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.
We didn’t have to travel very far to observe an interesting geologic exposure, on the outskirts of Melbourne. We went to Yarra Bend Park, situated along the banks of the Yarra River, where a small stone dam creates some rapidly flowing water.
Our location in Fig. 1 is at the confluence of the Yarra River and Merri Creek, the smaller stream on the left (western) side of the map, where the highway first enters the park area (colored in the map). The Yarra is a meandering river here and reverses direction, thus the name of the area (Yarra Bend). There is an exposure of interbedded sandstone and mudstone along the southern bank of the river, at the entrance to the park, near an observation deck. Figure 2, taken from the northern bank shows the exposure nicely.
A plaque mounted on the observation deck summarizes the geologic history of the area (Fig. 3).
The dark and light layers in Fig. 3 schematically represent the tilted sandstone/mudstone sequence seen across the river in Fig. 4. However, there’s more going on than that. Let’s look at the geologic map to get an idea (Fig. 5).
This report describes rocks ESE of Melbourne, where the orange (volcanic) rocks are in contact with the purple sedimentary rocks in Fig. 5. Yarra Bend park is located where the volcanic rock field forms a point that touches the main road (below the “o” in Melbourne). A previous post identified some older basalts along the coast to the SW, but those were about 30 MY old. These are much younger. They came from a volcanic field to the west, as suggested by the massive field of volcanic rocks west of Melbourne (see Fig. 5). In other words, Yarra Bend is at the eastern edge of a volcanic flow. There were no exposures of these basalts in Yarra Bend, however, because the rocks had eroded to form slopes and a black soil. Figure 6 shows some loose boulders used to reinforce the bank of Merri Creek under a highway overpass.
They ranged from a few feet across to the size of a small car.
Figure 7 is a close-up of a boulder. Note the holes (vesicles), suggesting this was a gaseous basalt. Large phenocrysts are not visible. The rusty surface suggests that the lava contained significant iron-bearing minerals (probably amphiboles or pyroxenes). There is no evidence of older sedimentary rocks on the north side of the river. In fact, elevations are much lower on the north side. Thus, the north and south banks of the Yarra River have completely different geology. According to the sign (Fig. 3), the basalts redirected the river against the bluffs to the south, implying that the different geomorphology existed at that time.
We drove to the south bank of the river and parked the car at the top of a ridge (seen in the background of Fig. 2), on a paved road less than a hundred feet above the river. We walked along the paved road that led down to the river, following a footpath (dash-dot line in Fig. 1) to the river at the extreme left of Fig. 1. Ridges of resistant rock formed the northward pointing headlands that defined the river’s meanders.
The ridge where we parked was directly underlain by thin-bedded sandstone/mudstone beds tilted in a generally SE direction (no strikes or dips were measured). However, small-amplitude folds were visible (Fig. 8, looking to the north).
The SE dip angle varied from about 45 degrees to vertical. The variation of bed orientation by so much over distances of less than twenty feet suggests that the sediments were very ductile during deformation. This inference is supported by remineralization, as seen in Fig. 9. Note that this thin bed was protruding from the layers shown in Fig. 8.
There was also evidence of brittle deformation, including hexagonal fracturing (not shown), which probably occurred during uplift and a reduction in temperature and pressure.
Without reviewing the geological literature on SE Australia, especially the Tasman fold belt, we can use our field data to construct a preliminary structural history of Yarra Bend Park. First, sand and mud were deposited about 420 MY ago, during the Ordovician period, along a coastline not that different than that existing today in the area.
Over an unknown time interval (certainly millions of years), the layers of sand and mud were buried 6 to 50 km beneath the surface (detailed mineralogy would limit this depth range substantially) and cemented to become rock. The entire region was then compressed along an approximately N-S axis and the lithified rocks seen here were folded. Only small folds are visible in this area (wavelengths less than 10 m and amplitudes less than 1 m), but they certainly would have been associated with larger scale folds during regional compression. The sediments were heated and compressed enough during this interval (lasting possibly millions of years) for the original sediment grains to remineralize as seen in Fig. 9.
The anticline seen in Fig. 8 was tilted more than 45 degrees in a generally easterly direction. A key observation for these rocks is that they were overturned after being folded; i.e., the fold itself was compressed along its axis and became nearly vertical. Rocks buried many kilometers beneath the surface rotated more than 90 degrees (overturned) indicates compression, usually associated with extreme folding. However, this small exposure reveals two compression directions, oriented at right angles, from N-S and E-W (rough estimates), which suggests a rotational stress regime. The rocks only recorded two members of what was probably continuous (but variable) compression while the tectonic plate carrying Yarra Bend Park rotated, dragged along by upper mantle convection.
