Why does western Oklahoma gypsum cause sinkholes?
In short: Western Oklahoma's Permian rocks include thick beds of gypsum, such as the Blaine Formation, and deeper layers of salt. Both dissolve in groundwater, creating caves, sinkholes and hidden voids, a landscape geologists call evaporite karst. On this ground, sudden settlement over a void is a different concern from ordinary clay movement.
What are the gypsum and salt beds?
Gypsum is a soft mineral (calcium sulfate) that forms when seawater evaporates. Rocks formed this way, including gypsum, anhydrite and rock salt, are called evaporites. Western Oklahoma has enormous gypsum reserves, mined for wallboard, plaster, cement and soil amendment 5. Thick rock salt underlies much of western Oklahoma at depths from about 30 to 3,000 feet, and natural brine springs feed several salt plains 5.
The Blaine Formation
The Permian Blaine Formation in southwestern Oklahoma is about 60 meters (roughly 200 feet) of gypsum with thin dolomite and shale beds between the gypsum layers 12. Near Mangum, it contains caves, sinkholes, disappearing streams and springs 12.
What is evaporite karst?
Karst is ground shaped by rock that dissolves in water. The Oklahoma Geological Survey notes that karst occurs in many areas of western Oklahoma where gypsum and shallow salt deposits are common 3. It describes the resulting sinkholes and caverns as potential hazards, with possible settlement or collapse of the land surface, and notes that contaminants can move quickly through karst groundwater with little filtering 3.
An Oklahoma Geological Survey compilation on evaporite karst includes Oklahoma studies on Blaine Formation karst, gypsum caves, cavity detection, a highway right-of-way, and regional karst mapping for avoiding engineering problems 4.
A case study: the Mangum dam sites
A proposed earth-fill dam near Mangum would have rested on the Blaine gypsum 1. Borings in 1999 found abundant open and clay-filled cavities in and near the abutments, and pressure tests lost large amounts of water 12. The geologist recommended abandoning the site because fixing the karst would be costly and still might not guarantee the dam 1. A site about 6 miles downstream on thick Flowerpot Shale showed favorable foundation conditions in testing 2.
What soils lie above gypsum country?
Woodward soils formed in residuum from soft, calcareous Permian sandstone or siltstone over a large area of western Oklahoma, with soft rock at 20 to 40 inches 6. Gypsum appears only in small amounts in Woodward profiles, up to about 3 percent in some subsoil layers and as thin seams in some deeper layers 6. A soil map alone therefore may not show gypsum or voids deeper below.
What does gypsum karst mean for foundations?
| Concern | Why it matters here |
|---|---|
| Hidden voids | Open and clay-filled cavities occur in Blaine gypsum 1 |
| Sudden collapse | Sinkholes and collapse are recognized hazards 3 |
| Water concentration | Groundwater flow drives dissolution, and fluids moved readily through cavities in tests 23 |
| Soil maps miss depth | Surface soils may show only minor gypsum 6 |
- Distinguish slow seasonal cracking from a sudden local drop. See sudden depression or sinkhole and karst subsidence and sinkholes.
- Circulating groundwater is what dissolves soluble rock 3, so where roof water, pool water and plumbing leaks go is part of the picture on gypsum ground.
- For voids under footings, see void and compaction grouting.
Related
- Gypsum karst and sinkholes
- Northwest (Gypsum Hills)
- Southwest (Lawton and Red Plains)
- Western Permian red beds
- Woodward soil
Common questions
What is the Blaine Formation?
It is a Permian rock unit in western Oklahoma made mostly of gypsum, about 60 meters thick in the Mangum area, with thin dolomite and shale beds between the gypsum layers.
Why does gypsum cause sinkholes?
Gypsum is soluble, so groundwater moving through gypsum beds can open cavities, caves and disappearing streams. The Oklahoma Geological Survey describes the resulting sinkholes and caverns as potential hazards, with possible settlement or collapse of the land surface.
Is there salt under western Oklahoma too?
Yes. Thick rock salt underlies much of western Oklahoma at depths from about 30 feet to 3,000 feet, and salt-rich springs feed salt plains in the region.
Has gypsum karst affected large projects in Oklahoma?
Yes. A proposed dam site near Mangum on the Blaine gypsum was recommended for abandonment after borings found abundant open and clay-filled cavities and large water losses during pressure tests.
Sources
- Gypsum Karst Leads to Abandonment of a Proposed Damsite in Oklahoma Geological Society of America (Oklahoma Geological Survey author)
- Gypsum Karst Is a Major Factor in Siting of a Proposed Dam in Southwestern Oklahoma Geological Society of America (Oklahoma Geological Survey author)
- Mapping of Karst Terranes in Oklahoma to Identify Potential Environmental and Hydrologic Problems AAPG Search and Discovery / Oklahoma Geological Survey
- Evaporite Karst and Engineering/Environmental Problems in the United States (Circular 109), excerpt and contents Oklahoma Geological Survey (copy hosted by U.S. DOE WIPP library)
- Industrial Minerals Oklahoma Geological Survey
- Official Soil Series Description: WOODWARD Series USDA Natural Resources Conservation Service
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