Twilight Zone karren - Chillagoe

At Chillagoe twilight zones are exensive in the Giant Grike areas. These grikes are up to 30 m deep and alternate with roofed sections of fissure cave passages.

The following text is an extract from the current draft of my chapter on Tropical Monsoon Karren in Australia for the "Karren Book".

Two types of horizontal solution ripple were described by Jennings (1982, p.23): on underhangs and in the twilight walls of cave entrances there are a sharp-ribbed and deeply recessed symmetrical form; whereas on steep slopes exposed by soil erosion of the pediment grikes there are more rounded and asymmetrical ripples that might have resulted from subsoil solution.

Jennings (1982, p.44-45) describes phototrophic karren which are oriented grooves, sticks and spines in the twilight zone of the caves and deep grikes (see photos). These are a type of phytokarst eroded by algae. Individual spikes and grooves are between 2-50mm across, but can be up to 400mm long! The orientation varies according to the direction to the light. Some spikes have cave coral growths on their tips, or along their full length. Jennings (1982, p.45) describes small needles, 10mm high and 1-2 mm thick, on the side of a rather deep pan on top of one tower.


Selected photographs and diagrams

To view full size images, click on the displayed thumbnail image.
D030806.jpg Giant Grike inside a tower.
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D030824.jpg Giant grike merging with a fissure cave passage.
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D030410.jpg Etched joints and breccia on a cave wall - twilight zone.
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D030408.jpg Solution ripples on cave wall - twilight zone.
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D030411.jpg Solution ripples on cave wall. Twilight zone.
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D030421.jpg D030420.jpg Solution ripples and some photo-trophic spikes.
On a cave wall, twilight zone.
Stereopair - view cross-eyed.
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D030414.jpg Solution ripples and phototrophic spikes on cave wall - twilight zone.
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D030419.jpg D030418.jpg Photo-trophic spikes, with corraloid growths on the tips.
Cave wall in twilight zone.
Stereopair - view cross-eyed.
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D030786.jpg D030787.jpg Photo-trophic spikes on cave wall, twilight zone.
Stereopair - view cross-eyed.
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D030813.jpg Photo-trophic spikes - twilight zone.
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D030797.jpg corraloid deposits on photo-trophic spikes.
Cave wall in twilight zone.
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(c) KG. Grimes, 2003