Imagine slapping two balls of play dough together and rolling them (slowly) in your palms, in a circular motion. That is what happened to these sediments/rocks. But we don’t know when, not from our observations. Such a complex squeezing explains the fabric of these rocks. They were first pushed (like a rug that folds up) from one direction, then another.
There is another problem, however. Did the second compression regime occur when the rocks were ductile (folding) or when they were brittle (faulting)? Answering this question would tell us a lot about the timing of the compression; e.g., whether it continued while these rocks were being unburied by erosion of overlaying rocks; or if the stress changed direction while the rocks were still deeply buried. Since our observations don’t include microfabric analysis, and no faults are evident, we have to use indirect methods to address the problem.
The top of the low hill seen in Fig. 4 reveals thin layers of sandstone (Fig. 10).
The apparent dip on these beds is consistently 30-40 degrees and there is no evidence of folds (compare Fig. 10 to Fig. 8). There is also no sign of the variability in dip angle that was seen further up the hill, much less overturned beds. Folding is not uniform throughout a thickness of sedimentary rock. Invariably, different kinds of sediment/rock compress at different rates, depending on lithology, water content, etc. Thus, there is always some slippage along low-angle delimitation planes (usually bedding surfaces) whenever a thickness of rock is compressed. The difference in deformation style cannot therefore be used as evidence of brittle deformation during the second phase of compression, although it is consistent with the substantial difference in dip angle, especially overturned beds.
Indirect support for inferring brittle deformation also comes from the difference in geology and geomorphology across the Yarra River. The absence of Paleozoic rocks on the north side, taken in concert with the lower elevations there, suggests the presence of a fault underneath the river. This could be a normal fault, with the northern side of the river dropping to form a graben. However, the lengthy compressional stress suggested by the deformation of the sediments is more consistent with a reverse fault, with the south bank being the hanging wall. There is no basalt on the south side, and only a few cobbles along the river bank.
Figure 11 shows a thick bed of massive sandstone exposed 100 feet further along the trail, maybe 20 feet down-section. Closer examination of the photograph reveals low-angle cross-bedding, i.e., generated by waves with very little transport of sand.
We can do a back-of-the-envelope calculation (aka guesstimate) to get an idea of the time between the massive nearshore sand and the later deposition of alternating sand and mud. I’m going to use SI units because they’re easier for estimating. If my field estimate of 7 m (about 20 feet) is okay (not likely), then we can easily estimate the time in years between the thick sandy layer in Fig. 11 and the interbedded sand/mud layers in Fig. 10. Using a conservative sedimentation rate for a deltaic environment (like the central Gulf of Mexico) of 0.002 m/year (2 mm/year), we can confidently calculate a reasonable age difference. Dividing 7 m of total sediment by a sedimentation rate of 0.002 m/year means that only 3500 years were required to go from the depositional environment represented by Fig. 11 to that seen in Fig. 10.
The sediments immediately below the massive bed (Fig. 12) are like those above.
That thick (more than 6 feet) layer of sand was deposited in a sandy nearshore environment, as indicated by thin bedding (visible by zooming in on the photo) and no mud at all. Figure 12 suggests that we should be wary of projecting a single stratigraphic section (one-dimensional) into three-dimensions, i.e., it is very likely that a massive bed as seen in Fig. 12 was deposited someplace else at any time during the time span represented by these rocks. The low-angle crossbedding suggests that this may have been a shoal or bar deposited on a mixed clastic coastline. It’s time to go further down section, into the past.
We followed a trail (including some cement steps cast into a flood-control levee) to river level. This short hike took us another 60 feet down section, which means we travelled ten-thousand years further back in time from Fig. 12. Here’s what we found.
This is a section that could be taken straight out of the northern Gulf of Mexico, in another ten-million years. Sand and mud are equally available. That only happens when there is active erosion in an upland with a variety of rocks available, and plenty of accommodation space for the sediment; i.e., relative sea level is rising, whether due to sea level increasing or subsidence. Yarra Bend Park was located in an active delta. Let’s look at those sediment layers closely.
Figure 14 is a close-up of the strata in Fig. 13, which reveals irregular bedding typical of nearshore marine sediments. Finer grained sedimentary particles have weathered out to reveal low-angle, planar cross-beds. There was plenty of mud available. I want to emphasize that this exposure is less than an acre in horizontal extent, so this entire thickness of maybe 50 m represents on the order of 25 thousand years of deposition and erosion of sand and mud on an acre of seafloor. In other words, the sedimentary environments represented by Figs. 11 and 12 were certainly found within a few kilometers of each other at ANY given time.
