Australian Capital Territory: Lake George
For details about the big picture, take a look at the first post in this series.
This post goes back to the beginning of the mountain building event, about 485 Ma in the past. We took a drive to Lake George, about 40 km NE of Canberra, just over the border in the state of New South Wales. Lake George is a shallow depression with no outlets and very little inflow. Thus, it is often dry and water depths are on-average about 1 m (Fig. 1).
It was mostly dry when we visited. Figure 1 looks across the lake to the east from the west side (location shown by red circle in Fig.2). We’ll be circumnavigating the basin in this post.
Lake George has a long geological history, including uplift of the west side of the basin within the last tens of millions of years. This faulting has brought Adaminaby Group (485-443 Ma) sedimentary rocks to the surface after more than 300 feet of uplift. This is a turbidite sequence of sandstone, mudstone, shale, greywacke, chert; quartzite, phyllite, slate (Fig. 3).
These beds consist of alternating fine grained sandstone and siltstone, with shales containing sand lenses. They are dipping steeply to the NE, but this faulting is not associate with the Paleozoic orogeny discussed in these posts.
Examining the rocks up closer (Fig. 4) reveals fine textures supporting the proposed turbidite depositional environment.
Figure 4A shows a bed ~6 inches thick, tilted 60 degrees to the right. Zooming in on it reveals thin cross-bedding intersecting joints. Figure 4B shows graded bedding, with slightly larger grains revealed in the white layers against the grey background of smaller grains to the right of the sample. Panel C reveals sets of laminae at low angles, intersected by joints filled with cement.
We followed a narrow asphalt road, which became gravel, to the NE side of Lake George (see Fig. 2), crossing through volcaniclastic and siliciclastic rocks deposited towards the end of the orogeny (433-410 Ma). There were no outcrops of these sedimentary rocks, however, and only a few sandstone beds were visible in ditches. The dominant exposed rocks were outcrops of small boulders of a gray, fine-grained igneous rocks on low hills (Fig. 5).
Figure 5 shows small outcrops (< 2 feet in length) of in-place fine-grained rock with large quartz crystals along joint surfaces (Fig. 5A) and as veins within joints (Fig. 5B and C). This fine-grained rock is part of a granitic pluton about 420 Ma in age, towards the end of the orogeny. The fine-grained rock may be overlaying volcanics into which the granitic rock was intruded at this location, although no intrusive rock was visible.
We followed a track to get as close to Lake George as possible (Fig. 6), where a small hill could be seen in the distance with outcrops visible. Most of the larger hills were covered with soil. This is a heavily eroded granitic pluton.
A few miles further east took us back into the oldest rocks again (~485 Ma), to the east of Lake George (Fig. 7A), where we found thin bedded siltstone and shale (Fig. 7B).
A short distance further finally revealed the granitic basement (Fig. 8).
Figure 8A shows the largest outcrop we found. It is about 4 feet across. A closer look reveals a granitic texture, with alkali feldspar and quartz visible as the pink-to-white and gray crystals, respectively (Fig. 8B). An even closer look (Fig. 8C) verifies the overall impression we have from panel B. However, this is not a homogeneous granite as seen in panel D, which shows contamination by lithic fragments (angular grains) and evidence of strain (shear displacement) as the magma was cooling, producing weak lamination within the darker part of Fig. 8D.
This post has circumnavigated a shallow basin surrounded by rocks from representative intervals during the orogeny. The oldest sedimentary rocks (~485 Ma) contain turbidites deposited on steep continental shelves where the land is rising. Deposition of these proximal (deposited near rising land) turbidites and greywackes continued as explosive volcanism began to dominate nearby. The sediments are from the ocean whereas the volcanics (Ignimbrites and tuffs) were deposited on land. This continued until granites were emplaced by about 420 Ma.
Next time, we’ll closer at the sediments deposited during this long interval.