center of transportation and tourism (Fig. 4).

Figure 4. Ferries crossing the Murray River at Mannum.

The Murray River is also a major source of water for the region. Large pipes followed the highway, leading in different directions, unimpeded by the Flinders Range (Fig. 5).

Figure 5. Water supply line from Murray River.

Now we’re ready to begin our journey, at the east side of the rectangle outlined in blue from Fig. 2. Erosion along the banks of the Murray River exposes Cenozoic (66-0 Ma) sedimentary rocks that are poorly consolidated but resistant in the low rainfall of this region (Fig. 6).

Figure 6. Section of Cenozoic rocks along Murray River.

Heading WNW from the “Starting Point,” our path took us over some rolling hills capped by rounded exposures of what turned out to be alkali granite (Fig. 7), which is about 500 Ma old.

Figure 7. Rounded block of granite near Palmer, SA.

Viewed up close, this rock consists of quartz and alkali feldspars (i.e. albite and orthoclase), with minor biotite (Fig. 8).

Figure 8. Close-up of granite from Fig. 7. The gray crystals are quartz; pink is orthoclase; white is albite; black is biotite. The field of view is less than one inch.

This rock appeared capping hills but formed no cliffs or ledges. Within a mile of this outcrop we found the country rock (Fig. 9), Neoproterozoic to Early Cambrian (541-509 Ma) sedimentary siliciclastic rocks.

Figure 9. Typical exposures of Keynes Subgroup near Palmer, SA. The lower panel is ~12 inches across.

These exposures appear to retain their original sedimentary texture, comprising thin bedding and lamination. However, they are very near exposures of younger granite, which suggests an intrusive relationship during metamorphism. Note the dip of the beds in Fig. 9.

A freshly cut block (to make room for a fence) reveals foliation that suggests high-pressure metamorphism (Fig. 10).

Figure 10. Details of exposed metasedimentary rocks from the Keynes Subgroup.

The foliation seen in Fig. 10 is bordering on gneiss, which isn’t formed until the minerals are near their melting point, which is consistent with the proximity of the Palmer Granite pluton (Figs. 7 and 8). Taken together, these rocks indicate deposition in a nearshore marine environment between ~540 and 510 Ma, burial to great depths, and intrusion of a granite magma about 500 Ma. This was an orogeny.

Traveling west (see blue-outlined inset in Fig. 2), we entered a canyon with exposures of metamorphosed sedimentary rocks (Fig. 11).

Figure 11A. Laminated to thin bedded siltstone and sandstone. Note the bedding plane is tilted to the left and away from camera, to reveal the bottom of beds. The image is ~3 feet in height.
Figure 11B. Exposure of indurate shale/slate. The view is of the bottom of a bedding plane, the rocks being tilted away from the camera. The image height is approx. 6 feet.
Figure 11C. Exposure of Barossa Complex sediments (2500-1000 Ma) along a fault. The bedding plane is almost vertical and nearly perpendicular to the camera angle. The white color in the center of the photo is remineralized from brittle fracture and grinding of the rock. The photo is ~12 feet in height.
Figure 11D. Close up of unaltered rock from Fig. 11C. Note the phenocrysts of pink mineral/rock in the fine matrix. The rock lacks foliation as seen in Fig. 10 but the presence of orthoclase feldspar (pink phenocrysts) suggest remineralization during metamorphism. The photo is ~3 inches in diameter.
Figure 11E. Close up of orthoclase crystal in fine-grained metasedimentary rock. This photo shows greater foliation than Fig. 11D, just as foliation and thus metamorphic grade were highly variable in rocks of the Keynes SubGroup (Figs. 9 and 10). The image is ~3 inches in diameter.

The range of sediments and their metamorphic grade seen in Fig. 11 span 1900 million years. They are exposed because of a major N-S fault running along the Flinders Range. The oldest (Figs. 11C, D, and E) are metasediments with foliation that were deposited between 2500 and 1000 MA.

The exposures in Fig. 11 A and B are from the Burra Group of Cryogenian age (720-635 Ma), long before the Keynes Subgroup sediments (Figs. 9 and 10) were deposited.

