Examination of thin sections under the microscope is a key part of any study
of carbonate sediments, as a companion to field or core logging, and as a
necessary precursor to geochemical analysis. This book is designed as a
laboratory manual to keep beside the microscope as an aid to identifying grain
types and textures in carbonates. For the newcomer to the subject, carbonates
can display a bewildering variety of grains, compared to sandstones, for
example, and we hope this book will help to give confidence to those initial
observations. By illustrating more than one example of common grains and
textures, we hope that the more experienced practitioner will also find
assistance in identifying the unfamiliar. However, such is the diversity of
carbonate sediments, that it is impossible to be completely comprehensive and
if we have omitted your favourite bioclast, then sorry! Throughout we have
tried to show good, but typical rather than exceptional, examples of each
feature. It has not been our intention to supply much interpretation except
where this is necessary to explain the origin of features illustrated.
Two comments that we have received about previous atlases (Adams et al., 1984;
MacKenzie & Adams, 1994) are worth mentioning here. Firstly, it is possible to
claim that some photographs are over- or underexposed. Photography of
carbonate sediments can be difficult, especially where there are micritic
grains, which are almost opaque, set in a coarsely crystalline, clear sparite
cement. The exposure has to be adjusted, such that, if the micritic grains are
the subject of the picture, the cement may well appear overexposed, and if the
cement is the subject, the grains will be underexposed and appear almost
black. When using the microscope, the eye makes adjustments depending on what
you are looking at, and in any case it is easy, and often necessary with
carbonates, to vary the light intensity.
Secondly, we have been asked why we have not supplied a full petrographic
description of a rock. We do not believe that this can be usefully done from
a photograph, typically showing a field of view a few millimetres
across. Carbonate rocks can vary such that no one field of view is
representative of the whole rock This is particularly true of coarser
packstones and grainstones with a mixture of grain types. Those wanting a
format for a full petrographic description are directed to Flügel (1982) and
Harwood (1988).
We have avoided using abbreviations in the text, but in each plate caption we
have used the abbreviation 'PPL' for plane-polarised light and 'XPL' for
pictures taken with polare crossed. Finally, we hope that readers whatever
their level of interest in carbonate sedimentology, will, by browsing through
this atlas, be able to appreciate just a little of the wonder of the
geological world as revealed under the microscope, and share our excitement at
the beauty and variety of natural textures seen in carbonates.
