Page 98 - Sedimentology and Stratigraphy

Summary: Facies and Environments 85

of reconstructing these palaeoenvironments depends and the global sea level changes. The record of all
on the integration of various pieces of sedimentological these events is contained within sedimentary rocks,
and palaeontological information. because the changes will affect environments that
will in turn determine the character of the sedimentary
rocks deposited. If we can establish that an area that
5.7.1 Palaeoenvironments in space had once been a coastal plain of peat swamps changed
to being a region of shallow sandy seas, then we can
The first prerequisite of any palaeoenvironmental infer that either the sea level rose or the land subsided.
analysis is a stratigraphic framework, that is, a Similarly if a lake that had been a site of mud deposi-
means of determining which strata are of approxi- tion became a place where coarse detritus from a
mately the same age in different areas, which are mountainside formed an alluvial fan, we may conclude
older and which are younger. For this we require that there might have been a tectonic uplift in the area.
some means of dating and correlating rocks, and Our palaeoenvironmental reconstructions therefore
this involves a range of techniques that will be con- provide a series of pictures of the Earth’s surface that
sidered in Chapters 19 to 23. However, once we have we can then interpret in terms of large- and small-scale
established that we do have rocks that we know to be events. When palaeoenvironmental analysis is com-
of approximately the same age across an area, we can bined with stratigraphy in this way, the field of study is
apply three of the techniques discussed in this chapter known as basin analysis and is concerned with the
and consider them together. behaviour of the Earth’s crust and its interaction with
First, there is the distribution of facies and facies the atmosphere and hydrosphere. This topic is consid-
associations. If we can recognise where there are the ered briefly in Chapter 24.
deposits of an ancient river, where the delta was and As statedabove, one of the objectives offaciesanalysis
the location of the shoreline on the basis of the char- is to determine the environment of deposition of succes-
acteristics of the sedimentary rocks, then this will sions of rocks in the sedimentary record. A general
provide most of the information we need to draw a assumption is made that the range of sedimentary
picture of how the landscape looked at that time. This environments which exist today (Fig. 5.13) have existed
information can be supplemented by a second techni- in the past. In broad outline this is the case, but it should
que, which is the analysis of palaeocurrent data, be noted that there is evidence from the stratigraphic
which can provide more detailed information about record of conditions that existed during periods of Earth
the direction of flow of the ancient rivers and the history that have no modern counterparts.
positions of the delta channels relative to the ancient
shoreline. Third, provenance data can help us estab-
lish where the detritus came from, and help confirm 5.8 SUMMARY: FACIES AND
that the rivers and deltas were indeed connected (if ENVIRONMENTS
they contained sands of different provenance it would
indicate that they were separate systems). An objective, scientific approach is essential for suc-
This sort of analysis is extremely useful in making cessful facies analysis. A succession of sedimentary
predictions about the characteristics of rocks that can- strata should be first described in terms of the litho-
not be seen because they are covered by younger strata. facies (and sometimes biofacies and ichnofacies) pres-
Palaeoenvironmental reconstructions are therefore ent, at which stage interpretations of the processes
more than just an academic exercise, they are a predic- of deposition can be made. The facies can then be
tive tool that can be used to assess the distribution of the grouped into lithofacies associations which can be
subsurface geology and help search for aquifers, hydro- interpreted in terms of depositional environments on
carbon accumulations and mineral deposits. the basis of the combinations of physical, chemical
and biological processes that have been identified
from analysis of the facies. There are facies associa-
5.7.2 Palaeoenvironments in time tions and sequences that commonly occur in particu-
lar environments and these are illustrated in the
Over thousands and millions of years of geological following chapters as ‘typical’ of these environments.
time, climate changes, plates move, mountains rise However, there is a danger of making mistakes by