The Burra Group has a diverse lithology: laminated siltstone; sandstone with heavy-mineral lamination (e.g. from a beach), comprised of quartz and feldspar, with cross-bedding; dolomite (a carbonate); blue-grey to pale pink; containing lots of clay and lenticular (i.e., many lens-shaped bedding structures). Note that all of these sediments are indicative of a passive margin, not an orogenic belt experiencing rapid uplift and the deposition of immature sediments like graywacke and turbidites.

We followed Kangaroo Creek Reservoir (Fig. 12) to its outlet, where a large cut had been made, exposing two sides of a major fault.

Figure 12. Location map for following photos. This inset is to the west of the blue-outlined inset in Fig. 2.

The road along the west side of Kangaroo Creek (Fig. 12) had been following the fault, taking us into sediments of the Emeroo Subgroup (720-635 Ma), the same age as the Burra Group but containing only quartzite (metamorphosed sandstone), sandstone, dolomite, and conglomerate. Road cuts reveal the rock textures associated with a fracture zone (Fig. 13).

Figure 13A. Road cut along Kangaroo Creek Reservoir, showing nearly vertical sediments of the Emeroo Subgroup and brittle failure identified by the diagonal light-colored zone.
Figure 13B. Road cut showing light-colored material created by grinding rock along a fault to form extremely fine-grained clay minerals. I tested it and in fact it had the consistency of flour.

The major fault that runs the length of Kangaroo Creek Reservoir is exposed at the reservoir’s outlet (Fig. 14).

Figure 14A. Cut at the outlet to Kangaroo Creek Reservoir (western limit of blue in Fig. 12). View looking south.
Figure 14B. Close-up of the road cut on the north side of the creek, showing strong lineation caused by shear along the fault.

Note that the rocks in Fig. 14B are darker than those on the south side. Both sides of the fault expose rocks from the same stratigraphic formation but within different units. The south side (Fig. 14A) could be either quartzite or possibly dolomite because both can have a similar color. No exposures were available for close examination, however, so I’m going to put my money on dolomite. Such a large exposure of quartzite, from my experience, would either show bedding or be massive, whereas there are irregular lineations in this exposure. Dolomite is a carbonate mineral, formed by the recrystallization of the original calcite that would have formed marine animal shells in the ancient seas. The transformation from calcite to dolomite during diagenesis is not well understood. Thus, it can take many forms whereas sandstone is quite limited.

No report in Australia would be complete without the final photo from a beach. We ended up on the main public beach serving the Adelaide region (Fig. 15), to discover that it is a disaster compared to the “unimproved” beaches we’ve seen elsewhere in our travels.

Figure 15. Public beach and pier in Adelaide.

The groin in the distance prevents sand being transported to this beach by waves and so it is starved of sediment. Thus, the nearshore bars and high berm to the left of the image. The beach face had a large component of fine-grained sediment as well. The beach was less than a mile long, terminating at the west end at an opening to a lagoon.

Summary

This is the last post from our trip to Adelaide, so I’ll summarize the geologic history briefly.

Between 2.5 and 1 billion years ago, South Australia was a passive margin (like the East Coast of the U.S.) and a variety of sediments accumulated in every imaginable coastal environment. Then there was a hiatus of about 300 million years, indicated by an unconformity between the Barossa Complex and Burra Group. A lot can happen in 300 million years, including an orogeny and subsequent uplift and erosion of the sedimentary record of such an event.

Between 720 and 635 million years ago, this region was a passive margin again, receiving similar sediments as a billion years earlier. These sediments were deposited on a surface that represented 300 million years of lost time. I didn’t find any dating of the orogenic event that produced the metamorphism seen in Figs. 11C, D, and E. Maybe it occurred during that missing interval or maybe…

The Delamerian Orogeny lasted from ~515 to 490 million years ago, long after the Burra group sediments had been buried and lithified.

The youngest Paleozoic sediments (the Keynes Subgroup, Figs. 9 and 10) were deposited between ~540 and 510 million years ago, overlapping slightly with the Delamerian Orogeny. However, the 105 million year hiatus between the Burra Group and the Keynes obscures a lot. For example, a little over 200 million years ago, North and South America were connected to Eurasia and Africa. Like I said, a lot can happen in a hundred million years.

