By A. E. Adams

Greater than half the world's petroleum is located in carbonate rocks, significantly within the heart East, the previous USSR, and North the USA. those rocks express a bewildering number of grains and textures, a result of wealth of other fossil organisms which give a contribution to their sedimentation and to the numerous diagenetic approaches which alter textures and vague the unique deposits and upholstery. cautious petrographic learn with a polarizing microscope is a key point for learning carbonate sediments, as a comparability to box or middle logging, and as a precursor to geochemical research. This atlas illustrates in complete colour quite a few gains of carbonate rocks and sediments, insurance way more broad than in any common textbook. it really is designed as a realistic advisor for opting for grain forms and textures in carbonates and should attraction alike to undergraduate and graduate scholars and to execs in either study and commercial laboratories.

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Extra info for A Colour Atlas of Carbonate Sediments and Rocks under the Microscope

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The straight frag­ ments are parts of bivalves and the chambered shell is a small gastropod. Foraminifera are sometimes confused with small gastropods, bur rhe former are dominantly calcitic and rarely preserved as casts in limestones. 64 Unstained thin section. Lower Carboniferous, South Wales, PPL, x 35. 65 Stained thin section, Lower Carboniferous, North Wales, PPL, x 25. 44 B io c la s t s In 64-66 no trace of the original shell structure is visible; the casts formed by complete solution of the aragonite shell, followed by precipitation of cement into the void at a later date.

53 Unstained thin section, 53 Lower Cretaceous, Middle East, PPL, x 18. 1 i‘ I|. I, ■ 54 Stained thin section, 54 Lower Cretaceous, Istrian Peninsula, PPL, x 17. 39 C a r b o n a t e S e d im e n t s a n d R o c k s U n d e r th e M ic r o s c o p e In pre-Quaternary limestones, most originally aragonitic bivalves are preserved as moulds or casts. 55 and 56 show two curvcd bivalve shells that have suffered from solution and were therefore likely to have been originally composed of aragonite. In the view taken with polars crosscd and the sensitive tint plate inserted, porosity shows up a violet colour.

Such structures look clear with plane-polar­ ised light, tend to stain an even, but fairly dense colour, and with polars crossed show an extinction shadow which moves through the fragment as the microscope stage is rotated. Apart from molluscs, the structure is found in trilobites, but there is unlikely to be confusion since trilobites are exclu­ sively Palaeozoic and bivalves with a homogeneous structure are essentially Mesozoic and Cenozoic. 49 and 50 show a bivalve with a two-layered structure.

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