Introduction

Fine-grained, siliciclastic sedimentary rocks, composed mainly of particles smaller than ∼ 62 microns (coarse silt and finer), make up approximately 50 percent of all sedimentary rocks in the stratigraphic record (Chapter 1). Thus, they are about twice as abundant as sandstones and conglomerates combined. These fine-grained rocks are known by a variety of names, including lutites, siltstones, mudstones, mudrocks, claystones, and shales. Tourtelot (1960) reviews in detail the history of fine-sediment terminology. He points out (p. 342) that historically the term shale has been used in two ways: (1) in a restricted sense to mean a laminated clayey rock and (2) as a broad, group name for all fine-grained siliciclastic rocks. He concludes that it is acceptable practice to include both these meanings for shale, and, therefore, that shale is the appropriate class name for fine-grained rocks, of equal standing with sandstones and limestones as group names.

Although some geologists (e.g. Potter et al., 1980, p. 15) have agreed with Tourtelot's conclusion that shale is an acceptable class name for all fine-grained rocks, others (e.g. Lundegard and Samuels, 1980; Spears, 1980; Stow and Piper, 1984) consider the dual use of the term in this way to be confusing.