Page 15 - Sedimentology and Stratigraphy

2 Introduction: Sedimentology and Stratigraphy

used to make estimates (to varying degrees of accu-
racy) of the physical, chemical and biological condi-
tions that existed at the time of sedimentation. These
conditions may include the salinity, depth and flow
velocity in lake or seawater, the strength and direction
of the wind in a desert and the tidal range in a shallow
marine setting.

1.2 SEDIMENTARY ENVIRONMENTS
AND FACIES

The environment at any point on the land or under the
sea can be characterised by the physical and chemical
processes that are active there and the organisms that
live under those conditions at that time. As an exam-
ple, a fluvial (river) environment includes a channel
confining the flow of fresh water that carries and
deposits gravelly or sandy material on bars in the
channel (Fig. 1.1). When the river floods, water
spreads relatively fine sediment over the floodplain
where it is deposited in thin layers. Soils form and
vegetation grows on the floodplain area. In a succes-
sion of sedimentary rocks (Fig. 1.2) the channel may
be represented by a lens of sandstone or conglomerate
that shows internal structures formed by deposition on
the channel bars. The floodplain setting will be repre-
sented by thinly bedded mudrock and sandstone with
roots and other evidence of soil formation.
In the description of sedimentary rocks in terms of Fig. 1.1 A modern depositional environment: a sandy
depositional environments, the term ‘facies’ is often river channel and vegetated floodplain.
used. A rock facies is a body of rock with specified
characteristics that reflect the conditions under
which it was formed (Reading & Levell 1996).
Describing the facies of a body of sediment involves
documenting all the characteristics of its lithology,
texture, sedimentary structures and fossil content
that can aid in determining the processes of forma-
tion. By recognising associations of facies it is possible
to establish the combinations of processes that were
dominant; the characteristics of a depositional envi-
ronment are determined by the processes that are
present, and hence there is a link between facies
associations and environments of deposition. The
lens of sandstone in Fig. 1.2 may be shown to be a
river channel if the floodplain deposits are found asso-
Fig. 1.2 Sedimentary rocks interpreted as the deposits of a
ciated with it. However, recognition of a channel form
river channel (the lens of sandstones in the centre right of the
on its own is not a sufficient basis to determine the view) scoured into mudstone deposited on a floodplain (the
depositional environment because channels filled darker, thinly bedded strata below and to the side of the
with sand exist in other settings, including deltas, sandstone lens).