Focus on Sedimentary Environments

Ancient marine carbonate sands

Caswell Bay, Gower, South Wales, UK

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thin section PPL
enlarged image
Thin section in plane-polarized light. Scale bar is 1 mm long. Move the cursor over the image to see the labels.

Detailed petrography

Take a close look at the thin section in plane- and cross-polarized light, and at different magnifications. Then try to answer the questions below

Identify the dominant grain shape(s) - remember you are seeing a 2-D section through a 3-D rock:

equant (cubic or spherical)
oblate (disc-shaped)
prolate (rod or roller shaped)

What controls the shape of the ooids? Why are they not all spherical (look, for example, at the difference between C and D)? Think about this, then check your answer.

What is the material between the grains (at E, for example), and under what conditions did it form?

Look at the contacts between ooids at A and B. What does the nature of the contacts tell us about the diagenetic history of the rock?

Page © ; last updated 13 August, 2002