A facies is a body of rock with specific characteristics. It is properly defined on the basis of color, bedding, composition, texture, fossils, and sedimentary structures. However, the term “facies” is also used in many other senses. It is used as a strict observational sense of a rock product (sandstone facies), in a genetic sense for the products of a process by which a rock is thought to have formed (turbidite facies), in an environmental sense for the environment in which a rock is thought to have formed (fluvial facies), and as a tecto-facies (post-orogenic facies). All these uses are acceptable, as long as you remember which concept is being used.
Facies distributions and changes are dependant on a number of interrelated influences, which include:
- Sedimentary Processes
- Sediment supply
- Sea Level changes
- Biological Activity
- Water Chemistry
A facies sequence is a series of facies that pass gradually from one into another. A sequence can have an abrupt or erosive boundary, or be bounded by a hiatus, as indicated by a rootlet bed, reworking, or early diagenesis.
Clastic environments have two important types of sequences. In one, there is a coarsening upward sequence that develops above a sharp or erosional base; a second type is the fining upward sequence. This is important in interpretation of depositional environments, because grain size is normally a measure of the hydraulic power at the time of deposition and a coarsening upward sequence indicates an increase in flow power. This may be due to shallowing as a delta, shoreline, or river crevasse builds out into deeper water, or to progradation of a submarine fan. Fining upward sequences can be formed by a migrating point bar in a river or by filling of an abandoned channel.
Walther's Law states that “the various deposits of the same facies area and, similarly, the sum of the rocks of different facies areas were formed beside each other in space, but in a crustal profile we see them lying on top of each other...it is a basic statement of far-reaching significance that only those facies and facies areas can be superimposed, without a break, that can be observed beside each other at the present time.” This is interpreted to mean that facies occurring in a conformable vertical sequence were formed in aerially adjacent environments and the facies in vertical contact must be the product of neighboring environments.