Deformed Ordovician Nearshore Marine Rocks in Melbourne

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.

  Fig. 1

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.

 Fig. 2

A plaque mounted on the observation deck summarizes the geologic history of the area (Fig. 3).

   Fig. 3      Fig. 4

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).

 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.

  Fig. 6

They ranged from a few feet across to the size of a small car.

 Fig. 7

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).

 Fig. 8

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.

 Fig. 9

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).

   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.

 Fig. 11

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.

 Fig. 12

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.

 Fig. 13

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.

  Fig. 14

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.

  Fig. 15

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.

 Fig. 16

A little further down-section (Fig. 17) shows crossbedding similar to Fig. 14, but with less mud.

     Fig. 17

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.