Return to Buchan limestone depth

The Pyramids Limestone Member

#836

Peter Freeman
Moderator

As an interesting and very relevant aside to this discussion, it should be pointed out that the Taravale Formation grades in its type and content from the south to the north of its range. In the south, for example near Buchan and leading up the Gelantipy Road towards the Potholes area, the Taravale is mudstone and marlstone. It is fairly calcareous, most frequently showing many calcareous nodules, up to several centimetres in diameter. This can be clearly seen in the road cutting opposite Homeleigh. In this state, the Taravale is non-caverniferous. However, in the north of its range, around the latitude of the Potholes Reserve and further north, the Taravale Formation consists of limestone, and in fact is often listed as the ‘Pyramids Limestone Member’. It is rather similar to the McLarty member of the Murrindal Limestone. The lowest (oldest) layers are muddy, but it gradually clears towards the top, and merges indiscernably into the McLarty. In the far north of the limestone exposure, past Murrindal, it has in some works been simply lumped with the McLarty.

The significance of this fact regarding the nature of the Taravale is that it does not form an insoluble base to the Murrindal limestone, and therefore does not confine the cave-forming processes. At the latitude of the Potholes Reserve there is continuous caverniferous rock all the way down to the Pyramids and the river. This is verified by the now-discovered existence of the Murrindal Potholes Eastern Master Cave, whose lower reaches penetrate into the Pyramids Limestone.

A characteristic that DOES influence the cave is the occurrence of insoluble bands, in both the McLarty and the Pyramids members. These increase in frequency with depth, and (I believe) are responsible for the gradually-descending profile of this cave. This profile is quite different from typical pothole forms, which in most places plunge steeply to the water table before joining a master cave that runs at, or close to, the water table for all of the remaining distance to its resurgence

The first clear occurence that I have seen of a strong insoluble chert band is on the floor of the Elk River Streamway. This band may be significant in the general tendency of the potholes to end, or go horizontal and too-tight, at around 200m ASL (though a water-table-lowering pause may alternatively have caused the depth-limit of the shafts). Above this level, the ‘dirtyness’ of the McLarty limestone member manifests only as muddy bands that are crumbly rather than strong and water-resistant.

Finally, related to the above and obvious from the discussion, beware of the distinction between limestone, marlstone and mustone! We have a tendency to think of one member or formation ceasing and suddenly being overlain by something different – eg McLarty resting on the Taravale; and we imagine that we’d see a clear line at the junction. In many, if not most, cases this is not so. These rock contents are a continuum, with a technical definition of ‘limestone’ being declared somewhat arbitrarily at 50% CaCO3. Really it’s all the same stuff, but some of it has more mixed-in crap than others.