The massive (almost 6 feet) thick bed in Fig. 11 can be examined at river-bank level because of the dip of the sedimentary rock layers (see Fig. 4). We went down-section and then turned to go back up section, but at a different angle, which is why I’m estimating distances and ages. These could all be much more accurate with some geometry. Figure 15 shows a 3-foot thick cross-bedded section at approximately a right angle to Fig. 11. This allows us to examine two different potential flow directions at the same time, albeit separated by maybe fifty feet.
Note how the dark/light lineations (laminae) are tilted slightly towards the left, the angle decreasing from the bottom of the “bed” to the top. Figure 15 suggests weak bedload transport with a leftward component. This transport weakened upwards. This is consistent with sand being transported along the bed by weak currents that varied irregularly, preventing consistent cross-bedding sets to develop. The more planar nature of lamination in Fig. 11 supports this conclusion because the flow would have been approximately “into” the rock surface in Fig. 11. This is an environment typically encountered below fair-weather wave base (e.g. deeper than 30 feet). The sediment size is uniform, suggesting continuous deposition rather than episodic. Storm and flood beds tend to have a lag deposit at the bottom and fine upward.
Using a “standard” deltaic sedimentation rate of 2 mm/year, we can [switch back to SI mode and] estimate that this meter-thick sand layer was deposited during five thousand years of continuous input from a land source. However, average sedimentation rates are meaningless for a sand body like Fig. 15 because sand will be concentrated by waves and currents into shoals at any given time. Furthermore, our estimate assumes a steady rate of subsidence; space had to be made between the ocean surface and bottom for this sand to accumulate.
Figure 16 reveals a sand layer with similar crossbedding to the overlaying bed but a different color and appearance; it is more rounded and less massive. This is probably an artifact of diagenesis that tells us nothing about its depositional environment.
A little further down-section (Fig. 17) shows crossbedding similar to Fig. 14, but with less mud.
I have described the sedimentary rocks of Yarra Bend Park in the order they were encountered in the field. Rather than rewrite this post, I’m going to summarize the geological history and refer to the photos in chronological sequence. I won’t make this mistake again.
Around 420 million years ago, the study area was receiving predominantly muddy sediment, with sand being deposited in thin beds (Figs. 13 and 14). Sand increased for several thousand years as the muddy layers decreased in thickness (Fig. 17), until an interval of major sand deposition occurs over a very short time span (Fig. 12), producing several massive, cross bedded layers (Figs. 4, 11, 15, and 16). These were probably shoals or barrier islands, possibly due to the river mouth migrating to the local area (delta switching). Subequal sand/mud deposition resumed in discrete beds (Figs. 8 and 10) for tens of thousands of years. If these rocks are 150 m in thickness, Yarra Bend Park was part of a stable clastic shelf for 75 thousand years.
Deposition continued and the sediments were buried to at least 6 km, where they were cemented to become sandstone and shale. This almost certainly took several million years. Compression, probably caused by collision with Gondwana, more than likely occurred during deposition of these sediments in a foreland basin. They were first folded from current south while still deeply buried but the direction of collision rotated ninety degrees clockwise as the rocks were unburied. With lower pressure and temperature, the deformation changed from ductile to brittle and faults would have occurred along planes of weakness, leading to a fault that defines the Yarra River’s modern channel.
Volcanism was continuous for the last 30 million years, culminating with the basalt flow that reached Yarra Bend Park about 700,000 years ago, pushing the river against this promontory of hard, Ordovician sedimentary rocks. They resisted erosion as the basalt weathered away from their flank, leaving us to ponder the amazing history of this unknown park on the fringe of civilization, buried in one of the most complex orogenic belts in the world.
This blog has been quiet for a while, not because I wasn’t traversing lands with a rich geological history; instead, my primary audience (off-road enthusiasts) had expressed no interest in geology. It turns out that they were typical Americans. Also, I’ve been busy writing novels and doing other literary actions, but now I’m writing this for myself. This is a record of my travels, with enough details to remind me of where I went and why it’s worth remembering.
This post reports on my first excursion since coming to Australia, a trip from Melbourne to Apollo Bay along the coastline of Bass Strait. Let’s get some perspective.
This is the SE tip of Australia. Melbourne is labeled, as is Tasmania and Bass Strait, which is 150 miles (~350 km) across. This post covers the very short distance from Melbourne to the tip of the continent to the SW. This was a day trip of only 200 km, which took about four hours because we stopped a lot.
Time for some geology. This trip is entirely located within the state of Victoria. The geologic map I’ll be using is from 1982 but I don’t think it’s very out of date because the geology of Australia has not been explored in detail. Here’s a portion of the Victoria geologic map, with city names. It is a rectangle bounded by Melbourne on the NE and the NW corner of Bass Strait on the SW.
