Geoscience Reference
In-Depth Information
The other common types of carbonate are:
•
Dolomite:
Dolomite has three fi eld characteristics that
distinguish it from other carbonates: (1) it tends to be a pale
yellowy-brown colour; (2) it reacts only slowly with dilute
hydrochloric acid; and (3) if ground between the teeth its
texture is softer than that of limestone and similar to that of
royal icing.
•
Siderite:
This is most easily distinguished by its distinctive
yellow-red colour (terracotta); it forms both bands and
nodules, particularly in mudrock successions.
6
There is no well-established classifi cation scheme for mixed
siliciclastic/carbonate successions and a combination of the
various classifi cation schemes is typically used. The type of
sediment, together with its depositional environment, governs
the synsedimentary and post-sedimentary structures, and
infl uences the preservation of associated features such as
fossils. The common features of each sedimentary rock type to
look for are summarized in Table 6.1 (pp. 110-111).
6.2.2 Recording sedimentary structures
Sedimentary structures are varied and complex. They are dealt
with in detail in many geological textbooks and in specialist
sedimentological textbooks (Section 6.6). A full coverage is
beyond the scope of this topic, so instead this section
concentrates on how to: (1) record and describe them;
(2) distinguish between structures that look similar; and
(3) decipher cross-cutting relationships.
Sedimentary structures provide very direct clues about the
processes responsible for deposition of a sedimentary rock, and
the processes that occur after its deposition. In a number of
cases they even provide specifi c information on the
depositional setting, e.g. hummocky cross-stratifi cation (HCS) is
diagnostic of the offshore transition zone, which lies between
fair-weather wave-base and storm wave-base, whereas swaley
cross-stratifi cation (SCS) forms close to fair-weather wave-base
in the shoreface zone.
In the fi eld you are likely to see structures that you don't
understand; this may be because they are unusual, poorly
developed, poorly preserved or exposed at a strange angle. So
don't worry if you can't identify every sedimentary structure
you see - this is not unusual. If you fi nd something you don't
understand, search uround for other, possibly better/more
revealing examples, preferably in the same unit but also in
adjacent units. Then spend more time on the best examples
fi rst, returning to the more enigmatic ones later if there is time.
