Page 93 - Sedimentology and Stratigraphy

80 Field Sedimentology, Facies and Environments

resources and for pure academic research purposes. hamper interpretation of the strata. There is, how-
When a hole is drilled it is not necessarily the case ever, a distinct advantage of core in that it usually
that core will be cut. Oil companies tend to rely on provides a continuity of vertical section over tens or
geophysical techniques to analyse the strata (22.4) hundreds of metres. Even some of the best natural
and only cut core if details of particular horizons are exposures of strata do not provide this 100% cover-
required. In contrast, an exploration programme for age, because beds are weathered away or covered by
coal will typically involve cutting core through the scree or vegetation.
entire hole because they need to know precisely Sedimentary data from core are recorded in the
where coal beds are and sample them for quality. same way as strata in outcrop by using graphic sedi-
After it has been cut, core is stored in boxes in mentary logs. Although recording field data is still
lengths of about a metre. The core cut by the oil important where it is possible to do so, it is also fair
companies is typically between 100 and 200 mm dia- to say that geologists in industry working on sedimen-
meter and is split vertically to provide a flat face tary rocks will probably spend more time logging core
(Fig. 5.11), but coal core is left whole, and is usually than doing fieldwork.
narrower, only 60 mm in diameter. When compared
with outcrop, the obvious drawback of any core is
that it is so narrow, and provides only a one-dimen- 5.6 INTERPRETING PAST
sional sample of the strata. Features that can be DEPOSITIONAL ENVIRONMENTS
picked out by looking at two-dimensional exposure
across a quarry or cliff face, such as river channels, Sediments accumulate in a wide range of settings that
reefs, or even some cross-bedding, can only be imag- can be defined in terms of their geomorphology, such
ined when looking at core. This limitation tends to as rivers, lakes, coasts, shallow seas, and so on. The
physical, chemical and biological processes that shape
and characterise those environments are well known
through studies of physical geography and ecology.
Those same processes determine the character of the
sediment deposited in these settings. A fundamental
part of sedimentology is the interpretation of sedimen-
tary rocks in terms of the transport and depositional
processes and then determining the environment in
which they were deposited. In doing so a sedimentol-
ogist attempts to establish the conditions on the sur-
face of the Earth at different times in different places
and hence build up a picture of the history of the
surface of the planet.

5.6.1 The concept of ‘facies’

The term ‘facies’ is widely used in geology, particu-
larly in the study of sedimentology in which sedimen-
tary facies refers to the sum of the characteristics of a
sedimentary unit (Middleton 1973). These character-
istics include the dimensions, sedimentary structures,
grain sizes and types, colour and biogenic content of
the sedimentary rock. An example would be ‘cross-
bedded medium sandstone’: this would be a rock con-
Fig. 5.11 When drilling through strata it is possible to sisting mainly of sand grains of medium grade, exhi-
recover cylinders of rock that are cut vertically to reveal the biting cross-bedding as the primary sedimentary
details of the beds. structure. Not all aspects of the rock are necessarily