Page 97 - Sedimentology and Stratigraphy

84 Field Sedimentology, Facies and Environments

(Fig. 5.12), these three facies may be considered to be on the basis of the facies characteristics, and perhaps
a facies sequence. Such a pattern may result from some biofacies information indicating that the fauna
repeated shallowing-up due to deposition on shoals are freshwater. This association of facies may there-
of bioclastic sands and muds in a shallow marine fore be referred to as a ‘lacustrine facies association’
environment (Chapter 14). Recognition of patterns of and be distinguished from other continental facies
facies can be on the basis of visual inspection of associations deposited in river channels (‘fluvial chan-
graphic sedimentary logs or by using a statistical nel facies association’) and as overbank deposits
approach to determining the order in which facies (‘floodplain facies association’).
occur in a succession, such as a Markov analysis It can be convenient to have shortened versions of
(Swan & Sandilands 1995; Waltham 2000). This the facies names, for example for annotating sedimen-
technique requires a transition grid to be set up with tary logs (Fig. 5.12). Miall (1978) suggested a scheme
all the facies along both the horizontal and vertical of letter codes for fluvial sediments that can be adapted
axis of a table: each time a transition occurs from one for any type of deposit. In this scheme the first letter
facies to another (e.g. from bioclastic wackestone to indicates the grain size (‘S’ for sand, ‘G’ for gravel, for
bioclastic packstone facies) in a vertical succession example), and one or two suffix letters to reflect other
this is entered on to the grid. Facies sequences/suces- features such as sedimentary structures: Sxl is ‘cross-
sions show up as higher than average transitions laminated sandstone’, for example. There are no rules
from one facies to another. for the code letters used, and there are many variants
on this theme (some workers use the letter ‘Z’ for silts,
for example) including similar schemes for carbonate
5.6.5 Facies names and facies codes rocks based on the Dunham classification (3.1.6). As a
general guideline it is best to develop a system that is
Once facies have been defined then they are given a consistent, with all sandstone facies starting with the
name. There are no rules for naming facies, but it letter ‘S’ for example, and which uses abbreviations
makes sense to use names that are more-or-less descrip- that can be readily interpreted.
tive, such as ‘bioturbated mudstone’, ‘trough cross- There is an additional graphical scheme for display-
bedded sandstone’ or ‘foraminiferal wackestone’. This ing facies on sedimentary logs (Fig. 5.12): columns
is preferable to ‘Facies A’, ‘Facies B’, ‘Facies C’, and so alongside the log are used for each facies to indicate
on, because these letters provide no clue as to the their vertical extent. An advantage of this form of pre-
nature of the facies. A compromise has to be reached sentation is that if the order of the columns is chosen
between having a name that adequately describes the carefully, for example with more shallow marine to the
facies but which is not too cumbersome. A general left and deeper marine on the right for shelf environ-
rule would be to provide sufficient adjectives to distin- ments, trends through time can be identified on the logs.
guish the facies from each other but no more. For
example, ‘mudstone facies’ is perfectly adequate if only
one mudrock facies is recognised in the succession. On 5.7 RECONSTRUCTING
the other hand, the distinction between ‘trough cross- PALAEOENVIRONMENTS IN SPACE
bedded coarse sandstone facies’ and ‘planar cross- AND TIME
bedded medium sandstone facies’ may be important in
the analysis of successions of shallow marine sandstone. One of the objectives of sedimentological studies is to try
Facies schemes are therefore variable, with definitions to create a reconstruction of what an area would have
and names depending on the circumstances demanded looked like at the time of deposition of a particular
by the rocks being examined. stratigraphic unit. Was it a tidally influenced estuary
The names for facies should normally be purely and, if so, from which direction did the rivers flow and
descriptive but it is quite acceptable to refer to facies where was the shoreline? If the beds are interpreted as
associations in terms of the interpreted environment lake deposits, was the lake fed by glacial meltwater and
of deposition. An association of facies such as ‘sym- where were the glaciers? Which way was the wind
metrically rippled fine sandstone’, ‘black laminated blowing in the desert to produce those cross-bedded
mudstone’ and ‘grey graded siltstone’ may have sandstones, and where were the evaporitic salt pans
been interpreted as having been deposited in a lake that we see in some modern desert basins? The process