We drove our trusty Toyota Yaris along Highway A1 and turned south, reaching the coast at Torquay, due south of the western tip of Port Phillip Bay. This is a geologically interesting area at the southern edge of a Quaternary volcanic zone (orange background with v’s scattered in the geologic map), overlain on Tertiary (25-65 MYA) marine sediments (tan area in center of above figure).
The underlying sediments of Tertiary age formed ridges along the shoreline covered by sand from the eroding poorly cemented sandstones, as seen in these photos from Spring Creek Park in Torquay.
Note the yellowish, non-marine coloration of the sediments in the right photo. They erode rapidly and don’t form bluffs. The absolute (radiometric) age of these sediments was unknown at the time of this writing, but this exposure is the top of the stratigraphic column in this area; in other words, we will be going further back in time as we head westward.
Many faults have been inferred for the region, as shown in a hypothesized cross-section along the coast.
Note the field geologists’ interpretation of faults, shown by nearly vertical lines, in the middle of the section. The green colors represent sections of the earth’s upper crust dominated by non-marine sediments whereas the red are volcanics from about 29 MYA. The slivers of orange in the center are the sediments shown in the previous photos. They are riding on older Cretaceous (green) sediments with volcanics systematically exposed and subsequently eroded, because of uplift along faults (vertical lines).
Our trip westward along the coast took us into the older, Cretaceous, sediments (shown in green in the previous maps) beginning southwest of Anglesea, where the Great Ocean Road entered a National nature preserve that resembles Southern California near San Diego. This photo was taken near the edge of a bluff overlooking the coast, but it was closed because of recent cliff collapse.
There are a number of rivers flowing to the coast from the inland uplands, but few of them reach their destination by late spring. The underlying rocky ridges form barriers that the river channels are forced to follow parallel to the coast for kilometers, forming structural lagoons with seasonal mixing with the ocean. The Anglesea River is a good example. The channel formed a lagoon that did not debouch into the ocean, although at high tide, during storms, there was inflow from the ocean. Here’s a look at this phenomenon.
The first photo shows an anatomizing river, with side channels and bars, as if reaching its destination. The second shows the berm that prevents the river’s flow from reaching the ocean. The berm is overtopped only during storms from sea, or high-water flows from land. All of the river’s flow is absorbed into the dry sand in this arid environment. The river flow is not sediment laden, as shown in this photo of birds bathing in the shallow water.
We tasted the water, and it was slightly brackish, from seawater overtopping the berm. The rivers don’t carry a lot of sediment from the arid, low-relief landscape through which they run. Thus, the substantial wave energy impacting this coastline is not spent redistributing sediment from rivers and instead erodes the coastline. The next two images show how this process occurs.
The first photo looks west from the Anglesea River, where the rocks are weaker and form a less-steep bluff. They also reveal changes in dip, from almost horizontal to dipping seaward at more than 30 degrees to the right of the photo, near the river’s mouth. This may well have been a fault zone during a regional uplift event. The photo on the right is from east of the river, where the bluffs are much stronger. However, rubble piles at the base of the cliff indicate where collapse has occurred. This is the area where the previous image was taken from the top of the cliff.
Note the dark coloration of the lower half of the bluff face in the second photo above. This region experienced extensive volcanism about 30 million years ago, when Australia is hypothesized to have passed over an upper mantle “hot spot.” There is an extensive volcanic field north of this area and basalt blocks have been brought in to reduce coastal erosion; however, the dark layer in the photo is a layer of ash that has formed a weak volcanic rock called tuff. Here’s an example of this tuff in place over older sandy sediments.
It is much less resistant to erosion by wave action than the blocks brought in for erosion control (seen in the left foreground of the previous photo). As the cliff retreats, the lower rock forms a table covered by a thin veneer of sand reworked continuously by waves. Slightly more resistant rock forms ledges that are exposed at low tide, as seen in this photo.
These exposed outcrops are visible along this entire section of coast, which is underlain by these same sandstone/conglomerate rocks. For example, here’s a photo taken at Aireys Inlet a few miles to the southwest. Note the tuff layer at the base of the pillar, seen clearly in the close-up photo below.
Because of the variable wave approach (from SW to SE), and heterogeneities in rock strength as well as structural factors such as faulting, many small pocket beaches have formed, many of which are the mouths of seasonal rivers and streams. Here is a good example.
Some of these bays are very large, as is this example from near Aireys Inlet.
