The previous post examined a couple of beaches along the eastern Tasmania coast and discussed coastal erosion in areas with very little sediment input. We finally reached our destination after a couple more hours, Freycinet National Park, located on an isolated peninsula (indicated by the pin in Fig. 1).
The peninsula is entirely composed of Devonian granite (393-359 MY). The best reason for this magmatism without major deformation (i.e., an orogeny) that I could find is that Tasmania was squeezed between Australia and Antarctica during assembly of Gondwana. The upper mantle was hot and would have produced granitic magma. It’s safe to assume that this area would have had relatively high elevations at that time as well, delivering sediment to the ocean.
This post isn’t going to discuss the granites in and around Coles Bay, but instead continue our examination of coastal geology. Great Oyster Bay is a relatively sheltered body of water. Coles Bay is even more protected from waves because it faces west (see Fig. 2) and the larger bay is only 10 miles across.
The nearby granite supplies crystals of minerals like quartz and alkali feldspars, which are more resistant to chemical weathering than the minerals comprising dolerite, so an ample supply of sand-sized grains is available from two rivers that enter Great Oyster bay at its northern end. This wasn’t the case at the beaches we visited in the last post. Consequently, a wide, shallow beach has been produced at the eastern margin of Coles Bay (Fig. 3).
The beach terminates to the south against the monolithic granite mountains that form the peninsula. Note how shallow the water is at approximately low tide in Fig. 3. Also take notice of the lack of surf because of the very shallow water. This is very interesting.
Looking to the north (Fig. 4) we see a berm at the high-tide line with a runnel (darker sand to the right) behind it. The tidal range is predicted to be 4.5-6 feet in Great Oyster Bay. Note the relatively steep beach face.
High tide laps at the toe of the dunes (Fig. 5), which consist of individual-to-merged primary dunes (created by sand blowing from the beach) with irregular spacing, rather than a single, continuous dune facing the beach. Plants appear to be important in building and maintaining the dunes.
There are some anomalies in Fig. 5, however; for example, there are black areas (organic matter), that should be buried, as well as wave-eroded scarps. We can take a look along the beach (Fig. 6) and notice some more features.
To the right, on the larger dune, we see the erosional surface stained dark. It is recovering, however, unlike at the previous beaches we observed. In fact, in front of the main dunes, incipient dunes are forming, as seen across most of the foreground of Fig. 6. High tide reaches and thus limits development of these dunes. There is a good supply of sand.
About 100 yards north of where Fig. 6 was taken, incipient dunes are not evident (Fig. 7) although sand is collecting at the toe of the main dunes. Note the low (~12 inches) scarp at the base (aka toe) of the dunes.
Near the northern extent of the beach, the dunes are low and irregular (Fig. 8). They are also more fully covered by plants.
Another interesting point in Fig. 8 is the lack of recovery of the beach toe. Dead grass covers the rough and steep cut in the sand. A line of debris marks a previous high-water event, possibly a spring tide or a storm.
The beach and dunes change character over the extent of this beach, which is less than a mile in length. How is that possible? The short answer is that there is more sand available at the southern end of the beach. More sand to build a berm and a robust dune system, and then to recover those dunes when they are eroded by storm waves. Let’s try and dig a little deeper.
Figure 9 is a view looking south from the northern end of the beach. Besides showing the granite peaks we’ll talk about next time, they show a wide, flat beach. It looks like it was created with a bulldozer, by the Corps of Engineers. It wasn’t. This beach hasn’t been modified in any way because it’s in a national forest.
This is an example of the limitations of models. Beaches have been classified by their dynamic behavior as either dissipative, with large waves expending their energy over a wide surf zone with several bars, or reflective. The latter type of beach has very low waves and a narrow surf zone on a steep beach face.
Do you see any waves in Fig. 9? There are nearshore bars visible in Fig. 3, however. So what’s up?
I have to rely on my experience with storm deposition on low-energy beaches in the Gulf of Mexico (see Keen and Stone, 2000). This is a topic not often addressed in the scientific literature because of the rarity of such situations. I have proposed in the past that a low-energy beach (small waves), with bars and a wide, shallow face (dissipative), is relict — leftover from a high-energy event. It’s a snapshot frozen in time because the low waves that usually occur are unable to move sand around. Such a beach, out of equilibrium, may persist for years before returning to its proper configuration. It’s even possible that it could last decades if storms are sufficiently rare.
What about the other question: Why is the northern end of the beach (Figs. 8 and 10) so different from the southern end (Fig. 3)?
The beach face is about as steep at the northern limit as at the southern terminus (compare Figs. 10 and 4), and both are much steeper than in the middle (Fig. 9). The northern end is becoming reflective (steep and narrow beach face) because sand is being removed, allowing the weak waves to do their wimpy thing. The sand has been transported to the southern end of Coles Bay by waves in a process called longshore drift. Waves never approach the coast at ninety degrees and thus they slowly roll sand particles in one direction or the other, up and down the beach face. This can be reinforced by tidal circulation, which is also an asymmetrical hydrodynamic process.
The southern end is becoming more reflective, despite the high sediment load, because of slightly higher wave energy and help from the large tidal range, which allows higher waves to approach closer to shore during high tide. The center of Coles Bay is not recovering and remains a wide flat beach typical of a “storm” profile.
Finally, let’s look across Coles Bay at the granite plutons we’ll be examining next. Figure 10 reveals that this beach, like so many pocket beaches in Tasmania and Australia, is a quantity of sand trapped between rocky headlands. The sediment has no where to go but back and forth, to and fro, for eons, until plate tectonics changes the situation drastically.
Which it always does…
The hectic pace of the last few posts is going to lighten up a little today. Not because there’s nothing going on; rather, there’s so much going on, spanning all of geologic time (the history of the earth), that we have to turn to general principles to stay on track.
I’m going to get a little philosophical here and talk about a basic tenet of geology: Uniformitarianism (yes, another ism) posits that chemical and physical processes have remained constant throughout earth’s history. Apparent irregularities are due to changes in past physical and chemical environments that we are unaware of; physics has not changed. For most classical geological processes, this means that we know more about the past than the present, even if in a somewhat static way.
One subdiscipline in which this is not the case is sedimentology, and other process-based disciplines that study processes that operate at standard pressure and temperature (e.g. 68 F and 1 bar used by NIST); I’m referring to things we can see with our eyes. Touch and not get burned or crushed.
For example, previous posts have discussed metamorphism and the ductile/brittle deformation of sedimentary rocks, without a shred of observational evidence that such processes exist. We haven’t seen any of these processes in action — not enough to convince a skeptic anyway. We extrapolate to geological scales from laboratory experiments or theory, but we are prevented from observing these processes directly by spatial and temporal constraints beyond our control.
For the record, I think geology is founded on a robust theory that is continuously tested with observations, and updated to reflect what is being learned as our observational skills improve.
The sedimentary rocks we examined in previous posts were originally deposited in familiar modern environments, like nearshore marine and rivers/lakes. Thus they look just like sands and muds we can examine in modern depositional environments, right down to the ripples and organisms found in them. The principle of uniformitarianism also applies to regional and global scales; in other words, we shouldn’t expect to find the same depositional environments in modern Tasmania that existed in the past.
