| Abstract |
The concept of facies is widely used in the study of sedimentary rocks and also provides a useful tool for the study of volcanics. The reason for this is that since fossils are rare in volcanic piles, and extensive use of radiometric dating and rock geochemistry is generally impractical, only lithostratigraphy is left by which volcanic piles can be stratigraphically mapped. However, volcanic piles contain lithological units of very similar appearance with extremely complex distributions. Standard stratigraphic methods break down because one lithological unit can suddenly disappear or cannot be readily distinguished from others. Facies on the other hand are more continuous and have more complex and hence more distinct identifying characteristics. A facies is a contiguous body of rock with clearly identifiable unifying characteristics. A single facies can range in scale from outcrop to regional levels but is most usefully defined on an intermediate scale. The concept is very similar to that of a formation in that one of the aims is to establish a mappable body. However, volcanic facies are also inter-related to each other as parts of a volcanic centre, and the identification of a facies has predictive value, whereas this need not be the case for different formations. Different facies may be part of one formation and be made up of numerous lithological units. Therefore there are two different sets of unifying characteristic for a facies, one compositional and one spatial. The set of compositional characteristics are: what particular lithological types are present, and in what proportion? The spatial characteristics are: what is the relationship with bordering facies, and what is their distribution with respect to the eruptive centre? |