Geology: every question answered
128 questions with short answers. Each links to the full, sourced explanation.
How do cherty limestone soils affect NE Oklahoma foundations?
Weathered cherty limestone leaves very gravelly soils, dense fragipans and some karst, so foundation concerns center on water, voids and rocky digging.
- Cherty limestone, Ozark Plateau: What is the rock?
- Chert is a dense, glassy rock made of silica that occurs as nodules and beds inside limestone. USDA soil descriptions for the Ozark region describe soils formed in residuum, or weathered-in-place material, from cherty limestone or cherty dolomite.
- What soils form on cherty limestone?
- Clarksville soils formed in hillslope sediments over clayey residuum from cherty dolomite or cherty limestone, on steep side slopes and narrow ridgetops. Key features:
- Is there karst in cherty limestone?
- Karst is ground shaped by dissolving rock, with features such as sinkholes, caves, springs and disappearing streams. In other parts of the Ozarks, soils formed in residuum from cherty limestone are described with karst topography in many areas.
- Cherty limestone, Ozark Plateau: What does this mean for foundations?
- Water held up by a fragipan. A very slowly permeable layer starting 14 to 27 inches down can slow drainage near footings and in crawl spaces after rain. See crawl space moisture and groundwater and perched water; Voids and sudden drops.
- Cherty limestone, Ozark Plateau: What is chert?
- Chert is a hard, glassy rock made of silica that forms nodules and layers in limestone. In northeast Oklahoma soils formed from cherty limestone, chert gravel can make up a large share of the subsoil.
- Do soils on cherty limestone swell?
- The main soils here, such as Clarksville and Nixa, are gravelly and loamy near the surface, with more clayey layers deeper down, such as below about 36 to 54 inches in Clarksville soils. Water, voids and hard digging are the more common issues.
- Cherty limestone, Ozark Plateau: What is a fragipan?
- A fragipan is a dense, brittle subsoil layer that roots and water pass through slowly. In Nixa soils it starts about 14 to 27 inches down, and permeability is very slow.
- Cherty limestone, Ozark Plateau: Are there sinkholes in northeast Oklahoma?
- The Oklahoma Geological Survey names the Ozark area of northeast Oklahoma as one of the state's two main limestone and dolomite karst regions, where sinkholes, caverns and subsidence are potential hazards.
How do Oklahoma river floodplain and terrace soils affect homes?
Floodplain and terrace deposits range from soft wet clay to sandy dunes and expansive claypans, so the soil series, not the river, predicts behavior.
- How do floodplains and terraces differ?
- Alluvium is sediment deposited by flowing water. The present floodplain is the low, flat land a river still floods. A terrace (or paleoterrace) is an older floodplain surface left higher than today's river as it cut downward.
- What soils form on floodplains?
- Osage is a Vertisol, a clay soil defined by shrink-swell, with intersecting slickensides from about 13 to 60 inches deep.
- What soils form on terraces?
- An Oklahoma State University study of five western Oklahoma soils found Kirkland had the greatest total shrink-swell across its profile, with a coefficient of linear extensibility up to about 0.090, and found Norge, on uplands of old alluvial origin, reached about 0.066.
- Floodplain and terrace soils: What does this mean for foundations?
- A thick clay floodplain soil with a water table within 6 to 18 inches for half the year can compress under load and stay wet beneath crawl spaces. See soft soil consolidation and crawl space moisture.
- What is the difference between alluvium and a terrace?
- Alluvium is sediment laid down by a river, usually on the present floodplain. A terrace is an older, higher floodplain left behind as the river cut down, so terrace soils are older, better developed and usually above normal flood levels.
- Are floodplain soils bad for foundations?
- It depends on the soil. Some, like Osage, are thick, poorly drained smectite clay with a water table within 6 to 18 inches in winter and spring. Others, like Port and Verdigris, are well-drained silty or loamy soils that still flood at times.