For example, most of the sedimentary rocks we’ve examined were part of thick sequences, sometimes miles thick, representing the erosion of large mountain ranges. As fascinating as the mountains of Tasmania are, they are less than 5000 feet in elevation. Furthermore, the rivers are small and carrying very little sediment, so neither sand nor mud are being delivered to the ocean. In other words, there are no swamps, wetlands, barrier islands, lagoons, river deltas, or any other coastal areas like in the northern Gulf of Mexico, the Bay of Bengal, or other areas with high sedimentation rates. What do we find instead?
Figure 1 shows a typical pocket beach/bay from the east coast of Tasmania, not very far north of Hobart (see Fig. 2 for location). The beach face is rather steep because of the coarse sand available (larger sand grains form steeper slopes). The beach is missing something, however; there is no berm at the top, even though the tidal range is about 3 feet here. This is like the beach in a lake.
Note that this location is in a small bay with a restricted mouth, which prevents most waves from entering. There is a small, tidal-dominated river that supplies no sediment. There isn’t even a bar at the mouth of the river.
Figure 3 is an aerial photo of Orford Bay from Google Earth.
This image reveals that the river was filling its estuary with sand, the beach propagating seaward, until the supply of sediment was exhausted, leaving a small bay. There has been very little sediment deposited after the groins were constructed at the river mouth.
Can we use the principle of uniformitarianism to apply what we see at Orford Bay to the sedimentary rocks we’ve encountered so far on our journey? The physical processes controlling erosion and deposition (i.e., waves, tides, currents) were present in the past but operating under different conditions. For one thing, Tasmania was not an island but part of a larger landmass; in fact, most of the area was under water and receiving sediment from adjacent land through much of geologic time. That’s why there are marine sediments mixed with volcanics in the older rocks we’ve examined. Nevertheless, we saw nearshore marine features in the Proterozoic rocks discussed in a previous post, which contained thin beds of mudstone intercalated with sandstone, as well as cross-bedding.
We can’t apply what we see here to the scale of rock sequences, however, because there is no active deposition occurring at Orford Bay. There will be no record of Orford Beach in the geological record because the sediment is relict; any particles in this area are being shuffled around endlessly, perhaps slowly moving offshore. There will be no future Orford Formation, the individual particles we see on this beach instead being dispersed as part of a larger layer of sediment created by weathering of igneous rocks within the region. However, as with any sediment, the destiny of this tiny, starving bay ultimately depends on plate tectonics.
What about those igneous rocks I alluded to?
Figure 4 shows a dark, highly fractured/jointed rock that has been worn down by the ravages of chemical and physical weathering — oh yeh, that’s Tasmanian Dolerite exposed (barely – Fig. 4 is less than 6 feet in height) along the coast.
I’m going to go out on a limb here (again) and say that the Jurassic dolerite in Fig. 4 is not contributing to the lovely, coarse-sand beach we saw in Fig. 1. Those sand grains came from the long-gone Triassic sandstones we saw in the last post. The Parmeener rocks, which are absent in this area, contain a lot of quartz and feldspar to form beaches.
For some reason, the Tasmanian Dolerite is pristine in this location and is photogenic (Fig. 5).
Careful examination of Fig. 5 (field of view ~2 inches) reveals white and gray particles which we can assume are mineral crystals, albeit tiny ones. However, they are not shaped like the crystals we see in museums and most rocks. Some of them look more like organisms than minerals.
Studying Fig. 5 has made me struggle to remember my mafic intrusive mineralogy to no avail. I’m not an expert but I’ll share my explanation of the irregular crystals seen in Fig. 5. Mafic magma, from which dolerite crystallizes, typically contains pyroxenes such as augite, which has a high iron content and thus weathers into hematite, a red mineral. Magma is not uniform however, and this sample could have cooled and crystallized in a part of the original magma with a more alkaline composition (i.e. sodium, potassium, and possibly rare earth elements). Alkaline pyroxenes like Aegirine can crystallize with the right chemical composition of the magma and cooling history.
I couldn’t find anything general on the topic of weathering products of these minerals, but it makes sense to me that Na and Ca will weather into whitish minerals at low temperature and pressure. Feldspars with these elements certainly weather to produce white byproducts because of the sodium and calcium.
Bottom line: this dolerite is contributing nothing but quartz, and little of that (hard to tell from Fig. 5), to the beaches of SE Tasmania. The other minerals in a dolerite are either dissolved or bound to the clay minerals formed from original olivine, feldspar and pyroxene minerals. Clay minerals have a mineral structure that accepts almost any element.
About twenty miles north, we came to Kelvedon Beach near the town of Swansea.
The foreshore, looking to the south (Fig. 6), shows an eroded dune and flat upper beach. Plants have regrown quite a bit, so it has probably been several years since the erosional event that removed half of the dune. That is a substantial volume of sand, possibly as much as 6 feet of dune.
The high-water line (limit of slightly darker sand in Fig. 6B) is marked by an increase in shell material but there is no sign of berm growth, which suggests a lack of sand to replenish the shoreface. The assumption of a catastrophic event is supported by relict shell beds on the upper beach (Fig. 6C), located on slightly elevated areas where a berm should be. Further evidence of a huge storm is the low volume of sand being returned to the beach by wave action, a natural process that restores beaches after storm erosion. The lack of beach recovery, during an interval long enough for seagrass to be reestablished, implies that the offshore flow during the event was truly catastrophic, taking sand out of reach of fair-weather waves.
Most of the shells seen in Fig. 6C were unbroken, as if deposited suddenly, rather than worked for months or years by waves, and broken into fragments. There were several patches of shells distributed irregularly along the shore, suggesting areas of concentrated wave power during the high-water event that eroded the dune. The lack of any burial or removal of the shells indicates that waves haven’t reached this high up the beach since the event. This beach is on a relatively open coast (pin in Fig. 6D). Waves interfere constructively to focus on sections of beach for various physical and meteorological reasons, and this is where erosion is greatest. The deposition of intact, disarticulated shells and excess sand in a low berm at hot spots, is what would be expected.
The beaches of eastern Tasmania can teach us so much.
For one thing, timing is everything. With waves and currents about the same in the past, this region was receiving vast quantities of sediment in a rich variety of coastal environments. Today, there isn’t enough sediment to stave off the storms that impact the coast and thus the shoreline is eroding, slowly but surely. Bad timing. There’s absolutely nothing we can do about continental drift, but that isn’t the only factor affecting coastal erosion.
Erosion is also what happens to most coastlines during when sea level rises, such as has been happening steadily for the last 10,000 years. Rising sea level requires a massive input of sediment to fill all that newly acquired accommodation space. Very few coastlines meet the criteria.
We’re certainly not driving the train, but we aren’t helping the situation either, pumping greenhouse gases into an already warming atmosphere.
Nor can we get off at the next stop…
Thus far on our Tasmanian field trip, we have seen glacial deposits and landforms, highly metamorphosed schists, remineralization and emplacement of granites, and deformed Proterozoic sedimentary rocks. This post is taking us from the continental divide to the southeast coast at Hobart. We’ll be encountering mostly Mesozoic rocks on this leg (Fig. 1). The extensive hydrothermal activity associated with volcanism and intrusion along the Dundas Trough didn’t occur in this part of proto-Tasmania.
