Broken River Karst - karren

The Broken River Karst Region

The Broken River Karst Region is a broad area of scattered limestone ridges and low towers in northern Queensland. The host rock is moderately steep-dipping beds of Palaeozoic limestone.

The following text is enlarged from a summary in Grimes, in prep: Tropical Monsoon Karren in Australia:

The region is similar to Chillagoe, but differs in that the limestones here are not as steeply dipping, typically 50-70 degrees, and rather than high abrupt towers, they form long linear ridges dissected by grikes, spitzkarren and larger sculptured pinnacles. Microkarren here are restricted to bleached patches where the biofilm has been removed, mainly as a result of Rock-wallaby defecation [photo]. They comprise well-developed microrills, micro-networks and micro-pits superimposed on rillenkarren and rainpits.

The Turtle Creek Tower (4BR-126) has some features of special interest. This broad, but steep-sided tower is topped by a bare plateau, including several broad solutional basins up to 100 m across, that are dissected into low spitzkarren and smoother areas of kamenitza, rainpits and rillenkarren and drained by an unusual set of flat-floored runnels.

The unusual set of "interconnected solution rivulets" was first recorded within the basins by Godwin (1988). The rivulets are large runnels that form a branching contributory system of small flat-floored, and locally meandering, stream channels incised into the limestone floors of the basins. The drainage of the largest basin leaves the tower via an increasingly deep channel with some 2 m waterfalls; in the smaller basins the runnels sink underground into small shafts. The channels have flat floors and steep sides which may be slightly undercut. Commonly they are from 0.3 to 2 m wide and from 10 to 80 cm deep. The floor is generally bare limestone, with a thin (1-5 mm) coating of clay and organic material (assayed by R.Wray at 46% acid-soluble carbonate, 19.5% organic and 34.5% insoluble, non-combustable, residue). There are several terraces visible on the channel floors with the presently active channel in places being a narrow slot within a broader channel. The higher terraces, which are commonly paired, now have small rillenkarren, rainpits and kamenitzas developing on them.

These channels appear to be dominantly solutional in origin. The wet season storms could produce sufficient runoff to allow some hydraulic erosion, though there is no sediment to provide abrasive tools. The clay and algal material on the floors may have favoured undercutting of the walls over down-cutting of the floor – as happens in kamenitzas.

Further Reading

  • Grimes, in press: Tropical Monsoon Karren in Australia. Chapter 30 in a proposed "Karren Book". This version was submitted in August 2007.
  • Godwin, M., 1988 [editor] Broken River Karst: a speleological field guide, North Queensland. Unpublished report by Chillagoe Caving Club and Queensland National Parks and Wildlife Service, Cairns. 134 pp.

    Selected photographs and diagrams

    To view full size images, click on the displayed image.

    In the Broken River Gorge

    This gorge cuts across a steep-dipping ridge of limestone.
    KG083937.JPG Wall of Broken River Gorge. Showing scallops & muddy flood-mark.
    File: KG083937.JPG
    KG083913.JPG KG083912.JPG Floor of gorge, showing scallops with deeply pitted floors where muddy water pools after floods.
    Stereopair - view cross-eyed
    File: KG083913.JPG & KG083912.JPG

    Turtle Creek Tower

    KG083959.JPG Part of the large summit basin on the tower. Showing the steep rim and part of the broad basin with a dendritic pattern of large flat-floored runnels wihin a field of low spitzkarren.
    File: KG083959.JPG
    KG084052P.JPG Detail of shallow branching flat-floored runnels. The wet channel is fed by a small spring at the base of the basin rim. Note undercuts, low terracing effects and thin sediment cover on floor.
    File: KG084052P.JPG
    KG084128.JPG A separate small basin on top of tower. A meandering and branching set of flat-floored runnels separates low spitzkarren.
    File: KG084128.JPG
    KG083988.jpg KG083987.jpg Slightly incised, flat-floored runnel, showing headward-eroding nick-points, undercut sides and terraces. See next photo for detail beyond hammer.
    Stereopair, view cross-eyed
    File: KG083988.jpg & KG083987.jpg
    KG084074.JPG Detail of flat-flored runnel shown above. Minor nick-point at left, paddy-field terracettes in centre and undercut walls.
    File: KG084074.JPG
    KG084029.jpg KG084030.jpg Incided runnel, showing multiple terraces with undercut walls. Basin rim in distance.
    Stereopair, view cross-eyed
    File: KG084029.jpg & KG084030.jpg
    KG084017.JPG KG084018.JPG Detail of an incised runnel with deeply undercut terraces.
    Stereopair, view cross-eyed
    File: KG084017.JPG & KG084018.JPG
    KG084143.JPG Bleached rock-wallaby trail across edge of the tower. The biofilm has been removed by urine.
    This is where the microkarren are most obvious.
    File: KG084143.JPG
    KG084127.JPG Bleached rillenkarren (with microkarren superimposed) at a wallaby camp-site. Note numerous faecal pellets.
    File: KG084127.JPG
    KG083992.JPG Microkarren (microrills) superimposed on rain-pits and rillenkarren in a bleached area. Note wallaby faecal pellet.
    File: KG083992.JPG
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