Fine-grained sediments are those rocks, both hard and soft, carbonate and clastic, that have a dominant grain-size in the clay or silt grades (<63 μm). They are the most abundant sediments on the earth's surface and form between 50 and 750f the geological column. Classifications of fine-grained sediments have been based most commonly on texture and fissility, and secondarily on mineralogy, chemical composition, colour, degree of metamorphism and depositional environment (Picard 1971). Different terminologies have, therefore, arisen but the state of confusion is less severe than that in, for example, the coarser-grained sediments that have been relatively more throughly studied and subdivided.
It is clearly important for improved communication within the earth sciences (for example, between sedimentologists and engineers) that a generally-accepted standard terminology is widely used. It is still appropriate for fine-grained sediments that the term should be simple and readily applied in the field. The results of more detailed laboratory or field analyses can then provide suitable descriptive modifiers to the basic terms. This is the aim of the terminology out-lined in Table 1.
The term mudrock implies a siliciclastic composition. Other fine-grained sediments can be designated carbonate mudrocks, silica mudrocks, and so on. A similar terminology is then appropriate (e.g. carbonate siltstone, silica mudstone) but a range of other terms, such as chalk, chert, diatomite and ooze, are also widely used. Classification of these sediments is not discussed here. The terminology presented in Table 1 is broadly in line with that proposed by many previous