These sedimentary rocks were deposited in both terrestrial and marine environments after mountain building and erosion during the previous 200 million years. Thus, we start out (see Fig. 1) in Triassic rocks of the Upper Parmeener Supergroup (252-201 MY), which includes sandstone, siltstone, and mudstone interbedded with coal and alkali basalt. The Triassic rocks in the NW part of our study area (along Hwy A10) are fine grained and exposed in borrow pits (Fig. 2).
Note the red color. Triassic sedimentary rocks throughout the world are red because they were deposited in oxygen-rich environments like rivers and lakes. They are thus called “red beds” colloquially. This was a time of high mountains and extensive land in Pangea and Gondwana that took a long time to erode.
After fifty million years, the Pangea supercontinent became unstable because of plate tectonics. When the crust thinned, mafic magma welled up and began to intrude into the thinned, terrestrial sedimentary rocks that had collected in the eastern part of proto-Tasmania. These weren’t deep rocks like gabbros, which have visible crystals in them, but were instead intruded near the surface and so formed small-grained rocks with some larger crystals that had already cooled. They are called dolerite. Dolerite is hard but variable in composition and, because of its mineral assemblage, it weathers quickly.
We encountered dolerite (diabase to American geologists) as a low bluff, about 10 feet hight (Fig. 3). This is known as the Tasmanian Dolerite. It is Jurassic in age, having been intruded between 201 and 145 MY ago. It underlies much of eastern Tasmania.
Note the blocky joint pattern that makes this look like a bedded sedimentary rock. Also, take notice of the red color, which has nothing to do with the red beds common to Triassic sedimentary rocks. The red comes from minerals like amphibole and augite, which have high concentrations of iron (Fe) and thus weather (i.e. rust) to form iron oxide.
Because of the variability in mineral composition and grain size, dolerite is often exposed only by road construction (Fig. 4). Thus, the same rock may form cliffs and mountains or low valleys.
For example, high ridges and mountains are typically underlain by dolerite in this part of Tasmania (Fig. 5).
We didn’t see any other examples of the contact between the Parmeener sediments and the dolerite, not even on the tops of mountains. They were either one or the other. It’s important to recall that because the dolerite was injected between layers of rock, it was originally in contact with the older sediments on either the top or the bottom of a sill or dike or as part of a laccolith. Thus, when we see Triassic sediments dipping slightly to form a cliff at the top of a hill (Fig. 6), we can’t be sure what’s beneath the resistant sandstone. However, the geologic map suggests that this is not sitting on top of Tasmanian Dolerite.
We will return to the Jurassic dolerite later. For now, let’s focus on the Triassic terrigenous sediments known as red beds. We didn’t see very many depositional sedimentary structures in the highly deformed rocks encountered so far on our field trip. It’s a little easier with these rocks, however, because they haven’t been subjected to mountain building (i.e., heating and compression). The first thing we notice is the high variability within a narrow window of time (Fig. 7).
The broad yellow line in Fig. 7A demarcates planar beds in an earlier time from irregular layers at a later date, presumably not too long (maybe a couple of years or the next day). Note that the view is slightly oblique looking down on the upper surface of the lower beds. As a general rule (be careful but still…), the planar beds were created when sand and silt were dominantly carried along by suspended sediment transport. Or at least by very gentle currents. Irregular beds as in the upper part of Fig. 7A are often indicators of channels, which are eroded into sediments, creating an irregular quiltwork of bedding.
Another complicating factor in sedimentary depositional environments is the presence of organic matter, like dead leaves, animals, plants, etc, which collects where currents are weak. This of course occurs with mud but sometimes there is more organic matter than fine-grained clay minerals. The several-inches of organic matter in Fig. 7B (outlined by white lines) could have collected over a few hours or months. We cannot know. But these layers are common in clastic sediments where clay, silt and sand are simultaneously transported by streams and rivers. Note the lack of small-scale layering in the lower unit of Fig. 7B.
We know these sediments were deposited in a river because all of the cross-beds dip in generally the same direction (indicated by black lines in Fig. 7C). The white lines separate different sets of cross-beds. Changes in cross-bedding style indicate alterations in the flow or sediment or water depth, something like that. They don’t have to be significant. One of the sets highlighted in the figure could have been created in a few minutes and was miraculously (low probability) buried before a strong current destroyed it.
We can’t identify the slope-building rock below the sandstone in Fig. 6 without physically sampling it; however, we can have more confidence (not total, without a physical sample) when we see a dark, intrusive igneous rock like dolerite on top of a lighter rock (Fig. 8A).
Figure 8B shows the location of the photo superimposed on a geological map of the area. The greenish map color to the left represents the distribution of Tasmanian Dolerite whereas the purplish area to the upper-right is the Parmeener Supergroup. The contact between them is (supposedly) covered by colluvium. It is very possible that some original geological data that contributed to the map was collected before modern GPS navigation; i.e., a contact could be exposed but mislocated or lost when the field data were used to create the contoured map available to me. This is always a problem with small exposures like Fig. 8A.
The dolerite appears black and fractured in road cuts (Fig. 9A) with irregular weathering, but it sometimes must be reinforced as it physically weathers in place (Fig. 9B). We will see this ubiquitous rock again and again as we continue our field trip.
As we approached the coast, and Hobart, there were more road cuts that exposed poorly consolidated clastic sediments laying unconformably over the dolerite (Fig. 10).
This light-colored sediment is Quaternary (< 2.5 MY), eroded from all of the older rocks as the surface of Tasmania continued rising in isostatic uplift after deep burial of the rocks (and subsequent erosion of overburden), as well as the recent removal of ice. From exposures elsewhere, there is a soil profile of ~2 feet at the exposed ground surface of the dolerite, which consists of clays, sand, and boulders that haven’t broken down.
I’ll leave you with a photo of a mountain that remains stubbornly upright, reminding us that erosion is slow in Tasmania and 300 MY old rocks are relatively young…
We headed west on C-132. The locations of photos will be indicated on a map using pins (red circle with a line extending downward).
We headed west in Cenozoic volcanics into Early Ordovician (500-470 MY) marine-to-nonmarine quartzose conglomerate-sandstone-siltstone sequences visible in low exposures along the road as seen in Fig. 2.
These rocks represent a smooth transition from nearshore to river and lake environments. Note the red color in outcrop (Fig. 3A). Note that the image is about 6 feet across.
Figure 3B shows the rock closer up, at which point irregular light-colored particles can be seen along with darker grains. The red color comes from oxidized iron in the cement. Figure 3C is as close as I can get with the iPhone camera. At this resolution, quartz and feldspar crystals can be seen. Most of them are rounded.
We drove a while across constantly changing Cenozoic volcanics and ~500 MY volcanics and volcaniclastic rocks, past several poorly visible volcanic craters that have become lakes. We eventually climbed out of the volcanic field into some felsic-to-intermediate volcanics (Fig. 4).
There were high ridges (Fig. 5A) supported by these rocks.
The outcrop in Fig. 5B is about 12 feet high. It shows fracturing and a solid appearance. However, near areas of probably hydrothermal alteration, this same rock weathers quickly to form an iron-rich soil (Fig. 5C). A major fault runs through the area where Fig. 5 was taken, as seen in Fig. 6.