The coast is lined with steep mountains eroded within Cretaceous, non nonmarine sedimentary rocks, forming very short streams that are prone to flash floods during heavy rain. The streams are so steep, that waterfalls can be found within a kilometer of the coast, such as Sheoak Fall, near Lorne, shown below.
The second image shows flood debris in the foreground, more than fifty feet above the stream bed seen in the distance, deposited during a flash flood. Another feature of these coastal sediments are the presence of calcareous concretions, formed during burial and diagenesis, which dissolve out when exposed to slightly acid rainwater. They leave large voids like this.
There are even caves higher up the slopes. The entire drive to Apollo Bay was similar to what I’ve described above, sandy beaches overlaying rock shelves, with seasonal rivers debouching into the ocean. We finished the drive by following a two-lane road through Otway National Park and seeing a spectacular primordial forest that stood out in sharp contrast against the semiarid coastal environment.
We descended from our last camp site in the San Juan Mts and discovered another environmental consequence of the weathering of the volcanic and intrusive rocks from this area. As the plagioclase feldspar weathers it forms the mud we see everywhere that is low, but it also forms a thin soil on the underlying rock that is weathering in place to form regolith. The trees cannot put roots very far into this broken and weathering rock so they have very shallow roots, which allow them to be blown over by a strong wind or just fall over when they get too large. We encountered a fallen tree as we headed for Dove Creek and the highway; we think it was an aspen but none of us were experts.
We didn’t have a saw (not too well prepared) but a couple of motorcyclists riding the TAT had a small hand axe and we had a hand saw, which together were used to cut about half-way through the trunk. The winch on the lead vehicle was then used to finish breaking the fallen tree and drag it out of the road.
Our path took us through some Paleozoic rocks that survived on the flanks of the uplift associated with both Laramide and Tertiary magmatism. These older sediments had been eroded from the top of the San Juan Mts before the Tertiary volcanism but they are very thick to the south and west in New Mexico and Utah. I am not going to discuss these areas in this series of posts because we didn’t really travel through it at a leisurely rate but just headed to Monticello, UT where we ended our trip.
Here is our afterward group shot in front of the Abajo Mountains as we were preparing to head east to Louisiana, New York, and New Hampshire.
I like to end these trip posts with a summary. This journey covered a long period of time, from the Proterozoic (~1.7 billion year ago) granites of the Wet Mts to the Miocene (~6 million years ago) basalts of the Raton Mesa in SE Colorado. It is typical of CO’s complex geologic history that these oldest and youngest rocks are near each other where our trip began on I-25.
These two regions not only represent the entire time interval we covered in this trip, but also the end members of the igneous rocks that are produced within the earth’s crust; the Proterozoic granites were formed deep beneath the surface from magma that originated from melting of continental crust during an ancient orogeny whereas the Miocene basalts flowed out onto the land from fissures. This basalt also represents a significant contribution of their source magma from ocean crust, which has led many geologists to propose that part of the Pacific Ocean’s crust was overridden by N. America and melted beneath the continental interior to produce basalt, which is typical of ocean crust only.
Between these two extremes of igneous rock, we find granitic rocks with intermediate composition from the Laramide orogeny (~60 million years ago), which began the slow process of raising the Rocky Mts to their present elevation, and both volcanic and plutonic igneous rocks that were produced as this event wound down between approximately 36 and 20 million years ago. Many of these igneous episodes produced gold and silver deposits that occur along deep fractures in the earth’s crust, originating from mountain building ~1.7 billion years ago.
My final comment on this trip refers back to the name of this blog; the most exciting and geologically interesting parts of this journey were only possible because of roads built over the last 150 years to extract gold and silver from these mountains. The few areas where we were able to travel over designated trails that weren’t much more than tracks were limited to meadows and mesas in the SW part of the state. For this journey, we might refer to the blog as “Rocks and Roads”.
We camped along a creek in Silverton across from a silver mine and mill, as shown in this satellite image; the mill is indicated by a white triangle. The mine and mill are now open for tours but I didn’t have the opportunity to visit them.
Here is a photo of the mill. The mine went into the mountain behind the mill, which appears to be a fault. Veins of silver and gold are frequently found along faults, which are weak lines in the crust where the high-pressure fluids can penetrate. Silverton was settled in 1874 and had a peak population of 5000 in the early 1900s. The mill is nestled against the ridge south of town.
Gold and silver milling technology was changing rapidly while Silverton was active but this mill probably first used an “arrastra” method with a horse (or maybe water power) to drag a heavy stone around a central post to crush the ore brought from the adjacent mine shaft. Water and chemicals like mercury would have been mixed with the crushed ore to form an amalgam (silver/gold plus mercury), which was heated to boil off the mercury. It is likely that this inefficient method was replaced with a Comstock mill, which used a heavy piston to crush the ore and a cyanide process would have been used to extract gold more efficiently.