- Can terrace soils be expansive?
- Yes. Kirkland and Tabler formed on old terraces and have clay subsoils with slickensides, a sign of repeated shrink and swell. Others, like Dougherty, are sandy.
- Floodplain and terrace soils: How thick is alluvium along Oklahoma rivers?
- It varies. In Oklahoma County, the North Canadian River alluvium generally averages about 50 feet thick and may be up to 150 feet thick in the river channel itself.
How do Ouachita Mountain rocks affect southeast Oklahoma homes?
Tightly folded sandstone, shale and chert form steep slopes with thin, rocky soils, so slope movement, erosion and fill matter more than clay swell.
- How did the Ouachita Mountains form?
- The Ouachita rocks were deeply buried, and the heat and pressure of mountain building squeezed and tightly folded them about 315 million years ago. The wooded mountains now rise roughly 500 to 1,500 feet above nearby valleys.
- Ouachita Mountains geology: What are the main rock types?
- The same survey described three kinds of layers that still frame how the region is understood:
- Ouachita Mountains geology: What soils form on the folded rocks?
- Ridges and mountainsides commonly carry thin soils over sandstone. The Hector series is an example:
- Ouachita Mountains geology: What does this mean for foundations?
- Hector soils occur on slopes up to 60 percent. On slopes like these, soil and fill on the downhill side of a house can creep slowly downslope over years; signs include leaning retaining walls, fences and trees.
- What kind of rock makes up the Ouachita Mountains?
- Mostly sandstone, shale and chert that were deeply buried and then squeezed into tight folds and faults about 315 million years ago. Hard sandstone forms the ridges and softer shale forms the valleys.
- Are soils deep in the Ouachitas?
- On ridges and mountainsides they are often thin. Hector soils, for example, reach hard sandstone at about 14 to 20 inches and occur on slopes up to 60 percent.
- Ouachita Mountains geology: What foundation problems are common on steep, rocky lots?
- Common concerns are downhill soil creep, erosion of thin soil around footings, and fill placed to level a lot.
- Ouachita Mountains geology: Why does rock type change so quickly across a lot?
- Because the layers are steeply tilted and faulted, hard sandstone and soft shale can both reach the surface within short distances.
How do Panhandle Ogallala and loess soils affect foundations?
Panhandle soils formed in Ogallala deposits capped by loess and sand. They are dry most of the year, so movement follows sudden moisture changes.
- What layers lie under the Panhandle?
- USDA soil descriptions identify the Ogallala Formation as calcareous, loamy alluvium (river-laid sediment) of Miocene-Pliocene age.
- Panhandle Ogallala and loess: What do these soils mean for foundations?
- Every series above has a mean annual precipitation of roughly 13 to 24 inches. Ulysses notes that most rain falls in high-intensity thunderstorms from late spring through early autumn.
- What is loess?
- Loess is silt that was carried and dropped by wind. In the Oklahoma Panhandle, soils such as Richfield formed in loess on high tablelands.
- What is the Ogallala Formation?
- It is a Miocene-Pliocene deposit laid down by rivers. USDA describes soils such as Potter as formed in calcareous, loamy alluvium from the Ogallala Formation in the Southern High Plains, including northwestern Oklahoma.
- Do Panhandle soils swell like central Oklahoma clay?
- Some do. Richfield has a smectitic clay subsoil, while Dalhart is loamy. Low rainfall means the soils are often dry, so a sudden wetting from a leak or poor drainage can be a bigger change than in wetter parts of the state.
- Panhandle Ogallala and loess: What is caliche?
- Caliche is a layer of soil cemented or enriched with calcium carbonate. Panhandle soils commonly have carbonate accumulations that start within a few inches to a couple of feet of the surface.
How do Pennsylvanian shale clays affect Oklahoma foundations?
Pennsylvanian shale under Tulsa and eastern Oklahoma weathers into poorly drained clays that hold perched water and often swell and shrink strongly.