The fault runs N-S next to Tullah. Note the numerous N-S faults running through Fig. 6. The photos in Fig. 5 were taken on A10 where it crosses a river south of Lake Mackintosh. The fault is expressed in cliffs on both sides of the river (Fig. 7).
This is mining country, as evidenced by the vertical mine lift shown in Fig. 7. There was no mine shaft under the original equipment on display however. The rock has changed significantly only a few miles further south (Fig. 8), even though it is still probably the Ordovician (~500 MY) volcaniclastic rocks we’ve been seeing before. Correlation is always a problem with field geology.
I can only speculate about Fig. 8A and say that this is either a contact, or near the contact of country rock (i.e. older sedimentary rock) with a granite (~500 MY) with porphyritic texture (Fig. 8). This is consistent with our path along the thin line of granite associated with the granite and quartz porphyry shown in the high-resolution 3D geologic model of this area (Fig. 9). You will have to zoom in to see the red areas. Also note the red squares that indicate mining areas. Granitic rocks are the primary source of hydrothermal fluids saturated with valuable elements like Cu, Au, and Ag, as well as a myriad of rare elements like niobium and titanium.
Another twenty miles through some very difficult terrane, finally following a twisting road through a break in the granite back of Tasmania, brought us to the other side of the mountain where there were no trees, but peaks that promised ore bodies, and wealth, ahead (Fig. 10).
The rock becomes even more saturated with the white color as we approach Queenstown (See Fig. 6), which was a major copper mine in the 19th and 20th centuries. Here, a thin-bedded sand/siltstone is permeated and a discontinuity (diagonal contact, possibly a fault, with the irregular and fractured rock to lower right) may have been the route for transport of hydrothermal fluids (Fig. 10).
I’m not certain about the time of hydrothermal activity (ore emplacement) because the granite porphyry is identified as ~500 My (during collision with Gondwana) and could have produced many of the necessary fluids to saturate surrounding rocks. However, the granite to the east of the mineral belt is Devonian in age (420-360 MY), a difference of ~100 MY. The orogeny could easily have lasted that long, so we shouldn’t fret about it. There is no central clearing house for geological data and its interpretation.
We made it to Queenstown and climbed one of the steepest roads you’ll find anywhere; we climbed to the copper mines that had put Queenstown, Tasmania on the map (Fig. 11).
A lot of copper ore was removed from the area over a century but now it looked pretty dead, probably played out. These were mostly underground mines, which require high-grade ore to be cost effective.
We followed the long and winding road to the pass that allowed us to turn eastward. Along the way we saw mining tailings piles (Fig. 12A) across the valley. We also noticed some quartz-rich hydrothermal activity (Fig. 12B), but very limited in extent. This wasn’t like the copper porphyry mines of the Americas with low-grade deposits renowned for huge open-pits.We passed near a number of old mines with no signs of activity as we headed east from the ridge. We failed to take any photos, however.
The road cut shown in Fig. 12C reveals the rich variety of color displayed by these remineralized rocks, as well as the beckoning omnipotence of the mountain tops, where the apparent bedding (from a distance) suggests a granitic magmatic body overlain by Paleozoic rocks infused with gold and silver. Alas, that wasn’t the case because there is no evidence of mines higher up the mountain than where these photos were taken.
Figure 12 D shows vertical lineations which suggest that this rock was subjected to a lot of shear after it had become relatively brittle. There are a lot of faults in the Queenstown region, as one would expect with the emplacement and later uplift of a massive granitic batholith (Fig. 13).
I would add that the striations seen in Fig. 12D are consistent with vertical shear along a fault running through the town of Queenstown.
Next time, we’ll go from the top of the world to the continental divide and (maybe) back to the sea…
This post is a continuation of the our first day in Tasmania. After looking at some glacial features, we headed south to our destination labeled DAY 1 in Fig. 1B. Note that the outline of Tasmania is indicated by a heavy black line in Fig. 1B. As before, the reader is recommended to examine Fig. 1A for any details of the geology because it is a high-resolution image.
We will be driving southward, starting from the north coast (to the left in Fig. 2) to Cradle Mountain, about half-way to the right, in the Central Volcanic Complex (orange in Fig. 2).
Once we entered the hills seen as we approached the coast, there were lots of low road cuts, which exposed intermediate volcanic and volcaniclastic rocks (Fig. 3).
The left image is a road cut about 10 feet high. The original bedding can be seen to be almost vertical, as indicated by the platy appearance. These rocks (whether volcanic or sedimentary) were originally laid down horizontal. It is difficult to date them because of subsequent burial and heating but they are thought to have been deposited (from radiometric dating) to between 1600 and 540 MY. This was when Tasmania was approaching a land mass to the (current) NE, the future Gondwana. Ocean crust was colliding with the land here at that time and creating volcanic rocks of a specific type.
The right photo of Fig. 3 shows a close up (the coin is 1/2 inch in diameter). Normally, a fresh break would be made on the rock to see the minerals and structures, but I didn’t do that. The photo does show fine structure, which could be either shallow or deeper water. There are a variety of coastal environments that would have collected fine sediment, including mud and silt. The location of Fig. 3 in the area is shown in Fig. 4A (below).
The background is from a phone app that shows an interactive geological map. The color labeled Fig. 3 and 5 has been identified from a lot of field work.
The rock is not as weathered in this location. The left image is an outcrop scale (~8 feet in height). It reveals more lighter colored sandy rock, presumably from a quartz-rich sediment. The darker color is from surface staining. The right image of Fig. 5 shows the coarser grain size compared to Fig. 3. We now have a reasonable answer to the question of where the dark and light cobbles we saw at Turners Beach in our last post (Fig. 7) originated. They were eroded by glacial action and transported along rivers to what was certainly not the coast at that time. The coastline would have been miles further offshore than today because of a much lower sealevel.
We crossed some volcanic rocks less than 60 MY old (yellow area in Fig. 4A), which tended to fill in lower areas, and entered another region of metamorphic rocks, these showing evidence of remineralization (Fig. 6). Here the rocks showed signs of chemical alteration and differential weathering.
The bedding plane surface shown in Fig. 6 reflects either muscovite or pyrite mineralization. Muscovite is a common metamorphic mineral that results from the burial and heating of mud whereas pyrite is injected by igneous rocks into the buried sedimentary rock. We drove a few miles further and discovered some vertical beds and an example of pyritization.
Figure 7 shows the original rock, tilted almost vertical (the heavy lines are bedding planes), with a sharp contact with a remineralized and weathered zone, which extends to the top of the exposure where soil is forming. This was probably a fault or plane of flow for hydrothermal liquids, which caused the deposition of pyrite on the left of the remineralized zone (Fig. 8).
The silvery (in the image) area is actually “fools gold,” or pyrite. The camera didn’t do a good job. This part of the exposure was covered with it. This is an indication of potential valuable minerals (e.g., gold, silver, copper) in the vicinity.
From Fig. 4A, we used our geologic map app to determine that these are Early Cambrian (509-485 MY) volcanic and volcaniclastic rocks, deposited when the collision with Gondwana beginning and metamorphosed and intruded when the collision was nearing completion, and granite-like rocks were being intruded. Mineral deposits are associate with these kinds of intrusions because they are more developed within the earth’s crust than basaltic rocks.
