MID-CONTINENT REGION

In the Mid-continent region (fig. 1), clays with high swelling potential (those that contain large amounts of smectite) occur in geologic units of Cretaceous, Tertiary, and Quaternary ages and are extensively exposed in areas bordering the Gulf Coast, and in Oklahoma, Nebraska, South and North Dakota, and western Kansas. In other parts of the region, swelling clays have slight to moderate swelling potentials; most are Paleozoic in age and contain large amounts of illite, varying amounts of mixed-layer clay minerals, and usually less than 15 percent montmorillonite.

In the Mid-continent region, swelling clays that have caused the greatest amount of damage are contained in stratigraphic: sequences ranging in age from Cretaceous to Quaternary, and are exposed in an area extending along the Gulf Coast from east-central Texas to Alabama. In this area, clay deposits with high swelling potential are thick, numerous, and extensive, and climatic conditions are conducive to large volume changes. Particularly troublesome are clays contained in the Washita, Woodbine, Eagle Ford, Taylor, and Navarro Groups of Cretaceous age in Texas (McDowell, 1959; Carothers, 1965; Bishop and Flanigan, 1972) and the Beaumont clay of Pleistocene age (Holtz, 1959; Vijayvergiya and Sullivan, 1974). Cretaceous clays with high swelling potential have caused damage to residences constructed on the South Bosque Formation shales in Waco and the Houston black clay deposits in San Antonio (Lytton and Dyke, 1980). In Alabama and Mississippi, clays with high swelling potential in formations of Late Cretaceous, Paleocene, and Eocene ages are reported to have caused considerable damage to pavements and other structures (Redus, 1962; Sowers and Kennedy, 1967; Green and Childress, 1974).

In the western part of the region, geologic units of Cretaceous to Quaternary ages also contain highly expansive clays; however, damage is much less serious than in the Gulf States because in much of the area they are covered by extensive surficial deposits not shown on the accompanying map, and also because the climate is so dry that the clays rarely absorb sufficient moisture to cause appreciable swelling. As noted by Johnson and McCasland (1971, p. 24), soils with high swelling potential occur throughout Oklahoma; however, in the western third of the state the climate is so dry that clays forming highway subgrades "...are seldom, if ever, wetted. Most troubles occur in the south and southeastern parts of the state where rainfall is high and the thick shale sections produce rather extensive areas of heaving soils."

Clays in the Quaternary alluvium of the lower Mississippi River valley in Louisiana are reported by Dixon (1967, p. 39-42) to be of the "montmorillonite type," and Dean (1977) notes that in southeastern Missouri, clayey soils of the alluvial valley have high "shrink-swell capacity," and that"... foundation problems in the area are associated with changing water levels and the instability of clayey soils." A foundation failure due to shrinkage of alluvial clay at Clarkesdale, Mississippi, has been described by Lusk (1963), who reports that foundation failures in alluvial deposits of the Mississippi River valley are common.

Swelling clays with low to moderate swelling potential are numerous and extensive in the sequence of Paleozoic rocks that underlie other parts of the Midcontinent region. Some of the units that have been reported to contain swelling clays that have caused damage to structures include the Decorah Formation (Parham and Austin, 1969) and the Maquoketa Shale (Lutzen and Rockaway, 1971; Gray, 1972; Dean, 1977) of Ordovician age, the Brassfield Formation and Crab Orchard Formation of Silurian Age, and the Bedford Shale of Devonian and Mississippian ages (Dobrovolny and Morris, 1965). Volume changes in swelling clays of Permian age have caused considerable damage to different types of structures in central Oklahoma (Means, 1959).