- What is Pennsylvanian shale?
- Shale is rock made of compacted clay and silt that splits into thin layers. In northeastern Oklahoma, the Nowata is an example: a series of bluish or greenish shales with a few sandstones and at least one coal bed, varying from about 50 to 600 feet thick at Tulsa.
- What clay soils form on shale?
- Soil scientists rate shrink-swell using the coefficient of linear extensibility (COLE), the fraction a soil's length changes from moist to dry. A COLE of 0.06 to 0.09 is high and 0.09 or more is very high.
- Pennsylvanian shale clays: What does this mean for foundations?
- Wet-season edge lift. Perched water from late fall to spring wets the clay at slab edges and under crawl spaces. See perimeter wetting heave and groundwater and perched water; Summer edge drop. As perched water disappears and clay dries, deep cracks open in soils like Summit.
- What is shale and why does it matter for foundations?
- Shale is a layered rock made of compacted clay and silt. When it weathers near the surface it turns back into clay soil, and in eastern Oklahoma many of those clay soils swell when wet and shrink when dry.
- Why is the ground so wet in winter and spring on shale soils?
- Unweathered shale lets very little water through, so rain collects above it as a perched water table. Dennis, Okemah, Parsons and Summit soils all hold perched water in roughly November through May.
- Which shale soils swell the most?
- Summit is a smectitic clay, the swelling clay family. It forms deep, wide summer cracks and has a coefficient of linear extensibility of 0.09 or more, which is in the very high class.
- How thick are these shale formations?
- Very thick. The Nowata shale varies from about 50 to 600 feet thick at Tulsa, and the Boggy Formation near McAlester is 610 to 1,220 meters.
How do Tulsa-area sandstone ridges affect Oklahoma foundations?
The sandstone forms shallow, loamy, well-drained soils, so slopes, fill, erosion and uneven rock matter more than clay swell on these lots.
- What is Pennsylvanian sandstone?
- In northeastern Oklahoma, sandstone beds alternate with shale. In Tulsa County, for example, the Coffeyville Formation was described as five zones, two of sandstone and three of shale, as much as 440 feet thick in all.
- What soils form on sandstone?
- A lithic contact is hard bedrock; a paralithic contact is soft, weathered rock.
- Tulsa-area sandstone soils: How does rock in the soil survey compare to pier depth?
- A soil series description covers the soil and the top of the rock, not what lies deeper. Coweta's description shows why that can mislead: soft sandstone at about 15 inches can be followed by hard sandstone farther down, and both are interbedded with shale.
- Tulsa-area sandstone soils: What does this mean for foundations?
- Slopes up to 30 to 60 percent on Collinsville, Coweta and Hector soils mean many lots are cut and filled to make a level pad. Fill that was not compacted can settle over time. See fill consolidation settlement and fill soils and lot history.
- Which soils form on Pennsylvanian sandstone?
- Common ones in eastern Oklahoma include Collinsville, Coweta and Bates. They are loamy, well drained and often shallow to sandstone.
- Tulsa-area sandstone soils: How shallow is the rock?
- Collinsville reaches hard rock at 4 to 20 inches, Coweta reaches soft sandstone at 10 to 20 inches, and Bates reaches soft sandstone or shale at 20 to 40 inches.
- Tulsa-area sandstone soils: If rock is that shallow, why do piers sometimes go much deeper?
- Sandstone often occurs as ledges with shale between them. Field records from Tulsa show steel piers stopping at a median of about 12 feet even where soil surveys show rock within a few feet, because a soil description covers only the soil and the top of the rock; soft sandstone can be followed by harder sandstone deeper down, interbedded with shale.
- Do sandstone soils swell?
- They are loamy rather than clayey, so seasonal swell is generally lower than on shale clays. Erosion, slope movement and fill are more common concerns.
How do western Oklahoma red beds affect foundation soils?