We continued until we came to Leven Canyon (Fig. 9), where more than 1000 feet of metamorphic rocks are exposed (see Fig. 4B for location). These are rocks originally deposited as more volcanic and sedimentary deposits at the same time (Early Cambrian) as the rocks from Figs. 6-8 (above).
Figure 9 extends more than 1000 feet from the top to below the image. It is looking obliquely into the canyon, showing the opposing wall. The black lines indicate approximate bedding planes, which are folded and tilted. These rocks are from the Tyndall Group (the same a Figs. 6-8).
We completed our day’s journey near Cradle Mountain (Fig. 4C), where we found Cenozoic (<60 MY) lava forming a flat surface with low falls in creeks (Fig. 10).
A short walk up the creek, which formed the contact between the younger basalts and older (509-485 MY) sedimentary rocks, revealed the older rocks. Note the location in Fig. 4C, which shows the location of Fig. 11, where Fig. 10 was taken as well.
That was the end of Day One. There’s a lot more to come…
The low hills of the north Tasman coast loom in the distance beyond a calm sea as we approach Devonport (Fig. 1).
Figure 1 is looking toward the left side of Fig. 2, just to the right of the word “coast.” Thus, chances are that the hills are associated with the volcanic sequences in Fig. 2.
We disembarked and headed west along the coast, passing a series of low hills (actually knolls) like those seen in Fig. 3.
The left image shows the abruptness of these knolls and the right photo shows how closely they were spaced. There are going to be different causes for any landform like this, but it is very likely that glacial erosion was significant in Tasmania. Figure 4 shows just how extensive ice was during the last glacial maximum; the entire island was buried under ice.
Shading indicates ice extent. The coast of Tasmania is shown by a thin line and several locations we will be referring to later are labeled, such as Cradle Mountain (Our destination for Day 1). I am not suggesting that the hilly features in Fig. 3 are moraines. We’ll see those features later. But they were probably partly created by the removal of weaker rock by ice action.
When rock is worn down by pieces of rock carried by the ice (rock is much harder than ice, but not rock embedded in ice), and then the ice sheet melts, a variety of materials are left behind. Rocks deposited in this way, which don’t fit into the region’s general geology are called erratics.
Figure 5 shows a block of quartzite about 3 feet in diameter. This was a very smooth, hard rock that was sitting next to the parking lot of a public beach called Turners Beach (Fig. 6).
Turners Beach is a cobblestone beach but it has an interesting feature seen in the right photo of Fig. 6; the swash zone (where the waves lap on the shore) is sandy. The cobbles are covered by sand in the wave-dominated zone. This suggests that the cobbles are being reworked and there is no current source, which of course there isn’t. There are no bluffs of conglomerate being eroded. There is no fast flowing river, fed by seasonal ice melt at the front of a glacier. There is no active source of sediment along this stretch of coast and everything is relict (leftover), so the fine-grained material is being winnowed on the shoreface as the cobbles look on with disinterest, piled into an armored berm by occasional storm waves.
Let’s take a closer look at the cobbles, to see what they tell us about the source of these sediments (Fig. 7).
Without thin-sections or a fresh hand sample, broken with a hammer and examined with at least 10x magnification, I can only speculate, but I’ll cheat a little. The left image is probably derived from a source like the boulder shown in Fig. 5. To go from a block to a rounded cobble about 4 inches in diameter requires ~10 miles of transport in a rocky, high-flow stream (I’m trusting my memory so this is an estimate). The second image shows a dark rock that is more irregular in shape. This is probably a metamorphosed volcanic rock.
Not all of the cobbles were sub-round, however; Fig. 8 shows elongate rocks, both light and dark colored, and a range of sizes. This suggests multiple sources, with respect to transport path, and different residence times in the transport system of steep creeks and rivers.
Keep in mind when looking at Figs. 5 through 8, that the world in which they were broken away from older rocks was exposed to thick ice that was spreading from the central part of the island (at higher elevation) and sliding towards the coast. Even during an ice age, there would have been catastrophic melting at the edge as the ice moved northward (see Fig. 4). This movement would have continued for millennia, with transport beneath the ice sheet in buried streams.
Figure 9 shows the resulting topography from thick ice flowing (glacially, excuse the pun) over rocks as old as 1.2 BY, which had already been metamorphosed and damaged by brittle deformation during several orogenies. There were weak points, which gave way to the relentless advance of the ice.
I’ll stop here before we get into the source of the cobbles we saw at Turners Beach.
It’s the eve of our trip to Tasmania, so I wanted to post something. This makes me spend some time learning a little about where we’re going. I didn’t budget enough time for a place as rich geologically as Tasmania. I won’t be scratching the surface of this remarkable land, most of it unreachable by road.
This preview is based on a report by the Tasmania Geological Survey (Seymour et al., 2006). We start with a very good geologic map (Fig. 1b).
All the brilliant colors around the edges are magnetic intensity, probably measured by an aircraft. The legend summarizes most of the geology. Note that the large area of light blue is undifferentiated Paleozoic rocks. The places we’ll be staying are labelled in Fig. 1a because, having been modified in Powerpoint, it didn’t have the resolution necessary to read the legend.
Day 1 will be spent going from Proterozoic (~1.2 BY) in the NW, to Cambrian (~600-500 MY) at the first stop. This is going to cover a lot of geology, including a major mountain building episode in the Cambrian. We will spend the night at Cradle Mountain, in the middle of Dundas Trough (Fig. 2).
Figure 2 is looking obliquely from the west. The figure is oriented with the southern end (right side of Fig. 2) located at the NW coast (light blue line behind the “Day 1” label) in Fig. 1. It approximately follows the gray area in Fig. 1 and ends below the “Tasmania” label. I should have a lot to report from this leg of our journey.
Day 2 will take us across the heart of Tasmania (See Fig. 1a), beginning with metamorphosed Cambrian volcanic and sedimentary rocks, coinciding with the Tyennan Orogeny. These rocks were deposited in the Dundas Trough (Fig. 3) and intruded by a granitic batholith as mountain building progressed.
Figure 3 shows the geologic regions of Tasmania. Details of the Dundas Trough are not included because they are shown in the inset map (Fig. 2). Our second day will examine Late Cambrian to Devonian (~500-360 MY) sedimentary rocks deposited during final uplift and then erosion of the mountains that had been created during the Tyennan Orogeny during the Early to Middle Cambrian. We will hopefully find metamorphic rocks produced by deep burial during this event in the Jubilee region.
If we’re lucky, we may find evidence of major faulting during a Devonian compression event in SE Tasmania, but I’m not counting on it. What we will see are Late Carboniferous to Triassic (~300-220 MY) sedimentary rocks as we approach Hobart, our destination at the end of Day 2. There was a hiatus (aka erosion) of 60 MY before sediments begin to collect again. They haven’t been described in detail, so I’ll do my best.
However, there is one other geologic feature I hope we’re able to visit on Day 2. Figure 4 shows an image from Seymour et al. (2006) of a Jurassic (~180 MY) diabase (dolerite aka microgabbro) dike injected into “basement” granite in the background and sedimentary rocks in the foreground.