After leaving Silverton, we headed west toward Ophir Pass (see photo above) where the landscape changed dramatically. This photo shows the view looking west from Ophir Pass; note the lack of vegetation in this rocky terrain. It would be extremely difficult for trees or shrubs to take root, or for lichen to grow on these constantly shifting fragments of the original volcanic rocks. It is noteworthy, however, that grass is growing at this same elevation in the background where the rocks are not as fragmented.
Note also the muddy matrix supporting the trail, which implies that these rocks contain minerals that weather rapidly to form clay, i.e., these rocks comprise much more Ca-bearing feldspars like plagioclase rather than the K-feldspars we saw early in our journey, which form gravel. This next photo shows that these rocks are very resistant to erosion where they are less fractured.
As an aside, this kind of scree is impossible to climb in a wheeled vehicle, and off-roaders avoid it like the plaque. There is no cohesion amongst these fragments and our tires just churn it up like water from a paddle wheel; of course, with enough horsepower and large enough tires it can be climbed but not consistently.
A close-up view of one of the larger fragments shows that this rock can be classified as an andesite porphyry.
The lighter colored cobble is a piece of host rock and not a mineral crystal. Zooming in shows many smaller particles of rock (not mineral) and an irregular surface texture that suggests a squeezing of the ash as it was deposited from the air and compressed as it cooled.
We continued traveling eastward until we encountered some sedimentary rocks containing sandstones and layers of coal. I have no photos of these formation however, but according to the USGS geologic map these are Cretaceous (85-60 mya) Mancos Shale, a nearshore marine deposit with coal beds, which probably contributed to the naming of Black Mesa. We chose a camp site among the Tertiary intrusive rocks as indicated by location 6 in this image.
Our campsite, among spruce and aspens, was apparently used by hunters and boy scouts. Here is a photo that includes a generic hand sample of an andesite and a 15x photo of a sample from our campsite.
The rocks (there were no exposures where we stayed) looked like this andesite porphyry from geology.com. The larger photo shows that the hand sample color of grey is probably caused by the areas of grey glassy material, which are not likely to be quartz as in the granites we saw earlier because quartz is not present in intermediate igneous rocks like this. The best general reference to understand the rocks where we stayed is the Roadside Geology of Colorado, suggests that intermediate intrusive magmas penetrated into the thick sequence of volcanic rocks to form diorite with unusually small grains because of rapid cooling so near the surface.
Here is another 15x photo of this rock from our campsite. This is a 15x magnification of a rock fragment at location 6.
If this hand sample had been light colored, the area indicated by circle A would be reasonably interpreted as quartz, as we saw in the sample from the Wet Mountains and the Sangre de Cristobal Mountains. However, the overall grey color of the sample, and the evidence of crystal terminations in A suggests to me, as a field geologist, that these are actually plagioclase feldspar with more Ca than Na (i.e. the more Ca the darker the plagioclase). Circle B is focusing on the white crystals that, though small, are clearly crystalline and not glass. These minerals are interlaced with the darker crystals and thus I interpret them to be a more Na-rich plagioclase feldspar (Na and Ca plagioclase form a continuous spectrum in this mineral). Area C shows a crystal of a dark mineral with much smaller crystals embedded or adjacent to it. I cannot tell if this is pyroxene or amphibole (a pyroxene-like mineral with water included in its crystal structure) but both contain Ca as well as Mg (magnesium). The most ambiguous (to me) mineral is indicated by circle D, which highlights an orange crystal; we saw pinkish to orange K-feldspar in the Proterozoic granites from the Wet Mountains but there should be no potassium in an andesite. However, some combinations of Na and Ca in the plagioclase series are this color and it is reasonable to assume (lacking thin-section analysis) that this is another phase of these common minerals.
If we estimate the composition of this sample as we did for the granites discussed in a previous post, we get the following: Ca-plagioclase = 50%; Na-plagioclase = 35%; mixed-plagioclase = 10%; amphibole = 5%. The fact that we can see crystal structures even in the smallest minerals in this sample suggests that this is one of the rapidly cooled diorites from the area rather than an andesite (volcanic), which would not have visible crystals at less than 100x magnification (e.g. under a microscope). The grey areas, in other words, are not glass by very small crystals formed near the surface.
We see the end result of this in the following photo of a local muddy area, where it was almost impossible from sliding sideways into the ruts.