At any rate, a series of granitic intrusions occurred, pushing their way into older sedimentary rocks, about 500 million years ago. This led to more metamorphism and deformation, probably associated with the Delamerian Orogeny.

Erosion followed until the Cenozoic (less than 68 million years ago), when terrigenous sediments accumulated everywhere, as revealed along the Murray River (Fig. 6).

Within the last couple of million years, Australia drifted over a mantle plume and volcanism began in Victoria, several hundred miles to the east near Melbourne, producing vast sheets of basalt.

This story is missing a lot, which we saw on previous trips. I’ll tie them together in a later post.

Drive to Adelaide, SA: Paleozoic Sedimentary Rocks

Today’s post covers the drive from Site 1 to Site 2 (Fig. 1), going from Quaternary volcanics to folded and faulted Paleozoic sedimentary rocks. The latter are presumably similar to the rocks that hosted the original magmatic gold that was eroded to form alluvial ore bodies like at Sovereign Hill.

Figure 1. Geologic map with anotation for eastern Victoria. The white line is the path taken. Site 1 was discussed in the last post.

Site 2 (from Fig. 1) can be expanded to show details of our movements within Grampians National Park (Fig. 2). Figure 2A shows the topography of the entire mountain range and the inset (Fig. 2B) is the first location we visited (Site 3).

Figure 2. Topographic maps of Grampians National Park, showing locations discussed below. See text for explanation.

The quaint tourist town of Halls Gap was entered through a narrow opening in the frontal ridge, seen to the extreme right side of Fig. 2B. Looking up from town, the ridges can be seen to form cliffs and dip generally westward (Fig. 3).

Figure 3. View looking upward and NW from Halls Gap. The ridge in the right background is the location of Site 3 (see Fig. 2B) for location.

Figure 2 suggests steep cliffs on the east side of the ridges, as indicated by darker shading, and lower slopes on the west. The yellow-filled area in Fig. 2B is a uniform block that is tilted westward, which will be shown in detail below. Our path took us west of this fault block. The road is indicated by a white line pointed westward. A circuitous path took us eventually to Site 3 and afforded an opportunity to examine the rock up close (Fig. 4).

Figure 4. Photo of a clean surface of the uniform rock that is exposed throughout Grampians National Forest. The can opener is for scale.

This is a well-sorted sandstone. According to RockD, it is the Grampians Group, marginal marine and fluvial sandstone with some red mud and conglomerate (444-427 Ma). This sample doesn’t show any crossbedding or other fluvial structures but it is fairly uniform in texture. Furthermore, zooming in on Fig. 4 reveals that the grains aren’t as rounded as we would expect from a beach or marine sand, which has been worked by wave action. The reddish hue is another indicator of a fluvial origin because marine waters have less oxygen and don’t oxidize iron within the cement. Oxidized iron (rust) comes in many hues of red.

The view from Site 3 (Fig. 5) reveals Lake Bellfield in background and the fault block shaded in yellow in Fig. 2B in the foreground. Note the uniform surface dipping to the west. This is a bedding plane and it was originally horizontal. Note also how the cliffs in the center-right of the photo are nearly horizontal. This suggests folding, which is associated with compressional tectonics. Keep in mind that these rocks were buried several miles beneath the surface when deformation occurred ~400 million years ago.

Figure 5. View SE from Site 3 (see Fig. 2B for location). Lake Bellfield is in the background.

As it turns out, the rock layers are not uniformly dipping to the west as suggested by the topographic map (Fig. 2). For example, at Site 4 (see Fig 2B for location), the following view (Fig. 6) shows the variability of the mountains as seen from the ground.

Figure 6. Panoramic photo of Grampians National Park from the west. The letters refer to panels in Fig. 7.

Looking first to the southern end of the range (e.g. Zumsteins in Fig. 2A) shows the kind of appearance that the westward dipping fault block in Fig. 5 would imply (Fig. 7A).

Figure 7. Close-up shots from Fig. 6.

