Page 43 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 8 1 , 82, 83
P i s o i d s a n d O n c o i d s

The nomenclature of carbonate grains which arc larger
than 2 mm in diameter and have an outer layer with
concentric laminae, depends to a certain extent on an
interpretation of their origin. Thus the term pisoid or
pi.wlith usually refers to grains presumed to have formed
inorganically, usually in a subaerial environment. On the
other hand, oncoids. or oncoliflrs, are presumed to be
biogenic. blue-green algae on the grain surfaces, trapping
and binding fine sediment particles.
81 is a photograph of a polished rock surface showing
oncoids. Note the size of the grains, the asymmetrical
growth and the wavy nature of many of the laminae, all
f e atures characteristic of oncoids. The bluish-grey areas
arc sparry calcite and the orange-brown areas arc stained
with iron oxides.
82 and 83 show concentrically-laminated grains whose
origins are more difficult to interpret. 82 shows grains
which arc about 2 mm in diameter. The outer surfaces arc
not as smooth as most ooids, although the concentric
lamination is very regular. Grains in the upper right show
irregular outer coats of micrite and some particles have
apparently grown together to form compound grains (e.g.
lower left). This latter f e ature is unlikely to occur in ooids,
where precipitated carbonate laminae are formed while
the grain is held in suspension. These grains are therefore
interpreted as oncoids. The cement is sparite. This
photograph is of a thin section made by Sorby in 1849 and
illustrates the high quality of his sections.
83 shows grains with a regular. well-defined concentric
layering. in grains up to 5 mm in diameter. This is typical
of inorganic growth and these grains may be pisoids.
Pisoid� are commonly fractured or broken. Broken pieces
can be seen towards the top right of the photograph.

81: Polis/red s w f ace. L/anel�r F o rmation. Lower Carbo­
niferous. Blaen Onneu, South Wales: magnification x I .8.
82: Unstai11ed thin section. Wenlock Limestone. Silurian,
Mafl•em Hills. En�land: magnification x 13. PPL.
83: Stai11ed tlli11 section, L01rer Jurassic. Greece: magn(/ic­
atioll x I I . PPI�.

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