The photo was taken about 50 miles by road from Hobart, but there is no road to Cape Surville, so I’m not counting on it. Maybe there are other exposures…
Day 3 will take us to Coles Bay in the Eastern Tasmania Terrane (See Fig. 1a), where we will encounter more Ordovician to Devonian (~500-360 MY) sediments as well as quite a bit of faulting and intrusion of granitic rocks during the Carboniferous (~360-300 MY). Figure 5 shows the extent of igneous intrusion after Tasmania had solidly grounded against Gondwana.
Throughout our adventure, we’ll see the sedimentary and igneous rocks that were created when Tasmania was torn away from the motherland (aka Gondwana).
It begins tomorrow evening when we board the ferry with our Toyota Yaris, ready to explore unknown lands…
Seymour et al., The Geology and Mineral Deposits of Tasmania: A Summary, Geological Survey Bulletin 72, 2006.
It was a beautiful day to go to the beach, so we joined our neighbors on the foreshore. We headed SE along the coast, looking for some cliffs that we read about. We found them a half-hour drive from home. The geology that will be presented in this post has been discussed knowledgeably on weekendgeology.com, which I recommend because I’m limiting my comments to only what I’ve seen with my own eyes.
It was a nice day for a drive along the coast. The bicyclists were out in droves but traffic was light and we made good time, finally arriving in the coastal town of Bayside, which overlooks the Southern Ocean from cliffs about 100 feet in height (field estimate). Figure 2 looks out on Beaumaris Bay/Half Moon Bay.
At the bottom of a paved path, we found a sandy beach filled with sun seekers. It was a well-sorted medium quartz sand (field estimate) with shell fragments in the swash zone, with dark, fine-grained sedimentary rocks exposed during low tide (Fig. 3).
According to weekendgeology, these rocks have been dated using fossils to about 10 million years ago (MYA), which is within the Upper Miocene epoch. They looked pretty horizontal at this point. Turning around, to face the cliff, we discovered that they are conformably overlain by a reddish sandstone that gives the location its name (Fig. 4).
The contact is weathering faster than either of the two lithologies in Fig. 4, a pattern not repeated in a slightly different location (Fig. 5).
The upper, yellow-reddish lithology is younger and there is no unconformity between them, so it’s only a little younger. Fossils within it have been used to place it within the late Miocene/early Pliocene (~5 MYA). A burrow is visible at the top of the photo. A trace fossil like this can be used to identify depositional environment and even age (roughly). This burrow was created by a worm of some kind, probably marine. The cross-bedding is typical for a nearshore marine setting, where waves are present but not dominant. This was not part of a submarine bar or a river, but an area influenced by both waves and steady currents. Possibly a sand bar in a river mouth.
The structures in Fig. 5 suggest that the sediment was being constantly reworked, possibly by tides and episodic flooding from a river. For example, Fig. 6 reveals irregular cross-beds that suggest more uniform transport to the left. But they do not occur in sets, only as individual beds, as if sediment were moving across the area briefly, followed by a quiet period.
Slumps of semi-firm sandy mud are outlined in Fig. 6 immediately above the primary color transition from green (marine) to reddish (terrigenous). The contact does not represent a sudden change over time but rather a horizontal shift; probably from a quiet, possibly lagoon, area represented by the green; to a more open channel flow where the river actually met the sea. Note the greenish areas in the younger rocks. Color in rocks has to be treated with caution, especially at scales less than the thickness of a major bed.
I’ve mentioned diagenesis several times in this blog. As sediments are buried slowly, over millions of years, and heated, the complex organic molecules they contain are slowly cooked. It’s like breaking down crude oil in a refinery; gunk goes in, and a variety of products come out at different stages of heating and pressure. Except, diagenesis isn’t as controlled, so the gunk produces byproducts of the diagenetic process intermittently and irregularly. The red-green color differentiation of terrestrial-marine sediments is like that. Red is due to oxidized iron and green is caused by reduced iron.
There’s a simple rule: red is land and green is sea. But it only works at large scale. When I first looked at the younger (red) rocks in Fig. 6, I assumed they were terrestrial, deposited in rivers. What probably happened is that sediment was deposited in a river delta, with sand spits, bars, and other transitional sand bodies.
Let’s return briefly to the slightly older “green” fine-sand sediments from ~10 MYA. They contained irregularly distributed, sub-round hollows where a dark inclusion may have existed (Fig. 7).
This is not a fossil, but a remnant of the irregular diagenetic chemical processes I alluded to above. They are very similar in origin to the concretions that we saw in an earlier trip. They are not like the burrow seen in Fig. 5.
Let’s estimate how fast sediment was accumulating here about 5 MYA. That’s not so easy to do because we’re not working with millions of years in this example; nevertheless, let us attempt to see what kind of depositional environment we’re looking at. First we observe that Fig. 6 is about 2 m (we’re using SI units now) in height. To constrain our estimate, let’s use a sedimentation rate of 2 mm/yr like before to calculate how much sediment would accumulate in a million years. We multiply 0.002 m/y and one million years, to find that a steady depositional rate of 2 mm/y would have produced 2 km of sediment. Obviously, that didn’t work. We’re looking at 2 m of sediment, so let’s estimate the apparent sedimentation rate using observations (treating the given ages of the rocks as observations because of the complex radiometric methodology used to constrain the geologic time scale), rather than guesses. We divide 2 m by 1 million years to estimate that (overall) there was very little sedimentation occurring (0.002 mm/y). A meaningless estimate for a geological process.
I alluded to accommodation space in previous posts. There doesn’t appear to have been much in this area about 5 million years ago. What was happening?
There was a lot of sediment available, as shown by the slumping sediments in Fig. 6. We get a clue from the lowest part of the exposure (Fig.8), near the contact with the reddish sediments in Figs. 4-6. The sunglasses (for scale) show that gravel particles (>2 mm in diameter) were sparsely mixed with fine sand. Note that the large grains in Fig. 8 are what is called sub-angular to sub-round in shape. These particles travelled along a river bed for less than 100 miles (wild guess); the point is that the source rock for these grains wasn’t hundreds or thousands of miles distant. There were mountains near the field site 5 MYA.
It isn’t just that there were mountains. This is a lot more complex than a developing foreland basin…no way…Australia was separating from Antarctica (according to current thinking) during the Tertiary period (Fig. 9).
Note that this map is dated at 94 MYA; the reason for including this model result is to show that Australia, Victoria to be specific, was moving away from other land masses (to the NW in the image) and there was no regional compression, like during the Paleozoic.
The sandy matrix seen throughout these rocks is consistent with the reworking of quartz-rich sediment for millions of years. However, Fig. 8 suggests that a new source of angular gravel was available in small amounts intermittently. There was uplift and erosion of rocks containing either igneous (plutonic) rocks or sediments deposited near a source in antiquity (e.g. during the Paleozoic). This probably occurred during isostatic adjustment, which would explain the low sedimentation rate we estimated.
Before addressing this question, I’d like to return to the rocks. Figure 10 is included in this post to make my point that stratigraphers deal with rocks at the scale of ten’s of meters but sedimentologists and geochemists are forced to get closer to the action.
I have nothing in particular to say about Fig. 10, other than this is an example of remineralization at small scale (image size is ~3 inches), which impacts observations at larger scales. Enough said. I debated whether or not to annotate Fig. 11…no reason to.
