A.2.3. Stratigraphy
Stratigraphy is the study of rock strata (layers of sedimentary rock or layered igneous rocks), particularly
the sequences of layers, and the information this provides on the geological history and relative ages of a
region.
Geologists tend to view rock strata in terms of a stratigraphic sequence. When interpreting a sequence of
sedimentary rocks, it is important to understand that each sedimentary layer represents an instant in time
and as such, each layer has a specific chronology or point in geological time.
This concept can be used as a rough gauge of the passage of time. For example, if it is observed that
muddy sediments accumulate at a rate of about 10 metres per million years, then 100 metres of mudstone
rock would have originally taken 10 million years to accumulate (100 metres ÷ 10 metres/million years =
10 million years).
Obviously there are errors associated with such an estimate because:
1. a constant rate of deposition of sediment cannot always be assumed. There may be periods where
sediment deposition is high or doesn't occur at all;
2. sediment might have been actively eroded from the sedimentary pile before it was lithified into rock;
and/or
3. deposition of large volumes of sediment causes compaction (squashing) of the sedimentary pile
resulting in an underestimate of the total amount of sediment accumulated.
Sedimentary rocks can be regarded like the pages of a book, with each sedimentary layer recording
details of Earth's environment at the time of deposition of the original sediment. Sediments will often
contain fossils, the evolution of which also provides the geologist with another valuable relative dating
tool.
Guiding Stratigraphic Principles
Rock layers or “strata” are deposited according to specific Geological Principles. It is these Principles that
help us figure out the relative order of events that occurred when the rocks were deposited.