Page 96 - Sedimentology and Stratigraphy

Interpreting Past Depositional Environments 83

attempting to establish associations of facies it is useful deposited adjacent to each other at any one time. This
to bear in mind the processes of formation of each. Of the means that sandstone beds formed in a desert by aeo-
four examples of facies just mentioned the ‘bioturbated lian dunes might be expected to be found over or under
mudstone’ and the ‘wave rippled medium sandstone’ layers of evaporates deposited in an ephemeral desert
both probably represent deposition in a subaqueous, lake because these deposits may be found adjacent to
possibly marine, environment whereas ‘medium sand- each other in a desert environment (Fig. 5.13). How-
stone with rootlets’ and ‘coal’ would both have formed ever, it would be surprising to find sandstones formed
in a subaerial setting. Two facies associations may in a desert setting overlain by mudstones deposited in
therefore be established if, as would be expected, the deep seas: if such is found, it would indicate that there
pair of subaqueously deposited facies tend to occur was a break in the stratigraphic succession, i.e. an
together, as do the pair of subaerially formed facies. unconformity representing a period of time when ero-
The procedure of facies analysis therefore can be sion occurred and/or sea level changed (2.3).
thought of as a two-stage process. First, there is the
recognition of facies that can be interpreted in terms
of processes. Second, the facies are grouped into facies 5.6.4 Facies sequences/successions
associations that reflect combinations of processes
and therefore environments of deposition (Fig. 5.12). A facies sequence or facies succession is a facies
The temporal and spatial relationships between association in which the facies occur in a particular
depositional facies as observed in the present day order (Reading & Levell 1996). They occur when
and recorded in sedimentary rocks were recognised there is a repetition of a series of processes as a
by Walther (1894). Walther’s Law can be simply response to regular changes in conditions. If, for
summarised as stating that if one facies is found example, a bioclastic wackestone facies is always
superimposed on another without a break in a strati- overlain by a bioclastic packstone facies, which is in
graphic succession those two facies would have been turn always overlain by a bioclastic grainstone

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Fig. 5.13 A summary of the principal sedimentary environments.