Shale
Shale is a sedimentary rock composed predominantly of clay-sized particles. Most of the particles are clay minerals, but other fine-grained clastic materials are normally present. Shale is formed by the lithification of clay or mud, commonly with admixed silt. Shales composed predominantly of clay minerals easily split into thin flat plates or sheets parallel to bedding (fissile). Arkansas shales, when freshly exposed, are commonly very dark gray or nearly black, but weather to shades of very light gray to buff. Black and gray shales owe their color to finely divided carbonaceous matter or pyrite. Less commonly, shale may be light gray, greenish, or reddish in color when fresh. Greenish shale owes it’s color to the presence of ferrous iron and/or chlorite, and reddish shale to the presence of iron oxide. Rock units consisting mostly of clay-sized particles of minerals other than clay are termed claystone or mudstone because they lack the fissility of shale.
Throughout most of the Interior Highlands, shale is the dominant sedimentary rock. Most shale occurs in very thick sequences of Mississippian and Pennsylvanian rocks. Units consisting almost entirely of shale may be more than 300 feet thick. Shale is more easily eroded than most other sedimentary rocks. Consequently, it underlies valley floors and the lower flanks of mountains where it is less likely to be exposed in outcrop than the more resistant sedimentary rocks occupying higher elevations. It is often exposed in creek beds, along the banks of the major rivers, and in road cuts.
Shale has several potential uses. It may be finely ground and used as a filler in paints, plastics, asphalt compounds, roofing cement, and some linoleum. In parts of Arkansas where clay is not locally available, shale is used to manufacture brick. Many county roads are based on shale and, if some harder aggregate such as thin-bedded sandstone or limestone is admixed, a relatively smooth and durable surface results. Shale is often used in paved highway construction as subbase fill material as it is readily available and less expensive to excavate than sandstone or limestone. Some shale units exhibit dramatic swelling when fired and have potential uses as lightweight aggregate. The Chattanooga Shale (Devonian) and younger black shales of the Ozark region of north Arkansas have not been investigated for oil shale potential. However, corresponding shale units in adjoining states yielded up to 15 gallons of oil per ton and averaged around 10 gallons per ton. Recovery was accomplished by distillation, instead of solvent extraction.
Shale, containing the mineral talc, developed from the alteration of shale adjacent to soapstone deposits in Saline County. Ground talcose shale has potential value as a filler in pottery clay. Baking of shale, which occurred in the zone of contact metamorphism at the Magnet Cove intrusion in Hot Spring County, created hornfels, a rock that is usable as a crushed stone product.
Slaty Shale
In parts of the Ouachita Mountains, especially the central core, much of the shale has undergone very low-grade regional metamorphism resulting in cleavage that has replaced fissility as the dominant planar structure. Cleavage permits the rock to be split easily into relatively thin slabs. The stone industry and many geologists this rock "slate." However, the degree of regional metamorphism is so low that other geologists prefer the term "slaty shale." Locally, in proximity to some major faults, shaly rocks have been sufficiently crystallized to be considered low-grade slate. The two types of rock can be used interchangeably for most purposes, but slaty shale has proven superior for roofing granules. Rough and cut blocks were previously used as shingles and for floors, patios, table tops, and interior and exterior covered walkways. This rock is good quality for interior applications, although most appears not to have undergone sufficient crystallization to stand up to prolonged exterior use.
The higher quality slaty shale in Arkansas is in portions of Polk, Montgomery, Garland, Pulaski, and Saline Counties. Deposits are in the Stanley Shale, Missouri Mountain Shale, Polk Creek Shale, Womble Shale, and Mazarn Shale formations (Paleozoic). Slaty shale in the Stanley Shale apparently has the best physical properties.
Deposits of Shale and Slaty Shale
Shale, used for local construction fill, has been mined from the Interior Highlands, and slaty shale from the core area of the Ouachita Mountains. Recent "slate" mining has been mostly restricted to Montgomery County. Slaty shale in the Stanley Shale north of Glenwood is hauled from the open pit to a preparation plant where it is crushed and ground into granules for roofing. Slaty shale of the Womble Shale was also mined from an open pit in northern Saline County. Broken rock was transferred to a grinding plant in Bryant. The rock was crushed, dried, ground, and bagged for rail shipment. It was used mainly as fillers and additives to paints and plastics. The company recently ceased operations in Arkansas.
Shale and slaty shale resources in Arkansas are considered inexhaustible. However, there can be shortages of particular types or colors. Markets for Arkansas slaty shale products are limited by competition from substitute materials and the relatively few industries utilizing slate granules and flour (fines).
References
Branner, G. C., 1940, Mineral resources of Benton, Carroll, Madison, and Washington Counties: Arkansas Geological Survey County Mineral Report 2, 55 p.
Miser, H. D., and Purdue, A. H., 1929, Geology of the DeQueen and Caddo Gap quadrangles, Arkansas: U. S. Geological Survey Bulletin 808, 195 p.
Nuelle, L. M., and Sumner, S. S., 1981, A preliminary evaluation of shale-oil resources in Missouri: Missouri Department of Natural Resources, Division of Geology and Land Survey, Information Circular 27, 31 p.
Purdue, A. H., 1909, The slates of Arkansas: Arkansas Geological Survey, 170 p.
Purdue, A. H., and Miser, H. D., 1923, Description of the Hot Springs district: U. S. Geological Survey Atlas, Folio 215, 12 p.
Stone, C. G., and Bush, W. V., 1984, General geology and mineral resources of the Caddo River watershed: Arkansas Geological Commission Information Circular 29, 32 p.
Swanson, V. E., 1960, Oil yield and uranium content of black shales: U. S. Geological Survey Professional Paper 356-A, 44 p.