Karst features in the coastal limestones of WA
Bob Gozzard Geological Survey of Western Australia Geological Survey of Western Australia
Outline of talk z
Geological setting (Phil Playford)
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Karst processes
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Karst morphology
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Karst hazards
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Engineering problems
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Engineering classification of karst
Geological Survey of Western Australia
Karst geology z
Areas of distinctive hydrology and landforms
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Soluble rocks – limestone, gypsum, chalk, salt
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Limestones are porous and prone to dissolution by weakly acidic groundwater and rainfall Waters circulate through cracks and pores and remove calcium carbonate by dissolution Landforms prone to rapid change by weathering process Areas characterised by swallow holes, caves, dolines, gorges, pinnacles Geological Survey of Western Australia
Karst in coastal limestones in Western Australia z
Peron and Edel peninsulas at Shark Bay
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Jurien - Nambung
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Scott Coastal Plain
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Lake Joondalup – Yanchep
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Naturaliste Region
Geological Survey of Western Australia
Karst processes in coastal limestones z z
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Concurrent speleogenesis and lithogenesis Caves and karst features formed at the same time as the loose limesand dunes were being cemented Evidence to show karst development was well advanced by the time the material became cemented But exposure to subaerial weathering and meteoric waters have resulted in further karst development
Syngenetic karst Eogenetic karst Geological Survey of Western Australia
Syngenetic karst z
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Percolating rainwater cements limesand to form limestone by dissolution and redeposition of CaCO3 Caprock and calcrete layers form on or in the dune as former paleosoils or levels of saturated groundwater
Geological Survey of Western Australia
Syngenetic karst z
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Within and below the calcrete solution pipes form by percolating groundwater becoming focussed Early cementation is localised around roots to form rhizomorphs and limestone becomes vuggy
Geological Survey of Western Australia
Syngenetic karst z
Dissolution of limestone at the water table takes place
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Local swamps can provide aggressive (organic) acidic water
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Clustering of caves along edge of dune ridges adjacent to swamps (flank-margin caves)
Geological Survey of Western Australia
Syngenetic karst z
Loose sand subsides into incipient cavities forming caves
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Beneath caprock shallow caves may form
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Subsidence dolines may form without caves
Geological Survey of Western Australia
Syngenetic karst z
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When bulk of rock is hardened to support a roof, caves can develop; buried caprocks assist cave development Interconnected horizontal cave systems form at the water table
Geological Survey of Western Australia
Syngenetic karst z
Collapse is ubiquitous and collapse domes are common
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Surface sculpturing is rare
Geological Survey of Western Australia
Speleogenesis at Yanchep
Geological Survey of Western Australia
Eogenetic karst z
Rainfall z z z
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Karren - small-scale surficial dissolution: flutes, pits, pans Infiltration causes widening of joints Seepage through thick cover sands cause suffosion depressions – sinkholes, dolines
Meteoric water z z z
Piping New caves and collapse domes Speleothems – stalactite, stalagmite, helictite, flowstone
Geological Survey of Western Australia
Eogenetic karst z
Surface water z
Gorges z z
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Cavern collapse Fluviokarstic – streams erode down to water table in more pluvial period
Weathering z z
Pinnacles Tafoni and other cavernous weathering
Geological Survey of Western Australia
Karst morphology z z
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Coastal limestones do not represent “classic” karst Have many features in common with “classic” karst: z Swallow holes, dolines, caves, gorges, pinnacles Have several distinctive features: z Suffosion dolines - subsidence as soil slumps and is removed from below by washing into limestone z Case-hardening – calcrete layers and caprock z Vertical dissolution pipes – cylindrical, soil filled pipes, point recharge z Dune swales – swamp waters undercut edges of dunes to form steep sided, flat floored depressions not unlike a polje z Collapse modification – rubble filled caves and inclined fissures z Limited surface sculpturing – only small-scale surficial dissolution Geological Survey of Western Australia
Geological Survey of Western Australia
Excursion stop
Geological Survey of Western Australia
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Steep groundwater gradient Easternmost interbarrier depression Between Yokine and Balcatta dune complexes
Association of prominent karst features Geological Survey of Western Australia
Caves
Geological Survey of Western Australia
Pinnacles
Geological Survey of Western Australia
The formation of pinnacles Sand dunes form — cross-bedding reflects different periods of dune development Plants colonise the dunes — their roots play a role in helping turn the dune sands into limestone Extensive dissolution of softer parts of the limestone has left pinnacles of limestone surrounded by quartz sand Wind blows away the quartz sand to reveal pinnacles of limestone Geological Survey of Western Australia
Suffosion sinkholes and dolines
Geological Survey of Western Australia
Gorges – Boomerang Gorge
Geological Survey of Western Australia
Geohazard mapping of karst
Geological Survey of Western Australia
Geohazard mapping of karst
Geological Survey of Western Australia
Engineering problems associated with caves in karst z
Roof collapse
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Cavity stoping
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Q value after Barton et al (1974) Labelled fields from Norwegian Tunnelling Method Geological Survey of Western Australia
Caves at shallow depth threaten foundation integrity Rule of thumb – ground is stable if thickness of rock ≥ span of cave roof Simple beam failures are a “worst case” scenario – Gracetown 1996
Engineering problems associated with sinkholes in karst z
Suffosion dolines or sinkholes
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Form in non-cohesive soil cover
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Varying rates – gradual to sudden
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Catastrophic sinkholes not known
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Geological Survey of Western Australia
Surface subsidence as soil slumps and is removed from below by washing into limestone
Smaller and more numerous in thinner soil profiles Greatest proportion in soil 2-10 m thick over fractured rockhead
Engineering problems associated with rockhead in karst z
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Irregular rockhead profiles
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Pinnacled rockhead
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Isolated pinnacles
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Geological Survey of Western Australia
Rockhead topography notably unpredictable
Undercut pinnacles supported only by surrounding soil
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Narrow, vertical soil-filled pipes
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Loose blocks
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Buried sinkholes
An engineering classification of karst z
Developed overseas (UK) z
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Used principally in Europe
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Extensive international literature
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Developed for “classic” karst
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Geological Survey of Western Australia
Waltham and Fookes
Limited use for syngenetic karst in limesand eolianites Characterise not classify Susceptibility mapping – spatial relationships between karst affected area and collapse-related factors
Summary z
Karst geology comprises distinctive hydrology and landforms
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West coast of WA from Shark Bay to Augusta
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Syngenetic karst in limesand eolianite followed by eogenetic karst and weathering Not “classic” karst – several distinctive features Association of prominent karst features with steep groundwater gradient
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Importance of geohazard mapping
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Engineering problems associated with caves, sinkholes, rockhead
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Widely used engineering classifications of limited use
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Characterise not classify
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Susceptibility mapping Geological Survey of Western Australia
Geological Survey of Western Australia
