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Buchan limestone depth

This topic contains 5 replies, has 4 voices, and was last updated by  Peter Freeman 7 years, 3 months ago.

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  • #380

    Anonymous

    I was flipping through a victorian geology book last night and it gave mention to Buchan limestone being approximately 230m deep. If that is the case, why do we get mostly shallow caves down there? It also mentioned Bindi having limestone to about a depth of 210m yet as far as I’m aware the caves there are seriously small.

    Is someone able to shed some light <img src=:” title=”Question” />

    #832

    tom-aberdeen
    Member

    Could it be height above sea level? I think Buchan is roughly around 200m ASL.

    Tom

    #833

    gbaddeley
    Member

    The cave development at Buchan was strongly influenced by the cutting down of the limestone valley by the ancestral Buchan/Snowy River (the Potholes) and action by the more recent Buchan River (the show caves, Wilsons Cave). It mostly depends on where the water table was and for how long. Thick limestone is no guarantee of deep caves. The conditions need to be right. 50 – 80 m deep is probably about the limit for the Potholes, depending on where it is on the hill.

    #834

    Peter Freeman
    Moderator

    The question first raised is very pertinent. It will have occurred, of course, to generations of inquisitive cavers at Buchan. The full answers to it are important to the prospects for more interesting cave discoveries.

    The answer is not a simple or single one – many factors are involved. Also, when we say that the Murrindal Potholes are around 50m deep, that is 50m to the points to which we can penetrate, not to the ultimate bottoms of the caves.

    The potholes area is at 250-300m ASL, while Scrubby Creek resurgence is at 125. There is therefore scope for a 175m deep cave. However, many complications might arise to make this water path non-negotiable as a cave. For example, I believe the horizontal level of Elk River’s development is set by a chert band lying at 200m ASL. However, that chert band may not be extensive, and penetration of it can be expected at some other place.

    By the way, the limestones of the Buchan Group are actually much more than 290m deep – I forget the figure now, I would have to look it up. The larger figure includes the Taravale Formation, which is the intervening mudstone. That mudstone is partly calcareous, and, although not normally caverniferous, it must be taken into account when predicting where and how the caves will form in Murrindal and Buchan-Caves limestones.

    I am working on an article (perhaps for publication eventually) on this general topic.

    #835

    Peter Freeman
    Moderator

    On checking the geological map, the figure of ~300m seems right for the thickness of Buchan Caves Limestone (BCL). The whole Buchan Formation (bottom-to-top, oldest-to-youngest) consists of BCL, Taravale Mudstone (TM), and Murrindal Limestones (ML). The Murrindal Limestones consist of the McLarty member and the less significant Rocky Camp member. The Murrindal Limestones also reach ~300m thickness, and the whole Buchan formation (BCL, TM, ML) exceeds 1000m in places. The surface relief of course is much lower, so that limits the caves.

    #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.

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