Page 94 - Sedimentology and Stratigraphy

Interpreting Past Depositional Environments 81

indicated in the facies name and in other instances it that the sediments were deposited in shallow, clear,
may be important to emphasise different characteris- warm, seawater. The analysis is more complicated if
tics. In other situations the facies name for a very the sediments are the products of processes that can
similar rock might be ‘red, micaceous sandstone’ if occur in a range of settings. For example, cross-
the colour and grain types were considered to be more bedded sandstone can form during deposition in
important than the grain size and sedimentary struc- deserts, in rivers, deltas, lakes, beaches and shallow
tures. The full range of the characteristics of a rock seas: a ‘cross-bedded sandstone’ lithofacies would
would be given in the facies description that would therefore not provide us with an indicator of a specific
form part of any study of sedimentary rocks. environment.
If the description is confined to the physical and Interpretation of facies should be objective and
chemical characteristics of a rock this is referred to as based only on the recognition of the processes that
the lithofacies. In cases where the observations con- formed the beds. So, from the presence of symmetrical
centrate on the fauna and flora present, this is termed ripple structures in a fine sandstone it can be deduced
a biofacies description, and a study that focuses on that the bed was formed under shallow water with
the trace fossils (11.7) in the rock would be a descrip- wind over the surface of the water creating waves
tion of the ichnofacies. As an example a single rock that stirred the sand to form symmetrical wave rip-
unit may be described in terms of its lithofacies as a ples. The ‘shallow water’ interpretation is made
grey bioclastic packstone, as having a biofacies of because wave ripples do not form in deep water
echinoid and crinoids and with a ‘Cruziana’ ichnofa- (11.3) but the presence of ripples alone does not
cies: the sum of these and other characteristics would indicate whether the water was in a lake, lagoon or
constitute the sedimentary facies. shallow-marine shelf environment. The facies should
therefore be referred to as ‘symmetrically rippled
sandstone’ or perhaps ‘wave rippled sandstone’, but
5.6.2 Facies analysis not ‘lacustrine sandstone’ because further informa-
tion is required before that interpretation can be
The facies concept is not just a convenient means of made.
describing rocks and grouping sedimentary rocks seen
in the field, it also forms the basis for facies analysis,
a rigorous, scientific approach to the interpretation of 5.6.3 Facies associations
strata (Anderton 1985; Reading & Levell 1996;
Walker 1992; 2006). The lithofacies characteristics The characteristics of an environment are determined
are determined by the physical and chemical pro- by the combination of processes which occur there.
cesses of transport and deposition of the sediments A lagoon, for example, is an area of low energy,
and the biofacies and ichnofacies provide information shallow water with periodic influxes of sand from
about the palaeoecology during and after deposition. the sea, and is a specific ecological niche where only
By interpreting the sediment in terms of the physical, certain organisms live due to enhanced or reduced
chemical and ecological conditions at the time of salinity. The facies produced by these processes will be
deposition it becomes possible to reconstruct muds deposited from standing water, sands with wave
palaeoenvironments, i.e. environments of the past. ripples formed by wind over shallow water and a
The reconstruction of past sedimentary environ- biofacies of restricted fauna. These different facies
ments through facies analysis can sometimes be a form a facies association that reflects the deposi-
very simple exercise, but on other occasions it may tional environment (Collinson 1969; Reading & Levell
require a complex consideration of many factors 1996).
before a tentative deduction can be made. It is a When a succession of beds are analysed in this way,
straightforward process where the rock has charac- it is usually evident that there are patterns in the
teristics that are unique to a particular environment. distribution of facies. For example, on Fig. 5.12, do
As far as we know hermatypic corals have only ever beds of the ‘bioturbated mudstone’ occur more com-
grown in shallow, clear and fairly warm seawater: the monly with (above or below) the ‘laminated siltstone’
presence of these fossil corals in life position in a or the ‘wave rippled medium sandstone’? Which
sedimentary rock may therefore be used to indicate of these three occurs with the ‘coal’ facies? When