Australian Capital Territory: Siliciclastic Sedimentary Rocks
This is the third post from the ACT. For an overview take a look at the first post.
This time I’m going to visit several locations from the Canberra area that show the variability of sediments during the orogeny. Figure 1 gives some idea of the complex history sand and mud particles have after being deposited in the ocean. We’ll talk more about that in the next post, but for now we’ll compare the sediments.
The rocks in Fig. 1 are estimated to have been deposited between 443 and 427 million years ago (Ma). The white rock has a lot of features that we’ll look into next time. For now, notice that the overlying thin bedded light-and-dark layers appear to be in continuous contact with the lighter rock. Ignore the displacement along faults.
This sediment consists of uniformly fine-sand-to-silt particles. The steep cross-bedding (Fig. 2) suggests this was a nearshore marine environment with strong wave action, producing submarine bars and probably a steep delta-front winnowed by wave turbulence.
The cross-bedding suggests transport from the right side of the photo, so fine sand was coming from an easterly source (e.g., NE to SE), probably near a river mouth. Imagine the mouth of the Columbia River, draining the Cascade Mountains. A lot of sand is deposited in giant spits and sand bars. The cross-bed sets (Fig. 2) are ~3-4 feet thick and dipping seaward (to the left). Note the near-horizontal orientation of the conformable sediments overlaying this unit. This is very close to the deposition angle.
What about the overall shape of the sand body? At this outcrop, we are able to see what appears to be a cross-section of this submarine feature (Fig. 3).
The photo is taken looking offshore along the paleo-shoreline, so the contact between the sand body and the overlaying sand/mud sequence dips landward and shows some irregularities (note the tongue protruding upwards to the right of the motorcycle). It reaches its maximum thickness in Figs 1 and 2 (left of the biker in Fig. 3) and is truncated by a fault. It looks like a fairly large submarine bar , maybe 200 yards across. Could have even been a barrier island or shoal that was buried by mud.
There is one last feature to mention in passing about this outcrop. The contact between the fine sand body and the overlaying mixed sediment indicates deformation of the sediments when they were still sediments, i.e., before burial (Fig. 4).
Figure 4A shows a pinching to the right of Yoko, forming narrow necks separated by lozenge shaped dark sediment. This structure is called “boudin” because it looks like a sausage. In sediments, it occurs when mud is stretched overlaying sandy sediments, which are very weak in extension. This conjecture is consistent with extension indicated by normal faults seen throughout the exposure. Figure 4B shows filling of channels by the underlying coarse sediment, analogous to a river flood channel being filled in. Note that the sediments are dipping ~10 degrees to the right in panel B and take that into account.
A closeup view of Fig. 2 reveals extensive dragging of sediment layers along both sides of the faulted contact between the two units. This cannot happen with deeply buried and partially cemented rocks.
The exposure we’ve been discussing is located near Site 22 in Fig. 5. The next stop takes us to Black Mountain, Site 4 in Fig. 5, where we’ll examine some turbidites deposited during the same broad interval (443-427 Ma).
The Black Mountain Sandstone is called a flysch, part of a sequence grading from deep water turbidity current sediments to shallow water shale and sandstone. We aren’t going to be able to determine that much detail from our field work. The beds vary from nearly horizontal (Fig. 6) to dipping at more than 30 degrees (Fig. 7).
Figures 6A and B were taken at a single exposure along a road cut. The thin sand layer (white in both panels) is continuous between them. This layer is less than 1 foot thick. A massive sand layer is separated from it by layer of mud.
Figure 7 is how the rocks typically appeared. They are jointed and brittle and it is difficult to discern any sedimentary structures in them (Fig. 8).
A close-up examination of Fig. 8A suggests that there are some larger particles present, but I wouldn’t bet on it. Discerning fine sediment structures in fine-sand turbidites requires breaking a lot of hand samples off from the exposure, and I don’t do that. I have to live with what’s visible at the outcrop. There is a hint of slight cross-bedding, dipping to the left in the lower-right corner of panel A. Figure 8B gives the impression of layering that dips to the left as well (these samples were in place but the photos are not oriented). There is also evidence of angular fragments consistent with a turbidity current, which transports larger grains as bed load in a predominantly fine matrix.
We had the opportunity to take a few photos of a spectacular road cut of the Canberra formation (also 443-427 Ma) located on the eastern edge of Fig. 5. This formation consists of finer clastic sediments and some volcaniclastics. We couldn’t stop for a close examination because of heavy weekend traffic and no place to pullover, so we drove slowly where there was a passing lane and took a lot of photos. Figure 9 is representative of what we saw (after the fact).
These were two identifiable (to me) folds with thrust faults. The area labeled “Fault Zone” had no recognizable bedding and tended to be fragmented. Other photos revealed vertical to overturned beds or breccia zones with no discernible structure.
The final example of siliciclastic sediments from this orogeny is from Site 10 (Fig. 5), a fine-grained, volcaniclastic sandstone deposited around 424 Ma.
Figure 10A gives more technical details than I can give, and it is a good example of how seriously Australian’s take geology. Anything accessible has a sign explaining its geological history. Panel B shows the anticline hinge that made the site famous. Figure 10C shows the fine lamination expected in a fine-sandstone, with indications of soft-sediment deformation as well (near the key fob). Panel D shows what’s left of the hinge point. Looking back at Fig. 10B, the hinge (greatest curvature) is to the left of Yoko. That’s what is shown in panel D. In other words, this anticline was folding over like so many others (e.g., Fig 9) when the rocks were deeply buried and being compressed. I walked all around the site but found no glaring evidence of the fault referred to in Fig. 10A.
That does it for this post. We’ve looked at sediments deposited over a 60 million-year interval during an orogeny, and how they’ve been deformed since. We’ll get back to that in a later post, when we put this road trip into the bigger picture.
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