Page 16 - Sedimentology and Stratigraphy

Stratigraphy 3

tidal environments and the deep sea floor: it is the oceans and where changes in the atmosphere affect
association of different processes that provides the full the climate, perhaps even on a human time scale. To
picture of a depositional environment. understand how these global systems work, we need a
record of their past behaviour to analyse, and this is
provided by the study of stratigraphy.
1.3 THE SPECTRUM OF Stratigraphy provides the temporal framework for
ENVIRONMENTS AND FACIES geological sciences. The relative ages of rocks, and
hence the events that are recorded in those rocks, can
Every depositional environment has a unique combi- be determined by simple stratigraphic relationships
nation of processes, and the products of these pro- (younger rocks generally lie on top of older, as Steno
cesses, the sedimentary rocks, will be a similarly recognised), the fossils that are preserved in strata and
unique assemblage. For convenience of description and by measurements of processes such as the radioactive
interpretation, depositional environments are classi- decay of elements that allow us to date some rock units.
fied as, for example, a delta, an estuary or a shoreline, At one level, stratigraphy is about establishing a
and subcategories of each are established, such as wave- nomenclature for rock units of all ages and correlating
dominated, tide-dominated and river-dominated del- them all over the world, but at another level it is about
tas. This approach is in general use by sedimentary finding the evidence for climate change in the past or
geologists and is followed in this book. It is, however, the movements of tectonic plates. One of the powerful
important to recognise that these environments of tools we have for predicting future climate change is
deposition are convenient categories or ‘pigeonholes’, the record in the rock strata of local and global changes
and that the description of them tends to be of ‘typical’ over periods of thousands to millions of years. Further-
examples. The reality is that every delta, for example, is more our understanding of evolutionary processes is in
different from its neighbour in space or time, that every part derived from the study of fossils found in rocks of
deltaic deposit will also be unique, and although we different ages that tell us about how forms of life have
categorise deltas into a number of types, our deposit is changed through time. Other aspects of stratigraphy
likely to fall somewhere in between these ‘pigeon- provide the tools for finding new resources: for exam-
holes’. Sometimes it may not even be possible to con- ple, ‘sequence stratigraphy’ is a predictive technique,
clusively distinguish between the deposits of a delta widely used in the hydrocarbon industry, that can be
and an estuary, especially if the data set is incomplete, used to help to find new reserves of oil and gas.
which it inevitably is when dealing with events of the The combination of sedimentology and stratigraphy
past. However, by objectively considering each bed in allows us to build up pictures of the Earth’s surface at
terms of physical, chemical and biological processes, it different times in different places and relate them to
is always possible to provide some indication of where each other. The character of the sedimentary rocks
and how a sedimentary rock was formed. deposited might, for example, indicate that at one
time a certain area was an arid landscape, with desert
dunes and with washes of gravel coming from a nearby
1.4 STRATIGRAPHY mountain range. In that same place, but at a later time,
conditions allowed the formation of coral reefs in a
Use of the term ‘stratigraphy’ dates back to d’Orbingy shallow sea far away from any landmass, and we can
in 1852, but the concept of layers of rocks, or strata, find the record of this change by interpreting the rocks
representing a sequence of events in the past is much in terms of their processes and environments of deposi-
older. In 1667 Steno developed the principle of super- tion. Furthermore, we might establish that at the same
position: ‘in a sequence of layered rocks, any layer is time as there were shallow tropical seas in one place,
older than the layer next above it’. Stratigraphy can be there lay a deep ocean a few tens of kilometres away
considered as the relationship between rocks and time where fine sediment was deposited by ocean currents.
and the stratigrapher is concerned with the observa- We can thus build up pictures of the palaeogeogra-
tion, description and interpretation of direct and tan- phy, the appearance of an area during some time in
gible evidence in rocks to determine the history of the the past, and establish changes in palaeogeography
Earth. We all recognise that our planet is a dynamic through Earth history. To complete the picture, the
place, where plate tectonics creates mountains and distribution of different environments and their