Soft red sandstone and siltstone form shallow silty soils, while claystone and old river clays form expansive clay soils.
- What are red beds?
- Red beds are sedimentary rocks colored red by iron oxide. Oklahoma's Permian red beds, about 265 million years old, include red shale and sandstone along with salt and gypsum. The state rock, the rose rock, comes from these Permian red beds.
- What soils form from the red beds?
- Both are well drained and of large extent in western Oklahoma, a few southern Kansas counties and northwestern Texas.
- Western Oklahoma red beds: What does this mean for foundations?
- On Hollister, Tillman and Vernon soils, the clay can shrink deeply in dry weather and swell when rain returns. Cracks reaching 30 to 48 inches or more show how deep seasonal drying can go. See perimeter drying settlement, center heave and the active zone.
- What are red beds?
- Red beds are sedimentary rocks such as sandstone, siltstone, shale and claystone colored red by iron oxide. In western Oklahoma they date to the Permian Period, about 265 million years ago, and are interlayered with salt and gypsum.
- Why are some western Oklahoma soils so expansive?
- Clays weathered from red-bed claystone, or washed off it into old terraces, can be highly expansive. Hollister, a smectitic clay, for example, has cracks more than 30 inches deep that stay open 150 or more days in most years.
- Western Oklahoma red beds: Are all red-bed soils clayey?
- No. Woodward and Quinlan formed from soft red sandstone or siltstone and are loamy or silty, with soft rock at 10 to 40 inches. They are less clayey and occur on slopes of up to 40 to 50 percent.
- Western Oklahoma red beds: Does gypsum show up in red-bed soils?
- It can. The Vernon soil may contain up to 2 percent gypsum, and shale in the red beds near Union City contains gypsum nodules and veins.
How do Wichita granite and outwash affect Lawton-area foundations?
Around the Wichita Mountains, soils formed in granite outwash over red clay, such as Foard, are sodium-rich, very slowly permeable and prone to shrink-swell.
- What is the Wichita granite?
- The Wichita Mountains are built of granite and similar igneous rocks that formed about 525 million years ago. They were uplifted into mountains about 315 million years ago, during the same mountain-building episode that raised the Arbuckle and Ouachita ranges.
- What is granite wash?
- As the mountains eroded, streams carried granitic sand and gravel outward onto the surrounding lowlands. This is called granitic outwash or granite wash.
- Wichita granite and outwash: What soils are found in the Lawton area?
- A natric horizon is a subsoil layer high in exchangeable sodium. In Foard, exchangeable sodium reaches roughly 15 to 25 percent in the lower subsoil. Smectitic means the clay is dominated by smectite, the most swelling clay family.
- Wichita granite and outwash: What does this mean for foundations?
- Smectitic clay with wide cracks and slickensides in Foard and Hollister, and deeply cracking clay in Tillman, shrinks during dry periods and swells when wet. In this setting, slab edges can drop in drought and rise in wet spells.
- How old is the Wichita Mountains granite?
- The granite formed about 525 million years ago and was uplifted into mountains about 315 million years ago, according to the Oklahoma Geological Survey.
- What is granite wash?
- Granite wash is coarse sediment worn off the granite mountains and carried outward by streams. Around the Wichitas, old granitic outwash lies over Permian red-bed clays, and soils like Foard formed in that mix.
- Wichita granite and outwash: Why are Foard soils a concern for foundations?
- Foard is a smectitic clay with wide cracks and slickensides and a high-sodium subsoil. Water moves through it very slowly, and it can shrink and swell strongly with seasonal moisture.
- Is the granite itself a foundation problem?
- Most homes near the Wichitas are built on the plains around the mountains, where the ground is outwash and red-bed clay rather than exposed granite. Those clay soils, such as Foard, Hollister and Tillman, are the main source of seasonal movement.
How does Arbuckle limestone affect Oklahoma foundations?
Hard Arbuckle limestone and dolomite surface in south-central Oklahoma. Shallow rock and karst, not clay swell, are the main foundation questions there.