Figure 7B, slightly to the north, reveals high cliffs somewhat further away, possibly east of Wartook (see Fig. 2A). The cliff is unbroken except for a saddle just to the right of the central tree. Further north (Fig. 7C), the ridge is in the foreground and a higher ridge, facing east, forms the skyline. This location is about where Laharum is located in Fig. 2A. Thus it would seem that faulting was not uniform. No where is this more evident than just north (to the left) of Fig. 6.

Figure 8. View looking towards Dadswells Bridge (see Fig. 2A for location), taken from Site 4.

The southern (right side) rocks tilt to the north whereas the northern (left side) tilt to the south. Glancing at Fig. 2A, we see that the V formed by these fault blocks is at the narrowest (and lowest) part of the ridge they form.

Taken together, these observations suggest that deformation was not a simple process here, just as observed in previous posts. The geologic map (Fig. 9) reveals several faults in Grampians National Park.

Figure 9. Detailed geologic map of Grampians National Park from Rock-D. Lines represent structural faults.

The map reveals several N-S trending faults along which the huge blocks seen in Fig. 2B and Fig. 5 tilted to the west. It also shows nearly orthogonal faults (e.g., south of Halls Gap), and curved ones near Dadswells Bridge. This suggests that folding may have occurred after faulting. One possibility is tectonic inversion, in which (for example) a stress field changes from extensional (e.g., before 450 Ma) to compressional (e.g., after 400 Ma). Such an inversion of tectonic stress would reactivate old faults and deform them, as well as allowing thrust faults (compression) to occur on previous normal faults (extension).

Just an idea…

Drive to Adelaide, SA: Quaternary Volcanics

The next few posts are going to document and discuss some of the geology we encountered on a drive from Melbourne to Adelaide, South Australia. The border between states is open now so it was no problem. The total trip is about 450 miles in length. We took three days to make it all the way. This post is going to present our observations from the morning of the first day, to keep the post shorter. I know I get carried away sometimes.

Figure 1 shows the geologic map of the part of Victoria covered on the first day. This post wlll discuss Location 1 only.

Figure 1. Geologic map of Victoria. The orange areas with small “v” symbols are volcanic rocks less than 2 million years old, from the Quaternary Period.

The volcanics were erupted over older rocks, in this case Paleozoic sedimentary rocks that had been intruded by granitic magma during the period of mountain building discussed in previous posts. Gold was deposited in fractures and faults at that time. Later, as the mountains eroded, the gold was deposited in rivers and because of its resistance to weathering, and collected in significant deposits. This kind of gold deposit is called alluvial. The Sovereign Hill mine was the site of the richest alluvial gold rush in the world, with one nugget weighing more than 150 lbs.

The location of the original shaft (Fig. 2) is now a tourist site. The underground mine play ran out and less expensive methods became employed to recover the ore.

Figure 2. Schematic of shafts under the original mine at Sovereign Hill. The shafts would have followed original river beds rather than faults as in magmatic gold mines.

Underground mining produces large piles of ore (Fig. 3), which mark the location of mine entrances throughout the world.

Figure 3. Tailings pile from the Sovereign Hill mine. The relatively small volume suggests that underground mining didn’t last long after the initial gold rush.

There was a mining project still going on, however, out of sight of the tourists, on the other side of the hill (Fig. 4).

Figure 4. Solution mining pond from the current Sovereign Hill operation.

I’m not sure if the operation is working directly on low-concentration sediments or simply created a large basin for the highly toxic waste products produced by acid-leaching methods. We didn’t see any evidence of large earth movers or dump trucks, however. This involves a lot of on-site chemical processing (Fig. 5).

Figure 5. Acid leaching equipment actively in use at Sovereign Hill.

As already mentioned, the gold was located in Paleozoic sedimentary rocks beneath the Quaternary volcanic rocks. As seen in Fig. 1, this was an extensive volcanic field, which implies that there should be volcanoes still to be found because of the young age (< 2 Ma). A topographic map of the area in fact reveals the presence of many volcanoes (Fig. 6).

Figure 6. Topographical map with several examples of cinder cones indicated. The photos shown in subsequent figures were taken from near the M8 label on the black line marked “Our Path.”

Here’s what those volcanoes look like from a distance.