The image size is less than 3 feet. These sediments were deposited during the change in location of the river mouth. The black layer that truncates cross-bedded sediments below and is overlain by them above, was an anomaly. A quiet period maybe. I have to look closer however.
These are trace fossils. The worms went crazy for a few years (maybe only months), burrowing and eating everything organic within this dark layer. They didn’t make it (at least not here), but that’s life. Spatiotemporal adjustment. The depositional environment became firmly planted in the terrestrial sphere (removed from the ocean) with no more quiet water, by the time poorly cemented and poorly sorted sands were deposited.
Figure 13 shows the thick layer of weak sandstone that supports the bluffs, and mostly produces steep slopes. Here, at Half Moon Bay, they are a little stronger. The white layer could be nothing more than salt being concentrated during burial. They are the same rocks. But, what do they look like?
Figure 14 shows the contact with the nearshore marine sediments we’ve been examining. Obviously, this is a different depositional environment. Guessing, I’d say it was already arid, from the white deposits (salt is the most common of a few white chemical deposits, whether depositional or diagenetic), but there were flood events, just as several million years earlier.
Compare Figs. 15 and Fig. 8 to see that this region (probably a delta near a source of sediment, i.e. mountains) was in dynamic equilibrium for several million years; sometimes there was more coarse sediment (from an unidentified source), but one has to ask: Why are the sediments so poorly mixed and the coarse grains showing no layering?
Figure 16 was taken from the same area as Fig. 15, but from a greater distance. The image is about 3 feet high. It has been marked up to show the general lineation of the angular gravel deposits. The lines are not meant to imply a clear bedding; not by any means, but they are real because diagenesis does not recrystallize silica into new crystals at the shallow burial depths these rocks have seen (maybe 5 km?). I have seen exactly the same distribution of coarse fragments in a sand matrix in both rocks and recent fluvial sediments. Each marked line in Fig. 16 indicates a weak episodic depositional event, strong enough to move some gravel around but not sufficient to form cross-bedding. Nevertheless, the (admittedly crude) analysis suggests that flow to the left (i.e. river transport) was important.
We made one more stop on our field trip into the Tertiary, another cove within Half Moon Bay. We had hoped to follow an unimproved path down to the shore, examining beds rich in fossils; but our plans had to be modified when we found the path closed because people had been destroying the cliff in a search for the very fossils that made it a treasure trove of life here 5-10 MYA. We drove further south (the dark-blue dot in Fig. 1) and hiked along a path as far as we could. Figure 17 shows the beach here.
This particular cove, for oceanographic reasons (e.g. waves and coastal currents) collects seaweed from throughout the eastern shores of Half Moon Bay. There was a sign announcing a project to “save” the shore from what is probably its natural state. Fig. 17 reveals the cause of my cynical analysis; the pathetically small amount of sand deposited (at great cost) has not and cannot prevent the algae (dark area larger than the beach) from collecting. Some beaches are not meant for human occupation.
We came here to get a look at evidence of deformation after these Miocene/Pliocene rocks were deposited. Let’s start with Fig. 18,
Figure 18 has been marked-up with yellow lines indicating bedding surfaces estimated from along the coast (see the inset map). This is the stretch of coast that we couldn’t access because of the trail closure. To my eye, there is a low-amplitude anticline visible in these Tertiary rocks. The orientation of the compression that would have created such a feature (in modern coordinates) was apparently from the SW. Study the inset map, assuming that the anticline marked in Fig. 18 is approximately orthogonal to the coast seen in the inset.
As noted above, Victoria was not in a compressional stress regime after these sediments were deposited. Nevertheless, we cannot argue with the rocks. I’ll add here that a map of anticlines is consistent with our observations. There was mild compression in this area at about the same time as basalts were being created and flowing over the land to the west. We discussed those already.
The deformation seen in these rocks occurred less than 5 million years ago, when Australia was in a tensional tectonic regime, being dragged by upper mantle convection to the NW (see Fig. 9). I would propose that the mild fold seen in Fig. 18, and reported en mass,are nothing more than drape folds. These coastal/fluvial sediments were being buried suddenly, as the upper cliff sediments were deposited, probably during a period of volcanism that was sporadic in Victoria about a million years ago.
It’s all consistent, which is a critical criterion in any geological study. These rocks record a transition from shallow marine to deltaic and then to riverine, the transition having accelerated when the upper, poorly consolidated, sands were deposited. In My Opinion (IMO), this was the result of Australia being picked up by a mantle plume, which lifted this area and created a lot of volcanism. Half Moon Bay slid off the elevated mantle plume before the sediments, produced by erosion of isostatically uplifted granites, could properly lithify.
This is a short note to show what we found along the rocky coast at the mouth of the Yarra River, on our return from Organ Pipes National Park. We didn’t have to go far as shown in Fig. 1.
The rocks on the south side of the Yarra River are slightly higher than on the north side and are exposed at all but the highest tide. Figure 2 shows what the shoreline looks like and Fig. 3 shows the sediment texture of poorly sorted sand and gravel.
A Lava Blister is defined as: “The surficial swelling of a plastic lava flow crust in response to the puffing up of gas or vapour from beneath the flow. Blisters may also form through hydrostatic or artesian forces in the lava. They are usually 1–150 m in diameter, with a maximum height of 30 m, and are hollow. Compare tumulus.” (Oxford Dictionary)
Here’s what the one we found looks like at mid-tide (Fig. 4). I have outlined the rim. Note the smooth surface of the fractured basalt lining the rim.
Apparently, this volcanic field is one of only three locations known where these features occur. I admit this isn’t a very good example, but it has been subjected to coastal erosion form almost a million years. Here are a couple other photos, now that you know what you’re looking at.
The title of this post, besides being a bad pun, refers to the fascinating secondary fabric we found in some basalt flows not far from our house in Melbourne. An hour-long car ride took us to Organ Pipes National Park, near the Melbourne International Airport. The sign at the park entrance gives an overview of the park (Fig. 1). We’re going to see some of this for ourselves.
The now-familiar Victoria geologic map (Fig. 2) reveals that the park is located well within the Pleistocene volcanic field, with radiometric ages of approximately 700 ka to 1 ma. Note also the thin slice of Paleozoic sedimentary rocks in the area (indicated by the purple color). We’ll see those as well today.
The terrain consists of a plateau dissected by streams like Jacksons Creek running through Organ Pipes National Park (OPNP). The flat surface was created by the lava flows that emanated from numerous volcanic cones. The landscape seen in Fig. 3 hasn’t changed that much since these flows covered the land, filling creeks and leveling the terrain.
The asphalt path seen in Fig. 3 leads down a steep slope to Jacksons Creek, ~150 feet below the pediment level. We’ll take a look at the basalt from the top of the plateau later. This week, we’re going to start from the oldest rocks and work our way upward. So let’s get going.
Jacksons Creek, about twenty feet wide and probably no more than a few feet deep, winds its way along the canyon. On the other bank, where we couldn’t get, we discovered Silurian sandstones (according to the park info about 420 million years old), with an apparent dip of more than 60 degrees to the NE (Fig. 4 is looking towards the SE).
