SYNGENETIC KARST

by Ken Grimes

The Concept of Syngenetic Karst

Syngenetic karst occurs where karst features, including caves, developed at the same time as a calcareous sand was being cemented into a rock. It is a special feature of the soft dune limestones found in the coastal regions of southern and western Australia; but also occurs in other parts of the world, e.g. South Africa and the Bahamas.

The dune limestones (or aeolianites) are a series of calcareous sand ridges which formed as dunes along the present or prior coastlines during the Quaternary (the last 2 million years, and pretty young by geological standards). The dune ridges are now partly consolidated calcarenites: that is, sandy limestones in which the sand grains are mainly fragments of marine shells that grew in the shallow seas, were washed up onto the beach, and then blown inland.

In these calcareous Quaternary dunes some karst features are "syngenetic", in that they have developed at the same time as the sand was being cemented into a rock. The term was coined by Joe Jennings in 1968, based partly on observations made by Lex Bastian in West Australia and Alan Hill in South Australia. The unconsolidated calcareous sand of the coastal dunefields is gradually converted to limestone by solution and redeposition of calcium carbonate by percolating water. This initially produces a caprock or calcrete layer which is capable of supporting the roof of a shallow cave. The downward percolating water also dissolves vertical solution pipes, and at the same time cements the surrounding sand. It then continues to carry dissolved calcium carbonate downward to slowly cement and strengthen the deeper sands. Simultaneously, enhanced mixing corrosion occurs where percolation water meets the water table - typically at the level of an adjacent swampy plain or at the sea. In the early stages the soft sand subsides immediately and no cavities remain, but once the rock is sufficiently hardened to support a roof, the solution at the water table results in the development of horizontal caves at that level.

Because the "rock" has limited strength, it collapses easily and breakdown is extensive, particularly in the early stages. As the rock becomes harder, and solution continues at the base of the cave (commonly beneath the rubble piles) larger chambers can form. In some West Australian cave areas, the water flow is concentrated along valleys in the surface of an impermeable basement and linear stream caves result.

Features of Syngenetic Karst

**Typical features of Syngenetic Karst on a dune limestone.**

The main characteristics of syngenetic karst are the development of a cemented (calcreted) caprock near the surface, of vertical solution pipes, and of low, wide, horizontal maze caves either beneath the caprock or at the level of the adjoining swampy plains (see Figures). The caves have multiple entrances (often via solution pipes or the collapse of the surface crust) and an irregular outline of chambers, pillars and short connecting passages, generally with a roof height less than one metre throughout. The walls are often difficult to see (and map) as they are out of reach where the roof slowly drops to floor level. Where the water movement has been concentrated into underground streams more linear passages may develop. Breakdown is extensive; in many cave systems much of the original solutional system at the water-table is now replaced by collapse domes - typically with the rubble floor rising and falling in parallel with the roof, leaving only a narrow gap for cavers to travel through. Particularly good examples of this can be seen at the Kelly Hill Caves on Kangaroo Island. If solution is continuing to dissolve the collapsing rubble pile, then larger, more roomy, collapse chambers can result.

Cave Map: 7Kb GIF

Recemented subsidence breccias are seen in the walls and roof of some syngenetic caves. They indicate instability while the cave was forming in the partly cemented sand, but in most cases have since been further cemented. None-the-less, one sometimes tends to wonder about their strength. Click the adjacent photo for a bigger view (100kb.JPG)

Subsidence and collapse dolines occur on the surface. In places mass subsidence can generate a chaotic surface of tumbled blocks and fissures.

Solution pipes are one of the most distinctive features of syngenetic karst. They are vertical cylindrical tubes with case-hardened walls, typically 0.5 to 1 m in diameter, which can penetrate down from the surface as far as 20 metres into the soft limestone. The pipes may contain roots (and root growth may have occurred hand-in-hand with solution of the pipe). They occur as isolated features, or in clusters with spacings as close as a metre or so. Many of the caves are entered via such pipes.

Solution Pipe entrance to a cave. Click on photo for a larger view (45kb.JPG) Solution Pipes with indurated rims. Click for a larger view (93kb.JPG)

Pinnacles such as those found in the Nambung National Park of West Australia are less common, and are thought by some to be an extreme case where the solution pipes have coalesced to leave only residual blocks of cemented rock.

Syngenetic karst development is typical of the Quaternary dune calcarenites; however, the Tertiary limestone at Mount Gambier, in South Australia, is also a relatively soft porous limestone, and consequently it shows some of the features of syngenetic karst, in particular the development of solution pipes and calcreted caprocks.


Solution Pipe entrance to a cave. Click on photo for a larger view (45kb.JPG)	Solution Pipes with indurated rims. Click for a larger view (93kb.JPG)

SYNGENETIC KARST

by Ken Grimes

The Concept of Syngenetic Karst

Features of Syngenetic Karst

Further Reading