SHALES AND FIRECLAYS

Shales and fireclays are among the most common rocks in the Southern Appalachian Coal Fields. They can be distinguished from sandy shales and sandy fireclays, which they resemble, by their smooth or only slightly gritty texture which can be easily detected by touching or rubbing the core. Some shales are similar in appearance to dark, fine-grained limestones but can be distinguished from them because they are much softer and less resistant to impact. Most shales and fireclays can be scratched easily with the fingernail and fail readily upon impact. Many black shales resemble bone but can be differentiated from it by density and the color of the powdered rock. Bone is considerably less dense than shale and, when scratched with a knife, powdered bone is orange brown or black but shale is gray.

The primary properties for differentiating shales and fireclays are bedding and fracture. These two properties are very closely related because both arise from the orientation of the tiny flakes which make up these rocks. Hence, the clay flakes in those shales which show distinct light and dark banding are oriented parallel to the banding, and the rock fractures parallel to both banding and grain orientation. Likewise the clay flakes in underclays which display irregularly oriented streaks and bands are also strongly disoriented and the rock fractures in an irregular fashion. In some shales and fireclays, parallel or irregularly oriented bands or streaks are not present; nevertheless the rocks do have a distinctive fracture which reflects grain orientation similar to that found in those where bedding is apparent.

These characteristics of bedding and fracture lead to four major divisions of the fine-grained rocks:

1. Shales which show parallel fracture but lack apparent layering (e.g. 70R).
2. Shales that are layered and show fractures parallel to the layering (e.g. 74L).
3. Fireclays which may or may not show irregularly oriented bedding features, but which fracture in an irregular fashion (e.g. 76R).
4. An intermediate variety in which apparent bedding is obscure but, when present, is somewhat disoriented and in which the fracture is somewhat irregular (e.g. 74R).

The second property used in differentiating among shales and fireclays is color. The colors range from black through dark gray to light gray green. Dark gray is the most common color and black is less abundant. Specimen 70R can be regarded as representing a dividing line between dark gray and black and specimen 72R can be used in separating dark gray from light gray. The latter color is characteristic of some fireclays and a few shales and, in some cases, a greenish or tan tint is also present.

Red, red and green, or bright green shales have been encountered in some core holes in the Southern Appalachian region, particularly those that have completely penetrated the coal bearing rocks. Because the total number of such occurrences is small and because these rock types are very conspicuous and readily recognized, they are not illustrated in this book. When encountered, however, their presence can be noted in the following form:

Green Shale 144 not illustrated
Red and Green Shale 154 not illustrated
Red Shale 164 not illustrated

A key for identifying the commonly occurring shales and fireclays is given in Table 4.