This is as close as we could get. These sedimentary rocks were tilted (folded and faulted) during collision with suspect terranes and finally Gondwana throughout the Paleozoic, ending in the early Mesozoic period (about 200 MY ago). The red color of these sediments suggest that they were nonmarine, probably deposited in a river floodplain, which is consistent with the nearshore marine depositional environment we saw in Ordovician rocks we encountered in a previous post. Those were of late Ordovician age, only a few million years before these nonmarine sediments were deposited. The land was rising (relative to sea level) about 400 MY ago, but not suddenly.
The Ordovician marine rocks from the last post were also tilted and even overturned. They showed evidence of compression shifting from SE early to SW during a later episode of deformation. We can’t be certain of the direction of compression of these Silurian rocks, but it appears to have been (in OPNP anyway) directed along a NE line, i.e., about 90 from the viewing direction of Fig. 4. Before we leave this fragment of the distant past, let’s see what else we can get out of these rocks. Figure 5 shows a very simple analysis of the compression directions I have estimated from these rocks and the Ordovician sediments from last week.
This analysis comes with (at least) two warnings: 1) deformation has nothing to do with the age of the rocks except that it couldn’t have occurred for millions of years after deposition of the youngest deformed rocks (Silurian); and 2) I have put an arrowhead on the stress line for Phase I based on what I know of the Tasman Orogeny. Phase I occurred first and was mild, based on low-angle anticlines seen at Yarra Bend Park. Phase II was later and stress was towards the NE (present coordinates) because of the steep dip angle of an anticline axis at Yarra Bend. This is for certain. The Silurian rocks at OPNP may reflect Phase I deformation, but we couldn’t see them, or any evidence, from across Jacksons Creek. However, they are consistent with strong compression towards the NE.
No younger sedimentary rocks are found in the area, so it is assumed that whatever rocks were deposited or erupted (volcanic) in this area between Silurian and Pleistocene were eroded away. So, we’ll skip over the Mesozoic and focus on this post’s titular rocks.
Figure 6 shows an unconformity between the tilted Silurian rocks and younger volcanic rocks that overlay them. This could probably be called an angular unconformity because lava is deposited in layers, just like sediments.
The viewing angle is different from Fig. 4, but the general bedding of the Silurian sedimentary rocks is indicated by yellow hand-drawn lines. The black line is the approximate contact between the older rocks and the Pleistocene volcanics. That was the approximate land surface ~one million years ago. Pretty cool (another bad pun). The white square indicates Fig. 7.
The viewing angle has changed again, but the bump of the black line in Fig. 6 is the point in Fig. 7. The angular fragments of basalt in Fig. 7 indicate the thin soil that has developed over a million years in this semi-arid climate. It isn’t very often that a contact like this can be seen, a hiatus of more than 300 million years unobscured by soil and vegetation. Alas, it was hidden from us by logistics and our unwillingness to find a way to get over/around the creek, in violation of the law, to get a closer look. Before we move on, I want to share an interesting photograph of this exposure (Fig. 8). It demonstrates how dangerous it is to use color in assessing a rock from a distance. The white exposure is the same as the yellow rock but something was different when it was buried and became cemented…diagenesis is a complex process.
Moving upstream, and higher into Pleistocene basalt, we see some interesting features of this particular lava flow in this particular location. This is tricky because we don’t know how thick the flow is; nevertheless, let’s go to the feature that gives the park its name to begin.
This is classic columnar jointing. It is thought to occur during uniform cooling of igneous rock. A famous example is Devil’s Tower, Wyoming. The rocks of OPNP are definitely extrusive, unlike other examples. We have seen the contact between this basalt and the surface it flowed over. If experts cannot agree on how an igneous rock that never saw the light of day formed this unique structure, they certainly aren’t going to agree on these basalts, especially the more spectacular fabric seen throughout the park.
This is a back-of-the-envelope estimation of the orientation of the columns visible at the main exposure along Jacksons Creek. I’m not going to talk about the mechanics of the analysis because there are some notes on the figure. The white ellipses, representing the ends of columns, tell a story too unbelievable to accept if the pictures weren’t telling the story. Within a few hundred yards (at most) from their contact with the land surface this basalt flowed over, they were cooling so uniformly that these elongate fractures formed. More unbelievable, the cooling joints are twisted. It gets more bizarre.
Figure 1 indicates a site called “Rosette Rock.” We went to take a look. Figure 11 shows what we found.
For scale, this is an exposure of basalt approximately 50 feet in height, along Jacksons Creek. The yellow lines represent the directions of the cooling joints. I didn’t use ellipses because there was very little variation out of the plane orthogonal to an axis pointing at the observer. In other words, this is a wheel, with the joints radiating from a central point, a hub if you like. I have no more explanation of this now than I did when I first saw it in person. I don’t think anyone does. This makes Devil’s Tower look like a stick figure. We followed the creek to see these curious structures up close.
About 500 yards upstream, around a meander (See Fig. 1), we came upon the “Tesselated Pavement” (Fig. 12), a “bedding plane” of the basalt probably created by the pounding of basalt boulders and pebbles over the millennia (Fig. 13).
This exposure seemed to be part of a resistant tongue protruding from the thick flows supporting the hill in the background of Fig. 13. There were several other such resistant tongues along a couple of hundred yards of Jacksons Creek’s quiet surface. The exposure in Fig. 13 was continuous with the massive outcrop seen in Fig. 14.
This rock wasn’t as columnar and it was fractured badly. It was threatening to collapse on the trail. The length of Jacksons Creek was a surreal geological experience. For example, the rocks shown in Fig. 14 were crumbling, yet there is no scree collected at the base of the low cliff. Very strange, especially in a protected national park. The smooth surface in Fig. 13 was less than 100 feet from Fig. 14 and at the same elevation, at creek level. Very strange. It’s as if someone built a scene for a movie but didn’t understand geological processes. Very strange, especially in a semiarid environment.
I looked for evidence of paleosol within the basalt, but all I found were irregular areas of remineralization (Fig. 15).
This was either a single flow, or a series that occurred before weathering had proceeded enough to leave evidence. However, the erosional terrace seen in Fig. 9 suggests that there were multiple flows. We retraced our steps back to the top of the basalt flow at OPNP and noted changes in the rock that support rapid deposition of all of the basalts observed in the area.
Figure 16 reminds us of the columnar jointing deeper within the flow. Note the top of the photo shows the present surface.
Here are some photos of exposed basalt from near the top of the sequence (Fig. 17). Note brittle fractures (left photo), and thin bedding reminiscent of the blocks seen in Fig. 14, but weathered.
A close-up view of the rocks seen in Fig. 17 reveals vesicles and a hint of depositional flattening, as if the lava were dense enough to condense (Fig. 18).
At the top of the plateau, the basalt contains more vesicles and shows no sign of compression under its own weight (Fig. 19).
To summarize this trip, which covered more than 400 million years, most of it unrecorded, what is today Organ Pipes National Park was a river plain that was buried before being deformed by the pressure of tectonic plates colliding to form Gondwana. This took several hundred million years. Whatever rocks were pushed up to form mountains were long gone, leaving an erosional surface not unlike that today, which was covered by lava flows that filled every gully, creek, and canyon in the area. Erosion had to begin again to erode Jacksons Creek anew. It seems to me that OPNP is filled with a single flow that ended with gaseous magma capping a fine-grained volcanic rock that cooled and later fractured by mechanisms that are not well understood.
The rocks tell us it is so.