- What is the Arbuckle rock?
- Carbonate rock means limestone (calcium carbonate) and dolomite (calcium-magnesium carbonate). The Arbuckle rocks were deposited in shallow seas.
- Arbuckle limestone: What is karst?
- Karst is ground shaped by rock that dissolves in water, producing features such as cavities, sinkholes, caves, disappearing streams and springs.
- What does Arbuckle rock mean for foundations?
- On this ground, the key questions are what lies directly under the footing (sound rock, soil, or a soil-filled opening) and whether roof or yard water is being concentrated into fractures in the rock; Sudden, localized drops are a sign worth separating from slow seasonal movement.
- How is deep Arbuckle rock tied to earthquakes?
- The same rock group extends deep underground in other parts of Oklahoma. That deep setting is discussed separately in induced seismicity in Oklahoma and seismic damage vs. soil movement.
- Arbuckle limestone: Where can I read more?
- The South-Central (Arbuckles and Texoma) region page covers the soils and other rock units around the Arbuckle uplift, and gypsum karst and sinkholes explains the different, faster-dissolving karst of western Oklahoma.
- What are the Arbuckle carbonates?
- They are thick beds of limestone and dolomite laid down in shallow seas during the Cambrian and Ordovician periods. They are exposed at the surface in the Arbuckle Mountains of south-central Oklahoma.
- Are there sinkholes in the Arbuckle Mountains area?
- The Arbuckle Mountains are one of the two main limestone and dolomite karst areas in Oklahoma. Small karst features are found across much of the outcrop, but large open caves are known in only a few places.
- Arbuckle limestone: Is the rock close to the surface?
- In the outcrop belt the rock is uniformly hard and crops out at the surface, where tilted limestone layers form rows of outcrops sometimes called tombstone topography.
- How does Arbuckle rock relate to Oklahoma earthquakes?
- The deep Arbuckle Group is a separate topic from the surface outcrop. See the induced seismicity guide for how deep rock layers relate to Oklahoma earthquakes.
How does Garber Sandstone affect Oklahoma foundations near OKC?
It weathers into loamy, well-drained soils often shallow to soft rock that swell less than red clays, so drainage, erosion and shallow rock matter most.
- What is the Garber Sandstone?
- The Garber Sandstone is an early Permian rock unit, deposited in a delta setting, made of lens-shaped beds of sandstone, shale and siltstone.
- What soils form on Garber sandstone?
- Darnell formed in Permian sandstone on ridge crests and upper hill slopes; Stephenville formed in material weathered from Permian sandstone on hill backslopes; Zaneis formed in residuum from interbedded Permian sandstone and shale on low hills, and its subsoil is fine-loamy.
- What does Garber sandstone mean for foundations?
- The Garber soils above are classed as loamy or fine-loamy, not clayey. However, the Garber itself contains shale and siltstone lenses, and Zaneis and Lucien formed partly from shale or clay layers.
- Where is the Garber Sandstone at the surface?
- It crops out on the east side of Oklahoma County. The rock layers dip west, so on the west side of the county the Garber lies under hundreds of feet of Hennessey shale.
- Do Garber Sandstone soils swell like red clay?
- Soils formed on Garber sandstone, such as Darnell, Lucien, Stephenville and Zaneis, are loamy and well drained. They generally move less than the clay soils on Hennessey shale, but clay and shale layers within the sandstone can still produce clayey spots.
- Is the Garber the same as the Garber-Wellington aquifer?
- The Garber Sandstone and the Wellington Formation together make up the main water-bearing part of the Central Oklahoma aquifer, often called the Garber-Wellington aquifer.
- How deep is rock on Garber soils?
- It varies by series. Darnell and Lucien reach soft sandstone at about 10 to 20 inches, Stephenville at 20 to 40 inches, and Zaneis at about 40 to 60 inches.