As mentioned in previous posts, minerals containing Ca weather rapidly and produce by-products like clays that form MUD; however, as we saw later in our travels, when these clays form a veneer over subjacent regolith, they can be negotiated with care.
After passing thru the narrow canyon along Mill Creek (see previous post), we spent the night at Deer Lakes in Grand Mesa-Uncompahgre-Gunnison Natl Forest before ascending the lofty passes of the San Juan Mts. It stopped raining long enough to set up camp and then continued intermittently throughout the night and next day.
We continued deeper into the middle phase (30-26 mya) volcanic rocks and the outcrops became darker colored, reflecting the chemical evolution of the magma source for these ash layers and flows. This general area is indicated as site 5 on this photo.
We continued following wide gravel roads and paved highways until we reached Lake San Cristobal (elevation 9000 feet), where the resistant welded tuffs and flows produced by this volcanic phase formed peaks such as this one overlooking the lake.
The road followed money to Lake San Cristobal, where condos are everywhere and there is even a marina with small sailboats and row boats. The pavement and new gold (money) stopped at the inlet to the lake, where Upper Lake Fork Rd became a single lane gravel trail with intermittent wide spots to let opposing traffic by.
There was a lot of traffic heading back to the lake because the trail eventually encounters andesite flows that form ledges that a bulldozer could not level. Cars were not going to Cinnamon Pass, which is where we ended up.
Here is a photo that suggests why there is a road to Cinnamon Pass and beyond. I snapped this image while trying to avoid going over the side, but it shows a vein of what is probably pyrite (fool’s gold) in the cliff overlooking the Gunnison River.
Cinnamon Pass was the second highest elevation we reached during our trip (12640 feet). The peaks, which weren’t much higher, were composed of the alternating layers of ash and lava flows. These photos show the appearance of these rocks; note the small angular shards that litter the slopes. These result from dense joint patterns that indicate extreme brittle deformation, probably from faulting that followed volcanism continually.
The headwaters of the Gunnison River ended in a shallow, flat meadow that appears to be another cirque (glacier origin area) from the recent ice ages. After crossing Cinnamon Pass, where the horizontal rain and fog/clouds made for miserable conditions, we dropped down into Animas Forks, which must have been a bustling mining community at one time. Today, however, it is represented by a few old mines (some for sale) and a USFS toilet.
This is where the road from Lake San Cristobal led. Apparently, these mines produced enough gold and silver to justify the road we followed to California Pass (elevation 12930 feet). There was snow on the ridges and their concave shapes suggests that glacial erosion was a significant factor here.
Note the sharp edges along the ridge, and the tilting of the bedding planes (original horizontal surface upon which the ash flows and lava were deposited); faulting was ubiquitous over the several million years during which middle phase volcanism took place. This was followed by the late phase volcanism associated with the Rio Grande Rift, which was discussed in the Prologue.
This image shows the irregular surface of the San Juan Mts but it cannot prepare you for the 3000 foot descent that lies ahead on the trail to Silverton.
I saw several veins of what looked like K-feldspar as we began our descent, such as this one that had not been explored by miners.
Further on, I noticed this area that had been dug in an exploratory manner but apparently it did not look promising.
This vein is more promising from an economic geology standpoint because it appears to contain much more quartz, which is associated with the last stage of magmatic fractionation that produces fluids high in elements that don’t fit well into most silicate minerals (e.g. gold, silver, rare earths).
It would appear, however, that none of these veins west of Animas Forks contained economic concentrations of gold and silver. We made our final descent to Silverton via the potentially deadly Corkscrew Gulch trail in the rain, which is why I have no photos of the trail or the rocks. As we approached US-550, the hills were dominated by light-colored talus that did not originate from mines, but from weathering of the volcanic rocks. This could be another example of dense fracture patterns associated with faulting after the original ash flows were deposited.
We camped at Marshall Pass (location 3 in the photo below) among what we thought were Aspens, which apparently like the rocky soil that probably originated from weathering of the shales we saw in the previous post. The rocks were angular and uniformly about 8 inches in length; their size and shape are consistent with the origin of Poncha Creek following a fault through Marshall Pass. I will discuss this later.
After descending from Marshall Pass, we followed some the TAT until we entered the San Juan volcanic field. Our route took us into the early phase of this volcanic period in CO’s history about 35 mya, when volcanoes in this area were producing large amounts of ash and a lava similar in composition to the granites we have seen in previous posts. This photo shows the sudden appearance of such a volcano as we entered the San Juan Mts.
Note the rounded ridges and small peaks around the base of the main cone. The ridges are weathered lava flows, which often originate from the bottom of volcanoes and the small peaks are evidence of minor eruptions around the base of the main volcano. The flows are more visible in this photo that shows several such flows. We had the opportunity to follow a trail along one such flow, which still formed a scarp that is ~5-15 feet high after 30 my of erosion.
