Geoscience Reference
In-Depth Information
changes in topography or sea-level will cause the system
to change its equilibrium profi le. By piecing together
evidence from terrestrial, shallow-marine and deep-marine
successions deposited at the same time we can gain a more
complete understanding of Earth processes.
6.5.1 Climate change
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Long-term climatic indicators include the presence or absence
of the following: coals, evaporites, palaeosols, plant megafossils
(Chapter 5), palynomorphs, foraminifers, diatoms, corals and
particular animal taxa that are latitudinally dependent. Three
main types of sedimentary rock are used to study and construct
short-term changes in climate through time : (1) fi ne-grained
marine sedimentary rocks deposited on the continental shelf
below storm wave-base or in the deep ocean; (2) fi ne-grained
lake deposits; and (3) other non-marine deposits such as
fi ne-grained wind-blown sediment (loess) and cave deposits.
These deposits are used because in general sedimentation is
continuous and fairly constant so they contain a fairly
unbroken record of past changes. Sedimentary deposits of the
same age can be used to determine, for example, the
temperature gradient over latitude or altitude. Because the
climate affects the chemistry and biota of the rocks laid down,
a range of geochemical proxies (e.g. C-, O-, Sr- and Os-isotopes,
Mg/Ca ratios and CaCO
3
content) has been developed to obtain
an indication of the changes in the climatic variables. Sections
6.2.1 and 13.1.3 provide information on obtaining a record
from this type of succession for these purposes.
6.5.2 Sequence stratigraphy and relative
sea-level change
The facies patterns, unconformity surfaces, geometry and
preservation of many shallow-marine and non-marine
sedimentary rock successions can be used to interpret changes
in relative sea-level. This works particularly well when it is
possible to correlate, with certainty, between shallow-marine
and deep-marine facies and even into non-marine successions
across sedimentary basins or wide regions. Sequence
stratigraphy provides a conceptual model for the detection of
past changes in relative sea-level change by predicting the
geometry and composition of sediment packages and surfaces
that form during a single cycle of relative sea-level change.
Sequence stratigraphic analysis is used widely to predict facies
distributions, to examine the sea-level history over a period of
time, to understand the genesis of the rock succession and
predict where gaps and condensed sections may be present. For