How does limestone north of Tulsa affect Oklahoma foundations?
Pennsylvanian limestone beds can weather into Apperson soil, a very expansive clay over hard limestone about 3.5 to 5 feet down.
- Pennsylvanian limestones: What are the rock layers?
- In northeastern Oklahoma, Pennsylvanian limestone and shale units alternate: the Oologah limestone is 80 to 145 feet thick, while the Nowata shale above it ranges from 50 to 600 feet.
- Pennsylvanian limestones: What is Apperson clay on limestone?
- The Apperson series is the main soil formed from this limestone:
- Pennsylvanian limestones: What does this mean for foundations?
- Seasonal shrink-swell. Smectite clay above the rock has cracks that open and close periodically, and clayey soils swell as they take on water. Soil under a slab edge can drop in drought and rise when rain returns.
- Pennsylvanian limestones: Is karst a concern?
- Limestone can dissolve and form karst features. The Oklahoma Geological Survey identifies the Ozark area in the northeast and the Arbuckle Mountains in the south as the state's main limestone and dolomite karst areas. See karst subsidence and sinkholes.
- Where are Pennsylvanian limestones near Tulsa?
- Limestone units such as the Oologah occur in the prairie country of northeastern Oklahoma, in a belt running south from the Kansas line. They alternate with shale formations such as the Nowata, reported at 50 to 600 feet thick.
- Pennsylvanian limestones: Can limestone produce expansive clay?
- Yes. The Apperson soil formed in calcareous residuum from Pennsylvanian limestone, and its subsoil is smectitic silty clay with cracks that open and close and slickensides. It is classed as a Vertisol, the soil order defined by shrink-swell.
- Pennsylvanian limestones: How deep is the rock under Apperson soils?
- Hard limestone is 40 to 60 inches below the surface, about 44 inches in the typical profile. That is shallow enough to matter for footings but still leaves several feet of very expansive clay above it.
- Do these limestones form sinkholes?
- The Oklahoma Geological Survey names the Ozark area in the northeast and the Arbuckle Mountains in the south as the state's main limestone and dolomite karst areas. Any sudden local depression should still be checked on its own merits.
How does the Wellington Formation affect Oklahoma foundations?
Wellington layers shift quickly from sand to clay, so soils range from shallow and loamy to deep and highly expansive and must be judged lot by lot.
- What is the Wellington Formation rock?
- The Wellington Formation is made of sands, shales and mudstones. A USGS review describes the Wellington and Garber as interbedded, lens-shaped sandstone, shale and siltstone deposited in similar delta environments during the early Permian.
- What soils form on the Wellington Formation?
- Vertic features, such as wide cracks and slickensides (polished shear faces in clay), are the soil survey's signs of strong shrink-swell. Renfrow's description requires cracks at least 5 millimeters wide within about 4 feet of the surface for part of most years.
- What does the Wellington Formation mean for foundations?
- On Renfrow and Kirkland, slow-draining clay with slickensides shrinks in drought and swells when wet. Renfrow has low saturated hydraulic conductivity and high runoff, so water tends to run off rather than soak in evenly.
- What is the Wellington Formation?
- It is an early Permian rock unit of sands, shales and mudstones in central Oklahoma. Together with the Garber Sandstone it forms the main water-bearing part of the Central Oklahoma aquifer.
- Are Wellington soils sandy or clayey?
- Both. Because the rock mixes sandstone and shale in lens-shaped beds, soils on it range from shallow, loamy soils like Lucien to deep, expansive clays like Renfrow.
- Wellington Formation: How can I tell what is under my lot?
- Check the USDA Web Soil Survey for the mapped soil series. A clay series such as Renfrow and a shallow loamy series such as Lucien behave very differently even a short distance apart.
What Cretaceous rocks and soils lie under southern Oklahoma?
Layers of clay, shale, marl, limestone and sand; the clay and shale weather into very expansive soils, while sand layers give loose, well-drained soils.
