Fractures, caves, and worm tubes??
Time.I realized, after posting my previous fracture photo (#23; Breccia in paleocavern) that we were looking at the end of time as far as that paleocavern was concerned.It has reached the end of its cycle of formation and destruction.I decided to begin the cycle anew; with the next few postings we’ll pursue the development of a cavern system, beginning in this photo showing its birth.
The limestone in this photo is part of Canning Basin’s Devonian reef complex, which crops out in Windjana Gorge National Park, in NW Australia.The rock is a microbial boundstone, with gently-dipping layers defined by stromatactis structures.At least two distinctly different fracture types may be seen here.
Two prominent, planar fractures cross-cut the outcrop, dipping 60 degrees to the right relative to bedding.A third similar, but near-vertical, fracture can be seen in the upper right of the photo.These multi-meter-tall fractures are decorated by spaced solution vugs – step 1 in the development of a cavern.In two dimensions these look like a string of pearls, although in 3D they might be more-appropriately described as worm tubes.
Based on their dip magnitude the two prominent fractures may be incipient faults.Incipient because there is no sign of shearing offset evident to the unaided eye.However their dip, plus the echelon nature of the fracture on the right, could indicate an origin in shear.What is clear is that they formed after the genetically distinct, and numerically more-abundant, set of fractures that pervade the rock. These short (approximately 20 cm tall), calcite- and debris-filled fractures are nearly normal to bedding and they have been ptygmatically folded during compaction of their host strata.They are most clearly evident just right of the vertical rusty stain on the left side of the photo, although they may be seen throughout. For a discussion of ptygmatically folded fractures see the Fifty Photos posting #13.
Based on their folded nature, and their marine-based fillings, these short, pervasive fractures formed syndepositionally.They were cemented and folded before the tall, planar, vuggy fractures propagated.The tall fractures show an apparent absence of compaction-induced deformation, hence they formed after the rock was largely compacted.
The presence of vugs along the tall, straight fractures indicates solutions that were corrosive and/or understaturated in calcite were able to percolate along these fractures preferentially relative to the bulk rock matrix.This would have established a feedback loop, whereby fluids moving along a fracture dissolved, and thus enlarged a passageway, thus allowing more solution to flow through and hence enlarging the passage further, etc.This process leads, eventually, to a connected cavern system controlled in part by the fracture system.The genetic linkage between fractures and cavern systems can be seen in the map geometry of most of the Earth’s cavern systems (see for example this map of Carlsbad and Slaughter Canyon caverns).
The next time you tour a cave, look up at the ceiling in any long, straight passageway and look for its nucleating fracture.Then think of these worm-tube vugs.