Figure 7 shows a large number of volcanoes, like where the most eastward arrow in Fig. 6 is pointing. They were covered in grass and were used for grazing sheep mostly. We couldn’t get access because they were all on private land and, surprisingly for Australia, there was no volcano park or anything…

Figure 7. Several cinder cones seen from the SE (see Fig. 6 for location).

As seen in Fig. 1, in some locations the older Paleozoic rocks were present at the surface (Dark blue in Fig. 1). The terrain here consisted of rolling hills, occasional ridges, and volcanoes. The ridges may be the surface expression of folds in the older rocks, created when the intrusion responsible for the gold mineralization occurred. That was back about 400 Ma in the past, however. At any rate, Fig. 8 was taken on a downhill slope from one of these ridges, showing an unusually large volcano in the distance, possible the complex near the town of Ercildoune in Fig. 6.

Figure 8. View looking NW towards large volcanic complex. See Fig. 1 for location near town of Ercildoune.

That’s it for this post. We finally drove through the volcanic complex we’d heard about and, although we couldn’t visit any volcanoes ourselves, it was inspiring to drive through so many volcanoes, to imagine what it was like when they were erupting the massive volumes of lava seen in Fig. 1.

Next time, we’ll continue to the west to examine some Paleozoic sedimentary rocks that have been folded and faulted.

Geology of Australia: a Summary

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.

Australian Capital Territory: Orogenic Volcanism

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.

 

 

 

 

 

 

 

 

Australian Capital Territory: Sedimentology Homework

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.

Australian Capital Territory: Siliciclastic Sedimentary Rocks

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.

Australian Capital Territory: Lake George

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.

 

 

 

 

 

 

 

 

 

 

 

 

 

Australian Capital Territory: The Great Dividing Range

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

The rocks from this area are dated between 485 and 407 million years ago (Ma). The oldest rocks are greywacke, deposited in the ocean from a rapidly rising landmass (labeled D in Fig. 2A). The youngest are rhyolite tuff and associated volcaniclastic sedimentary rocks (labeled C in Fig. 2A). That’s a long time span, but it gives one an idea of how slow geological processes occur. However, these rocks record an orogeny in its entirety, from the first sediments deposited from rapidly rising and eroding mountains, to the emplacement of plutons and  volcanism.  Of course, it’s difficult to identify the exact date an orogeny begins or ends; after all, when does an orogenic belt become an eroding mountain range?

A geologic map of the area (Fig. 2) shows the complex geology associated with an orogeny lasting more than 70 million years.

Fig. 2

  A

  B

Figure 2A covers a larger area and indicates some potential locations to examine as well as several common rock units. Figure 2B is a closeup map of the Canberra area. (Note that rock units do not use the same references in Figs. 2 A and B.)

There are many recognized field sites in the ACT and they are listed with descriptions and access instructions in a publication of the Australian Geological Society.

The numbers on Fig. 3 refer to site numbers, each of which has a separate document.

Fig. 3

A B  C

We’ll be visiting several of these locations on Friday and Saturday.

39 – Lake George: Late Ordovician (485-443 Ma) quartz turbidite and black slate (Adaminaby Group). Silurian (~420 Ma) turbidites, volcanics, and granite.

04 – Black Mountain: Siliciclastic sequence (443-427 Ma); proximal flysch sequence.

00 – Canberra: Canberra Formation (443-427 Ma): Mudstone, siltstone, minor sandstone; dacitic ignimbrite; volcaniclastic sediments, ashstone. State Circle Shale/Black Mountain Sandstone (443-427 Ma). (This is a shared post with 04 but in a different location, which is not in the guide book.)

16 – Mount Ainslie: Mount Ainslie Volcanics within the Hawkins Volcanic Suite; erupted 428-424 Ma.

26 – War Memorial Conglomerate: Dacitic agglomerate within the lower part of the Ainslie Volcanics (428-424 Ma). The agglomerate is composed of angular clasts of greyish dacitic tuff.

10 – Deakin Anticline: A well developed anticline, with a fault between two rock outcrops in volcanoclastic fine sandstone and siltstone of the Yarralumla Formation (424-423 Ma).

31 – Botanical Garden: A conglomerate formed on an alluvial fan, thought to be of Pliocene (2-3 Ma).

 

 

 

 

 

 

 

 

 

Kangaroo Point

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.