- What are these rocks and where are they?
- The Cretaceous Period ran from about 145 to 66 million years ago, and in the Late Cretaceous most of Oklahoma was covered by a shallow sea. Rocks from that time are preserved in southern Oklahoma, including the Tishomingo and Atoka areas.
- Cretaceous rocks, south OK: Why do they form two very different soils?
- The Durant soil formed in clayey material weathered from Cretaceous shale and claystone, with its typical profile in Bryan County, Oklahoma. Its clay is smectitic, meaning it is dominated by smectite, the clay mineral family that swells most when it takes up water.
- Cretaceous rocks, south OK: What are the practical points for homeowners?
- Look up the soil series for the lot. A clay series like Durant and a sandy series like Bernow call for different explanations of the same crack; On the clay soils, keeping moisture even around the foundation is the main lever.
- Where are Cretaceous rocks in Oklahoma?
- They occur across southern Oklahoma. Early USGS mapping around Tishomingo and Atoka described them as nearly flat layers dipping gently south.
- Why are some soils on Cretaceous rock so expansive?
- Clayey Cretaceous shale weathers to smectite clay, the mineral family that swells most when wet. The Durant soil, formed from this material, develops cracks wider than 1 centimeter and about 30 inches deep in dry periods.
- Are all Cretaceous soils clayey?
- No. Sandstone and sand layers weather to well-drained sandy and loamy soils such as Bernow, which have much lower swell potential but can erode.
- Cretaceous rocks, south OK: How can I tell which one is under my house?
- The USDA Web Soil Survey maps soil series by address. A clay series like Durant and a sandy series like Bernow can occur within a short distance of each other because the rock layers alternate.
What is Hennessey shale and how does it affect Oklahoma foundations?
Red Permian shale west of OKC that weathers into swelling red clay soils, which shrink in drought and swell when wet, driving seasonal foundation movement.
- What is Hennessey Group rock?
- The Hennessey is a group, meaning several related rock formations mapped together. It is early Permian in age and named for exposures at Hennessey in Kingfisher County.
- How does the shale behave near the surface?
- The unweathered shale is very tight and nearly impermeable. Near the surface it is different:
- What red clay soils form on the Hennessey?
- An Oklahoma State University study measured the coefficient of linear extensibility (COLE), a lab measure of how much a soil shrinks as it dries. It found values up to about 0.090 in a Kirkland-like soil and 0.095 in Bethany, tied to large amounts of smectite clay.
- Hennessey shale red clay: What does this mean for foundations?
- Because the clay changes volume with moisture, foundations on Hennessey soils commonly show:
- What is the Hennessey Shale?
- It is a group of early Permian rock layers, mostly red-brown shale with siltstone, some sandstone and thin gypsum beds. It is named for exposures at Hennessey in Kingfisher County and forms the surface across much of western Oklahoma County.
- Why do houses on Hennessey red clay crack?
- The shale weathers into clay soils with a lot of smectite, a swelling clay. Those soils shrink and crack when dry and swell when wet, so the ground under a foundation moves up and down with the seasons.
- How deep does the weathered shale go?
- A central Oklahoma groundwater study describes weathered zones in the shale about 10 to 15 feet deep, fractured by expansion, contraction and leaching. Below that, the shale is very tight.
- Which soils form on the Hennessey?
- Common ones include Renfrow, Kirkland and Bethany. Renfrow formed in weathered Permian clayey shale and has vertic features such as wide cracks and slickensides.
Why does western Oklahoma gypsum cause sinkholes?
Gypsum beds like the Blaine Formation, and deeper salt, dissolve in groundwater and form caves, sinkholes and voids, a different risk from 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.
- 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.
- 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.
- What does gypsum karst mean for foundations?
- Distinguish slow seasonal cracking from a sudden local drop.
- Gypsum karst and sinkholes: 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.
- Gypsum karst and sinkholes: 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.