This photo shows one of the many well-preserved cones from the early phase of volcanism.
These volcanoes were producing mostly ash and debris that formed immobile layers that did not flow; however, they also produced ahs flows like Mt. St. Helens that would have moved rapidly down the slopes and destroyed anything in their path. These photos were taken at the approximate location indicated as “4” in the base map below.
I have some photos of these rocks that show some of the textures of these volcanic layers. This photo shows a tuff that does not appear to have melted together after the material was deposited from the air. If the fragments are hot enough and are buried under later volcanic material, they can become fused to form a “welded tuff” but that is not the case for this sample.
It has the coloration of the orthoclase (K-Feldspar) we saw in the granitic rocks from previous posts but it also has many holes (vesicles), which originally contained gas that later escaped; some of these holes are left from weathered minerals and rock fragments that were entrained during eruption. I suggest using your browser to zoom in on this image to see the remarkable number of different particles it contains, many of which are probably from the sedimentary rock through which it passed during extrusion. Here is another sample that has a different weathering color but is essentially the same.
The bulk of these tuffs is silica glass that didn’t have time to cool and form quartz crystals; it is about the same as the glass we produce for windows. I think I should again suggest that anyone looking at this post should click on these photos and zoom in with the browser to appreciate their composition.
The next photo shows a sample with easily identified quartz (the gray areas) and areas that show one of the weathering products, as shown by the white coloration. My first guess about the white material is a salt or carbonate formed from rapid decomposition of the glass to release common rock elements like Ca or Na, which formed minerals like NaCl (salt) or carbonates like calcite (CaCO3); possibly with metals included, e.g., Fe or Mg. Only laboratory analyses can identify these weathering products as well as the particles included in the original tuff.
This photo shows what an exposure of these tuffs looks like. Note the cliff at the top and the substantial debris at its base because of erosion of these relatively weak ash flows.
As we travelled to the SW, however, we entered a region where volcanism occurred several million years later, the middle phase of volcanism in the San Juan Mts. These volcanic rocks include layers of more resistant lava and welded tuffs. Here is a photo of a canyon formed from mixed layers of tuff, welded tuff, and lava flows.
The nearest outcrop also shows what could be termed a volcanic breccia because it contains large (~12 inch) cobbles in a fine-grained matrix. This photo also shows the variability of the different flows in the cliff. Note the irregularity of the layers as if they were being deposited on an eroded terrain.
As a volcano releases pressure and material from a magma chamber, a number of different types of effusives are produced; for example, we have just seen examples of tuffs that are formed from exploding liquid magma and any material it picks up as it exits the magma chamber thru channels in the rock. Here is an example of a tuff that appears to have been so hot that this liquid rock melted together to form a welded tuff.
Again, I urge the reader to zoom in on this photo, which shows fewer and smaller vesicles, as well as a layering and white minerals that I cannot identify from the photo.
The previous hand sample shows obvious layering but no evidence of flow; i.e., this material was deposited from the air and formed separate layers, possibly over a short period of time (e.g. days to weeks). I think this because there is some evidence of mixing between layers, which would not occur if it flowed or solidified between layers. This relationship is better seen in the following photo, which shows a kind of settling of an upper layer into a subjacent one.
There is no geologic reason to expect to find useful minerals (valuable) in these volcanic layers because they do not show any evidence of veins or remineralization like we saw on the previous post. Nevertheless, as we passed thru the flows of the middle phase, we crossed “Mineral Creek”, which was an opaque green as it flowed from the adjacent mountains. I stopped and took a panoramic photo of the valley and zoomed in on the opposing hills to discover an interesting symmetry across the valley.
The panorama photo has white outcrops circled. The lower photos show close-ups of the opposing sides of the valley; similar exposures occur on both sides of the valley. These volcanic layers are horizontal and there is no obvious faulting (e.g. dipping layers). There were no trails to either exposure so any mining reconnaissance did not suggest economic remineralization (i.e. gold or silver) worth road building. Until I can find the time to collect samples from both exposures and compare them, this remains a geological mystery to me.
I have a footnote to this post. We drove the dirt roads in this area under both wet and dry conditions. The roads were all wide and smooth with very little washboard texture but they created a lot of dust when dry. Weathering of the tuffs produces silt-sized glass particles that drain moisture fast but stay in the air when disturbed by truck tires, especially offroad tires. It was quite different than the gravel roads we encountered in the Wet Mountains and at Marshall Pass, both of which were based on weathered granitic rocks that produce sand grains as they erode.