What bedrock is under Oklahoma City homes?
In short: The Oklahoma City metro sits on Permian "red beds": mainly the Hennessey shale and the Garber sandstone 13. Where Hennessey shale is near the surface, public borings found rock within a few feet to about 16 feet 2; on Garber sandstone at a north Edmond site, soil extended 20 to more than 90 feet down 3. The clay soils formed from these rocks, not the rock itself, cause most shrink-swell movement 14.
What are the main rock units?
| Unit | What it is | Example site |
|---|---|---|
| Hennessey Group | Red-brown shale interbedded with siltstone, with some greenish-gray shale, sandstone beds and gypsum lenses 1 | Lake Hefner area (northwest OKC) 1; I-35/I-240 interchange 2 |
| Garber Sandstone | Red, massive, cross-bedded sandstones in lens shapes, with red clay shales and sandy shales 3 | I-35 over Waterloo Road, north Edmond 3 |
| Alluvium | River-laid sand, silt and clay over rock | Covers Hennessey shale at the I-35/I-240 site 2 |
The Hennessey is about 90 meters (roughly 300 feet) thick in the USGS study area, and its near-surface layers in western Oklahoma County dip gently west-southwest toward the Anadarko Basin 1. An ODOT geotechnical report describes the Hennessey unit as red, platy to blocky clay shales and mudstones about 400 feet thick 2. See the geology pages for the Hennessey shale and Garber sandstone.
How deep is rock?
On Hennessey shale (I-35/I-240): soil of lean clay, silt and sand extended about 2 to 16 feet, and bedrock was reached at 16, 5.5, 2, 2 and 2 feet in five borings 2. The report notes that shale weathering is erratic and can change over short distances 2.
On Garber sandstone (I-35 at Waterloo Road): soil of sand and moderate- to high-plasticity clay extended about 20 to 91.5 feet 3. Competent rock was found at depths from 20.5 to 95 feet across eight borings 3.
| Setting | Depth to rock in public borings |
|---|---|
| Hennessey shale, south OKC | about 2 to 16 ft 2 |
| Garber sandstone, north Edmond | about 20 to 95 ft 3 |
These are highway sites, so they show the range rather than the depth under any particular house. More boring data: depth to rock in Tulsa and OKC.
Where do groundwater and perched water occur?
At the Hennessey site, most borings were dry, but lab tests showed high saturation in some soil as shallow as 3 feet, and the report warns of perched water in the soil or at the soil-rock contact that changes with the seasons 2. At the Garber site, water levels after drilling ranged from about 15 to 35 feet 3. Perched water is water held up on top of a tight layer, above the main water table. See groundwater and perched water.
How does the rock differ from the soil on top?
The Hennessey shale in a USGS core taken below the soil was mostly illite and kaolinite, clay minerals with low shrink-swell potential 1. The soils that form on Permian shales tell a different story. An Oklahoma State University study of central and western Oklahoma soils found high proportions of 2:1 swelling clays, with Bethany and Kirkland soils reaching shrink-swell (COLE) values of 0.095 and 0.090; Kirkland's fine clay was mainly montmorillonite 4. Statewide, more than 75% of Oklahoma contains bedrock units that can serve as sources for expansive soils 5. This is why the weathered soil zone, not the rock, drives most shrink-swell movement.
What does it mean for foundations?
- Shallow Hennessey rock usually means shorter piers, but rock depth can change sharply between neighboring borings 2.
- On thick soil over Garber sandstone, competent rock at the boring site (20.5 to 95 feet) 3 lay well below the 8 to 16 foot depths typical of Oklahoma push pier installation records 6, so piers there may stop in dense soil rather than on rock.
- At the I-35/I-240 site, alluvium lies over the Hennessey shale 2.
Related
- Soil series in the OKC metro
- OKC metro region
- Oklahoma geologic provinces
- Shale weathering and bearing strata
- Western Permian red beds
Common questions
What rock is under Oklahoma City?
Mostly Permian red beds. The Hennessey Group, a red-brown shale with siltstone and some sandstone, underlies large parts of the metro, and the Garber Sandstone, a red cross-bedded sandstone with red shales, underlies other areas including north Edmond borings.
How deep is bedrock in Oklahoma City?
It varies widely. Highway borings on Hennessey shale near I-35 and I-240 found rock between about 2 and 16 feet, while borings on Garber sandstone near I-35 and Waterloo Road found overburden 20 to more than 90 feet thick.
Is the Hennessey shale itself expansive?
A USGS study found the Hennessey shale cored below the soil at a northwest Oklahoma City site was mostly illite and kaolinite with low shrink-swell potential; the swelling problems are mainly in the clay soils that form from these rocks.
Why does depth to rock matter for my house?
Driven and drilled piers get much of their support from firm soil or rock, so shallow rock usually means shorter piers, while thick soil over deep rock means piers may stop on friction in dense soil instead.
Sources
- Geologic and Hydraulic Characteristics of Selected Shaly Geologic Units in Oklahoma (Water-Resources Investigations Report 96-4303) U.S. Geological Survey, with the Oklahoma Geological Survey
- Report of Geotechnical Engineering Services, Proposed Retaining Wall C2, Crossroads Interchange Reconstruction, I-35/I-240 Oklahoma Department of Transportation
- Geotechnical Engineering Report: Three Span Bridge, Interstate 35 over Waterloo Road Oklahoma Department of Transportation
- Shrink-Swell and Other Characteristics of Five Benchmark Soils of Western Oklahoma (Technical Bulletin T-150) Oklahoma State University Agricultural Experiment Station
- Geologic Hazards in Oklahoma Geological Society of America (abstract by K. V. Luza, Oklahoma Geological Survey)
- Oklahoma steel push pier installation records, 2025 (unpublished field data) TerraBrain field data
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